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Choristodera

Choristodera

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Choristodera is an order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 of semi-aquatic diapsid
Diapsid
Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...

 reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s which ranged from the Middle Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

, or possibly Late Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

, to at least the early Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

. Choristoderes have been found in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

, and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

. The most common fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s are typically found from the Late Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 to the lower Eocene. Cladists
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

 have placed them between basal diapsid
Diapsid
Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...

s and basal archosauromorphs
Archosauromorpha
Archosauromorpha is an infraclass of diapsid reptiles that first appeared during the late Permian and became more common during the Triassic. Included in this infraclass are the groups Rhynchosauria, Trilophosauridae, Prolacertiformes, Archosauriformes, and, tentatively, Choristodera...

 but the phylogenetic position of Choristodera is still uncertain. It has also been proposed that they represent basal lepidosauromorphs
Lepidosauromorpha
Lepidosauromorpha is a group of reptiles comprising all diapsids closer to lizards than to archosaurs . The only living sub-group is the Lepidosauria: extant lizards, snakes, and tuatara...

. Most recently, workers have placed Choristodera within Archosauromorpha.

Description and phylogeny



Champsosauridae is the best-known family of the Choristodera and typifies the group. Champsosaurus
Champsosaurus
Champsosaurus is an extinct genus of diapsid reptile belonging to the order Choristodera. It grew to about 1.50 m long....

was first described from Late Cretaceous strata of Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...

 by Cope
Edward Drinker Cope
Edward Drinker Cope was an American paleontologist and comparative anatomist, as well as a noted herpetologist and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested in science; he published his first scientific paper at the age of nineteen...

 in 1876. Champsosaurs resembled modern gharial
Gharial
The gharial , , also called Indian gavial or gavial, is the only surviving member of the once well-represented family Gavialidae, a long-established group of crocodilians with long, slender snouts...

s (gavials) or false gharial
False gharial
The false gharial , also known as the Malayan gharial, false gavial, or Tomistoma is a freshwater crocodile of the Crocodylidae family with a very thin and elongated snout...

s. The skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

 of these animals have a long, thin snout filled with small, sharp conical teeth. This is due to champsosaurs and gharials occupying similar niches
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...

: hunting small aquatic prey in rivers and swamps. This is a classic example of evolutionary relay. More primitive choristoderes have shorter, broader snouts.

There are major differences to the gharials and other crocodilia
Crocodilia
Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...

ns, however, particularly in the skull. The orbits are found well forward on the skull, and the rear of the skull is bulbous, hugely expanded and consists of complex bony arches surrounding empty space. These spaces probably contained massive jaw muscles. Other hypotheses for the spaces, such as an otic sensory organ housing, have been tossed around with little support. The external nares are found on the tip of the rostrum. This indicates that champsosaurs breathed while submerged by extending their rostrum through the water surface while their body rested on the bed of the lake or stream. Crocodylians and phytosaur
Phytosaur
Phytosaurs are an extinct group of large semi-aquatic Late Triassic archosaurs. Phytosaurs belong to the family Phytosauridae and the order Phytosauria. They were long-snouted and heavily armoured, bearing a remarkable resemblance to modern crocodiles in size, appearance, and lifestyle, an example...

s have their nares located dorsally on their rostrum or skull respectively. This position allows them to rest submerged just below the surface.

Champsosaur skulls are actually very similar to lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...

 skulls, though heavily modified. This has led some researchers to consider champsosaurids as lepidosauromorphs. However, champsosaurs lack the complex quadrate
Quadrate bone
The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids , and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint .- Evolutionary variation :In snakes, the quadrate bone has become elongated...

 of lepidosauria
Lepidosauria
The Lepidosauria are reptiles with overlapping scales. This subclass includes Squamata and Sphenodontidae. It is a monophyletic group and therefore contains all descendents of a common ancestor. The squamata includes snakes, lizards, tuataras, and amphisbaenia. Lepidosauria is the sister taxon...

ns. With features of both diapsid groups, the phylogenetic position of Choristodera is highly confused.

