? Choristodera
A Champsosaur Hunting, Mary Parrish
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Sauropsida Subclass: Diapsida (unranked) Archosauromorpha? (unranked) Lepidosauromorpha? (unranked) Choristodera Cope, 1884
Groups
Cteniogenidae Simoedosauridae Champsosauridae Image File history File links cahmp pic File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Scientific classification or biological classification is how biologists group and categorize extinct and living species of organisms. ... Phyla Porifera (sponges) Ctenophora (comb jellies) Cnidaria (coral, jellyfish, anenomes) Placozoa (trichoplax) Subregnum Bilateria (bilateral symmetry) Acoelomorpha (basal) Orthonectida (flatworms, echinoderms, etc. ... Typical Classes Subphylum Urochordata - Tunicatas Ascidiacea Thaliacea Larvacea Subphylum Cephalochordata - Lancelets Subphylum Myxini - Hagfishes Subphylum Vertebrata - Vertebrates Petromyzontida - Lampreys Placodermi (extinct) Chondrichthyes - Cartilaginous fishes Acanthodii (extinct) Actinopterygii - Ray-finned fishes Actinistia - Coelacanths Dipnoi - Lungfishes Amphibia - Amphibians Reptilia - Reptiles Aves - Birds Mammalia - Mammals Chordates (phylum Chordata) include the vertebrates, together with... Clades Subclass Anapsida Subclass Diapsida Infraclass Lepidosauromorpha Infraclass Archosauromorpha Sauropsida is an amniote clade that includes all recent and all or almost all extinct reptiles (excluding the Synapsida), and birds. ... Classes Ichthyosauria Sauropterygia Lepidosauria Archosauria Diapsids (two arches) are a group of tetrapod animals that developed two holes (temporal fenestra) in each side their skulls, about 300 million years ago during the late Carboniferous period. ... Groups Order Rhynchosauria Order Trilophosauria Order Prolacertiformes Plesion     Archosauriformes Division        Archosauria Archosauromorpha (Greek for ruling reptiles form) are a diverse group or Infraclass of diapsid reptiles that first appeared during the late Permian, but only became common during the Triassic. ... Orders Sphenodontia Squamata Eosuchia Conservation status: Fossil The Lepidosauria are a subclass of reptiles comprising the orders : Squamata Sphenodontia or Rhynchocephalia Eosuchia Conservation status: Fossil Lepidosaurians are the most successful of modern reptiles. ... Edward Drinker Cope Edward Drinker Cope (July 28, 1840–April 12, 1897) was an American paleontologist and comparative anatomist. ... 1884 is a leap year starting on Tuesday (click on link to calendar). ...

Choristodera is an order of semi-aquatic diapsid reptiles which ranged from the Middle Jurassic, or possibly Late Triassic, to upper Eocene, or upper Oligocene. Choristoderes have been found in North America, Asia, and Europe. The most common fossils are typically found from the Late Cretaceous to the lower Eocene. Cladists have placed them between basal diapsids and basal archosauromorphs but the phylogenetic position of the Choristodera is still uncertain. It has also been proposed that they represent basal lepidosauromorphs. Most recently, workers have placed Choristodera within Archosauromorpha. Scientific classification or biological classification refers to how biologists group and categorize extinct and living species of organisms. ... Classes Ichthyosauria Sauropterygia Lepidosauria Archosauria Diapsids (two arches) are a group of tetrapod animals that developed two holes (temporal fenestra) in each side their skulls, about 300 million years ago during the late Carboniferous period. ... Orders  Crocodilia - Crocodilians  Rhynchocephalia - Tuataras  Squamata   Suborder Sauria- Lizards   Suborder Serpentes - Snakes   Suborder Amphisbaenia - Worm lizards Testudines - Turtles Superorder Dinosauria  Saurischia  Ornithischia Reptiles are tetrapods, and also are amniotes, animals whose embryos are surrounded by an amniotic membrane. ... The Jurassic period is a major unit of the geologic timescale that extends from about 200 Ma (million years ago) at the end of the Triassic to 146 Ma at the beginning of the Cretaceous. ... The Triassic is a geologic period that extends from about 245 to 202 Ma (million years ago). ... The Eocene epoch (56-34 MYA) is a major division of the geologic timescale and the second epoch of the Palaeogene period in the Cenozoic era. ... The Oligocene epoch is a geologic period of time that extends from about 34 million to 23 million years before the present. ... World map showing North America A satellite composite image of North America North America is a continent in the northern hemisphere bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the south by the Caribbean Sea, and on the west by the... World map showing Asia (geographically) Asia is the central and eastern part of Eurasia and worlds largest continent. ... A satellite composite image of Europe Europe is the worlds second-smallest continent in terms of area, with an area of 10,600,000 km² (4,140,625 square miles), making it larger than Australia only. ... A fossil Ammonite Fossils are the mineralized remains of animals or plants or other traces such as footprints. ... The Cretaceous period is one of the major divisions of the geologic timescale, reaching from the end of the Jurassic period, about 146 million years ago (Ma), to the beginning of the Paleocene epoch of the Tertiary period (65. ... This cladogram shows the relationship among various insect groups. ... Classes Ichthyosauria Sauropterygia Lepidosauria Archosauria Diapsids (two arches) are a group of tetrapod animals that developed two holes (temporal fenestra) in each side their skulls, about 300 million years ago during the late Carboniferous period. ... Groups Order Rhynchosauria Order Trilophosauria Order Prolacertiformes Plesion     Archosauriformes Division        Archosauria Archosauromorpha (Greek for ruling reptiles form) are a diverse group or Infraclass of diapsid reptiles that first appeared during the late Permian, but only became common during the Triassic. ... A phylogeny (or phylogenesis) is the origin and evolution of a set of organisms, usually of a species. ... Orders Sphenodontia Squamata Eosuchia Conservation status: Fossil The Lepidosauria are a subclass of reptiles comprising the orders : Squamata Sphenodontia or Rhynchocephalia Eosuchia Conservation status: Fossil Lepidosaurians are the most successful of modern reptiles. ...

