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Choristodera (from the
Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...
χωριστός ''chōristos'' + δέρη ''dérē'', 'separated neck') 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 ...
order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...
of
semiaquatic In biology, being semi-aquatic refers to various macroorganisms that live regularly in both aquatic and terrestrial environments. When referring to animals, the term describes those that actively spend part of their daily time in water (in ...
diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The earliest traditionally identified diapsids, the araeosc ...
reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...
s that ranged from the Middle
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. ...
, or possibly
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
, to the
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
(168 to 20 or possibly 11.6 million years ago). Choristoderes are morphologically diverse, with the best known members being the crocodile-like neochoristoderes such as '' Champsosaurus''. Other choristoderans had lizard-like or long necked morphologies. Choristoderes appear to have been confined to the Northern Hemisphere, having been found in
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 ...
,
Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...
, and
Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...
, and possibly also
North Africa North Africa (sometimes Northern Africa) is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region. However, it is sometimes defined as stretching from the Atlantic shores of t ...
. Choristoderes are generally thought to be derived neodiapsids that are close relatives or members of Sauria.


History of discovery

Choristodera was erected in 1876, originally as a
suborder Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and recognized ...
of
Rhynchocephalia Rhynchocephalia (; ) is an order of lizard-like reptiles that includes only one living species, the tuatara (''Sphenodon punctatus'') of New Zealand. Despite its current lack of diversity, during the Mesozoic rhynchocephalians were a speciose g ...
by
Edward Drinker Cope Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontology, paleontologist, comparative anatomy, comparative anatomist, herpetology, herpetologist, and ichthyology, ichthyologist. Born to a wealthy Quaker fam ...
to contain '' Champsosaurus,'' which was described from
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 ...
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
Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...
by Cope in the same paper. A year later, in 1877, '' Simoedosaurus'' was described by
Paul Gervais Paul Gervais (full name: François Louis Paul Gervais) (26 September 1816 – 10 February 1879) was a French palaeontologist and entomologist. Biography Gervais was born in Paris, where he obtained the diplomas of doctor of science and of medic ...
from Upper
Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...
deposits at Cernay, near
Rheims Reims ( ; ; also spelled Rheims in English) is the most populous city in the French department of Marne, and the 12th most populous city in France. The city lies northeast of Paris on the Vesle river, a tributary of the Aisne. Founded by ...
, France. These remained the only recognised choristoderes for over a century, until new taxa were described in the late 20th century. Beginning in the late 1970s, additional taxa were described by Soviet-Mongolian teams from Lower Cretaceous sediments in Mongolia. In studies from 1989 to 1991,
Susan E. Evans Susan Elizabeth Evans is a British palaeontologist and herpetologist. She is the author or co-author of over 100 peer-reviewed papers and book chapters. She received a BSc in Zoology at Bedford College (London), Bedford College in 1974, and i ...
described new material of '' Cteniogenys'' 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 Britain. The genus had first been described by Charles W. Gilmore in 1928 from the
Late Jurassic The Late Jurassic is the third Epoch (geology), epoch of the Jurassic Period, and it spans the geologic time scale, geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic stratum, strata.Owen ...
of the western United States, and had previously been enigmatic. The studies revealed it to be a small, lizard-like choristodere, different from the crocodile-like forms previously known.


Description

Choristoderes vary substantially in size, the smallest genera like '' Cteniogenys'' and ''
Lazarussuchus ''Lazarussuchus'' (meaning " Lazarus's crocodile") is an extinct genus of amphibious reptile, known from the Cenozoic of Europe. It is the youngest known member of Choristodera, an extinct order of aquatic reptiles that first appeared in the Mi ...
'' had a length of only around , and the largest known choristoderan, '' Kosmodraco dakotensis'' is estimated to have had a total length of around . Neochoristoderes such as ''Champsosaurus'' are the best-known group of the Choristodera. They resembled modern
crocodilia Crocodilia () is an order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchia ...
ns, especially gharials. The
skull 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 ...
of these animals have a long, thin snout filled with small, sharp conical teeth. Other choristoderes are referred to collectively as "non-neochoristoderes", which are mostly small lizard-like forms, though ''
Shokawa ''Shokawa'' is an extinct genus of choristoderan diapsid reptile, known from the Lower Cretaceous of Japan. It is only known from one species, ''Shokawa ikoi.'' The only known remains are a postcranial specimen lacking the skull, discovered at th ...
, Khurendukhosaurus'' and '' Hyphalosaurus'' possess long
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 ...
like necks. The grouping of "non-neochoristoderes" is
paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
(not containing all descendants of a common ancestor), as the lizard-like bodyform represents the ancestral morphology of the group.


