page_title: Choristodera
section_title: Fossil record
hierarchical_section_title: Choristodera / Fossil record
caption_attribution_description: English: Monjurosuchus splendens fossil in lacustrine rock of the Late Jurassic/Early Cretaceous Yixian Formation of the Liaoning Province, displayed in Hong Kong Science Museum
context_page_description: Choristodera is an extinct order of semiaquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Late Triassic, to at least the early Miocene. It was named by Edward Drinker Cope in 1884. Choristoderes have been found in North America, Asia, and Europe, and possibly also North Africa and East Timor. 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 Choristodera is still uncertain. It has also been proposed that they represent basal lepidosauromorphs. Most recently, workers have tentatively placed Choristodera within Archosauromorpha, with their ear bone anatomy more closely correlating to those of archosaurs and turtles than to those of lepidosaurs.
context_section_description: 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.
  Cteniogenys is the most basal choristodere. Like Monjurosuchus, it is a small-bodied, lizard-like animal. The webbed feet of Monjurosuchus reflect its aquatic lifestyle. Cteniogenys is known from North America and Europe, while Monjurosuchus is known from China.
  Lazarussuchus was considered the most basal choristoderan when first described, and hence a late-surviving basal choristodere. However, Matsumoto et al. (2013) recovered Lazarussuchus in a clade along with hyphalosaurids and monjurosuchids, as did the cladistic analysis of Coeruleodraco by Matsumoto et al. (2019). This taxon is known from Cenozoic deposits in France and the Czech Republic. One species, L. dvoraki, persisted into the early Miocene in the Czech Republic.
  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 plesiosaurs or nothosaurs. Shokawa is known from Japan, and Hyphalosaurus is known from China.
  The named clade Neochoristodera includes Champsosaurus, Tchoiria, Simoedosaurus, and Ikechosaurus. 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–Paleogene 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 meters in length (10 ft) versus 1.5 meters (5 ft) for the largest Cretaceous champsosaurs.
  Simoedosaurus is known from Europe, western Asia 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 choristodere. If a member of Choristodera, 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 thalattosaur, 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, such as falling temperatures and habitat loss. Competition with crocodilians has occasionally been cited as a possible source of decline, but the co-existence of large choristoderes with aquatic crocodilians, as well as a fairly broad range of morphology in both clade
caption: Monjurosuchus splendens
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