Choristodera (from the Ancient Greek χωριστός chōristos + dérē, 'separated neck'
History of discovery
Choristodera was erected in 1876, originally as a suborder of
Rhynchocephalia by Edward Drinker Cope to contain
Champsosaurus, which was described from
Late Cretaceous strata of
Montana by Cope in the same paper.
A year later, in 1877,
Simoedosaurus was described by
Paul Gervais from Upper
Paleocene deposits at Cernay, near
Reims, 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 described new material of
Cteniogenys from the
Middle Jurassic of Britain. The genus had first been described by Charles W. Gilmore in 1928 from the
Late Jurassic 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 had a length of only around , and the largest known choristoderan,
Kosmodraco is estimated to have had a total length of around .
such as
Champsosaurus are the best-known group of the Choristodera. They resembled modern
, especially
gharials. The
skull 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, Khurendukhosaurus and
Hyphalosaurus possess long
Plesiosauria like necks. The grouping of "non-neochoristoderes" is
Paraphyly (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 (shared traits characteristic of the group), including a median contact of the elongated
of the skull separating the
nasal bones from the
, the dorsal flange of the
maxilla is inflected
Medially rotate (toward the midline of the body), the
Pineal foramen are absent, the
are expanded behind (posterior to) the occipital condyle, the teeth are conical and sub-thecodont (located in shallow sockets), the
Mandible are slender with elongated grooves running along the labial (outward facing) surface of the bone, additional
Sacrum are present, expanded "spine tables" are present on the vertebrae, and the surfaces of both ends of vertebral centra 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.
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
gastralium (rib-like bones situated in the abdomen) like
tuatara and
.
Internal skull anatomy
The internal skull anatomy of choristoderes is only known for
Champsosaurus. The
Neurocranium 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
Endocast (space occupied by the brain in the
cranial vault) is proportionally narrow in both lateral and dorsoventral axes, with an enlarged
Pineal gland and
. The optic lobes and
Flocculus 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
Olfaction capabilities (sense of smell). The nasal passages lack bony
turbinates. The semicircular canals of the
inner ear are most similar to those of other aquatic reptiles. The expansion of the
Saccule indicates that
Champsosaurus likely had an increased sensitivity to low frequency sounds and vibrations.
Dentition
Most choristoderes have rather simple undifferentiated (
homodont) teeth, with striated
Tooth 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
Palate 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 bone,
Palatine bone and
vomer, 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
Reptile scale 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 foot.
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
were present.
The Menat specimen of
Lazarussuchus 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
, fish,
,
and
crocodyliformes. They appear to have been almost exclusively found in warm temperate climates, with the range of neochoristoderes extending to the high
Northern Canada during the
Coniacian-
Santonian 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, 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
Piscivore.
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.
and
Lazarussuchus have been suggested to have fed on invertebrates.
Preserved gut contents of a
Monjurosuchus specimen appear to show arthropod cuticle 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.
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
Viviparity,
but another specimen shows that
Hyphalosaurus baitaigouensis also possessed soft-shelled eggs, similar to those of
Lepidosauria.
A possible explanation for this is that
Hyphalosaurus was
Ovoviviparity, 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 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.
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 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.
Tracks
Tracks from the
Early Cretaceous (
Albian) of South Korea, given the
ichnotaxon name
Novapes 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 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 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
Lacertilia due to the shape of the cervical vertebrae.
Louis Dollo in 1891 returned Choristodera to Rhynchocephalia, but in 1893 suggested a close relationship with
Pareiasaurus.
Alfred Romer in publications in 1956 and 1968 placed Choristodera within the paraphyletic or
Polyphyly 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
Archosauromorpha, lepidosauromorphs or members of Diapsida
incertae sedis. In a 2016 analysis of neodiapsid relationships by Martín Ezcurra they were recovered as members of the advanced neodiapsid group
Sauria, in a
polytomy 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 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 (
Ladinian)
Erfurt Formation of
Southern Germany, known from both cranial and postcranial material, which he claimed represented the oldest known choristodere.
from the
Late Triassic (
Rhaetian) of Britain was historically suggested to be a choristodere,
but was later demonstrated to be a member of the marine reptile group
Thalattosauria.
The oldest unequivocal choristoderan is the small lizard-like
Cteniogenys, the oldest known remains of which are known from the late Middle Jurassic (
Bathonian ~168-166 million years ago) Forest Marble and Kilmaluag formations of Britain, with remains also known from the Upper Jurassic Alcobaça Formation of Portugal and the Morrison Formation of the United States, with broadly similar remains also known from the late Middle Jurassic (
Callovian) Balabansai Formation of Kyrgyzstan in
Central Asia,
the Bathonian
Itat Formation of western Siberia,
as well as possibly the Bathonian aged
Anoual Formation in Morocco, North Africa.
Choristoderes underwent a major evolutionary radiation in Asia during the Early Cretaceous, 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 crocodyliformes.[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-Maastrichtian), 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 on the island of Laramidia.
Champsosaurus survived the K-Pg extinction, and together with fellow neochoristoderes Kosmodraco
Further reading
