Megacephalosaurus Eulerti MP
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''Megacephalosaurus'' (; "great-headed lizard") is an
extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
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 short-necked
pliosaur 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 ...
that inhabited the
Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...
of
North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
about 94 to 93 million years ago during the
Turonian The Turonian is, in the International Commission on Stratigraphy, ICS' geologic timescale, the second age (geology), age in the Late Cretaceous epoch (geology), Epoch, or a stage (stratigraphy), stage in the Upper Cretaceous series (stratigraphy), ...
stage of the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...
, containing the single species ''M. eulerti''. It is named after its large head, which is the largest of any
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 ...
in the continent and measures up to in length. ''Megacephalosaurus'' was one of the largest marine reptiles of its time with an estimated length of . Its long snout and consistently sized teeth suggest that it preferred a diet of smaller-sized prey. Remains representing the pliosaur include two fossil skulls, three ribs, and a neural arch. The fossils have been found in deposits of the
Carlile Shale The Carlile Shale is a Turonian age Upper Cretaceous, Upper/Late Cretaceous series shale geologic Formation (geology), formation in the central-western United States, including in the Great Plains region of Colorado, Kansas, Nebraska, New Mexico ...
and
Greenhorn Limestone The Greenhorn Limestone or Greenhorn Formation is a geologic formation in the Great Plains Region of the United States, dating to the Cenomanian and Turonian ages of the Late Cretaceous period. The formation gives its name to the Greenhorn cycle ...
in
Kansas Kansas ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the west. Kansas is named a ...
and elsewhere in the
midwestern United States The Midwestern United States (also referred to as the Midwest, the Heartland or the American Midwest) is one of the four census regions defined by the United States Census Bureau. It occupies the northern central part of the United States. It ...
. First discovered in 1950, they were originally thought to have been giant fossils of a closely related pliosaur known as ''
Brachauchenius lucasi ''Brachauchenius'' (meaning 'short neck') is an extinct genus of pliosaurs that lived during the Late Cretaceous in what are now North America and North Africa. Only one species is known, ''B. lucasi'', initially described by Samuel Wendell Willis ...
''. But by 2013, paleontologists understood that such fossils belonged to a distinct animal. ''Megacephalosaurus'' was among the last known pliosaurs.


Discovery and history


Initial identification as ''Brachauchenius lucasi''

