孙氏新铺龙(爬行纲:海龙目)一新幼年个体研究

A new juvenile specimen of Xinpusaurus suni(Reptilia:Thalattosauria)from the Upper Triassic of southwestern China

新铺龙是产于我国西南地区的海龙目的一个属,最早发现于贵州省关岭晚三叠世卡尼期小凹组地层。虽然新铺龙材料众多,但其未成年个体骨骼学特征及个体发育模式并未被详细地记述。笔者等报道了一个保存完好的孙氏新铺龙未成年标本,是目前发现最小的完整的孙氏新铺龙个体,标本产自贵州省关岭县新铺乡。依据其体长小于正型标本的一半,头骨长度和躯干比值大,头骨骨缝清晰等特征,判断该标本为孙氏新铺龙的幼年个体。新标本补充了孙氏新铺龙幼年个体的形态学信息,如上颞骨内侧突短于外侧突,翼骨无牙齿发育,神经弓与椎体未融合,尺骨两端微凸,五边形坐骨等。此外,结合已发表的新铺龙标本数据进行统计分析发现,该物种吻部相对头部呈负异速生长,证明了吻部占头骨比例的大小属于新铺龙种内个体发育差异,而并非之前所认为的新铺龙种间差异。

Objective:Xinpusaurus is one of the thalattosaurs found in southwestern China.It was first discovered in the Carnian Xiaowa Formation,Guanling County,Guizhou Province.Although abundant Xinpusaurus materials are known,the morphology of juvenile Xinpusaurus and its ontogeny have not been well studied.Here we report a wellpreserved and mostly articulated juvenile skeleton of Xinpusaurus suni from Yaoshantian,Xinpu,Guanling County,Guizhou Province.The new specimen is nearly completely preserved,being the smallest reported Xinpusaurus suni individual so far.Methods:Morphological observation and quantitative morphological analysis were conducted to understand the ontogeny of Xinpusaurus suni.The snout/skull ratios of all available Xinpusaurus suni specimens were analyzed linearly and shown by scatter plot.Results:The body length of the new specimen is less than half of the holotype,the skull length is no more than one-third of the holotype skull,the ratio of the skull/trunk length is large,and the sutures of the skull are clear.These lines of morphological evidence support that the new specimen is a juvenile individual.The statistical analysis of all available Xinpusaurus suni specimens reveals that the allometric growth of the snout is significant during the ontogeny.Conclusions:The new specimen supplements the known morphological information of juvenile Xinpusaurus suni.It differs from the adult Xinpusaurus suni by its anterolateral processes longer than anteromedial processes of the supratemporal,pterygoid without teeth,neural arches and centra of the dorsal vertebrae unfused,about 72 caudal vertebrae,distal clavicle significantly shrunk,coracoid nearly round with coracoid foramen,pentagonal ischium,proximal and distal articulation surface of ulna convex.The allometric growth of the snout suggests that the proportion of snout/skull shall not be used to differentiate species in Xinpusaurus.