''Peloneustes'' (meaning ) is a
genus
Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of
pliosaurid plesiosaur
The Plesiosauria or plesiosaurs are an Order (biology), order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia.
Plesiosaurs first appeared in the latest Triassic Period (geology), Period, possibly in the Rhaetian st ...
from the
Middle Jurassic
The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...
of England. Its remains are known from the Peterborough Member of the
Oxford Clay Formation, which is
Callovian
In the geologic timescale, the Callovian is an age and stage in the Middle Jurassic, lasting between 165.3 ± 1.1 Ma (million years ago) and 161.5 ± 1.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the ...
in age. It was originally described as a
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
of ''
Plesiosaurus
''Plesiosaurus'' (Greek: ' ('), near to + ' ('), lizard) is a genus of extinct, large marine sauropterygian reptile that lived during the Early Jurassic. It is known by nearly complete skeletons from the Lias of England. It is distinguishable by ...
'' by palaeontologist
Harry Govier Seeley
Harry Govier Seeley (18 February 1839 – 8 January 1909) was a British paleontologist.
Early life
Seeley was born in London on 18 February 1839, the second son of Richard Hovill Seeley, a goldsmith, and his second wife Mary Govier. When his fa ...
in 1869, before being given its own genus by naturalist
Richard Lydekker
Richard Lydekker (; 25 July 1849 – 16 April 1915) was a British naturalist, geologist and writer of numerous books on natural history. He was known for his contributions to zoology, paleontology, and biogeography. He worked extensively in cata ...
in 1889. While many species have been assigned to ''Peloneustes'', ''P. philarchus'' is currently the only one still considered valid, with the others moved to different genera, considered ''
nomina dubia
In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application.
Zoology
In case of a ''nomen dubium,'' it may be impossible to determine whether a ...
'', or
synonymised with ''P. philarchus''. Some of the material formerly assigned to ''P. evansi'' has since been reassigned to
"''Pliosaurus''" ''andrewsi''. ''Peloneustes'' is known from many specimens, including some very complete material.
With a total length of , ''Peloneustes'' is not a large pliosaurid. It had a large, triangular skull, which occupied about a fifth of its body length. The front of the skull is elongated into a narrow
rostrum
Rostrum may refer to:
* Any kind of a platform for a speaker:
**dais
**pulpit
** podium
* Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects
* Rostrum (ship), a form of bow on naval ...
(snout). The mandibular
symphysis
A symphysis (, : symphyses) is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint.
# A symphysis is an amphiarthrosis, a slightly movable joint.
# A growing together o ...
, where the front ends of each side of the
mandible
In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla).
The jawbone i ...
(lower jaw) fuse, is elongated in ''Peloneustes'', and helped strengthen the jaw. An elevated ridge is located between the tooth rows on the mandibular symphysis. The teeth of ''Peloneustes'' are conical and have circular cross-sections, bearing vertical ridges on all sides. The front teeth are larger than the back teeth. With only 19 to 21
cervical (neck) vertebrae, ''Peloneustes'' had a short neck for a plesiosaur. The limbs of ''Peloneustes'' were modified into flippers, with the back pair larger than the front.
''Peloneustes'' has been interpreted as both a close relative of ''
Pliosaurus
''Pliosaurus'' (meaning 'more lizard') is an extinct genus of thalassophonean pliosaurid known from the Late Jurassic (Kimmeridgian and Tithonian stages) of Europe and South America. This genus has contained many species in the past but recent ...
'' or as a more
basal (early-diverging) pliosaurid within
Thalassophonea
Pliosauridae is a family of plesiosaurian marine reptiles from the Latest Triassic to the early Late Cretaceous (Rhaetian to Turonian stages). The family is more inclusive than the archetypal short-necked large headed species that are placed in ...
, with the latter interpretation finding more support. Like other plesiosaurs, ''Peloneustes'' was well-adapted to aquatic life, using its flippers for a method of swimming known as
subaqueous flight. Pliosaurid skulls were reinforced to better withstand the stresses of feeding. The long, narrow snout of ''Peloneustes'' could have been swung quickly through the water to catch fish, which it pierced with its numerous sharp teeth. ''Peloneustes'' would have inhabited an
epicontinental (inland) sea that was around deep. It shared its habitat with a variety of other animals, including
invertebrate
Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...
s, fish,
thalattosuchians,
ichthyosaur
Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides.
Ichthyosaurians thrived during much of the Mesozoic era; based on fo ...
s, and other plesiosaurs. At least five other pliosaurids are known from the Peterborough Member, but they were quite varied in anatomy, indicating that they would have eaten different food sources, thereby avoiding competition.
History of research

The
strata
In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...
of the Peterborough Member of the
Oxford Clay Formation have long been mined for
brickmaking
A brick is a type of construction material used to build walls, pavements and other elements in masonry construction. Properly, the term ''brick'' denotes a unit primarily composed of clay. But is now also used informally to denote building un ...
. Ever since the late 19th century, when these operations began, the
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
s of many marine animals have been excavated from the rocks.
Among these was the specimen which would become the
holotype
A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
of ''Peloneustes philarchus'', discovered by geologist Henry Porter in a
clay pit
A clay pit is a quarry or Mining, mine for the extraction of clay, which is generally used for manufacturing pottery, bricks or Portland cement. Quarries where clay is mined to make bricks are sometimes called brick pits.
A brickyard or brickwor ...
close to
Peterborough
Peterborough ( ) is a City status in the United Kingdom, cathedral city in the City of Peterborough district in the Ceremonial counties of England, ceremonial county of Cambridgeshire, England. The city is north of London, on the River Nene. A ...
, England. The specimen includes a
mandible
In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla).
The jawbone i ...
, the front part of the upper jaw, various
vertebrae
Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...
from throughout the body, elements from the
shoulder girdle
The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans, it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists o ...
and
pelvis
The pelvis (: pelves or pelvises) is the lower part of an Anatomy, anatomical Trunk (anatomy), trunk, between the human abdomen, abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also c ...
,
humeri
The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of ...
(upper arm bones),
femora
The femur (; : femurs or femora ), or thigh bone is the only bone in the thigh — the region of the lower limb between the hip and the knee. In many four-legged animals the femur is the upper bone of the hindleg.
The top of the femur fits in ...
(upper leg bones), and various other limb bones.
In 1866, geologist
Adam Sedgwick
Adam Sedgwick FRS (; 22 March 1785 – 27 January 1873) was a British geologist and Anglican priest, one of the founders of modern geology. He proposed the Cambrian and Devonian period of the geological timescale. Based on work which he did ...
purchased the specimen for the
University of Cambridge
The University of Cambridge is a Public university, public collegiate university, collegiate research university in Cambridge, England. Founded in 1209, the University of Cambridge is the List of oldest universities in continuous operation, wo ...
's
Woodwardian Museum
The Sedgwick Museum of Earth Sciences, is the geology museum of the University of Cambridge. It is part of the Department of Earth Sciences and is located on the university's Downing Site in Downing Street, central Cambridge, England. The Sedg ...
(now the Sedgwick Museum of Earth Sciences, Cambridge),
with the specimen being catalogued as CAMSM J.46913 and stored in the university's lecture room within cabinet D.
Palaeontologist
Harry Govier Seeley
Harry Govier Seeley (18 February 1839 – 8 January 1909) was a British paleontologist.
Early life
Seeley was born in London on 18 February 1839, the second son of Richard Hovill Seeley, a goldsmith, and his second wife Mary Govier. When his fa ...
described the specimen as a new species of the preexisting genus ''Plesiosaurus'', ''
Plesiosaurus
''Plesiosaurus'' (Greek: ' ('), near to + ' ('), lizard) is a genus of extinct, large marine sauropterygian reptile that lived during the Early Jurassic. It is known by nearly complete skeletons from the Lias of England. It is distinguishable by ...
philarchus'', in 1869.
The specific name means , possibly due to its large, powerful skull.
Seeley did not describe this specimen in detail, mainly just giving a list of the known material.
While later publications would further describe these remains, CAMSM J.46913 remained poorly described.
Alfred Leeds and his brother Charles Leeds had been collecting fossils from the Oxford Clay since around 1867, encouraged by geologist
John Phillips of the
University of Oxford
The University of Oxford is a collegiate university, collegiate research university in Oxford, England. There is evidence of teaching as early as 1096, making it the oldest university in the English-speaking world and the List of oldest un ...
, assembling what became known as the Leeds Collection. While Charles eventually left, Alfred, who collected the majority of the specimens, continued to gather fossils until 1917. Eventually, after a visit by
Henry Woodward of the
British Museum of Natural History
The Natural History Museum in London is a museum that exhibits a vast range of specimens from various segments of natural history. It is one of three major museums on Exhibition Road in South Kensington, the others being the Science Museum (Lo ...
(now the Natural History Museum in London) to Leeds' collection in
Eyebury in 1885, the museum bought around of fossils in 1890. This brought Leeds' collection to wider renown, and he would later sell specimens to museums throughout Europe, and even some in the United States.
The carefully prepared material was usually in good condition, although it quite frequently had been crushed and broken by geological processes. Skulls were particularly vulnerable to this.

