Neochoristodera
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Neochoristodera is a lineage of specialised
crocodile Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to inclu ...
-like fully aquatic
choristodere Choristodera (from the Greek χωριστός ''chōristos'' + δέρη ''dérē'', 'separated neck') is an extinct order of semiaquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Triassic, to the late Miocene (168 to ...
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s. Noted for their long jaws and large size, these animals were predominant across the Northern Hemisphere, occurring in freshwater and coastal environments across the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
and early
Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...
.


Systematics

Neochoristoderes form a monophyletic group, however there is no consensus about the relationships of the genera, which have been recovered as 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 tr ...
in recent studies. Neochoristodera contains the named genera ''
Champsosaurus ''Champsosaurus'' is an extinct genus of crocodile-like choristodere reptile, known from the Late Cretaceous and early Paleogene periods of North America and Europe (Campanian-Paleocene). The name ''Champsosaurus'' is thought to come from , () ...
'', '' Ikechosaurus'', ''
Kosmodraco ''Kosmodraco'' is a genus of large bodied choristodere from the Paleocene of North America. Originally described as a species of '' Simoedosaurus'', it was found to represent a distinct genus in 2022. Multiple fossil skulls show a relatively shor ...
'', ''
Liaoxisaurus ''Liaoxisaurus'' is a genus of choristodere, a type of crocodile-like aquatic reptile. It is known from a partial specimen found in Aptian-age Lower Cretaceous rocks of the Jiufotang Formation, Chaoyang, Liaoning, China. ''Liaoxisaurus'' was n ...
'', '' Mengshanosaurus'', ''
Simoedosaurus ''Simoedosaurus'' is an extinct reptile known from the Paleocene of North America, Europe and western Asia, and a member of the Choristodera, a group of aquatic reptiles that lived in the Northern Hemisphere from the Jurassic to the early Cenoz ...
'' and ''
Tchoiria ''Tchoiria'' is a genus of simoedosaurid choristodere, a type of crocodile-like aquatic reptile. Fossils of this genus have been found in Early Cretaceous-age rocks in Mongolia. Four species have been named, but two have been given their own ...
''. Various taxa of uncertain affinities within this group are known, including a partial femur of a choristodere, possibly of a neochoristodere from the
Cedar Mountain Formation The Cedar Mountain Formation is the name given to a distinctive sedimentary geologic formation in eastern Utah, spanning most of the early and mid-Cretaceous. The formation was named for Cedar Mountain (Utah), Cedar Mountain in northern Emery Coun ...
of the United States and an indeterminate partial skeleton from the
Kuwajima Formation The Kuwajima Formation is an Early Cretaceous geologic formation in Japan. Its precise age is uncertain due to a lack of identifying fossils, and it was previously considered likely Valanginian to Hauterivian in age. However, it is now considered ...
of Japan.


Evolution

Neochoristoderes first appear in the
Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145  Ma to 100.5 Ma. Geology Pro ...
of
Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...
, where they co-exist with other choristodere groups like monjurosuchids and hyphalosaurids. Here, a regional absence of aquatic
crocodilian Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...
s, possibly due to colder temperatures, seems to have opened the ecological niche for these choristoderes to occupy a similar ecological niche. The oldest known neochoristodere is '' Mengshanosaurus'', from 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 145.0 ± 4.0 Ma and 139.8 ± 3.0 Ma (million years ago) ...
-
Valanginian In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 139.8 ± 3.0 Ma and 132.9 ± 2.0 Ma (million years ago). The Valanginian Stage succeeds the Berriasian Stage of the Lower Cretaceou ...
aged
Mengyin Formation The Meng-Yin or Mengyin Formation () is a geological formation in Shandong, China, whose strata date back to the Berriasian and Valanginian stages of the Early Cretaceous.Cedar Mountain Formation The Cedar Mountain Formation is the name given to a distinctive sedimentary geologic formation in eastern Utah, spanning most of the early and mid-Cretaceous. The formation was named for Cedar Mountain (Utah), Cedar Mountain in northern Emery Coun ...
, a large gap occurs between these Early Cretaceous faunas and the Late Cretaceous ones. There are no fossils of neochoristoderes in the Asian Late Cretaceous, but the subsequent distribution of ''
Simoedosaurus ''Simoedosaurus'' is an extinct reptile known from the Paleocene of North America, Europe and western Asia, and a member of the Choristodera, a group of aquatic reptiles that lived in the Northern Hemisphere from the Jurassic to the early Cenoz ...
'' in the
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), E ...
and
Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
implies that there were Asian taxa around this time, seeing as Simoedosauridae is predominantly Asian and ''
Simoedosaurus ''Simoedosaurus'' is an extinct reptile known from the Paleocene of North America, Europe and western Asia, and a member of the Choristodera, a group of aquatic reptiles that lived in the Northern Hemisphere from the Jurassic to the early Cenoz ...
'' only propagated widely after the KT event. Choristoderes reappear in the fossil record in the
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. Campanian s ...
of
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
under the genus ''
Champsosaurus ''Champsosaurus'' is an extinct genus of crocodile-like choristodere reptile, known from the Late Cretaceous and early Paleogene periods of North America and Europe (Campanian-Paleocene). The name ''Champsosaurus'' is thought to come from , () ...
''. This genus survives the KT event unscathed and diversifies in the aftermath, being soon after joined by ''
Simoedosaurus ''Simoedosaurus'' is an extinct reptile known from the Paleocene of North America, Europe and western Asia, and a member of the Choristodera, a group of aquatic reptiles that lived in the Northern Hemisphere from the Jurassic to the early Cenoz ...
''. Both taxa remain at high latitudes in
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
and
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
until the Eocene, when they mysteriously disappear. In 2021, fossil remains of indetermine neochoristoderes were described from several sites from the
Navesink Formation The Navesink Formation is a 66 to 70 mya greensand glauconitic marl and sand geological formation in New Jersey. It is known for its Cretaceous period fossil shell beds and dinosaur bones. Description The Navesink Formation, named after Navesin ...
of
New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...
, marking the first known occurrence of neochoristoderes from the former landmass
Appalachia Appalachia () is a cultural region in the Eastern United States that stretches from the Southern Tier of New York State to northern Alabama and Georgia. While the Appalachian Mountains stretch from Belle Isle in Newfoundland and Labrador, Ca ...
. Niche modeling based on the presumed niche of ''Champsosaurus'' indicates that neochoristoderes may have had a widespread distribution in Appalachia, but the majority of this habitat was not located in areas conducive for Cretaceous fossilization, leading to only a small margin of optimal habitat in New Jersey that preserved choristodere remains. Competition from
crocodilia Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...
ns has been at times implicated in the extinction of neochoristoderes. There appears to be a niche partitioning between neochoristoderes and long-snouted crocodilians such as
gharials Gavialidae is a family of large semiaquatic crocodilians with elongated, narrow snouts. Gavialidae consists of two living species, the gharial (''Gavialis gangeticus'') and the false gharial (''Tomistoma schlegelii''), both occurring in Asia. Man ...
, which are absent from freshwater sites in Laurasia until well after neochoristoderes disappear, but competition between both groups, if it even existed, is currently unaccounted for. Ultimately, both ''
Champsosaurus ''Champsosaurus'' is an extinct genus of crocodile-like choristodere reptile, known from the Late Cretaceous and early Paleogene periods of North America and Europe (Campanian-Paleocene). The name ''Champsosaurus'' is thought to come from , () ...
'' and ''
Simoedosaurus ''Simoedosaurus'' is an extinct reptile known from the Paleocene of North America, Europe and western Asia, and a member of the Choristodera, a group of aquatic reptiles that lived in the Northern Hemisphere from the Jurassic to the early Cenoz ...
'' co-existed with a variety of broad-snouted species like alligatorids and ''
Borealosuchus ''Borealosuchus'' (meaning "boreal crocodile") is an extinct genus of crocodyliforms that lived from the Late Cretaceous to the Eocene in North America. It was named by Chris Brochu in 1997 for several species that had been assigned to ''Leidyos ...
''.


