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m redirect is duplicate of Choristodera Undid revision 394157864 by 68.106.255.1 (talk)
 
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#redirect [[Choristodera]]
{{Taxobox
| name = Champsosaurs
| fossil_range = [[Middle Jurassic]] - [[Miocene]], {{fossilrange|165|20|earliest=200}} <small>Possible [[Late Triassic]] record</small>
| image = Champsosaurus BW.jpg
| image_width = 220px
| image_caption = Artist's impression of ''[[Champsosaurus]]''
| regnum = [[Animal]]ia
| phylum = [[Chordate|Chordata]]
| classis = [[Reptile|Reptilia]]
| subclassis = [[Diapsid]]a
| ordo = [[extinction|†]]'''Champsosauria'''
| ordo_authority = [[Edward Drinker Cope|Cope]], 1884
| subdivision_ranks = [[Family (biology)|Families]]
| subdivision =
* [[Champsosauridae]]
* [[Cteniogenidae]]
* [[Hyphalosauridae]]
* [[Monjurosuchidae]]
* [[Simoedosauridae]]
}}

'''Champsosauria''' also known as [[Choristodera]], is an [[Order (biology)|order]] of semi-aquatic [[diapsid]] [[reptile]]s which ranged from the Middle [[Jurassic]], or possibly Late [[Triassic]], to at least the early [[Miocene]]. Champsosaurs have been found in [[North America]], [[Asia]], and [[Europe]]. The most common [[fossil]]s are typically found from the Late [[Cretaceous]] to the lower Eocene. [[Cladistics|Cladists]] have placed them between basal [[diapsid]]s and basal [[Archosauromorpha|archosauromorphs]] but the [[phylogenetic]] position of Champsosauris is still uncertain. It has also been proposed that they represent basal [[Lepidosauromorpha|lepidosauromorphs]]. Most recently, workers have placed Champsosauria within Archosauromorpha.

== Description and phylogeny ==

[[File:Champsosaur.JPG|left|thumb|''Champsosaurus'' fossil at the [[Royal Tyrrell Museum]].]]
Champsosauridae is the best-known family of the Champsosauria and typifies the group. ''[[Champsosaurus]]'' was first described from Late Cretaceous [[strata (geology)|strata]] of [[Montana]] by [[Edward Drinker Cope|Cope]] in 1876. Champsosaurs resembled modern [[gharial]]s (gavials) or [[false gharial]]s. The [[skull]] of these animals have a long, thin snout filled with small, sharp conical teeth. This is due to champsosaurs and gharials occupying similar [[Ecological niche|niches]]: hunting small aquatic prey in [[rivers]] and [[swamps]]. This is a classic example of [[evolutionary relay]]. More primitive champsosaurs have shorter, broader snouts.

There are major differences to the gharials and other [[crocodilia]]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 [[wikt:otic|otic]] sensory organ housing, have been tossed around with little support. The external [[nares]] are found on the tip of the [[wikt:rostrum|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]]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]] skulls, though heavily modified. This has led some researchers to consider champsosaurids as lepidosauromorphs. However, champsosaurs lack the complex [[quadrate bone|quadrate]] of [[lepidosauria]]ns. With features of both diapsid groups, the phylogenetic position of Champsosauria is highly confused.

Other features found in choristoderes include heavily ossified [[gastralium|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 impression]]s 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]]).

=== Classification ===
(after [http://www.helsinki.fi/~mhaaramo/metazoa/deuterostoma/chordata/reptilia/choristodera.html Mikko's Phylogeny Archive],<ref name=MPA>{{cite web |url=http://www.helsinki.fi/~mhaaramo/metazoa/deuterostoma/chordata/reptilia/choristodera.html |title=†Choristodera |accessdate=16 March 2008 |last=Haaramo |first=Mikko |date=12 November 2006 |work=Mikko's Phylogeny Archive }} (date is for last update to contents)</ref> ranks per [http://paleodb.org/cgi-bin/bridge.pl?action=checkTaxonInfo&taxon_no=37784 The Paleobiology Database]<ref name=PBD>{{cite web |url=http://paleodb.org/cgi-bin/bridge.pl?action=checkTaxonInfo&taxon_no=37784 |title=The Paleobiology Database |accessdate=16 March 2008 |work=The Paleobiology Database }} </ref>; all names in italics are genera)