Naturalist
Richard Lydekker
Richard Lydekker (; 25 July 1849 – 16 April 1915) was a British naturalist, geologist and writer of numerous books on natural history. He was known for his contributions to zoology, paleontology, and biogeography. He worked extensively in cata ...
was informed of a plesiosaur skeleton in the British Museum of Natural History by geologist
George Charles Crick, who worked there. The specimen, catalogued under NHMUK PV R1253,
had been discovered in the Oxford Clay Formation in
Green End, Kempston, near
Bedford
Bedford is a market town in Bedfordshire, England. At the 2011 Census, the population was 106,940. Bedford is the county town of Bedfordshire and seat of the Borough of Bedford local government district.
Bedford was founded at a ford (crossin ...
. While Lydekker speculated that the skeleton was once complete, it was damaged during excavation. The limb girdles had been heavily fragmented when the specimen arrived at the museum, but a worker named Lingard in the Geology Department managed to restore much of them. In addition to the limb girdles, the specimen also consists of a partial mandible, teeth, multiple vertebrae (although none from the neck), and much of the limbs. Lydekker identified this specimen as an individual of ''Plesiosaurus philarchus'' and published a description of it in 1889. After studying this and other specimens in the Leeds Collection, he concluded that
plesiosaurs
The Plesiosauria or plesiosaurs are an Order (biology), order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia.
Plesiosaurs first appeared in the latest Triassic Period (geology), Period, possibly in the Rhaetian st ...
with shortened necks and large heads could not be classified as species of ''Plesiosaurus'', meaning that ''"P." philarchus'' belonged to a different genus. He initially assigned it to ''
Thaumatosaurus'' in 1888,
but later decided that it was distinct enough to warrant its own genus, which he named ''Peloneustes'' in his 1889 publication.
The name ''Peloneustes'' comes from the Greek words , meaning or , in reference to the Oxford Clay Formation, and , meaning .
Seeley, however, lumped ''Peloneustes'' into ''
Pliosaurus
''Pliosaurus'' (meaning 'more lizard') is an extinct genus of thalassophonean pliosaurid known from the Late Jurassic (Kimmeridgian and Tithonian stages) of Europe and South America. This genus has contained many species in the past but recent ...
'' in 1892, claiming that the two were insufficiently different to warrant separate genera.
Seeley and Lydekker could not agree on which genus to classify ''P. philarchus'' in, representing part of a feud between the two scientists. However, ''Peloneustes'' has since become the accepted name.

The Leeds Collection contained multiple ''Peloneustes'' specimens.
In 1895, palaeontologist
Charles William Andrews
Charles William Andrews (30 October 1866 – 25 May 1924) F.R.S., was a British palaeontologist whose career as a vertebrate paleontologist, both as a curator and in the field, was spent in the services of the British Museum, Department of Ge ...
described the anatomy of the skull of ''Peloneustes'' based on four partial skulls in the Leeds Collection.
In 1907, geologist Frédéric Jaccard published a description of two ''Peloneustes'' specimens from the Oxford Clay near Peterborough, housed in the Musée Paléontologique de Lausanne,
Switzerland
Switzerland, officially the Swiss Confederation, is a landlocked country located in west-central Europe. It is bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. Switzerland ...
. The more complete of the two specimens includes a complete skull preserving both jaws; multiple isolated teeth; 13
cervical (neck), 5 pectoral (shoulder), and 7 caudal (tail) vertebrae; ribs; both scapulae, a
coracoid
A coracoid is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is present as part of the scapula, but this is n ...
; a partial interclavicale; a complete pelvis save for an
; and all four limbs, which were nearly complete. The other specimen preserved 33 vertebrae and some associated ribs. Since the specimen Lydekker described was in some need of restoration, and missing information was filled in with data from other specimens in his publication, Jaccard found it pertinent to publish a description containing photographs of the more complete specimen in Lausanne to better illustrate the anatomy of ''Peloneustes''.
In 1913, naturalist Hermann Linder described multiple specimens of ''Peloneustes philarchus'' housed in the Institut für Geowissenschaften,
University of Tübingen
The University of Tübingen, officially the Eberhard Karl University of Tübingen (; ), is a public research university located in the city of Tübingen, Baden-Württemberg, Germany.
The University of Tübingen is one of eleven German Excellenc ...
and
State Museum of Natural History Stuttgart
The State Museum of Natural History Stuttgart (), abbreviated SMNS, is one of the two state of Baden-Württemberg's natural history museums. Together with the State Museum of Natural History Karlsruhe (Staatliches Museum für Naturkunde Karls ...
, Germany. These specimens had also come from the Leeds Collection.
Among the specimens he described from the former institution was a nearly complete mounted skeleton, lacking two cervical vertebrae, some caudal vertebrae from the end of the tail, and some
distal
Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...
phalanges. Only the rear part of the
cranium
The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate.
In the human, the skull comprises two prominent ...
was in good condition, but the mandible was mostly undamaged. Another of the specimens Linder described was a well-preserved skull (GPIT RE/3409),
also from the University of Tübingen, preserving a
sclerotic ring
The scleral ring or sclerotic ring is a hardened ring of plates, often derived from bone, that is found in the eyes of many animals in several groups of vertebrates. Some species of mammals, amphibians, and crocodilians lack scleral rings. The rin ...
(the set of small bones that support the eye), only the fourth time these bones had been reported in a plesiosaur.
Andrews later described the
marine reptile
Marine reptiles are reptiles which have become secondarily adapted for an aquatic or semiaquatic life in a marine environment. Only about 100 of the 12,000 extant reptile species and subspecies are classed as marine reptiles, including mari ...
specimens of the Leeds Collection that were in the British Museum of Natural History, publishing two volumes, one in 1910 and the other in 1913. The anatomy of the ''Peloneustes'' specimens was described in the second volume, based primarily on the well-preserved skulls NHMUK R2679 and NHMUK R3808 and NHMUK R3318, an almost complete skeleton. NHMUK R3318 was so well preserved that it could be rearticulated and mounted, although the missing parts of the pelvis and limbs had to be filled in. The mounted skeleton was put on display in the museum's Gallery of Fossil Reptiles.
Andrews had described this mount in 1910, remarking that it was the first skeletal mount of a
pliosaurid, thus providing important information about the overall anatomy of the group.
In 1960, palaeontologist
Lambert Beverly Tarlo published a review of pliosaurid species that had been reported from the
Upper Jurassic
The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987.
In European lithostratigraphy, the name ...
. Many pliosaurids species had been named based on isolated fragments, creating confusion. Tarlo also found that inaccurate descriptions of the material and palaeontologists ignoring each other's work only made this confusion worse. Of the 36 species he reviewed, he found only nine of them to be valid, including ''Peloneustes philarchus''.
In 2011, palaeontologists Hilary Ketchum and Roger Benson described the anatomy of the skull of ''Peloneustes''. Since the previous anatomical studies of Andrews and Linder, more specimens had been found, including NHMUK R4058, a skull preserved in three dimensions, providing a better idea of the skull's shape.
Formerly assigned species and specimens
Many further species have been assigned to ''Peloneustes'' throughout its
taxonomic
280px, Generalized scheme of taxonomy
Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme of classes (a taxonomy) and the allocation ...
history, but these have all since been reassigned to different genera or considered invalid.
In the same publication in which he named ''P. philarchus'', Seeley also named another
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
of ''Plesiosaurus'', ''P. sterrodeirus'' based on seven specimens in the Woodwardian Museum consisting of cranial and vertebral material.
When Lydekker erected the genus ''Peloneustes'' for ''P. philarchus'', he also reclassified ''"Plesiosaurus" sterrodeirus'' and ''"
Pleiosaurus" aequalis'' (a species named by John Phillips in 1871)
as members of this genus.
In his 1960 review of pliosaurid
taxonomy
image:Hierarchical clustering diagram.png, 280px, Generalized scheme of taxonomy
Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme o ...
, Tarlo considered ''P. aequalis'' to be invalid, since it was based on
propodials (upper limb bones), which cannot be used to differentiate different pliosaurid species. He considered ''Peloneustes sterrodeirus'' to instead belong to ''Pliosaurus'', possibly within ''P. brachydeirus''.