Description/Phylogeny

Champsosauridae is the most common family of the Choristodera and typifies the group. Champsosaurus was first described from Late Cretaceous strata of Montana by Cope in 1876. Champsosaurs resembled modern gharials or false gharials. The skull of these animals have a long, thin rostrum filled with small, sharp conical teeth. This is due to champsosaurs and gharials occupying similar niches: hunting small aquatic prey in rivers and swamps. This is a classic example of evolutionary relay. More primitive choristoderes have shorter, broader snouts. State nickname: Treasure State Other U.S. States Capital Helena Largest city Billings Governor Brian Schweitzer (D) Senators Max Baucus (D) Conrad Burns (R) Official language(s) English Area 381,156 km² (4th)  - Land 377,295 km²  - Water 3,862 km² (1%) Population (2000)  - Population 902,195 (44th)  - Density 2. ... Edward Drinker Cope Edward Drinker Cope (July 28, 1840–April 12, 1897) was an American paleontologist and comparative anatomist. ... 1876 is a leap year starting on Saturday. ... Binomial name Gavialis gangeticus (Gmelin, 1789) // Taxonomy The Gharial (Gavialis gangeticus) is the only surviving member of the family Gavialidae, a long-established group of crocodile-like reptiles with long, narrow jaws. ... The false gharial (Tomistoma Schlegelii) is a fresh-water reptile, resembling a crocodile with a very thin and elongated snout resembling that of the gharial, hence its name. ... A hippopotamuss skull A skull, or cranium, is a bony structure of Craniates which serves as the general framework for a head. ... Rostrum can mean one of several different things: A rostrum (Latin beak) is an anatomical structure resembling a birds beak, such as the snout of crocodiles or dolphins or the part of the carapace of a crustacean. ... In ecology, a niche is a term describing the relational position of a species or population in an ecosystem. ... For the Second World War frigate class, see River class frigate The Murray River in Australia A waterfall on the Ova da Fedoz, Switzerland A river is a large natural waterway. ... A freshwater swamp A swamp is a wetland that features permanent inundation of large areas of land by shallow bodies of water, generally with a substantial number of hummocks, or dry-land protrusions. ... In evolutionary biology, evolutionary relay describes how independent species acquire similar characteristics as a result of their evolution in similar ecosystems, but not at the same time. ...

There are major differences to the gharials and other crocodilians, 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 phytosaurs have their nares located dorsally on their rostrum or skull respectively. This position allows them to rest submerged just below the surface. This article needs to be cleaned up to conform to a higher standard of quality. ... Nares are a birds nostrils. ... Phytosaurs - family Phytosauridae or Parasuchidae - were a group of large (2 to 12 meters long - average size 3 to 4 meters) semi-aquatic predatory thecodonts that flourished during the Late Triassic period. ...

Champsosaur skulls are actually very similar to lizard skulls, though heavily modified. This has lead some researchers to consider champsosaurids as lepidosauromorphs. However, champsosaurs lack the complex quadrate of lepidosaurians. With features of both diapsid groups, the phylogenetic position of Choristodera is highly confused. Families Many, see text. ... The quadrate is a jaw bone in all jawed vertebrates except mammals (in whom it has become a middle-ear bone, the incus). ... Orders Sphenodontia Squamata Eosuchia Conservation status: Fossil The Lepidosauria are a subclass of reptiles comprising the orders : Squamata Sphenodontia or Rhynchocephalia Eosuchia Conservation status: Fossil Lepidosaurians are the most successful of modern reptiles. ...

Other features found in choristoderes include heavly ossified gastralia 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-overlaping scales of very small size and the skin containing no scutes (like that found in crocodylia, see crocodile exoskeleton). A scute (Latin scutum, shield) is a horny, chitinous, or bony external plate or scale, as on the shell of a turtle or the underside of a snake. ... This article needs cleanup. ...

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. In phylogenetics, a group is monophyletic (Greek: of one stem) if all organisms in that group are known to have developed from a common ancestral form, and all descendants of that form are included in the group. ...