Skeletal anatomy

According to Matsumoto and colleagues (2019), choristoderes are united by the presence of nine
synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to ...
(shared traits characteristic of the group), including a median contact of the elongated
prefrontal bone The prefrontal bone is a bone separating the lacrimal and frontal bones in many tetrapod skulls. It first evolved in the sarcopterygian clade Rhipidistia, which includes lungfish and the Tetrapodomorpha. The prefrontal is found in most modern and ...
s of the skull separating the nasal bones from the
frontal bone In the human skull, the frontal bone or sincipital bone is an unpaired bone which consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bo ...
s, the
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 ...
flange of 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 ...
is inflected medially (toward the midline of the body), the parietal foramen are absent, the
squamosal bone The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral ...
s are expanded behind ( posterior to) the
occipital condyle The occipital condyles are undersurface protuberances of the occipital bone in vertebrates, which function in articulation with the superior facets of the Atlas (anatomy), atlas vertebra. The condyles are oval or reniform (kidney-shaped) in shape ...
, the teeth are conical and sub- thecodont (located in shallow sockets), the dentaries are slender with elongated grooves running along the labial (outward facing) surface of the bone, additional
sacral vertebrae The sacrum (: sacra or sacrums), in human anatomy, is a triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part of the pelvic cavity, ...
are present, expanded "spine tables" are present on the vertebrae, and the surfaces of both ends of
vertebral centra 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 ...
are flat (amphiplatyan). All known choristoderans possess or are inferred to possess a novel skull bone not found in other reptiles, referred to as the "neomorphic bone" or neomorph, which is a component of the
dermatocranium The dermatocranium is the portion of the cranium that is composed of dermal bone, as opposed to the endocranium and splanchnocranium, which are composed of endochondral bone. The dermatocranium comprises the skull roof, the facial skeleton (usu ...
. Ancestrally, the skull of choristoderes possess elongated upper and lower temporal fenestrae (openings of the skull behind the eye socket), these are greatly expanded in neochoristoderes, most extremely in ''Champsosaurus'', giving the skull a cordiform (heart shaped) appearance when viewed from above. In many "non-neochoristoderes" the lower temporal fenestrae are secondarily closed. Choristoderes possessed
gastralia Gastralia (: gastralium) are dermal bones found in the ventral body wall of modern crocodilians and tuatara, and many prehistoric tetrapods. They are found between the sternum and pelvis, and do not articulate with the vertebrae. In these reptil ...
(rib-like bones situated in the abdomen) like
tuatara The tuatara (''Sphenodon punctatus'') is a species of reptile endemic to New Zealand. Despite its close resemblance to lizards, it is actually the only extant member of a distinct lineage, the previously highly diverse order Rhynchocephal ...
and
crocodilia Crocodilia () is an order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchia ...
ns.


Internal skull anatomy

The internal skull anatomy of choristoderes is only known for ''Champsosaurus.'' 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 ...
of ''Champsosaurus'' is poorly ossified at the front of the skull (anterior), but is well ossified in the rear (posterior) similar to other diapsids. The cranial endocast (space occupied by the brain in the
cranial vault The cranial vault is the space in the skull within the neurocranium, occupied by the brain. Development In humans, the cranial vault is imperfectly composed in newborns, to allow the large human head to pass through the birth canal. During bir ...
) is proportionally narrow in both lateral and dorsoventral axes, with an enlarged
pineal body The pineal gland (also known as the pineal body or epiphysis cerebri) is a small endocrine gland in the brain of most vertebrates. It produces melatonin, a serotonin-derived hormone, which modulates sleep patterns following the diurnal cycles. ...
and
olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OF ...
s. The optic lobes and flocculi are small in size, indicating only average vision ability at best. The olfactory chambers of the nasal passages and olfactory stalks of the braincase are reasonably large, indicating that ''Champsosaurus'' probably had good
olfactory The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, it ...
capabilities (sense of smell). The nasal passages lack bony turbinates. The
semicircular canals The semicircular canals are three semicircular interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the lateral, anterior and posterior semicircular canals. They are the part of the bony labyrinth, ...
of the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
are most similar to those of other aquatic reptiles. The expansion of the sacculus indicates that ''Champsosaurus'' likely had an increased sensitivity to low frequency sounds and vibrations.