Another of the species described by Seeley in 1869 was ''Pliosaurus evansi'', based on specimens in the Woodwardian Museum.
These consisted of cervical and
dorsal (back) vertebrae, ribs, and a coracoid. Due to it being a smaller species of ''Pliosaurus'' and its similarity to ''Peloneustes philarchus'', Lydekker reassigned it to ''Peloneustes'' in 1890, noting that it was larger than ''Peloneustes philarchus''.
He also thought that a large mandible and paddle attributed to ''Pleiosaurus ?grandis'' by Phillips in 1871
belonged to this species instead.
In 1913, Andrews assigned a partial skeleton of another large pliosaur found by Leeds to ''Peloneustes evansi'', noting that while the mandible and vertebrae were similar to other ''Peloneustes evansi'' specimens, they were quite different from those of ''Peloneustes philarchus''. Consequently, Andrews considered it possible that ''P. evansi'' really belonged to a separate genus that was morphologically intermediate between ''Peloneustes'' and ''Pliosaurus''.
In his 1960 review of pliosaurids, Tarlo
synonymised ''Peloneustes evansi'' with ''Peloneustes philarchus'' due to their cervical vertebrae being identical (save for a difference in size). He considered the larger specimens of ''Peloneustes evansi'' distinct, and assigned them to a new species of ''Pliosaurus'', ''
P. andrewsi'' (although this species is no longer considered to belong in ''Pliosaurus''
).
Hilary F. Ketchum and Roger B. J. Benson disagreed with this synonymy, and in 2011 considered that since the holotype of ''Peloneustes evansi'' is nondiagnostic (lacking distinguishing features), ''P. evansi'' is a ''
nomen dubium
In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application.
Zoology
In case of a ''nomen dubium,'' it may be impossible to determine whether a ...
'' and therefore an indeterminate pliosaurid.
Palaeontologist
Ernst Koken described in 1905 another species of ''Peloneustes'', ''P. kanzleri'', from the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
Wealden Group
The Wealden Group, occasionally also referred to as the Wealden Supergroup, is a group (stratigraphy), group (a sequence of rock strata) in the lithostratigraphy of southern England. The Wealden group consists of wiktionary:paralic, paralic to c ...
of northern Germany.
In 1960, Tarlo reidentified this species as an
elasmosaurid
Elasmosauridae, often called elasmosaurs or elasmosaurids, is an extinct family of plesiosaurs that lived from the Hauterivian stage of the Early Cretaceous to the Maastrichtian stage of the Late Cretaceous period (c. 130 to 66 mya). The taxo ...
.
In 1913, Linder created a subspecies of ''Peloneustes'', ''P. philarchus'' var. ''spathyrhynchus'', differentiating it based on its spatulate mandibular
symphysis
A symphysis (, : symphyses) is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint.
# A symphysis is an amphiarthrosis, a slightly movable joint.
# A growing together o ...
(where the two sides of the mandible meet and fuse).
Tarlo considered it to be a synonym of ''Peloneustes philarchus'' in 1960,
and the mandibular symphysis of ''Peloneustes'' is proportionately wider in larger specimens, making this trait more likely to be due to
intraspecific variation
Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species.
Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organism ...
(variation within species). Crushing makes accurate measurement of these proportions difficult.
In 1948, palaeontologist
Nestor Novozhilov
Nestor Ivanovich Novozhilov was a Soviet paleontologist. In 1948, Novozhilov described a pliosaur specimen discovered on the banks of Russia's Volga Riveras a new species, '' Pliosaurus rossicus''. The specimen, while large, was damaged during the ...
named a new species of ''Peloneustes'', ''P. irgisensis'', based on PIN 426, a partial skeleton consisting of a large, incomplete skull, vertebrae, and a partial hind limb, with stomach contents preserved. The specimen was unearthed in the
Lower Volga Basin in Russia.
In his 1960 review, Tarlo considered this species to be too different from ''Peloneustes philarchus'' to belong to ''Peloneustes'', tentatively placing it in ''Pliosaurus''. He speculated that Novozhilov had incorrectly thought ''Peloneustes'' to be the sole long-snouted pliosaurid, hence the initial assignment.
In 1964 Novozhilov erected a new genus, ''
Strongylokrotaphus'', for this species,
but further studies concurred with Tarlo and reassigned the species to ''Pliosaurus'',
possibly a synonym of ''Pliosaurus rossicus''.
More recent studies considered the taxon as a ''nomen dubium'' given its currently undiagnostic character, but nevertheless recommend a new description of the holotype.
However, this suggestion risks being tainted by the current state of conservation of specimen PIN 426, the skull and notably the mandible having since been seriously damaged by
pyrite
The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic luster and pale brass-yellow hue ...
decay, and the associated elements being noted as lost.

In 1998, palaeontologist Frank Robin O'Keefe proposed that a pliosaurid specimen from the Lower Jurassic
Posidonia Shale
The Posidonia Shale (, also called Schistes Bitumineux in Luxembourg) geologically known as the Sachrang Formation, is an Early Jurassic (Early to Late Toarcian) geological formation in Germany, northern Switzerland, northwestern Austria, souther ...
of Germany might represent a new species of ''Peloneustes''.
However, in 2001, he considered it to belong to a separate genus and species, naming it ''
Hauffiosaurus zanoni''.
Palaeontologists
Zulma Gasparini and Manuel A. Iturralde-Vinent assigned a pliosaurid from the Upper Jurassic
Jagua Formation of
Cuba
Cuba, officially the Republic of Cuba, is an island country, comprising the island of Cuba (largest island), Isla de la Juventud, and List of islands of Cuba, 4,195 islands, islets and cays surrounding the main island. It is located where the ...
to ''Peloneustes'' sp. in 2006.
In 2009, Gasparini redescribed it as ''
Gallardosaurus iturraldei''.
In 2011, Ketchum and Benson considered ''Peloneustes'' to contain only one species, ''P. philarchus''. They recognised twenty one definite specimens of ''Peloneustes philarchus'', all from the Peterborough Member of the Oxford Clay Formation. They considered some specimens from the Peterborough Member and
Marquise
A marquess (; ) is a nobleman of high hereditary rank in various European peerages and in those of some of their former colonies. The German-language equivalent is Markgraf (margrave). A woman with the rank of a marquess or the wife (or wid ...
, France previously assigned to ''Peloneustes'' to belong to different, currently unnamed pliosaurids.
In 1972, paleontologist
Teresa Maryańska
Teresa Maryańska (1937 – 3 October 2019) was a Polish paleontologist who specialized in Mongolian dinosaurs, particularly pachycephalosaurians and ankylosaurians. She is considered not only as one of Poland's but also one of the world's leadin ...
attributed cranial fragments to ''Peloneustes'' sp. which were discovered in an
Oxfordian quarry in
Załęcze Wielkie, Poland.
In 2011, Ketchum and Benson reidentified this specimen as actually coming from an undetermined marine
teleosaurid crocodylomorph
Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. Extinct crocodylomorphs were considerably mor ...
.
Description