The first specimen of ''Megacephalosaurus'' was discovered by two teenage brothers named Frank and Robert Jennrich while collecting fossil shark teeth near
Fairport, Kansas Fairport is an unincorporated community in Paradise Township, Russell County, Kansas, United States. It is located about 12 miles north of Gorham near the west county line and next the Saline River. History In 1879, Knight & Bradshaw built ...
. The skull was found bottom-side up and completely crushed into a thin layer. Recognizing the significance of their discovery, the brothers corresponded with paleontologist George F. Sternberg and led him to the fossil during the fall of 1950. Sternberg identified the locality to likely be either the
Greenhorn Limestone The Greenhorn Limestone or Greenhorn Formation is a geologic formation in the Great Plains Region of the United States, dating to the Cenomanian and Turonian ages of the Late Cretaceous period. The formation gives its name to the Greenhorn cycle ...
or
Graneros Shale The Graneros Shale is a geologic formation in the United States identified in the Great Plains as well as New Mexico that dates to the Cenomanian Age of the Cretaceous Period. It is defined as the finely sandy argillaceous or clayey ''near-shor ...
with nearby fossils including that of a ''
Xiphactinus ''Xiphactinus'' (from Latin and Greek for "sword-ray") is an extinct genus of large predatory marine ray-finned fish that lived during the late Albian to the late Maastrichtian. The genus grew up to in length, and superficially resembled a garga ...
''. The three men, with the help of a local cow-herder named Jim Rouse, excavated the skull by late October of the same year. The land on which the fossil was found was owned by Otto C. Eulert (making the fossil his property), who subsequently donated the specimen to the Sternberg Museum of Natural History. This specimen was cataloged as FHSM VP-321. Sternberg wrote to Eulert on November 2, 1950, that he initially thought the fossil belonged to a large mosasaur. However, he quickly changed his identification to that of a plesiosaur. After conferring with
Samuel Paul Welles Samuel Paul Welles (November 9, 1909 – August 6, 1997) was an American palaeontologist. Welles was a research associate at the Museum of Palaeontology, University of California, Berkeley. He took part in excavations at the Placerias Quarry in ...
of
UC Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California), is a public land-grant research university in Berkeley, California, United States. Founded in 1868 and named after the Anglo-Irish philosopher George Berkele ...
and other paleontologists during the 1950
Society of Vertebrate Paleontology The Society of Vertebrate Paleontology (SVP) is a professional organization that was founded in the United States in 1940 to advance the science of vertebrate paleontology around the world. Mission and activities SVP has about 2,300 members inte ...
meeting in
Albuquerque, New Mexico Albuquerque ( ; ), also known as ABQ, Burque, the Duke City, and in the past 'the Q', is the List of municipalities in New Mexico, most populous city in the U.S. state of New Mexico, and the county seat of Bernalillo County, New Mexico, Bernal ...
, Sternberg identified FHSM VP-321 as the Late Cretaceous pliosaur ''Brachauchenius lucasi''. Although additional associated fossils of the specimen were recovered during its excavation, Sternberg had the skull and left lower jaw embedded on a mount of plaster and displayed it in the museum's public exhibits by 1951. The identification of the supposed ''Brachauchenuis lucasi'' skull that is FHSM VP-321 was unquestioned for decades. Even a study done in 1996 on the mounted skull identified it as such as the diagnostic features of the dorsal sides of the skull, which was the only parts of the skull visible, were still characteristic of ''B. lucasi''.


Reexamination and redescription

In November 2003, Robert Jennrich led staff members of the Sternberg Museum to the original locality where he and his brother discovered the specimen. Scientists found that this locality was not of the Greenhorn Limestone or Ganeros Shale as Sternberg initially presumed, but instead of the Fairport Chalk Member of the