Dentition

Most choristoderes have rather simple undifferentiated (
homodont 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 where ...
) teeth, with striated enamel covering the tooth crown but not the base. Neochoristoderes have teeth completely enveloped in striated enamel with an enamel infolding at the base, labiolingually compressed and hooked, the exception being '' Ikechosaurus'' which has still rather simple teeth aside from the start of an enamel infolding. Teeth implantation is subthecodont, with teeth being replaced by erosion of a pit in the lingual (side of the tooth facing the tongue) surface of the tooth base. There is some tooth differentiation among neochoristoderes, with the anterior teeth being sharper and more slender than posterior teeth. Choristoderes retain
palatal 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 sepa ...
teeth (teeth present on the bones of the roof of the mouth). Unlike most diapsid groups, where palatal teeth are reduced or lost completely, the palatal teeth in choristoderes are extensively developed indicating food manipulation in the mouth, probably in combination with the tongue. In most choristoderes, longitudinal rows of palatal teeth are present on the pterygoid,
palatine A palatine or palatinus (Latin; : ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since Roman Empire, Roman times.
and
vomer The vomer (; ) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones. The vomer forms ...
, as well as a row on the pterygoid flange. In some neochoristoderes the palatal tooth rows are modified into tooth batteries on raised platforms. The morphology of the palatal teeth is identical to that of the marginal teeth of non-neochoristoderes, and the replacement of palatal teeth is nearly identical to the replacement of marginal teeth.


Skin

An exceptionally preserved specimen of '' Monjurosuchus'' preserves pleated skin, which indicates that in life it was probably thin and soft. The preserved
scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number ...
are small and overlapping, and are smaller on the ventral underside of the body than the dorsal surface. A double row of larger ovoid scales runs along the dorsum (upper midline) of the body. The fossil also preserves webbed feet. ''Hyphalosaurus'' was covered in scales of varying shape, depending on their position on the body, with at least one and possibly multiple rows of large ovoid scales running down sides of the trunk and tail. The feet display evidence of webbing, and the tail probably had additional tissue at the top and bottom, allowing it to be used as a fin to propel ''Hyphalosaurus'' through the water. Skin impressions of ''Champsosaurus'' have also been reported, they consist of small (0.6-0.1 mm) pustulate and rhomboid scales, with the largest scales being located on the lateral sides of the body, decreasing in size dorsally, no
osteoderm Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinct amph ...
s were present. The Menat specimen of ''
Lazarussuchus ''Lazarussuchus'' (meaning " Lazarus's crocodile") is an extinct genus of amphibious reptile, known from the Cenozoic of Europe. It is the youngest known member of Choristodera, an extinct order of aquatic reptiles that first appeared in the Mi ...
'' preserves some remnants of soft tissue, but no scales, which shows that the hindfoot (pes) was not webbed, and a dark stained region with a crenellated edge is present above the caudal vertebrae of the tail, suggestive of a crest similar to those found in some living reptiles, like the tuatara, lizards and crocodiles.


Paleobiology

Choristoderes are exclusively found in freshwater deposits, often associated with
turtle Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...
s, fish,
frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...
s,
salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...
s and
crocodyliformes Crocodyliformes is a clade of crurotarsan archosaurs, the group often traditionally referred to as "crocodilians". They are the first members of Crocodylomorpha to possess many of the features that define later relatives. They are the only pseu ...
. They appear to have been almost exclusively found in warm
temperate climate In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ra ...
s, with the range of neochoristoderes extending to the high
Canadian Arctic Northern Canada (), colloquially the North or the Territories, is the vast northernmost region of Canada, variously defined by geography and politics. Politically, the term refers to the three territories of Canada: Yukon, Northwest Territories a ...
during the
Coniacian The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series and spans the time between 89.8 ± 1 Ma and 86.3 ± 0.7 Ma (million years ago). The Coniacian is preceded by ...
-
Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya ( million years ago) and 83.6 ± 0.7 m ...
stages of the Late Cretaceous (~89-83 Million years ago), a time of extreme warmth. Due to the morphological similarities between choristoderes and crocodyliformes, it has often been assumed that they existed in competition. However "non-neochoristoderes" were smaller than adult aquatic crocodyliformes and were more likely in competition with other taxa. For the more crocodile-like neochoristoderes, there appears to have been
niche differentiation 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 ...
, with gharial-like neochoristoderans occurring in association with blunt snouted crocodyliformes, but not in association with narrow snouted forms.