''Peloneustes'' is a small-
to medium-sized member of Pliosauridae.
NHMUK R3318, the mounted skeleton in the Natural History Museum in London, is long,
while the mounted skeleton in the Institut für Geowissenschaften, University of Tübingen measures in length.
Plesiosaurs typically can be described as being of the small-headed, long-necked "plesiosauromorph" morphotype or the large-headed, short-necked "pliosauromorph" morphotype.
''Peloneustes'' is of the latter morphotype,
with its skull making up a little less than a fifth of the animal's total length.
''Peloneustes'', like all plesiosaurs, had a short tail, massive torso, and all of its limbs modified into large flippers.
Skull

While the holotype of ''Peloneustes'' lacks the rear portion of its cranium, many additional well-preserved specimens, including one that has not been crushed from top to bottom, have been assigned to this genus. These crania vary in size, measuring in length. The cranium of ''Peloneustes'' is elongated, and slopes upwards towards its back end.
Viewed from above, the cranium is shaped like an
isosceles triangle
In geometry, an isosceles triangle () is a triangle that has two Edge (geometry), sides of equal length and two angles of equal measure. Sometimes it is specified as having ''exactly'' two sides of equal length, and sometimes as having ''at le ...
,
with the back of the cranium broad and the front elongated into a narrow
rostrum
Rostrum may refer to:
* Any kind of a platform for a speaker:
**dais
**pulpit
** podium
* Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects
* Rostrum (ship), a form of bow on naval ...
. The rearmost part of the cranium has roughly parallel sides, unlike the tapering front regions. The external nares (openings for the nostrils) are small and located about halfway along the length of the cranium. The kidney-shaped
eye sockets face forwards and outwards and are located on the back half of the cranium. The sclerotic rings are composed of at least 16 individual elements, an unusually high number for a reptile. The
temporal fenestrae
Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit (eye socket). These openings have historically been used to track the evolution and affinities of reptiles. Temporal fenestrae are commonly ( ...
(openings in the back of the cranium) are enlarged, elliptical, and located on the cranium's rearmost quarter.

Characteristically, the
premaxillae
The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals has ...
(front upper tooth-bearing bones) of ''Peloneustes'' bear six teeth each, and the
diastemata (gaps between teeth) of the upper jaw are narrow. While it has been stated that ''Peloneustes'' had
nasals (bones bordering the external nares), well-preserved specimens indicate that this is not the case. The
frontals (bones bordering the eye sockets) of ''Peloneustes'' contact both the eye sockets and the external nares, a distinctive trait of ''Peloneustes''. There has been some contention as to whether or not ''Peloneustes'' had
lacrimals (bones bordering the lower front edges of the eye sockets), due to poor preservation. However, well preserved specimens indicate that the lacrimals are distinct bones as in other pliosaurids, as opposed to extensions of the
jugal
The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species.
Anatomy ...
s (bones bordering the lower rear edges of the eye sockets). The
palate
The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity.
A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly sep ...
of ''Peloneustes'' is flat and bears many openings, including the internal nares (the opening of the nasal passage into the mouth). These openings are contacted by palatal bones known as
palatines
Palatines () were the citizens and princes of the Palatinates, Holy Roman States that served as capitals for the Holy Roman Emperor. After the fall of the Holy Roman Empire in 1806, the nationality referred more specifically to residents of the ...
, a configuration used to identify this genus. The
parasphenoid
The parasphenoid is a bone which can be found in the cranium of many vertebrates. It is an unpaired dermal bone which lies at the midline of the roof of the mouth. In many reptiles (including birds), it fuses to the endochondral (cartilage-derived ...
(a bone that forms the lower front part of the
braincase
In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...
) bears a long cultriform process (a frontwards projection of the braincase) that is visible when the palate is viewed from below, another distinctive characteristic of ''Peloneustes''. The
occiput
The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone lies over the occipital lobes of the ...
(rear part of the cranium) of ''Peloneustes'' is open, bearing large fenestrae.
''Peloneustes'' is known from many mandibles, some of which are well-preserved. The longest of these measures . The mandibular symphysis is elongated, making up about a third of the total mandibular length. Behind the symphysis, the two sides of the mandible diverge before gently curving back inwards near the hind end. Each
dentary
In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla).
The jawbone ...
(the tooth-bearing bone in the mandible) has between 36 and 44 teeth, 13 to 15 of which are located on the symphysis. The second to seventh
tooth sockets (tooth sockets) are larger than those located further back, and the symphysis is the widest around the fifth and sixth. In addition to the characteristics of its mandibular teeth, ''Peloneustes'' can also be identified by its
coronoids (upper inner mandibular bones), which contribute to the mandibular symphysis. Between the tooth rows, the mandibular symphysis bears an elevated ridge where the dentaries meet. This is a
unique feature of ''Peloneustes'', not seen in any other plesiosaurs. The mandibular glenoid (socket of the
jaw joint) is broad, kidney-shaped, and angled upwards and inwards.

The teeth of ''Peloneustes'' have circular cross sections, as seen in other pliosaurids of its age.
The teeth have the shape of recurved
cone
In geometry, a cone is a three-dimensional figure that tapers smoothly from a flat base (typically a circle) to a point not contained in the base, called the '' apex'' or '' vertex''.
A cone is formed by a set of line segments, half-lines ...
s. The
enamel of the
crowns bears regularly-spaced vertical ridges of varying length on all sides. These ridges are more concentrated on the concave edge of the teeth. Most of the ridges extend to one half to two-thirds of the total crown height, with few actually reaching the tooth's apex.
The dentition of ''Peloneustes'' is
heterodont
In anatomy, a heterodont (from Greek, meaning 'different teeth') is an animal which possesses more than a single tooth morphology.
Human dentition is heterodont and diphyodont as an example.
In vertebrates, heterodont pertains to animals wher ...
, that is, it has teeth of different shapes. The larger teeth are
caniniform
In mammalian oral anatomy, the canine teeth, also called cuspids, dogteeth, eye teeth, vampire teeth, or fangs, are the relatively long, pointed tooth, teeth. In the context of the upper jaw, they are also known as ''fangs''. They can appear mo ...
and located at the front of the jaws, while the smaller teeth are more sharply recurved,
stouter, and located further back.
Postcranial skeleton