Carlile Shale The Carlile Shale is a Turonian age Upper Cretaceous, Upper/Late Cretaceous series shale geologic Formation (geology), formation in the central-western United States, including in the Great Plains region of Colorado, Kansas, Nebraska, New Mexico ...
, dated approximately 92.9 . Paleontologists began to doubt the identification of FHSM VP-321 when in 2007 the Sternberg Museum curator Mike Everhart discovered photos of the skull's underside taken prior to its mounting, which revealed underside features that were different from those found in other ''B. lucasi'' skulls. To confirm the perceived differences, FHSM VP-321 was arranged to be removed from its mount and Everhart collaborated with paleontologists Bruce A. Schumacher of the USDA Forest Service and
Kenneth Carpenter Kenneth Carpenter (born 21 September 1949) is an American paleontologist. He is the former director of the USU Eastern Prehistoric Museum and author or co-author of books on dinosaurs and Mesozoic life. His main research interests are armore ...
of the USU Eastern Prehistoric Museum in a study to examine the underside of the skull. The study, which was published in 2013, found the skull bore several unique features that made it distinct from ''B. lucasi'' and concluded that FHSM VP-321 was a distinct genus of pliosaur. It was named ''Megacephalosaurus eulerti'', with the generic name being a
portmanteau In linguistics, a blend—also known as a blend word, lexical blend, or portmanteau—is a word formed by combining the meanings, and parts of the sounds, of two or more words together.
of the
Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...
(, "great") and (, "head") prefixed onto (, "lizard") in reference to the size of the skull, and the specific epithet being an honor of Eulert for his donation of the fossil. The history of the paratype skull UNSM 50136 was unclear when it was first examined by Schumacher in a 2008 study. When it was recovered from the collections of the
University of Nebraska State Museum The University of Nebraska State Museum, also known as Morrill Hall, founded in 1871, is a natural history museum featuring Nebraska biodiversity, paleontology, and cultural diversity, located on the University of Nebraska–Lincoln City Campus n ...
for study, there was strangely no contextual information or records pertaining to the large fossil. The few reputed information on the skull included one that simply stated it originated from Kansas, leading Schumacher to speculate that it may have originally been within the collections of the Sternberg Museum and was obtained by the University of Nebraska during a fossil exchange. No stratigraphic data was recorded for USNM 50136, but tests on a piece of
matrix Matrix (: matrices or matrixes) or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the m ...
extracted from the fossil identified nannofossil assemblages that are most associated with deposits of the Greenhorn Limestone likely dated around 93.9 mya at earliest. Schumacher concluded that USNM 50136 was a species with affinities to ''B. lucasi'' and that it could either expand the definition of the taxon or be of an entirely new species, assigning the
open nomenclature Open nomenclature is a vocabulary of partly informal terms and signs in which a taxonomist may express remarks about their own material. This is in contrast to synonymy lists, in which a taxonomist may express remarks on the work of others. Common ...
''aff. Brachauchenius lucasi''. When FHSM VP-321 was found to be a distinct genus, USNM 50136 was identified to be conspecific with it and was assigned to be its paratype. Additional fossils currently identified as ''B. lucasi'' may actually belong of ''Megacephalosaurus''. For example, Everhart expressed possibility that UNSM 112437, a partial skull recovered from the Graneros Shale and labeled as ''B. lucasi'', is actually another possible representative of ''Megacephalosaurus''.