Diet

Neochoristoderans are presumed to have been
piscivorous 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 rept ...
. ''Champsosaurus'' in particular is thought to have fed like modern gharials, sweeping its head to the side to catch individual fish from shoals, while ''Simoedosaurus'' is thought to have been more generalist, being able to take both aquatic and terrestrial prey. ''Cteniogenys'' and ''Lazarussuchus'' have been suggested to have fed on invertebrates. Preserved gut contents of a ''Monjurosuchus'' specimen appear to show
arthropod cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non-homology (biology), homologous, differing in th ...
fragments. Another specimen of ''Monjurosuchus'' has been found with preserved skulls of seven juvenile individuals within the abdominal cavity. This has been proposed to represent evidence of
cannibalism Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is also well document ...
. However, this proposal has been criticised by other authors, who suggest it is more likely that they represent late-stage embryos. A specimen of ''Hyphalosaurus'' has been found with small rib bones in its abdominal cavity, suggesting that it took vertebrate prey at least on occasion.


Reproduction

A specimen of ''Hyphalosaurus'' ''baitaigouensis'' has been found with 18 fully developed embryos within the mother's body, suggesting that they were
viviparous In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juve ...
, but another specimen shows that ''Hyphalosaurus baitaigouensis'' also possessed soft-shelled eggs, similar to those of
lepidosaurs The Lepidosauria (, from Greek meaning ''scaled lizards'') is a Order (biology), superorder or Class (biology), subclass of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata also includes Lizard, lizards and Snake, snakes. Sq ...
. A possible explanation for this is that ''Hyphalosaurus'' was
ovoviviparous Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a "bridging" form of reproduction between egg-laying oviparity, oviparous and live-bearing viviparity, viviparous reproduction. Ovoviviparous animals possess embryos that develo ...
, with the thin-shelled eggs hatching immediately after they were laid, presumably on land, though it has also been suggested that the species employed both viviparous and
oviparous Oviparous animals are animals that reproduce by depositing fertilized zygotes outside the body (i.e., by laying or spawning) in metabolically independent incubation organs known as eggs, which nurture the embryo into moving offsprings kno ...
reproductive modes. An embryo of ''Ikechosaurus'' has been found preserved within a weakly mineralised parchment-shelled egg, suggesting that ''Ikechosaurus'' was oviparous, and laid their eggs on land. ''Monjuruosuchus'' has been suggested to have been viviparous. In ''Champsosaurus'', it has been suggested that adult females could crawl ashore to lay eggs on land, with males and juveniles appearing to be incapable of doing so, based on the presumably
sexually dimorphic Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...
fusion of the sacral vertebrae and possession of more robust limb bones in presumed females. A skeleton of '' Philydrosaurus'' has been found with associated post-hatchling stage juveniles, suggesting that they engaged in post-hatching
parental care Parental care is a behavioural and evolutionary strategy adopted by some animals, involving a parental investment being made to the evolutionary fitness of offspring. Patterns of parental care are widespread and highly diverse across the animal k ...
.


Tracks

Tracks from the
Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...
(
Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...
) of South Korea, given the
ichnotaxon An ichnotaxon (plural ichnotaxa) is "a taxon based on the fossilized work of an organism", i.e. the non-human equivalent of an artifact. ''Ichnotaxon'' comes from the Ancient Greek (''íchnos'') meaning "track" and English , itself derived from ...
name '' Novapes ulsanensis'' have been attributed to choristoderans, based on the similarity of the pentadactyl (five fingered) preserved tracks to the foot morphology of ''Monjurosuchus''. The tracks preserve traces of webbing between the digits. The authors of the study proposed based on the spacing of the prints, that choristoderans could " high walk" like modern crocodilians. Tracks attributed to neochoristoderans dubbed '' Champsosaurichnus parfeti'' have also been reported from the Late Cretaceous
Laramie Formation The Laramie Formation is a geologic formation (geology), formation of the Late Cretaceous (Maastrichtian) age, named by Clarence King in 1876 for exposures in northeastern Colorado, in the United States.King, C. 1876. Report of the Geological Exp ...
of the United States, though only two prints are present and it is not possible to distinguish between a manus (forefoot) or pes (hindfoot).


