四川中侏罗世的龟化石

本文对产自四川自贡中侏罗世的三件龟甲标本作了鉴定,认为均可归成渝龟属(Chengyuchelys)。一为自贡成渝龟,新种(Chengyuchelys zigongensis,sp.nov.),一为似贝氏成渝龟(C.baenoides, Young et Chow),一为成渝龟未定种(Chengyuchelys sp.)。在形态描述的基础上,讨论了成渝龟属的系统分类。

MIDDLE JURASSIC TURTLES FROM SICHUAN, SW. CHINA

Three turtle shells were unearthed together with dinosaurs from Lower Sha-Xi-Miao Series of Zigong County, Sichuan Province, south-western China. The geological age of the Series is generally considered as Middle Jurassic. It is the earliest record of turtles so far known in our country, and is actually sparse in other countries. The present paper is a determination of the shells, and the classification of genus Chengyuchelys is also briefly discussed. CVOO678 (specimen number of Chongqing Museum) is a specimen including a carapace and a united plastron. In view of the structures of the shell, such as the form of neurals, the wide and fan-shaped bridge, the presence of intergulars, inframarginals and mesoplastra, especially the anal seute invaded to the hypoplastron at middle line, and the hind border of third vertebral scute notched posteriorly and passing through the sixth neural plate rather than the fifth one as seen in most turtles, the present specimen is essentially comparable with genus Chengyuchelys which was proposed by Young and Chow in 1953 for a specimen from Sichuan too. But the new shell differs still from Chengyuchelys baenoides, the single species of the genus, in following characters: 1) sulci of second and third vertebral seutes curved in form, 2) maximal width of fourth vertebral scute nearly twice as wide as its maximal length, 3) second costal seute extremely broad, 4) entoplastron leaf-like in form and with zigzag suture, and 5) most important of all, mesoplastron not narrowed at middle line and situated on both pectoral and abdominal seutes. By these different points, a new species of the genus, Chengyuchelys eigongensis, is thus proposed. V6507 (specimen number of IVPP) represents only by an anterior one-third shell and an isolated buttress which is very wide and stout. With the exception of entoplastron. all the features of this shell are much similar to those of C. baenoides, particu- larly the structure of mesoplastron which narrowed at middle line and limited only in abdominal scute. It seems reasonable to include the present specimen to C. baenoides. CV00679 (specimen number of Chongqing Museum) is a badly damaged specimen though most part of cgrapace and plastron is in preservation. According to the general feature, it is determined as Chengyuchelys sp. Up to now, the Jurassic turtles with a mesoplastron known in China are included in a single genus——Chengyuchelys, but the systematic position of the genus is still in hesitation, though it has been reported temporarily by the original authors as a member of Baenidae. In view of the shell, the genus, more exactly the G. zigongensis, is somewhat similar to Dinochelys of Upper Jurassic of North America, especially to that shell was presumed by Gaffney as juvenile shell possibly belonging to D. whitei, of course, they differ Still in many main characters. In the absence of skull, and waiting for more materials, the present author rather temporarily considers Chengyuchelys a member of incertae sedis of Cryptodira. It seems not impossible that Chengyuchelys may be a new group of Cryptodira and represents a branch of early evolution of turtles.