Biology

Neochoristoderes are thought to be fully aquatic. They possess laterally flattened, muscular tails and paddle-like limbs, their torsos are dorsoventrally flattened with short but massive ribs and their
gastralia Gastralia (singular 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 ...
are heavily ossified, facilitating sinking. While they have their nostrils at the end of the snout as in crocodiles, they are oriented towards the tip instead of dorsally; their eye sockets are similarly forward-facing, implying that these animals did not surface often and probably simply rose the snout in a snorkel-like fashion. As in most choristoderes the skin lacks
scute A scute or scutum (Latin: ''scutum''; plural: ''scuta'' "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterior po ...
s, instead having small, non-overlapping scales. In ''
Champsosaurus ''Champsosaurus'' is an extinct genus of crocodile-like choristodere reptile, known from the Late Cretaceous and early Paleogene periods of North America and Europe (Campanian-Paleocene). The name ''Champsosaurus'' is thought to come from , () ...
'' adult females are thought to have particularly ossified limbs and pelvises, implying that they could crawl unto land, while adult males and juveniles could not. As most choristoderes are thought to have been
ovoviviparous Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals possess embryos that develop insi ...
or
viviparous Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ...
, it is likely that at least some neochoristoderes were too. Most neochoristoderes have exceptionally large temporal fenestrae, suggesting powerful bite forces; ''
Champsosaurus ''Champsosaurus'' is an extinct genus of crocodile-like choristodere reptile, known from the Late Cretaceous and early Paleogene periods of North America and Europe (Campanian-Paleocene). The name ''Champsosaurus'' is thought to come from , () ...
'' is estimated to have a bite force around 1194 to 1910 N, as opposed to the modern
gharial The gharial (''Gavialis gangeticus''), also known as gavial or fish-eating crocodile, is a crocodilian in the family Gavialidae and among the longest of all living crocodilians. Mature females are long, and males . Adult males have a distinct b ...
's 310-497 N. In spite of this, most are thought to have had a diet similar to that of modern gharials due to the long and slender jaws, though ''
Simoedosaurus ''Simoedosaurus'' is an extinct reptile known from the Paleocene of North America, Europe and western Asia, and a member of the Choristodera, a group of aquatic reptiles that lived in the Northern Hemisphere from the Jurassic to the early Cenoz ...
'' has a more robust and shorter snout and could have fed on larger prey. Compared with other choristoderes, which have rather simple teeth, 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. There is some tooth differentiation, with the anterior teeth being larger than the posterior ones. As with most choristoderes, neochoristoderes have palatal teeth, indicating food manipulation in the mouth. Nearly all neochoristodere remains occur at high latitudes, suggesting a preference for
temperate climates In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...
. ''
Champsosaurus ''Champsosaurus'' is an extinct genus of crocodile-like choristodere reptile, known from the Late Cretaceous and early Paleogene periods of North America and Europe (Campanian-Paleocene). The name ''Champsosaurus'' is thought to come from , () ...
'' fossils are known in the
Canadian Canadians (french: Canadiens) are people identified with the country of Canada. This connection may be residential, legal, historical or cultural. For most Canadians, many (or all) of these connections exist and are collectively the source of ...
Arctic The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...
and
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is t ...
.R. Matsumoto and S. E. Evans. 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberian Geology 36(2):253-274


References

{{Taxonbar, from=Q2405206 Choristodera