Classification and phylogeny


Internal systematics

Historically, the internal phylogenetics of Choristodera were unclear, with the neochoristoderes being recovered as a well-supported clade, but the relationships of the "non-neochoristoderes" being poorly resolved. However, during the 2010s, the "non-neochoristoderes" from the Early Cretaceous of Asia (with the exception of '' Heishanosaurus'') alongside ''Lazarussuchus'' from the
Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...
of Europe were recovered (with weak support) as belonging to a monophyletic clade, which were informally named the "Allochoristoderes" by Dong and colleagues in 2020, characterised by the shared trait of completely closed lower temporal fenestrae, with ''Cteniogenys'' from the Middle-Late Jurassic of Europe and North America being consistently recovered as the basalmost choristodere. The long necked "non-neochoristoderes" ''Shokawa'' and ''Hyphalosaurus'' have often been recovered as a clade, dubbed the Hyphalosauridae by Gao and Fox in 2005. The finding of more complete material of the previously fragmentary ''Khurendukhosaurus'' shows that it also has a long neck, and it has also been recovered as part of the clade. Phylogeny from the analysis of Dong and colleagues (2020):


Relationships to other reptiles

Choristoderes are universally agreed to be members of Neodiapsida, but their exact placement in the clade is uncertain, due to their mix of primitive and derived features, and a long ghost lineage (absence of a fossil record) after their split from other reptiles. After initially being placed in Rhynchocephalia, Cope later suggested a placement in Lacertilla due to the shape of the cervical vertebrae.
Louis Dollo Louis Antoine Marie Joseph Dollo (; 7 December 1857 – 19 April 1931) was a Belgian palaeontologist, known for his work on dinosaurs. He also posited that evolution is not reversible, known as Dollo's law. Together with the Austrian Othenio Ab ...
in 1891 returned Choristodera to Rhynchocephalia, but in 1893 suggested a close relationship with ''
Pareiasaurus ''Pareiasaurus'' (from , "cheek" and , "lizard") is an extinct genus of Pareiasauromorpha, pareiasauromorph reptile from the Permian period. It was a typical member of its family (biology), family, the pareiasaurids, which take their name from th ...
''. Alfred Romer in publications in 1956 and 1968 placed Choristodera within the paraphyletic or
polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...
grouping of " Eosuchia", describing them, as "an offshoot of the basic eosuchian stock", a classification which was widely accepted. However, the use of computer based cladistics in the 1980s demonstrated the non-monophyly of "Eosuchia", making the classification of choristoderes again uncertain. Subsequent studies either suggested placement as archosauromorphs, lepidosauromorphs or members of Diapsida ''
incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...
''. In a 2016 analysis of neodiapsid relationships by
Martín Ezcurra Martín Daniel Ezcurra (born April 23, 1987) is an Argentine palaeontologist naming many extinct genera such as '' Aerotitan'', '' Lophostropheus'' and '' Powellvenator''. Biography Ezcurra was born on April 23, 1987. He first got into paleonto ...
they were recovered as members of the advanced neodiapsid group Sauria, in 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 ...
with Lepidosauromorpha and Archosauromorpha, with being the earliest diverging members of either group also being plausible. A position as basal archosauromorphs is supported by the ossification sequence of their embryos.


