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ecause they ¿ire srili living, turtles are commonplace objects to us; were they entirely extinct, their shells — the most remarkable defensive armor ever assumed by a tetrapod—would be a cause for wonder." — A. S. Romer, 1945.

Turtles are unique among living animals. Their trunk is embedded in the bony shell. The shell can be divided into dorsal and ventral parts (carapace and plastron), and usually includes inner and outer layer: the outer one is horny, made up by many scutes; the inner one is bony, made up by many plates. Not only do the vertebrae grow together with shell, but also the carapace bony plates are fused with ribs. They differ from all other terrestrial vertebrates by the ribs outside the shoulder and pelvic bones, which enables them to retract their heads and limbs into shells for protection and self-defense.

The turtles are one kind of specialized reptiles; they also are one branch of ancient reptiles. They used to be classified as primitive anapsids (i.e. reptiles with no temporal opening on the skull), but also regarded as diapsids (i.e. reptiles with two temporal openings) by other people. The turtles primarily can be divided into two groups: the Pleurodira and the Cryptodira. These names refer to the manner in which the living members of these groups retract their necks. The pleurodires do so by lateral flexure of the cervical vertebrae and rhe cryptodires by vertical flexure. All the living pleurodires inhabit fresh water of the southern continents (Africa, Australian and South America), but extinct ones may have lived in a marine environment. This group has a worldwide distribution in the Cretaceous and Paleogene. Cryptodires are much more diverse in the modern fauna than are the pleurodires. Modern cryptodires may be classified into three groups: the Testudinoidea, including the tortoise anil most freshwater turtles; the Chelonioidea, the sea turtles, with the limbs specialized as flippers and the shell reduced; and the Trionychoidea, the soft-shelled turtles.

Where did the turtles come from? How the shell and rhe strange structure were formed? The most primitive turtle known by now, Proganocbetys, which was found in the Upper Triassic of Germany, had a completely "normal" shell. The Jurassic turtles almost had rhe structure of modern turtles. Eunotosaurus from the Permian of South Africa was once thought to be the ancestor of the turtles, but in fact it is not related to turtles at all. Among reptiles preceded the turtles, pareisaurs. procolophonoids, captorhinids had once been regarded as the ancestor group of the turtles, bur none of these animals has convincing derived characters shared with turtles. Some paleontologists think the turtles may be closest to the marine reptile sauropterygian. but current evidence is nor strong enough to support this view. The origin of turtles still is an unsolved puzzle.

Many well-preserved turtle specimens have been found in western Liaoning. Most of them can be referred to Manchj/rocheJys belonging to the family Sinemydidae. Three species have been described so far of the genus: Maltcbi/rochelys manchouktwensist Al. donghai and AJ. liaoxiensis (Fig. 116). .VI. manchoukuomsis was named by Riuji Fndo and Tokio Shikama in 1942, the holotype of which was lost during WWII, Al .donghai was from the coalmine in Jixi (possibly from a higher horizon than thejehol Group), Heilongjiang Province, named by Shao-Iiang Ma in 1986. Al. liaoxiensis was established on a specimen from Jianshangou village, Beipiao, Liaoning Province by Shu-an Ji in 1995. Some smaller-sized turtles were also collected from the Yixian and Jiufotang Formations of the Jehol Group, whereas their systematic position remains to be studied (Fig. 117).

Characteristic features of Maitcburochelys include: skull very low, nasal small, prefrontal in contact with vomer, a paired pir present on ventral surface of basisphenoid, the supramarginal scales absent, shell very flat, plastron cruciform, mesoplastra absent. Based on these characters, Manchurochelys could be classified into Sinemydidae, Cryptodira. And it is the closest to Dracochelys, a group of turtles from the Early Cretaceous of Xinjiang, China.

Turtles are cold-blooded like most other reptiles; their modern groups are mainly distributed in the temporal, torrid zones. Most turtles live in terrestrial environment, normally in river, lake or swamp. Only a few groups are completely terraneous. There were many lakes in western Liaoning during the time of Manchurocbelys, and this kind of turtle-may have lived in the lake areas. Perhaps its lifestyle is similar to that of modern freshwater turtles.

1117 A small turtle (about 7 cm long) from Shangheshou locality (Jiufotang Formation) in Cliaoyang, Liaoning. [Photo: IVPP)

1117 A small turtle (about 7 cm long) from Shangheshou locality (Jiufotang Formation) in Cliaoyang, Liaoning. [Photo: IVPP)

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