Other features found in choristoderes include heavily ossified gastralia
Gastralium
Gastralia are dermal bones found in the ventral body wall of crocodilian and Sphenodon species. They are found between the sternum and pelvis, and do not articulate with the vertebrae...

 and modified distal limbs, not just the manus and pes, used as paddles. In addition, champsosaur ribs are short and massive, as in other aquatic reptiles. The thorax is dorso-ventrally flattened, and the tail is laterally compressed to aid in swimming. Skin impressions found with champsosaur fossils show non-overlapping scales of very small size and the skin containing no scutes like those found in crocodilians (see crocodile exoskeleton
Crocodile exoskeleton
The crocodile exoskeleton consists of the protective dermal and epidermal components of the integumentary system in animals of the order Crocodilia. It is a form of armour.-Structure and anatomy:...

).

Classification


(after Mikko's Phylogeny Archive, ranks per The Paleobiology Database; all names in italics are genera)

Fossil record



Long considered to be a morphologically conservative group, recent phylogenetic analyses and descriptions of new taxa have revolutionized understanding of this taxon.

The order Choristodera comprises two monophyletic groups and three basal taxa. Primitive choristoderes are characterized by small body size, a large, dorsally directed orbit and closed lower temporal fenestrae.

Lazarussuchus is the most basal choristodere. It creates a ghost lineage of about 11 million years from the last champsosaur. Cladistic analyses indicate Lazarussuchus is more primitive than Pachystropheus, the possible Triassic choristoderian. Therefore if the cladistic analyses are correct, Lazarussuchus implies a ghost lineage extending back from the Oligocene to the Cretaceous at the very least. This taxon is known from France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 and the Czech Republic
Czech Republic
The Czech Republic is a landlocked country in Central Europe. The country is bordered by Poland to the northeast, Slovakia to the east, Austria to the south, and Germany to the west and northwest....

. One species, L. dvoraki, persisted into the early Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

 in the Czech Republic.

Cteniogenys is another basal choristodere. Like Monjurosuchus, it is a small bodied, lizard like animal. The webbed feet of Monjurosuchus reflect its aquatic lifestyle. The former is known from North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, while the latter is known from China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

.

The two small-bodied choristoderes, Shokawa
Shokawa
Shokawa ikoi was a 1.8 m long choristoderan diapsid reptile closely resembling and closely related to the smaller choristoderan, Hyphalosaurus. It lived during the Lower Cretaceous in what is now Japan....

and Hyphalosaurus
Hyphalosaurus
Hyphalosaurus is a genus of freshwater aquatic reptile, and a major part of the Jehol Biota. It lived during the early Cretaceous period , about 122 million years ago. It contains two species, H. lingyuanensis and H. baitaigouensis, both from the Yixian Formation of Liaoning Province, China...

form the clade Hyphalosauridae within Choristodera. Their elongate necks and tails represent a unique shared derived condition. They resemble small plesiosaur
Plesiosaur
Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...

s or nothosaur
Nothosaur
Nothosaurs were Triassic marine sauropterygian reptiles that may have lived like seals of today, catching food in water but coming ashore on rocks and beaches. They averaged about in length, with a long body and tail. The feet were paddle-like, and are known to have been webbed in life, to help...

s. Shokawa is known from Japan, and Hyphalosaurus is known from China.

The named clade Neochoristodera includes Champsosaurus, Tchoiria, Simoedosaurus
Simoedosaurus
Simoedosaurus is an extinct reptile known from the Paleocene of North America and Europe, and a member of the Choristodera, a group of semiaquatic reptiles that lived in the Northern Hemisphere from the Jurassic to the early Cenozoic. It resembles the better-known Champsosaurus in having long,...

