Amazing Reptile Fossils from the Marine Triassic of China

BCAS Vol.24 No.2 2010

LI Chun* Institute of Vertebrate Paleontology and Paleoanthropology, CAS, Beijing 100044, China

nown as the “true terrestrial epicontinental seas. So far their some of these taxa were proved tetrapod,” reptiles mainly fossils are found in three major faunal to be the nomen dubium (invalid Klive on land. However, some provinces, i.e the Western Tethyan name), such as Shingyisaurus, which reptile groups made a secondary Province (nowaday Europe and the is now referred to as Nothosaurus. adaptation to their life in water. Middle East), the Eastern Tethyan sp. Great changes happened during Generally called “marine reptiles,” they Province (or by some authors, the the following decades. Since 1999, are mostly extinct, and their fossils are Western Pacific Province, nowaday hundreds of complete skeletons have found in Mesozoic marine deposits southern China) and the Eastern been excavated from Guizhou and around the world. Ichthyosaurs and Pacific Province (North America). Yunnan provinces, southwestern Plesiosaurs are the most famous marine The collecting and the study of marine China with the discovery of at least reptiles that lived in the “dinosaur age,” reptile fossils in Europe have a long 24 new genera. Among these new namely the Jurassic and Cretaceous. history of nearly 200 years, while in collections are some important reptile Many more “sea monsters” were ruling China, the first specimen, named as groups never reported before from the ocean in the Triassic period, when Keichousaurus hui, was not published the Eastern Tethyan Province, such dinosaurs had just arisen from its until 1956. By 1999, only a few genera as the placodont, the thalattosaur, and archosaur ancestors. and species had been known. All the protorosaur. Besides, some more In the Triassic, marine these specimens, mainly ichthyosaurs amazing taxa came to light, including reptiles were inhabitants of a near- and nothosaurs, are incomplete and the first marine “thecodonts” and the shore environment and shallow poorly preserved. Furthermore, most primitive turtle.

Suction in the sea

By the Middle Triassic time, along the margins of parts of middle Triassic marine deposits a number of reptile lineages had Pangea, including the giraffe- about 230 Million years old in diversified in shallow epicontinental necked protorosaurid reptile southwestern China. This taxon seas and intraplatform basins Dinocephalosaurus, collected from represents the first unambiguous

* To whom correspondence should be addressed at [email protected].

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evidence for a fully aquatic induced resistance to locomotion. protorosaur. Further, its extremely The slender neck would also position elongated neck is explained as an the head well in front of the sturdy adaptation to aquatic life, for example, body, such that Dinocephalosaurus to increase the feeding efficiency in could closely approach potential preys water. The adaptive significance of before its profile became apparent neck elongation in Tanystropheus has in dimly lit water. Given the length generated many speculations, but in and slenderness of the cervical ribs, Dinocephalosaurus, it presumably moderate lateral flexion of the neck served a functional role. Extending must have been possible. Contraction the head vertically in order to gulp of muscles originating from cervical air at the surface would have been ribs and bridging the intervertebral impossible; the hydrostatic pressure joints would rapidly straighten the would have simply prevented the neck while the ribs simultaneously lung inflation. To breathe, this animal splayed outward. The consequent Fig.1 Dinocephalosaurus orientalis Li would have had to move with the neck increase of the esophageal volume 2003, a marine protorosaur with an held nearly horizontal on the air-water could create suction, thereby the extremely long neck. interface, the head emerging from the animal could essentially swallow the water. Such a posture could increase pressure waves created by its head resulted in an almost perfect strike at a its “hull length” to reduce the wave- lunging forward. This would have prey in water.