Evolutionary history

Choristoderes must have diverged from all other known reptile groups prior to the end of the
Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
period, over 250 million years ago, based on their primitive phylogenetic position. In 2015, Rainer R. Schoch reported a new small (~ 20 cm long) diapsid from the
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...
(
Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic ...
) Lower Keuper of
Southern Germany Southern Germany (, ) is a region of Germany that includes the areas in which Upper German dialects are spoken, which includes the stem duchies of Bavaria and Swabia in present-day Bavaria, Baden-Württemberg, and the southern portion of Hesse ...
, known from both cranial and postcranial material, which he claimed represented the oldest known choristodere. '' Pachystropheus'' from the
Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...
(
Rhaetian The Rhaetian is the latest age (geology), age of the Triassic period (geology), Period (in geochronology) or the uppermost stage (stratigraphy), stage of the Triassic system (stratigraphy), System (in chronostratigraphy). It was preceded by the N ...
) of Britain was historically suggested to be a choristodere, but was later demonstrated to be a member of the marine reptile group
Thalattosauria Thalattosauria ( Greek for "sea lizards") is an extinct order of marine reptiles that lived during the Triassic Period. Thalattosaurs were diverse in size and shape, and are divided into two superfamilies: Askeptosauroidea and Thalattosauroide ...
. The oldest unequivocal choristoderan is the small lizard-like ''Cteniogenys,'' the oldest known remains of which are known from the late Middle Jurassic (
Bathonian In the geologic timescale the Bathonian is an age (geology), age and stage (stratigraphy), stage of the Middle Jurassic. It lasted from approximately 168.2 ±1.2 annum, Ma to around 165.3 ±1.1 Ma (million years ago). The Bathonian Age succeeds ...
~168-166 million years ago) Forest Marble and Kilmaluag formations of Britain, with remains also known from the Upper Jurassic
Alcobaça Formation The Alcobaça Formation, previously known as the Guimarota Formation and also known as the Consolação Unit, is a geological Formation (geology), formation in Portugal. It dates back to the Kimmeridgian stage of the Late Jurassic. It is an impor ...
of Portugal and the
Morrison Formation The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltston ...
of the United States, with broadly similar remains also known from the late Middle Jurassic (
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 ...
) Balabansai Formation of Kyrgyzstan in
Central Asia Central Asia is a region of Asia consisting of Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, and Uzbekistan. The countries as a group are also colloquially referred to as the "-stans" as all have names ending with the Persian language, Pers ...
, the Bathonian
Itat Formation The Itat Formation (Russian: итатская свита) is a geologic formation in western Siberia. It was deposited in the Bajocian to Bathonian ages of the Middle Jurassic. Dinosaur remains are among the fossils that have been recovered from ...
of western Siberia, as well as possibly the Bathonian aged Anoual Formation in Morocco, North Africa. Choristoderes underwent a major
evolutionary radiation An evolutionary radiation is an increase in taxonomic diversity that is caused by elevated rates of speciation, that may or may not be associated with an increase in morphological disparity. A significantly large and diverse radiation within ...
in Asia during the
Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...
, which represents the high point of choristoderan diversity, including the first records of the gharial-like Neochoristodera, which appear to have evolved in the regional absence of aquatic
neosuchia Neosuchia is a clade within Mesoeucrocodylia that includes all modern extant crocodilians and their closest fossil relatives. It is defined as the most inclusive clade containing all Crocodylomorpha, crocodylomorphs more closely related to ''Croc ...
n crocodyliformes. A partial femur of an indeterminate choristodere is known from the Yellow Cat Member of the Cedar Mountain Formation in North America. They appear to be absent from the well sampled European localities of the
Berriasian In the geological timescale, the Berriasian is an age/ stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 143.1 ±0.6 Ma and 137.05 ± 0.2 (million years ago) ...
aged
Purbeck Group The Purbeck Group is an Upper Jurassic to Lower Cretaceous lithostratigraphy, lithostratigraphic group (stratigraphy), group (a sequence of rock strata) in south-east England. The name is derived from the district known as the Isle of Purbeck in ...
, Great Britain and the
Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 125.77 Ma (million years ago) and 121.4 ± 1.0 Ma (Historically, this stage was placed at 129.4 million to approximately 125 million years ago) It is a ...
aged