, Ikechosaurus and possibly Pachystropheus. This group of large-bodied reptiles is characterized by elongate snouts and relatively small orbits. Although it reflects our early understanding of the Choristodera as a whole, it in fact represents a highly derived condition.

The neochoristoderan taxa Champsosauridae and Simoedosauridae are found from the Late Cretaceous to the middle Eocene, indicating that the group survived the Cretaceous–Tertiary extinction event. Champsosaurus teeth, vertebrae, and other bones are common fossils in Cretaceous deposits such as the Dinosaur Park Formation
Dinosaur Park Formation
The Dinosaur Park Formation is the uppermost member of the Judith River Group, a major geologic unit in southern Alberta. It was laid down over a period of time between about 76.5 and 75 million years ago. The formation is made up of deposits of a high-sinuosity fluvial system, and is capped...

 of Alberta
Alberta
Alberta is a province of Canada. It had an estimated population of 3.7 million in 2010 making it the most populous of Canada's three prairie provinces...

 and the Lance Formation
Lance Formation
The Lance Formation is a division of Late Cretaceous rocks in the western United States. Named after Lance Creek, Wyoming, the microvertebrate fossils and dinosaurs represent important components of the latest Mesozoic vertebrate faunas...

 of Wyoming
Wyoming
Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...

. The presence of champsosaurs as far north as the North Slope of Alaska implies warmer temperatures during the Cretaceous. The Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

 species, Champsosaurus gigas, is the largest known champsosaur. The most complete skeleton was found at the Wannagan Creek site
Wannagan Creek site
The Wannagan Creek site is a fossil site found just west of the South Unit of Theodore Roosevelt National Park of North Dakota, USA. The site is Paleocene in age, approximately 60 million years old. Paleontologists of the Science Museum of Minnesota have studied the site for nearly thirty years. ...

 in North Dakota
North Dakota
North Dakota is a state located in the Midwestern region of the United States of America, along the Canadian border. The state is bordered by Canada to the north, Minnesota to the east, South Dakota to the south and Montana to the west. North Dakota is the 19th-largest state by area in the U.S....

, but is found throughout North American Paleocene strata in the Dakotas, Wyoming, and Montana. C. gigas is unusual among Paleocene reptiles in that it is far larger than known Mesozoic ancestors, at about 3 meters in length (10 ft) versus 1.5 meters (5 ft) for the largest Cretaceous champsosaurs.

Simoedosaurus is known from Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 and North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, Tchoria is known from Mongolia
Mongolia
Mongolia is a landlocked country in East and Central Asia. It is bordered by Russia to the north and China to the south, east and west. Although Mongolia does not share a border with Kazakhstan, its western-most point is only from Kazakhstan's eastern tip. Ulan Bator, the capital and largest...

 and Ikechosaurus is known from China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

. Champsosaurus is known from North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, where its distinctive, hourglass-shaped vertebral centra are important biostratigraphic indicators.

Pachystropheus rhaeticus may or may not be a neochoristodere. If belongs to that taxon, it extends the fossil record of the choristoderes from the Middle Jurassic back 45 mya and implies a significant ghost lineage. However, Pachystropheus lacks any cranial material, and all of the postcranial adaptations that link it to choristoderes may merely reflect convergent
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

 adaptations to an aquatic lifestyle. All unambiguous choristoderes were freshwater animals, while Pachystropheus was recovered from European marine transgressive sequences. Pachystropheus may be a choristodere, or it may be a type of thallatosaur, a large bodied, long necked group of marine reptiles. More complete fossil material is needed in order to resolve the placement of Pachystropheus.

The ultimate extinction of choristoderes may have been due to a number of possible causes: falling temperatures in the Oligocene, increased competition from crocodilians, habitat loss, or some combination of these factors.

In 2006, the UK Royal Society announced that it had discovered a two-headed fossil of a choristodere, the first recorded time that such a reptile has been found fossilized. The chances of finding such a fossil are extremely low, as is shown by the current proportion of reptiles born with two heads.

External links