Description

Two specimens pertaining to ''Megacephalosaurus'' are known. Both represent fossil skulls. 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 ...
is FHSM VP-321, which consists of a nearly complete skull, three
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, and a cervical neural arch. The second specimen, USNM 50136, is designated as the
paratype In zoology and botany, a paratype is a specimen of an organism that helps define what the scientific name of a species and other taxon actually represents, but it is not the holotype (and in botany is also neither an isotype (biology), isotype ...
specimen. It consists of a partial skull containing parts of the upper jaw and some cranial bones. The skull of ''Megacephalosaurus'' is the largest known for any plesiosaur from North America to date. FHSM VP-321 measures in length when measuring along the midline of the cranium and the length of the lower jaw measures . UNSM 50136, although incomplete, is much larger than FHSM VP-321 and is estimated to have measured when it was complete. Based on understandings from fossils of other pliosaurs the skull likely equated to about one-fifth to one-fourth of total body length. This would yield a range of in total body length for ''Megacephalosaurus''. The snout is very elongate, making up around two-thirds of the total skull length. It tapers to a point with a small rostrum before the first teeth. The
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 ...
, which houses the lower teeth, also takes up around two-thirds of the total length of the lower jaw. The upper jaws possess twenty-two functional teeth on the right side and twenty-three teeth on the left; four of the frontmost pairs are located on the
premaxilla 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 h ...
while the rest are located on the
maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...
. Unlike in pliosaurs such as ''
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 ...
'' or ''
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 ...
'', the maxillary teeth in ''Megacephalosaurus'' do not reduce in size towards the base of the skull and are all consistently large. A pair of small pits measuring less than long is present just before the frontmost pair of functional teeth. It is believed that pits represent vestigial
dental alveoli Dental alveoli (singular ''alveolus'') are sockets in the jaws in which the roots of teeth are held in the alveolar process with the periodontal ligament. The lay term for dental alveoli is tooth sockets. A joint that connects the roots of the t ...
that once held an additional pair of teeth in ancestral species but devolved in ''Megacephalosaurus''. Almost all pliosaurs normally have five pairs of premaxillary teeth and this feature of reduction to four pairs is fairly unique among pliosaurs, indicating that the reduction to four pairs of premaxillary teeth may have been a novel adaptation introduced by Cretaceous pliosaurs. The lower jaws bear twenty-three pairs of functional teeth. Only the five frontmost pairs are relatively large, all teeth subsequent of them get progressively smaller as they progress towards the base of the dentary in a smooth transition. The sixth to fifteenth lower jaw teeth are considered to be intermediate-sized, and all teeth posterior to such are considered small. The teeth are conical, ending at a pointed tip, and are slightly curved towards the tongue and away from the midline of the jaw. The teeth are fairly large and the largest teeth known in ''Megacephalosaurus'' measure up to approximately in crown height. The enamel features patterns of long, thin, and convex ridges known as apicobasal ridges that run in the direction from the crown base to tip. Approximately half of these ridges, spaced 13–16 per centimeter of crown circumference, reach the full length of the crown. The function of apicobasal ridges in the teeth has been proposed to be to improve gripping and puncturing prey. The ribs are similar in general shape with those in typical
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
pliosaurs. However, ribs of ''Megacephalosaurus'' are double-headed, meaning that the head of the rib is structured for attachment at two points of a vertebra. This is a unique feature among Cretaceous pliosaurs, of which all but ''Megacephalosaurus'' lack. Double-headed ribs have been present in some Jurassic pliosaurs and it was previously thought that this feature disappeared with their extinction.