In 1913, Andrews reported that ''Peloneustes'' had 21 to 22 cervical, 2 to 3 pectoral, and around 20
dorsal
Dorsal (from Latin ''dorsum'' ‘back’) may refer to:
* Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism
* Dorsal, positioned on top of an aircraft's fuselage
The fus ...
vertebrae, with the exact number of sacral (hip) and caudal vertebrae unknown, based on specimens in the Leeds Collection.
However, in the same year, Linder reported 19 cervical, 5 pectoral, 20 dorsal, 2 sacral, and at least 17 caudal vertebrae in ''Peloneustes'', based on a specimen in the Institut für Geowissenschaften, University of Tübingen.
The first two cervical vertebrae, the
atlas
An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets.
Atlases have traditio ...
and
axis
An axis (: axes) may refer to:
Mathematics
*A specific line (often a directed line) that plays an important role in some contexts. In particular:
** Coordinate axis of a coordinate system
*** ''x''-axis, ''y''-axis, ''z''-axis, common names ...
, are fused in adults, but in juveniles they are present as several unfused elements.
The intercentrum (part of the vertebral body) of the axis is roughly rectangular, extending beneath the
centrum (vertebral body) of the atlas.
The cervical vertebrae bear tall
neural spine
Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...
s that are compressed from side to side.
The cervical centra are about half as long as wide. They bear strongly concave articular surfaces, with a prominent rim around the lower edge in the vertebrae located towards the front of the series. Each cervical centrum has a strong keel along the midline of its underside.
Most of the
cervical rib Cervical ribs are the ribs of the neck in many tetrapods. In most mammals, including humans, cervical ribs are not normally present as separate structures. They can, however, occur as a pathology. In humans, pathological cervical ribs are usually no ...
s bear two heads that are separated by a notch.
The pectoral vertebrae bear articulations for their respective ribs partially on both their centra and neural arches. Following these vertebrae are the dorsal vertebrae, which are more elongated than the cervical vertebrae and have shorter neural spines. The sacral and caudal vertebrae both have less elongated centra that are wider than tall. Many of the ribs from the hip and the base of the tail bear enlarged outer ends that seem to articulate with each other. Andrews hypothesised in 1913 that this configuration would have stiffened the tail, possibly to support the large hind limbs. The terminal (last) caudal vertebrae sharply decrease in size and would have supported proportionately larger
chevrons than the caudal vertebrae located further forwards. In 1913, Andrews speculated that this morphology may have been present to support a small
tail fin-like structure.
Other plesiosaurs have also been hypothesised to have tail fins, with impressions of such a structure possibly known in one species.
The shoulder girdle of ''Peloneustes'' was large, although not as heavily built as in some other plesiosaurs. The coracoids are the largest bones in the shoulder girdle, and are plate-like in form. The shoulder joint is formed by both the
scapula
The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...
(shoulder balde) and the coracoid, with the two bones forming a 70° angle with each other. The scapulae are typical in form for a pliosaurid and triradiate, bearing three prominent projections, or rami. The dorsal (upper) ramus is directed outwards, upwards, and backwards.
The underside of each scapula bears a ridge directed towards the front edge of its ventral (lower) ramus.
The ventral rami of the two scapulae were separated from each other by a triangular bone known as the
interclavicle
An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals ( marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In ...
. As seen in other pliosaurs, the pelvis of ''Peloneustes'' bears large and flat ischia and
pubic bones
In vertebrates, the pubis or pubic bone () forms the lower and anterior part of each side of the hip bone. The pubis is the most forward-facing (ventral and anterior) of the three bones that make up the hip bone. The left and right pubic bones ar ...
. The third pelvic bone, the
ilium, is smaller and elongated, articulating with the ischium. The upper end of the ilium shows a large amount of variation within ''P. philarchus'', with two forms known, one with a rounded upper edge, the other with a flat upper edge and more angular shape.
The hind limbs of ''Peloneustes'' are longer than its forelimbs, with the femur being longer than the humerus, although the humerus is the more robust of the two elements.
The
radius
In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...
(one of the lower forelimb bones) is approximately as wide as it is long, unlike the
ulna
The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...
(the other lower forelimb bone), which is wider than long.
The radius is the larger of these two elements.
The
tibia
The tibia (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...
is larger than the
fibula
The fibula (: fibulae or fibulas) or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. ...
(lower hindlimb bones) and longer than wide, while the fibula is wider than long in some specimens.
The
metacarpal
In human anatomy, the metacarpal bones or metacarpus, also known as the "palm bones", are the appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and the carpal bones ( wrist bones), which articulate ...
s,
metatarsal
The metatarsal bones or metatarsus (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges ( toes). Lacking individual names, the metatarsal bones are ...
s, and the proximal manual
phalanges
The phalanges (: phalanx ) are digit (anatomy), digital bones in the hands and foot, feet of most vertebrates. In primates, the Thumb, thumbs and Hallux, big toes have two phalanges while the other Digit (anatomy), digits have three phalanges. ...
(some of the bones making up the outer part of the paddle) are flattened. Most of the phalanges in both limbs have rounded cross-sections, and all of them have prominent constrictions in their middles. The number of phalanges in each digit is unknown in both the fore- and hind limbs.
Classification

Seeley initially described ''Peloneustes'' as a species of ''Plesiosaurus'', a rather common practice (at the time, the scope of genera was similar to what is currently used for
families
Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...
).
In 1874, Seeley named a new family of plesiosaurs, Pliosauridae, to contain forms similar to ''Pliosaurus''.
In 1890, Lydekker placed ''Peloneustes'' in this family,
to which it has been consistently assigned since.
Exactly how pliosaurids are related to other plesiosaurs is uncertain. In 1940, palaeontologist
Theodore E. White considered pliosaurids to be close relatives of
Elasmosauridae
Elasmosauridae, often called elasmosaurs or elasmosaurids, is an extinct family of plesiosaurs that lived from the Hauterivian stage of the Early Cretaceous to the Maastrichtian stage of the Late Cretaceous period (c. 130 to 66 mya). The taxo ...
based on shoulder anatomy.
Palaeontologist
Samuel P. Welles, however, thought that pliosaurids were more similar to
Polycotylidae
Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae. They are known as false pliosaurs. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widesprea ...
, as they both had large skulls and short necks, among other characteristics. He grouped these two families into the superfamily
Pliosauroidea
Pliosauroidea is an extinct clade of plesiosaurs, known from the earliest Jurassic to early Late Cretaceous. They are best known for the subclade Thalassophonea, which contained crocodile-like short-necked forms with large heads and massive toot ...
, with other plesiosaurs forming the superfamily
Plesiosauroidea
Plesiosauroidea (; Greek: 'near, close to' and 'lizard') is an extinct clade of carnivorous marine reptiles. They have the snake-like longest neck to body ratio of any reptile. Plesiosauroids are known from the Jurassic and Cretaceous perio ...
.
Another plesiosaur family,
Rhomaleosauridae, has since been assigned to Pliosauroidea,
while Polycotylidae has been reassigned to Plesiosauroidea.
However, in 2012, Benson and colleagues recovered a different topology, with Pliosauridae being more closely related to Plesiosauroidea than Rhomaleosauridae. This pliosaurid-plesiosauroid clade was termed
Neoplesiosauria.

Within Pliosauridae, the exact phylogenetic position of ''Peloneustes'' is uncertain.
In 1889, Lydekker considered ''Peloneustes'' to represent a transitional form between ''Pliosaurus'' and earlier plesiosaurs, although he found it unlikely that ''Peloneustes'' was ancestral to ''Pliosaurus''.
In 1960, Tarlo considered ''Peloneustes'' to be a close relative of ''Pliosaurus'', since both taxa had elongated mandibular symphyses.
In 2001, O'Keefe recovered it as a
basal (early-diverging) member of this family, outside of a group including ''
Liopleurodon
''Liopleurodon'' (; meaning 'smooth-sided teeth') is an extinct genus of carnivorous pliosaurid pliosaurs that lived from the Callovian stage of the Middle Jurassic to the Kimmeridgian stage of the Late Jurassic period (c. 166 to 155 mya). T ...
'', ''Pliosaurus'', and ''
Brachauchenius''.
However, in 2008, palaeontologists Adam S. Smith and Gareth J. Dyke found ''Peloneustes'' to be the
sister taxon
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and ...
of ''Pliosaurus''.
In 2013, Benson and palaeontologist Patrick S. Druckenmiller named a new clade within Pliosauridae,
Thalassophonea
Pliosauridae is a family of plesiosaurian marine reptiles from the Latest Triassic to the early Late Cretaceous (Rhaetian to Turonian stages). The family is more inclusive than the archetypal short-necked large headed species that are placed in ...
. This clade included the "classic", short-necked pliosaurids while excluding the earlier, long-necked, more gracile forms. ''Peloneustes'' was found to be the most basal thalassophonean.
Subsequent studies have uncovered a similar position for ''Peloneustes''.
The following
cladogram
A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
follows Ketchum and Benson, 2022.
Palaeobiology