Lazarussuchus inexpectatus 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 and 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 and Europe, while the latter is known from China. World map showing North America A satellite composite image of North America North America is a continent in the northern hemisphere bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the south by the Caribbean Sea, and on the west by the... A satellite composite image of Europe Europe is the worlds second-smallest continent in terms of area, with an area of 10,600,000 km² (4,140,625 square miles), making it larger than Australia only. ...

The two small-bodied choristoderes, Shokawa and Hyphalosaurus form an unnamed clade within Choristodera. Their elongate necks and tails represent a unique shared derived condition. They resemble small plesiosaurs or nothosaurs. Shokawa is known from Japan, and Hyphalosaurus is known from China. Families Cryptoclididae Elasmosauridae Plesiosauridae Pliosauridae Plesiosaurs (PLEE-see-oh-SORES) were large, carnivorous aquatic reptiles. ... A placodont prehistoric reptile that lived like seals of today, catching food in water and spending some time on land. ...

The named clade Neochoristodera includes Champsosaurus, Tchoiria, Simoedosaurus, 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 KT event. Champsosaurus teeth, vertebrae, and other bones are common fossils in Cretaceous deposits such as the Dinosaur Park Formation of Alberta and the Lance Formation of Wyoming. The presence of champsosaurs as far north as the North Slope of Alaska implies warmer temperatures during the Cretaceous. Champsosaurus gigas, found in the Paleocene, is the largest champsosaur. The most complete skeleton was found at the Wannagan Creek site, but is found thoughout 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; 3 m in length versus 1.5 m for the largest Cretaceous champsosaurs. The Cretaceous-Tertiary (KT) extinction event, also known as the KT boundary (from German: Kreide-Tertiär-Grenzschicht), was a period of massive extinction of species, about 65. ... Motto: Fortis et Liber (Strong and free) Other Canadian provinces and territories Capital Edmonton Largest city Calgary Lieutenant-Governor Norman Kwong Premier Ralph Klein (PC) Area 661,848 km² (6th) • Land 642,317 km² • Water 19,531 km² (2. ... State nickname: Equality State Other U.S. States Capital Cheyenne Largest city Cheyenne Governor Dave Freudenthal (D) Senators Craig Thomas (R) Mike Enzi (R) Official language(s) English Area 253,554 km² (10th)  - Land 251,706 km²  - Water 1,851 km² (0. ... The Paleocene epoch (65-56 MYA) (early dawn of the recent) is the first geologic epoch of the Palaeogene period in the modern Cenozoic era. ... The Wannagan Creek site is a fossil site found in the South Unit of Theodore Roosevelt National Park of North Dakota, USA. The site is Paleocene in age, approximately 60 million years old. ... The Dakotas is a collective term used in the United States to refer to the states of North and South Dakota together. ... The metre (or meter) (symbol: m) is the SI base unit of length. ...

Simoedosaurus is known from Europe and North America, Tchoria is known from Mongolia and Ikechosaurus is known from China. Champsosaurus is known from North America, where its distinctive, hourglass-shaped vertebral centra are important biostratigraphic indicators. A satellite composite image of Europe Europe is the worlds second-smallest continent in terms of area, with an area of 10,600,000 km² (4,140,625 square miles), making it larger than Australia only. ... World map showing North America A satellite composite image of North America North America is a continent in the northern hemisphere bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the south by the Caribbean Sea, and on the west by the... World map showing North America A satellite composite image of North America North America is a continent in the northern hemisphere bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the south by the Caribbean Sea, and on the west by the...

Pachystropheus rhaeticus may or may not be a neochorostodere. 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 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. For a discussion of convergence and convergent series, see limit (mathematics). ...

The ultimate extinction of choristoderes may have been the result of any number of factors: falling temperatures in the Oligocene, increased competition from crocodylians, habitat loss, or any combination of factors.

External links

• Palæos Choristodera page

• Mikko's Phylogeny Achive CHORISTODERA-champsosaurs

• Suite101 Champsosaur article

• UC BerkelyArchosauria: Systematics

References

• deBraga, M & O Rieppel (1997), 'Reptile phylogeny and the interrelationships of turtles'. Zoology. J. Linnean Soc, 120: 281-354

• Erickson, B. R. (1972). 'The Lepidosaurian Reptile Champsosaurus in North America'. Science Museum of Minnesota, Volume 1: Paleontology, Monograph.

• Evans, SE, and MK Hecht. 1993, A history of an extinct reptilian clade, the Choristodera: Longevity, Lazarus taxa, and the fossil record. Evolutionary Biology, 27:323–338

• Gao, K & RC Fox (1998), 'New choristoderes (Reptilia: Diapsida) from the Upper Cretaceous and Palaeocene, Alberta and Saskatchewan, Canada, and phylogenetic relationships of Choristodera'. Zoology. J. Linnean Soc, 124: 303-353.

• Ksepka D, K Gao and MA Norell. (2005), "A new choristodere from the Cretaceous of Mongolia." American Museum Novitates 3468: 1-22.

• Storrs G.W. & D.J. Gower (1993), 'The earliest possible choristodere (Diapsida) and gaps in the fossil record of semi-aquatic reptiles', Journal of the Geological Society, GSA, 150: 1103-1107