Classification

''Megacephalosaurus'' is a member of the subfamily
Brachaucheninae Pliosauridae is a family (biology), family of plesiosaurian marine reptiles from the Late Triassic, Latest Triassic to the early Late Cretaceous (Rhaetian to Turonian stages). The family is more inclusive than the archetypal short-necked large he ...
, which consists of pliosaurs that are currently only known during the Cretaceous period. Phylogenetic analyses of all known Cretaceous pliosaurs have consistently recovered them within the subfamily, which led to a hypothesis that it was the only lineage of pliosaurs that crossed the boundary between the Jurassic and Cretaceous. However, these patterns may have been due to large gaps in the fossil record, particularly an absence of pliosaur fossils during many stages of the Early Cretaceous. Recent discoveries of Early Cretaceous pliosaur teeth exhibiting distinct teeth characteristics now challenge the hypothesis and alternatively suggests that at least one more lineage crossed the Jurassic-Cretaceous divide. Members of the Brachaucheninae are variable and only one uniting characteristic between all is known; the possession of somewhat circularly-shaped teeth rather than full or somewhat trihedral-shaped teeth seen in some Jurassic pliosaurs. Some characteristics that are shared by most brachauchenines like ''Megacephalosaurus'' includes skull features (such as an elongated snout, gracile rostrum, and consistently sized teeth) that are better adapted for a general evolutionary shift towards smaller prey. However, there are notable exceptions such as ''
Kronosaurus ''Kronosaurus'' ( ) is an extinct genus of large short-necked pliosaur that lived during the Aptian to Albian Stage (stratigraphy), stages of the Early Cretaceous in what is now Australia. The first known specimen was received in 1899 and consis ...
'', which has teeth that are each shaped differently. A 2018 study by Daniel Madzia of the
Polish Academy of Sciences The Polish Academy of Sciences (, PAN) is a Polish state-sponsored institution of higher learning. Headquartered in Warsaw, it is responsible for spearheading the development of science across the country by a society of distinguished scholars a ...
, Sven Sachs of the
Natural History Museum, Bielefeld The Natural History Museum in Bielefeld () is a natural history museum in the city of Bielefeld in North Rhine-Westphalia, Germany. Since 2003, it was given the additional name namu, which stands for the German words Natur (nature), Mensch (man), ...
, and Johan Lindgren of
Lund University Lund University () is a Public university, public research university in Sweden and one of Northern Europe's oldest universities. The university is located in the city of Lund in the Swedish province of Scania. The university was officially foun ...
hypothesized that the presence of these inconsistencies indicates that the trait for consistently-sized teeth evolved independently within the Brachaucheninae three times; these occurrences being independently in '' Luskhan'', ''
Stenorhynchosaurus ''Stenorhynchosaurus'' is an extinct genus of pliosaurid plesiosaurs which lived in the Early Cretaceous of South America. The type species and only known is ''Stenorhynchosaurus munozi''. Etymology The genus name is derived from the Greek w ...
'', and in a clade that includes ''Megacephalosaurus'' and ''Brachauchenius''. An early phylogenetic attempt including a ''Megacephalosaurus'' specimen was performed in a 2012 study led by Roger Benson 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 ...
. It recovered FHSM VP-321 in a clade in which it is basal to another clade that includes ''Brachauchenius'' and ''Kronosaurus''. In 2013, Benson led another study that attempted another phylogenetic analysis using a better description of FHSM VP-321 made by Schumacher ''et al.'' (2013), being named as ''Brachauchenius eulerti''. This time, the study recovered FHSM VP-321 from a
polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tree ...
clade shared by ''Brachauchenius'' and a pliosaur specimen cataloged as DORK/G/1-2 but doubtfully labeled as'' ' Polyptychodon interruptus, with one outgroup consisting of ''Kronosaurus'', under a strict reduced consensus. In an alternate strict consensus analysis, the study recovered the FHSM VP-321 from a polytomy clade that includes ''
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 ...
'' and ''
Gallardosaurus ''Gallardosaurus'' is a genus of pliosaurid plesiosaur from the Caribbean seaway. It contains the single species ''Gallardosaurus iturraldei''. ''Gallardosaurus'' was found in middle-late Oxfordian-age (Late Jurassic) rocks of the Jagua Formati ...
''. Another study published in 2015 by Andrea Caua and Federico Fanti of the
University of Bologna The University of Bologna (, abbreviated Unibo) is a Public university, public research university in Bologna, Italy. Teaching began around 1088, with the university becoming organised as guilds of students () by the late 12th century. It is the ...
yielded similar phylogenetic results in a strict consensus method. Madzia ''et al.'' (2018) attempted multiple phylogenetic analyses with different methods of consensus. All strict consensus methods recovered ''Megacephalosaurus'' from a polytomy clade only shared by ''Brachauchenius'', ''Kronosaurus'', and DORK/G/1-2, while some
majority rule In social choice theory, the majority rule (MR) is a social choice rule which says that, when comparing two options (such as bills or candidates), the option preferred by more than half of the voters (a ''majority'') should win. In political ...
consensus methods saw ''Megacephalosaurus'' fall in a clade that had a polytomy with DORK/G/1-2 but derived from a clade shared with ''Kronosaurus'' and basal to a clade that includes ''Brachauchenius''. The cladogram below is modified from Madzia ''et al.'' (2018).