Plesiosaurs were well-adapted to marine life.
They grew at rates comparable to those of birds and had high
metabolism
Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...
s, indicating
homeothermy
Homeothermy, homothermy, or homoiothermy () is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate envir ...
or even
endothermy
An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inste ...
.
The
bony labyrinth
The bony labyrinth (also osseous labyrinth or otic capsule) is the rigid, bony outer wall of the inner ear in the temporal bone. It consists of three parts: the vestibule, semicircular canals, and cochlea. These are cavities hollowed out of the ...
, a hollow within the skull which held a sensory organ associated with balance and orientation, of ''Peloneustes'' and other plesiosaurs is similar in shape to that of
sea turtle
Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerh ...
s. Palaeontologist James Neenan and colleagues hypothesised in 2017 that this shape probably evolved alongside the flapping motions used by plesiosaurs to swim. ''Peloneustes'' and other short-necked plesiosaurs also had smaller labyrinths than plesiosaurs with longer necks, a pattern also seen in
cetacean
Cetacea (; , ) is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively c ...
s.
Additionally, ''Peloneustes'' probably had
salt gland
The salt gland is an organ (anatomy), organ for excreting excess salt (chemistry), salts. It is found in the cartilaginous fishes subclass elasmobranchii (sharks, rays, and skates), seabirds, and some reptiles. Salt glands can be found in the r ...
s in its head to cope with excess amount of salt within its body. However, ''Peloneustes'' appears to have been a predator of
vertebrate
Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain.
The vertebrates make up the subphylum Vertebra ...
s, which contain less salt than
invertebrate
Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...
s, therefore leading palaeontologist Leslie Noè to suggest in a 2001 dissertation that these glands would not have had to be especially large.
''Peloneustes'', like many other pliosaurs, displayed a reduced level of
ossification
Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...
of its bones. Palaeontologist Arthur Cruickshank and colleagues in 1966 proposed that this may have helped ''Peloneustes'' maintain its
buoyancy
Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...
or improved its manoeuvrability.
A 2019 study by palaeontologist Corinna Fleischle and colleagues found that plesiosaurs had enlarged
red blood cell
Red blood cells (RBCs), referred to as erythrocytes (, with -''cyte'' translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cel ...
s, based on the morphology of their
vascular canals, which would have aided them while diving.
Plesiosaurs such as ''Peloneustes'' employed a method of swimming known as
subaqueous flight, using their flippers as
hydrofoil
A hydrofoil is a lifting surface, or foil, that operates in water. They are similar in appearance and purpose to aerofoils used by aeroplanes. Boats that use hydrofoil technology are also simply termed hydrofoils. As a hydrofoil craft gains sp ...
s. Plesiosaurs are unusual among marine reptiles in that they used all four of their limbs, but not movements of the vertebral column, for propulsion. The short tail, while unlikely to have been used to propel the animal, could have helped stabilise or steer the plesiosaur.
The front flippers of ''Peloneustes'' have
aspect ratio
The aspect ratio of a geometry, geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side—the ratio of width to height, when the rectangl ...
s of 6.36, while the rear flippers have aspect ratios of 8.32. These ratios are similar to those of the wings of modern
falcon
Falcons () are birds of prey in the genus ''Falco'', which includes about 40 species. Some small species of falcons with long, narrow wings are called hobbies, and some that hover while hunting are called kestrels. Falcons are widely distrib ...
s. In 2001, O'Keefe proposed that, much like falcons, pliosauromorph plesiosaurs such as ''Peloneustes'' probably were capable of moving quickly and nimbly, albeit inefficiently, to capture prey.
Computer modelling by palaeontologist Susana Gutarra and colleagues in 2022 found that due to their large flippers, a plesiosaur would have produced more
drag than a comparably-sized cetacean or
ichthyosaur
Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides.
Ichthyosaurians thrived during much of the Mesozoic era; based on fo ...
. However, plesiosaurs counteracted this with their large trunks and body size.
Due to the reduction in drag by their shorter, deeper bodies, palaeontologist
Judy Massare proposed in 1988 that plesiosaurs could actively search for and pursue their food instead of having to lie in wait for it.
Feeding mechanics

In a 2001 dissertation, Noè noted many adaptations in pliosaurid skulls for predation. To avoid damage while feeding, the skulls of pliosaurids like ''Peloneustes'' are highly akinetic, where the bones of the cranium and mandible were largely locked in place to prevent movement. The snout contains elongated bones that helped to prevent bending and bears a reinforced junction with the facial region to better resist the stresses of feeding. When viewed from the side, little tapering is visible in the mandible, strengthening it. The mandibular symphysis would have helped deliver an even bite and prevent the mandibles from moving independently. The enlarged coronoid eminence provides a large, strong region for the anchorage of the jaw muscles, although this structure is not as large in ''Peloneustes'' as it is in other contemporary pliosaurids. The regions where the jaw muscles were anchored are located further back on the skull to avoid interference with feeding. The kidney-shaped mandibular glenoid would have made the jaw joint steadier and stopped the mandible from dislocating. Pliosaurid teeth are firmly rooted and interlocking, which strengthens the edges of the jaws. This configuration also works well with the simple rotational movements that pliosaurid jaws were limited to and strengthens the teeth against the struggles of prey. The larger front teeth would have been used to impale prey while the smaller rear teeth crushed and guided the prey backwards toward the throat. With their wide gapes, pliosaurids would not have processed their food very much before swallowing.

The numerous teeth of ''Peloneustes'' rarely are broken, but often show signs of wear at their tips. Their sharp points, slightly curved, gracile shape, and prominent spacing indicate that they were built for piercing. The slender, elongated snout is similar in shape to that of a dolphin. Both the snout and tooth morphologies led Noè to suggest that ''Peloneustes'' was a
piscivore
A piscivore () is a carnivorous animal that primarily eats fish. Fish were the diet of early tetrapod evolution (via water-bound amphibians during the Devonian period); insectivory came next; then in time, the more terrestrially adapted repti ...
(fish eater). To catch its prey, ''Peloneustes'' would have quickly swung its head to the side. The gracile snout's roughly circular cross-section would have minimised drag, while the long jaws were suited for quickly snapping up mobile prey. The flat, enlarged palate and reinforced braincase of ''Peloneustes'' would have reduced the torsion, flexing, and shearing caused by the long snout. The reinforced braincase would have reduced the shock absorption in the area of the brain. Since the tip of the snout was further away from the jaw joint, it would have exerted a weaker bite force than the regions further back. The front regions of the jaws of ''Peloneustes'' are elongate, indicating that they would have been used to quickly strike at and apprehend prey. These adaptations indicate a preference for smaller prey that, while agile, would have been easier to incapacitate and less powerful. However, ''Peloneustes'' would still have been capable of attacking fish of moderate size. While a skeleton, catalogued as NHMUK R3317, with
belemnite
Belemnitida (or belemnites) is an extinct order (biology), order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous (And possibly the Eocene). Unlike squid, belemnites had an internal skeleton that made up the cone ...
remains in its stomach was assigned to ''Peloneustes'' by Andrews in 1910,
it is very incomplete and may actually belong to the contemporaneous pliosaurid ''
Simolestes
''Simolestes'' (meaning "snub-nosed thief") is an extinct pliosaurid genus that lived in the Middle to Late Jurassic. The type specimen, NHMUK PV R 3319 is an almost complete but crushed skeleton diagnostic to ''Simolestes vorax'', dating back ...
'' instead, as suggested by Noè.
''Peloneustes'' has also been suggested to have inflicted bite marks upon a specimen of ''
Cryptoclidus'' by Bruce Rothschild and colleagues in 2018.
Palaeoenvironment