An early near-shore archosaur

The Archosauria originated in the ago, and have long been considered specializations normally associated Late Permian (about 250 million years terrestrial. Archosaurian remains had with aquatic reptiles. Together with ago) and later gave rise to dinosaurs been reported from Triassic marine fishes, nothosaurs, protorosaurs and (including birds), pterosaurs, and strata before, but none showed aquatic ichthyosaurs, it was collected from crocodylomorphs (including extant adaptation and were believed to be the the Middle Triassic marine limestone crocodylians). Its early members, remains of terrestrial animals washed in west Guizhou, China. The new traditionally called “thecodonts,” out to sea. In 2006, we reported a archosaurian is well represented by flourished worldwide during the new Triassic archosaurian named articulated skeletons, indicating a Triassic, from 250 to 205 million years Qianosuchus mixtus, which has several mosaic anatomy of both aquatic and terrestrial adaptation. This, combined with the specimen taphonomy, suggests that Qianosuchus may have lived in a coastal–island environment, a situation similar to some recent saltwater crocodiles. Qianosuchus probably used the strong hind limbs to walk on land or in shallow water and the long and laterally compressed tail to swim in deeper water. With dagger-like teeth and a large size, Qianosuchus may have dominated the fauna and preyed on whatever it could reach, such as near-shore nothosaurian sauropterygians and protorosaurs, and even mixosaurid ichthyosaurs and fishes, most of which commonly Fig. 2 As an early kin of the dinosaur, Qianosuchus mixtus Li et al 2006 is the only occur in the same beds of the new known “thecodonts” to have invaded into the sea. archosaurian.

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How did the turtle get its shell

The origin of the turtle body plan of reptile evolution. The anatomy the special body plan of the turtle. remains one of the great mysteries of turtles is highly derived, which Briefly speaking, a ventral plastron renders it difficult to establish their is fully developed in this species, but relationships with other groups of the dorsal carapace consists of neural reptiles. The oldest turtle in textbook, (central) plates only. The dorsal Proganochelys from the Late ribs are expanded, and osteoderms Triassic period in Germany has a absent. The new species shows that fully formed shell and offers no clue the plastron might have evolved as to its origin. While in 2008, we before the carapace and that the first described a new 220-million-year- step of carapace formation might be old turtle from Guizhou Province, the ossification of the neural plates southwestern China, somewhat coupled with broadening of the ribs. older than Proganochelys. This This corresponds to early embryonic documents an intermediate step in stages of carapace formation in the evolution of the turtle shell and extant turtles, and shows that the associated structures, and is thus turtle shell is not derived from a seen as a “missing link” for turtle fusion of osteoderms. Phylogenetic origin, just as the Archaeopteryx analysis places the new species basal seen for bird evolution. We named to all known turtles, fossil or extant. it “Odontochelys semitestacea,” The marine deposits that yielded the Fig. 3 Odontochelys semitestacea Li et al which means “turtle with teeth and fossils indicate that this primitive 2008, the most primitive turtle serves as a half-shell.” Odontochelys is of turtle inhabited marginal areas of the a “missing link” for turtle origin. great importance to the origin of sea or river deltas.

References

Li Chun, Wu Xiaochun, Rieppel O,Wang Liting & Zhao Lijun, 2008. An ancestral turtle from the Late Triassic of Southwestern China. Nature, 456: 497−501. Li Chun, Zhao Lijun & Wang Liting, 2007. A new species of Macrocnemus (Reptilia: Protorosauria) from the Middle Triassic of southwestern China and its palaeogeographical implication. Science in China Series D: Earth Sciences, 50(11): 1601−1605. Li Chun, Wu Xiaochun, Cheng Yannian & Sato T, 2006. An unusual archosaurian from the marine Triassic of China. Naturwissenschaften, 93: 200−206. Li Chun, Rieppel O et al, 2004. A Triassic protorosaur with an extremely long neck. Science, 305: 1931. Li Chun, Rieppel O, 2002. A new cyamodontoid placodont from Triassic of Guizhou, China. Chinese Science Bulletin, 47(2): 156−159. Li Chun, 2000. Placodont (Reptilia, Placodontia) from Upper Triassic of Guizhou, southwest China. Vertebrata PalAsiatica, 38(4): 314−317. Zhao Lijun, Wang Liting & Li Chun, 2008. A review of advances in Triassic Marine Reptile research in China. Acta Palaeontologica Sinica, 2008, 47(2): 99−106. (In Chinese)

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