Paleoecology

''Megacephalosaurus'' was among the last of the pliosaurs. It inhabited the
Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...
that spanned the middle of North America and cut it in two during the Turonian stage of the Late Cretaceous. The pliosaur was present during the
Cenomanian-Turonian boundary event The Cenomanian-Turonian boundary event, also known as the Cenomanian-Turonian extinction, Cenomanian-Turonian Oceanic Anoxic Event ( OAE 2), and referred to also as the Bonarelli Event or Level, was an anoxic extinction event in the Cretaceous p ...
, a period marked with significant worldwide faunal turnovers and extinctions caused by an abnormally intense increase in underwater
volcanism Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...
, which ushered a global
anoxic event An anoxic event describes a period wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic ( anoxic and sulfidic) waters. Although anoxic events have not happened for millions of years, the geol ...
that acidified the oceans, increased global temperatures, and caused a mass extinction that led to the disappearance of 26% of the entire marine fauna. Despite this, the vertebrate assemblages in the Western Interior Seaway remained stable throughout the ordeal as many of the taxa are also known from deposits before and after the boundary. The fossil assemblages of the Turonian-aged portions of the Fairport Chalk and Greenhorn Shale are considered small in terms of the number of present species but nevertheless encompassed a wide ecological diversity. Sharks constituted the majority of vertebrate diversity, of which ''
Cretoxyrhina ''Cretoxyrhina'' (; meaning 'Cretaceous sharp-nose') is an extinct genus of large mackerel shark that lived about 107 to 73 million years ago during the late Albian to late Campanian of the Late Cretaceous. The type species, ''C. mantelli'', is m ...
'' and ''
Squalicorax ''Squalicorax'', commonly known as the crow shark, is a genus of extinct lamniform shark known to have lived during the Cretaceous period. The genus had a global distribution in the Late Cretaceous epoch. Multiple species within this genus are co ...
'' were the most common. Other types of sharks that were present included the
mackerel sharks The Lamniformes (, from Greek ''lamna'' "fish of prey") are an order of sharks commonly known as mackerel sharks (which may also refer specifically to the family Lamnidae). It includes some of the most familiar species of sharks, such as the grea ...
''
Cretalamna ''Cretalamna'' is a genus of extinction, extinct Otodontidae, otodontid shark that lived from the latest Early Cretaceous to Eocene epoch (about 103 to 46 million years ago). It is considered by many to be the ancestor of the largest sharks to ha ...
'', ''
Cardabiodon ''Cardabiodon'' (; meaning 'Cardabia tooth') is an extinct genus of large mackerel shark that lived about 95 to 91 million years ago (Ma) during the Cenomanian to Turonian of the Late Cretaceous. It is a member of the Cardabiodontidae, a family ...
'', ''
Cretodus ''Cretodus'' is an extinct genus of large mackerel sharks belonging to the proposed family Pseudoscapanorhynchidae. ''Cretodus'' lived during the Late Cretaceous, ranging from the CenomanianCicimurri, D. J. (2001). "Fossil selachians from the B ...
'', ''
Archaeolamna ''Archaeolamna'' (from Greek ''arche'' which turned into ''archaeo'' and '' Lamna'', an extinct shark genus) is an extinct genus of mackerel sharks that lived during the Cretaceous. It contains three valid species (one with two subspecies) which ...
'' and ''
Dallasiella ''Dallasiella'' is an extinct genus of mackerel sharks that lived during the Late Cretaceous. It contains two valid species, ''D. willistoni'' and ''D. brachyodon'', which have been found in North America and Europe. While formerly placed in t ...
'', the
durophagous Durophagy is the eating behavior of animals that consume hard-shelled or exoskeleton-bearing organisms, such as corals, shelled mollusks, or crabs. It is mostly used to describe fish, but is also used when describing reptiles, including fossil t ...
''
Ptychodus ''Ptychodus'' (from 'fold' and 'tooth') is a genus of extinct large Durophagy, durophagous (shell-crushing) lamniformes, lamniform sharks from the Cretaceous period, spanning from the Albian to the Campanian. Fossils of ''Ptychodus'' teeth ar ...
'', and the sand shark ''
Johnlongia ''Johnlongia'' is an extinct genus of sand shark from the Cretaceous period. It contains two described species, ''J. parvidens'' and ''J. allocotodon'', and possibly a third unnamed species from the Niobrara Chalk. It is presumed piscivorous; ho ...