''Peloneustes'' is known from the Peterborough Member (formerly known as the Lower Oxford Clay) of the Oxford Clay Formation.
While ''Peloneustes'' has been listed as coming from the Oxfordian stage (spanning from about 164 to 157 million years ago
) of the Upper Jurassic,
the Peterborough Member actually dates to the
Callovian
In the geologic timescale, the Callovian is an age and stage in the Middle Jurassic, lasting between 165.3 ± 1.1 Ma (million years ago) and 161.5 ± 1.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the ...
stage (spanning from about 166 to 164 million years ago
) of the
Middle Jurassic
The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...
.
The Peterborough Member spans from the late lower Callovian to the early upper Callovian, occupying the entirety of the Middle Callovian.
It overlays the
Kellaways Formation and is overlain by the Stewartby Member of the Oxford Clay Formation.
The Peterborough Member is primarily composed of grey
bituminous
Bitumen ( , ) is an immensely viscous constituent of petroleum. Depending on its exact composition, it can be a sticky, black liquid or an apparently solid mass that behaves as a liquid over very large time scales. In American English, the m ...
(asphalt-containing)
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...
and
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...
rich in
organic matter
Organic matter, organic material or natural organic matter is the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have come fro ...
.
These rocks are sometimes
fissile
In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal Nuclear chain reaction#Fission chain reaction, chain reaction can only be achieved with fissil ...
(splittable into thin, flat slabs).
The member is about thick, stretching from
Dorset
Dorset ( ; Archaism, archaically: Dorsetshire , ) is a Ceremonial counties of England, ceremonial county in South West England. It is bordered by Somerset to the north-west, Wiltshire to the north and the north-east, Hampshire to the east, t ...
to the
Humber
The Humber is a large tidal estuary on the east coast of Northern England. It is formed at Trent Falls, Faxfleet, by the confluence of the tidal rivers River Ouse, Yorkshire, Ouse and River Trent, Trent. From there to the North Sea, it forms ...
.
The Peterborough Member represents an
epicontinental sea during a time of
rising sea levels
The sea level has been rising from the end of the last ice age, which was around 20,000 years ago. Between 1901 and 2018, the average sea level rose by , with an increase of per year since the 1970s. This was faster than the sea level had e ...
.
When it was deposited, it would have been located at a latitude of
35°N.
This sea, known as the Oxford Clay sea, was largely encircled by islands and continents, which provided the seaway with
sediment
Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...
.
Its proximity to land is demonstrated by the preservation of
terrestrial
Terrestrial refers to things related to land or the planet Earth, as opposed to extraterrestrial.
Terrestrial may also refer to:
* Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on o ...
fossils such as
driftwood
Driftwood is a wood that has been washed onto a shore or beach of a sea, lake, or river by the action of winds, tides or waves. It is part of beach wrack.
In some waterfront areas, driftwood is a major nuisance. However, the driftwood provides ...
in the Oxford Clay, in addition to a
clastic dike
A clastic dike is a seam of sedimentary material that fills an open fracture in and cuts across sedimentary rock strata or layering in other rock types.
Clastic dikes form rapidly by fluidized injection (mobilization of pressurized pore fluids) ...
in the lower levels of the Peterborough Member, with the dike's formation being facilitated by
rainwater
Rain is a form of precipitation where water droplets that have condensed from atmospheric water vapor fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Eart ...
.
The southern region of the Oxford Clay Sea was connected to the
Tethys Ocean
The Tethys Ocean ( ; ), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean during much of the Mesozoic Era and early-mid Cenozoic Era. It was the predecessor to the modern Indian Ocean, the Mediterranean Sea, and the Eurasia ...
, while it was connected to more boreal regions on its northern side. This allowed for
faunal interchange to occur between the Tethyan and boreal regions. This sea was approximately deep within of the shoreline.

The surrounding land would have had a
Mediterranean climate
A Mediterranean climate ( ), also called a dry summer climate, described by Köppen and Trewartha as ''Cs'', is a temperate climate type that occurs in the lower mid-latitudes (normally 30 to 44 north and south latitude). Such climates typic ...
, with dry summers and wet winters, though it was becoming increasingly
arid
Aridity is the condition of geographical regions which make up approximately 43% of total global available land area, characterized by low annual precipitation, increased temperatures, and limited water availability.Perez-Aguilar, L. Y., Plata ...
. Based on information from
δ18O isotopes in
bivalves
Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consis ...
, the water temperature of the
seabed
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds.
The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
of the Peterborough Member varied from due to seasonal variation, with an average temperature of . Belemnite fossils provide similar results, giving a water temperature range with a minimum to a maximum between or , with an average temperature of .
While traces of
green sulphur bacteria indicate
euxinic
Euxinia or euxinic conditions occur when water is both wikt:anoxic, anoxic and wikt:sulfidic, sulfidic. This means that there is no oxygen (O2) and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly strat ...
water, with low oxygen and high
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...
levels, abundant traces of
benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...
(bottom-dwelling) organisms suggest that the bottom waters were not
anoxic
Anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts:
* Anoxic waters, sea water, fresh water or groundwater that are depleted of dissolved ox ...
.
Oxygen levels appear to have varied, with some deposits laid down in more aerated conditions than others.
Contemporaneous biota
There are many kinds of invertebrates preserved in the Peterborough Member. Among these are
cephalopods
A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...
, which include
ammonites
Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...
, belemnites, and
nautiloids
Nautiloids are a group of cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Fossil nautiloids are diverse and species rich, with over 2,500 recorded species. Th ...
.
Bivalve
Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...
s are another abundant group, while
gastropod
Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda ().
This class comprises snails and slugs from saltwater, freshwater, and fro ...
s and
annelid
The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...
s are less so but still quite common.
Arthropod
Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
s are also present.
Brachiopod
Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear e ...
s and
echinoderms
An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larv ...
are rare. Despite not being known from fossils,
polychaete
Polychaeta () is a paraphyletic class of generally marine Annelid, annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called c ...
s probably would have been present in this ecosystem, due to their abundance in similar modern environments and burrows similar to ones produced by these worms.
Microfossils
A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...
pertaining to
foraminifera
Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...
ns,
coccolithophoroids, and
dinoflagellates
The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...
are abundant in the Peterborough Member.
A wide variety of fish are known from the Peterborough Member. These include the
chondrichthyans
Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which have skeletons ...
''
Asteracanthus'', ''
Brachymylus'', ''
Heterodontus'' (or ''
Paracestracion''),
''
Hybodus
''Hybodus'' (from , 'crooked' and 'tooth') is an extinct genus of Hybodontiformes, hybodont. Species closely related to the type species ''Hybodus reticulatus'' lived during the Early Jurassic epoch. Numerous species have been assigned to ''Hyb ...
'', ''
Ischyodus
''Ischyodus'' (from , 'power' and 'tooth') is an extinct genus of chimaera. It is the most diverse and long-lived chimaera genus, with over 39 species found worldwide spanning over 140 million years from the Middle Jurassic to the Miocene. Alm ...
'', ''
Palaeobrachaelurus'', ''
Pachymylus'', ''
Protospinax
''Protospinax'' is an extinct genus of cartilaginous fish from the Early Jurassic to Early Cretaceous of Europe and Russia. The type species, ''P. annectans'', was found in the Solnhofen limestones of southern Bavaria. Formerly known from only t ...
'', ''
Leptacanthus'', ''
Notidanus'', ''
Orectoloboides'', ''
Spathobathis'', and ''
Sphenodus''.
Actinopterygians
Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of skin ...
were also present, represented by ''
Aspidorhynchus'', ''
Asthenocormus'', ''
Caturus
''Caturus'' (from , 'down' and 'tail') is an extinct genus of predatory marine fishes in the family Caturidae in the order Amiiformes, related to modern bowfin. It has been suggested that the genus is non-monophyletic with respect to other c ...
'', ''
Coccolepis'', ''
Heterostrophus'', ''
Hypsocormus'', ''
Leedsichthys
''Leedsichthys'' is an extinct genus of pachycormid fish that lived in the oceans of the Middle to Late Jurassic.Liston, JJ (2004). An overview of the pachycormiform ''Leedsichthys''. In: Arratia G and Tintori A (eds) Mesozoic Fishes 3 - Systema ...
'', ''
Lepidotes
''Lepidotes'' (from , 'covered with scales') (previously known as ''Lepidotus'') is an extinct genus of Mesozoic ray-finned fish. It has long been considered a wastebasket taxon, characterised by "general features, such as thick rhomboid scales ...
'', ''
Leptolepis'', ''
Mesturus'', ''
Osteorachis'', ''
Pachycormus'', ''
Pholidophorus
''Pholidophorus'' (from , 'horny scale' and , 'to bear') is an extinct genus of Stem group, stem-teleost fish. Numerous species were assigned to this genus in the past, but only the type species ''Pholidophorus latiusculus'', from the Late Tria ...
'', and ''
Sauropsis''.
These fish include surface-dwelling, midwater, and benthic varieties of various sizes, some of which could get quite large. They filled a variety of niches, including invertebrate eaters, piscivores, and, in the case of ''Leedsichthys'', giant
filter feeder
Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a s ...
s.