{{clade| style=font-size:95%;line-height:80%
|1={{clade
|label1=order&nbsp;'''Champsosauria'''&nbsp;
|1={{clade
|1=?''[[Irenosaurus]]''
|2=?''[[Khurendukhosaurus]]''
|3=?''[[Lazarussuchus]]''
|label4=<font color="white">unnamed</font>
|4={{clade
|1=''[[Cteniogenys]]''/family&nbsp;'''[[Cteniogenidae]]'''
|label2=<font color="white">unnamed</font>
|2={{clade
|1=''[[Pachystropheus]]''
|label2=&nbsp;family&nbsp;'''[[Hyphalosauridae]]'''&nbsp;
|2={{clade
|1=''[[Hyphalosaurus]]''
|2=''[[Shokawa]]''
}}
|label3=&nbsp;family&nbsp;'''[[Monjurosuchidae]]'''&nbsp;
|3={{clade
|1=''[[Monjurosuchus]]''
|2=''[[Philydrosaurus]]''
}}
|label4=&nbsp;suborder&nbsp;'''[[Neochoristodera]]'''&nbsp;
|4={{clade
|label1=&nbsp;family&nbsp;'''[[Champsosauridae]]'''&nbsp;
|1={{clade
|1=''[[Champsosaurus]]''
|2=''[[Eotomistoma]]''
}}
|label2=&nbsp;family&nbsp;'''[[Simoedosauridae]]'''&nbsp;
|2={{clade
|1=''[[Liaoxisaurus]]''<ref name=GCetal05>{{cite journal |author=Gao Chunling |coauthors=Lu Junchang; Liu Jinyuan; and Ji Qiang |year=2005 |title=New choristodera from the Lower Cretaceous Jiufotang Formation in Chaoyang area, Liaoning China |journal=Geological Review |volume=51 |issue=6 |pages=694–697 |language=Chinese}}</ref>
|2=''[[Tchoiria]]''
|label3=<font color="white">unnamed</font>
|3={{clade
|1=''[[Ikechosaurus]]''
|2=''[[Simoedosaurus]]''
}}
}}
}}
}}
}}
}}
}}
}}

== Fossil record ==
[[File:Hyphalosaurus baitaigouensis 2.JPG|thumb|''[[Hyphalosaurus]]'']]
Long considered to be a morphologically conservative group, recent phylogenetic analyses and descriptions of new taxa have revolutionized understanding of this taxon.

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

''Lazarussuchus'' is the most basal champsosaur. 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]]. One species, ''L. dvoraki'', persisted into the early [[Miocene]] in the Czech Republic.<ref name=EK05>{{cite journal |last=Evans |first=Susan E. |coauthors=and Klembara, Jozef |year=2005 |title=A choristoderan reptile (Reptilia: Diapsida) from the Lower Miocene of northwest Bohemia (Czech Republic) |journal=Journal of Vertebrate Paleontology |volume=25 |issue=1 |pages=171–184 |doi=10.1671/0272-4634(2005)025[0171:ACRRDF]2.0.CO;2 }}</ref>

''Cteniogenys'' is another basal champsosaur. 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]].
[[Image:Monjurosuchus splendens 2.JPG|thumb|left|''Monjurosuchus splendens'']]
The two small-bodied choristoderes, ''[[Shokawa]]'' and ''[[Hyphalosaurus]]'' form the clade [[Hyphalosauridae]] within Choristodera. Their elongate necks and tails represent a unique shared derived condition. They resemble small [[plesiosaur]]s or [[nothosaur]]s. ''Shokawa'' is known from Japan, and ''Hyphalosaurus'' is known from China.

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 [[Cretaceous–Tertiary extinction 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. The [[Paleocene]] species, ''Champsosaurus gigas'', is the largest known champsosaur. The most complete skeleton was found at the [[Wannagan Creek site]] in [[North Dakota]], 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&nbsp;meters in length (10&nbsp;ft) versus 1.5&nbsp;meters (5&nbsp;ft) for the largest Cretaceous champsosaurs.

''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.

''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 evolution|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''.

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 ==
* [http://www.palaeos.com/Vertebrates/Units/270Archosauromorpha/270.100.html#Champosauridae Palæos] Choristodera page
* [http://www.helsinki.fi/~mhaaramo/metazoa/deuterostoma/chordata/reptilia/choristodera.html Mikko's Phylogeny Archive] CHORISTODERA-champsosaurs
* [http://www.ucmp.berkeley.edu/diapsids/archosy.html UC Berkely] Archosauria: Systematics
* [http://news.bbc.co.uk/2/hi/science/nature/6195345.stm BBC News] Two-Headed Reptile Fossil Found
* [http://www4.ncsu.edu/~dtksepka/Dan%20Ksepka%20/Choristoderes.html Choristodere research at NCSU]
* [http://www.dinosauria.com/jdp/evol/ghost.html Ghost lineages] - brief article on the fossil record of choristoderes

== References ==
{{Reflist}}

* 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

* Gao, K-Q. and D.T. Ksepka. 2008. Osteology and taxonomic revision of Hyphalosaurus (Diapsida: Choristodera) from the Lower Cretaceous of Liaoning, China. Journal of Anatomy 212: 747-760.

* Matsumoto R, Suzuki S, Tsogtbaatar K, Evans SE. 2009. New material of the enigmatic reptile Khurendukhosaurus (Diapsida: Choristodera) from Mongolia. Naturwissenschaften 96 (2): 233–242

{{Archosauromorpha}}

[[Category:Choristodera| ]]

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Latest revision as of 13:02, 1 November 2010

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