La Huérguina Formation The La Huérguina Formation (also known as the Calizas de La Huérguina Formation, La Huérguina Limestone Formation or as the Una Formation) is a geological Formation (geology), formation in Spain whose strata date back to the Barremian stage of ...
, Spain, though there is a record of a small ''Cteniogenys''-like taxon from the Berriasian aged
Angeac-Charente bonebed The Angeac-Charente bonebed is a fossil deposit located near Angeac-Charente in western France. It dates to the Berriasian stage of the Early Cretaceous, and is coeval with the Purbeck Group of Southern England. It has amongst the most diverse asse ...
in France.Ronan Allain, Romain Vullo, Lee Rozada, Jérémy Anquetin, Renaud Bourgeais, et al.
Vertebrate paleobiodiversity of the Early Cretaceous (Berriasian) Angeac-Charente Lagerstätte (southwestern France): implications for continental faunal turnover at the J/K boundary
Geodiversitas, Museum National d’Histoire Naturelle Paris, In press. ffhal-03264773f
In the latter half of the Late Cretaceous (
Campanian The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...
-
Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...
), the neochoristodere ''Champsosaurus'' is found in Utah, Wyoming, Montana, North Dakota, Alberta and Saskatchewan, which were along the western coast of 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 ...
on the island of
Laramidia Laramidia was an island continent that existed during the Late Cretaceous period (99.6–66 Year#SI prefix multipliers, Ma), when the Western Interior Seaway split the continent of North America in two. In the Mesozoic era, Laramidia was an island ...
. Indeterminate remains of neochoristoderes are also known from the Canadian High Arctic, dating to the early Late Cretaceous (
Coniacian The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series and spans the time between 89.8 ± 1 Ma and 86.3 ± 0.7 Ma (million years ago). The Coniacian is preceded by ...
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), ...
) and from the Navesink Formation of New Jersey from the latest Cretaceous (Maastrichtian), which formed the separate island of
Appalachia Appalachia ( ) is a geographic region located in the Appalachian Mountains#Regions, central and southern sections of the Appalachian Mountains in the east of North America. In the north, its boundaries stretch from the western Catskill Mountai ...
. Vertebrae from the
Cenomanian The Cenomanian is, in the International Commission on Stratigraphy's (ICS) geological timescale, the oldest or earliest age (geology), age of the Late Cretaceous epoch (geology), Epoch or the lowest stage (stratigraphy), stage of the Upper Cretace ...
of Germany and the Campanian aged Grünbach Formation of Austria indicate the presence of choristoderes in Europe during this time period. The only record of choristoderes from Asia in the Late Cretaceous is a single vertebra from the Turonian of Japan. Fragmentary remains found in the Campanian aged Oldman and Dinosaur Park formations in
Alberta Alberta is a Provinces and territories of Canada, province in Canada. It is a part of Western Canada and is one of the three Canadian Prairies, prairie provinces. Alberta is bordered by British Columbia to its west, Saskatchewan to its east, t ...
, Canada, also possibly suggest the presence of small bodied "non-neochoristoderes" in North America during the Late Cretaceous. ''Champsosaurus'' survived the K-Pg extinction, and together with fellow neochoristoderes ''Kosmodraco'' and ''Simoedosaurus'' are present in Europe, Asia and North America during the Paleocene, however they became extinct during the early
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
. Their extinction coincides with major faunal turnover associated with the elevated temperatures of the
Paleocene–Eocene Thermal Maximum The Paleocene–Eocene thermal maximum (PETM), alternatively ”Eocene thermal maximum 1 (ETM1)“ and formerly known as the "Initial Eocene" or “Late Paleocene thermal maximum", was a geologically brief time interval characterized by a ...
. Small bodied "non-neochoristoderes", which are absent from the fossil record after the Early Cretaceous (except for possible North American remains), reappear in the form of the lizard-like ''Lazarussuchus'' from the late Paleocene of France. The European endemic ''Lazarussuchus'' is the last known choristodere, surviving the extinction of neochoristoderes at the beginning of the Eocene, with the youngest known remains being those of ''L. dvoraki'' from the Early
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
of the Czech Republic and as well as possible indeterminate remains of ''Lazarussuchus'' reported from the late Miocene (~11.6 million years ago) of southern Germany.


References


Further reading

* * * * {{Authority control Prehistoric tetrapod orders Callovian first appearances Middle Jurassic taxonomic orders Late Jurassic taxonomic orders Early Cretaceous taxonomic orders Late Cretaceous taxonomic orders Paleocene taxonomic orders Eocene taxonomic orders Oligocene taxonomic orders Miocene taxonomic orders Aquitanian extinctions Tertiary extinctions of vertebrate taxa Taxa named by Edward Drinker Cope