Plesiosaurs are common in the Peterborough Member, and besides pliosaurids, are represented by
cryptoclidids, including ''Cryptoclidus'', ''
Muraenosaurus
''Muraenosaurus'' (from the Latin "''Muraena''" meaning "eel" and "''Sauros''" meaning lizard) is an extinct genus of cryptoclidid plesiosaur reptile from the Oxford Clay of Southern England. The genus was given its name due to the eel-like appe ...
'', ''
Tricleidus
''Tricleidus'' is an extinct genus of cryptoclidid plesiosaur known from only specimen (BMNH R3539) from the middle Jurassic of United Kingdom. It was first named by Andrews in 1909 and the type species is ''Tricleidus seeleyi''.
One species ...
'', and ''
Picrocleidus
''Picrocleidus'' is an extinct genus of plesiosaur from the Middle Jurassic Oxford Clay Formation ( Callovian stage) of the United Kingdom.
Discovery and naming
The holotype was discovered in the Oxford Clay Formation in Peterborough, Eng ...
''.
They were smaller plesiosaurs with thin teeth and long necks,
and, unlike pliosaurids such as ''Peloneustes'', would have mainly eaten small animals.
The ichthyosaur ''
Ophthalmosaurus
''Ophthalmosaurus'' (Greek ὀφθάλμος ''ophthalmos'' 'eye' and σαῦρος ''sauros'' 'lizard') is a genus of ichthyosaur known from the Middle-Late Jurassic. Possible remains from the earliest Cretaceous, around 145 million years ago, a ...
'' also inhabited the Oxford Clay Formation. ''Ophthalmosaurus'' was well adapted for deep diving, thanks to its streamlined,
porpoise
Porpoises () are small Oceanic dolphin, dolphin-like cetaceans classified under the family Phocoenidae. Although similar in appearance to dolphins, they are more closely related to narwhals and Beluga whale, belugas than to the Oceanic dolphi ...
-like body and gigantic eyes, and probably fed on cephalopods.
Many genera of crocodylomorphs are also known from the Peterborough Member. These include the
gavial-like
teleosauroids ''
Charitomenosuchus'', ''
Lemmysuchus'', ''
Mycterosuchus'', and ''
Neosteneosaurus''
and the
mosasaur
Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains wer ...
-like
metriorhynchids ''
Gracilineustes'', ''
Suchodus'', ''
Thalattosuchus'',
and ''
Tyrannoneustes''.
While uncommon, the small piscivorous
pterosaur
Pterosaurs are an extinct clade of flying reptiles in the order Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 million to 66 million years ago). Pterosaurs are the earli ...
''
Rhamphorhynchus
''Rhamphorhynchus'' (, from Ancient Greek ''rhamphos'' meaning "beak" and ''rhynchus'' meaning "snout") is a genus of long-tailed pterosaurs in the Jurassic period. Less specialized than contemporary, short-tailed pterodactyloid pterosaurs such ...
'' was also part of this marine ecosystem.

More pliosaurid species are known from the Peterborough Member than any other assemblage.
Besides ''Peloneustes'', these pliosaurids include ''Liopleurodon ferox'', ''Simolestes vorax'', ''"Pliosaurus" andrewsi'', ''
Marmornectes candrewi'',
''
Eardasaurus powelli'', and, potentially, ''
Pachycostasaurus dawni''.
However, there is considerable variation in the anatomy of these species, indicating that they fed on different prey, thereby avoiding competition (
niche partitioning
In ecology, a niche is the match of a species to a specific environmental condition.
Three variants of ecological niche are described by
It describes how an organism or population responds to the distribution of resources and competitors (for e ...
).
The large, powerful pliosaurid ''Liopleurodon ferox'' appears to have been adapted to take on large prey, including other marine reptiles and large fish.
The long-snouted ''Eardasaurus powelli'' like ''Liopleurodon'' also has teeth with cutting edges and may have also taken large prey.
''Simolestes vorax'', with its wide, deep skull and powerful bite, appears to have been a predator of large cephalopods.
''"Pliosaurus" andrewsi'', like ''Peloneustes'', possesses an elongated snout, an adaptation for feeding upon small, agile animals.
However, its teeth are suited for cutting, indicating a preference for larger prey, while those of ''Peloneustes'' are better adapted for piercing.
''"Pliosaurus" andrewsi'' is also larger than ''Peloneustes''.
''Marmornectes candrewi'' is also similar to ''Peloneustes'', bearing a long snout, and perhaps also fed on fish.
''Pachycostasaurus dawni'' is a small, heavily built pliosaur that probably fed on benthic prey. It has a weaker skull than other pliosaurids and was more stable, so it probably used different feeding methods to avoid competition.
Unlike the other pliosaurids of the Oxford Clay, ''Pachycostasaurus'' was rather rare, perhaps mainly living outside of the depositional area of the Oxford Clay Formation, possibly inhabiting coastal regions, deep water, or even rivers instead.
While several different types of pliosaurids were present in the Middle Jurassic, the long-snouted piscovorous forms such as ''Peloneustes'' died out at the Middle-Upper Jurassic boundary. This seems to have been the first phase of a gradual decline in plesiosaur diversity. While the cause of this is uncertain, it may have been influenced by changing ocean chemistry, and, in later phases, falling sea levels.
See also
*
List of plesiosaur genera
This list of plesiosaurs is a comprehensive listing of all genera that have ever been included in the order Plesiosauria, excluding purely vernacular terms. The list includes all commonly accepted genera, but also genera that are now considered inv ...
*
Timeline of plesiosaur research
This timeline of plesiosaur research is a chronologically ordered list of important fossil discoveries, controversies of interpretation, Taxonomy (biology), taxonomic revisions, and cultural portrayals of plesiosaurs, an order of marine reptiles ...
References
External links
*
{{Taxonbar, from=Q242046
Pliosauridae
Callovian life
Middle Jurassic plesiosaurs of Europe
Jurassic England
Fossils of England
Oxford Clay
Fossil taxa described in 1889
Taxa named by Richard Lydekker
Sauropterygian genera