''. Several bony fish are also known. Many of their fossils are too fragmentary to be properly identified, but known Turonian taxa include but not limited to ''
Enchodus ''Enchodus'' (from , 'spear' and 'tooth') is an extinct genus of aulopiformes, aulopiform Actinopterygii, ray-finned fish related to alepisaurus, lancetfish and lizardfish. Species of ''Enchodus'' flourished during the Late Cretaceous, where t ...
'', ''
Pachyrhizodus ''Pachyrhizodus'' is an extinct genus of ray-finned fish that lived during the Late Cretaceous, Cretaceous to Paleocene in what is now Europe, North America, South America, and Oceania. Many species are known, primarily from the Cretaceous of Eng ...
'', and the ichthyodectids ''
Ichthyodectes ''Ichthyodectes'' is an extinct genus of ichthyodectid ray-finned fish which lived during the Late Cretaceous. Fossils of the species included have been found from Canada to Texas. Description ''Ichthyodectes ctenodon'' ("fish biter with comb ...
'' and ''
Xiphactinus ''Xiphactinus'' (from Latin and Greek for "sword-ray") is an extinct genus of large predatory marine ray-finned fish that lived during the late Albian to the late Maastrichtian. The genus grew up to in length, and superficially resembled a garga ...
''. While consistent in fauna, the abundance of fish fossils from the Turonian-aged Fairport Chalk and Greenhorn Shale are significantly lower than such from older deposits such as an earlier deposit of the Greenhorn Shale below the Cenomanian-Turonian boundary that has yielded hundreds to thousands of fish fossils. However, this may simply be a case of collecting bias. Of the marine reptiles, plesiosaurs – including the
polycotylid 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 ...
''
Trinacromerum ''Trinacromerum'' is an extinct genus of sauropterygian reptile, a member of the polycotylid plesiosaurs. It contains two species, ''T. bentonianum'' and ''T. kirki''. Specimens have been discovered in the Late Cretaceous fossil deposits of what ...
'', pliosaurs such as ''Brachauchenius'' and ''Megacephalosaurus'' itself, and indeterminate elasmosaurs –
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, the
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 ...
''
Terminonaris ''Terminonaris'' is a genus of extinct pholidosaurid crocodyliforms that lived during the Late Cretaceous (Cenomanian and TuronianWu X-C, Russell AP, & Cumbaa SL. 2001. ''Terminonaris'' (Archosauria: Crocodyliformes): new material from Saskatchew ...
'', the marine
squamate Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest Order (biology), order of reptiles; most members of which are commonly known as Lizard, lizards, with the group also including Snake, snakes. With over 11,991 species, it i ...
''
Coniasaurus ''Coniasaurus'' is an extinct genus of Late Cretaceous marine squamates that range in age from Cenomanian to Santonian. It was first described by Richard Owen in 1850 from lower Cenomanian chalk deposits in South East England (Sussex). Two spec ...
'', and
plioplatecarpine Plioplatecarpinae is a subfamily of mosasaurs, a diverse group of Late Cretaceous marine squamates. Members of the subfamily are informally and collectively known as "plioplatecarpines" and have been recovered from all continents, though the occu ...
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 ...
s have been found within the Turonian Fairport Chalk and/or Greenhorn Shale. The Turonian also marked the beginning of the radiation of mosasaurs, coinciding with the temporal range of ''Megacephalosaurus'', which would have encountered such marine reptiles. The temporal range of ''Megacephalosaurus'' extends past the Cenomanian-Turonian boundary and after the anoxic event. By this point, only three known pliosaur taxa remained: itself, ''Brachauchenius'', and ''
Polyptychodon ''Polyptychodon'' (meaning 'many-folded tooth') is a genus of pliosaurid found in Middle-Late Cretaceous marine deposits in southern England, France and Argentina. It has been considered a ''nomen dubium'' in a 2016 review. History of discover ...
''. All three pliosaurs disappeared during the progression of the Turonian stage, marking the extinction of the pliosaurs. It has been hypothesized that this final demise may have been linked with the rise of polycotylids, which have been rapidly diversifying around the same time.


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 ...


Notes


References


External links

* {{Taxonbar, from=Q12981281 Late Cretaceous plesiosaurs of North America Pliosauridae Fossil taxa described in 2013 Paleontology in Kansas Sauropterygian genera