三叠纪关岭动物群邓氏萨斯特鱼龙的两性异形

The sexual dimorphism of Shastasaurus tangae(Reptilia:Ichthyosauria) from the Triassic Guanling Biota,China

贵州关岭晚三叠世早期法郎组瓦窑段迄今为止已产出了大量大型长吻鱼龙化石骨架,其中见于正式文献的有14件之多。头骨结构的相似性指示它们均归属于同一种:邓氏萨斯特鱼龙(Shastasaurus tangae)。对比已发表标本和尚未发表的本种新材料的头后骨骼发现,这些大型长吻的邓氏萨斯特鱼龙化石可以粗略地划分为两种类型:一种类型(A型)前肢和后肢大多不发育附生指列,且后肢第Ⅱ远端跗骨、蹠骨和第2趾前端1～2个趾节骨明显小于正常尺寸,甚至缺失;另一种类群(B型)前肢多发育一列后附生指,后肢发育一列前附生指。与此相对应,A型头骨顶脊前端分叉角度大,头骨略粗壮;B型头骨顶脊前端分叉角度小,头骨相对略纤长。鉴于这些化石均产自同一地区的相同层位,每一类型的骨骼数量大体相等,身体大小范围接近,虽然也可以据两种类型的微小差异划分为两个形态种,但依据头骨和头后骨骼结构的相似性本文更倾向于将这些标本置于同一种,而将彼此间区别判断为两性之间的差异。依据躯干和尾部长度比等推测A型鱼龙可能为雌性个体,B型鱼龙可能为雄性个体。

Marine reptiles, especially large-sized long-snout ichthyosaurs, are highly controversial in taxonomy in Guanling Biota （early Camian, Guizhou, SW China）. A large number of large- sized long-snout ichthyosaur skeletons have been excavated from Guanling and adjacent areas since 2000. Totally 14 specimens were used for taxonomical research and morphological description in previous works, which were originally named as different genera and species （Yin et al., 2000： Gmr 009, Gmr 015; Li and You, 2002： IVPP V 11865, V 11869; Chen and Cheng, 2003： TR 00001, SPCV 30014; Maisch et al., 2006： GNP dq-46, D-41, dq-22; Pan et al., 2006： GMPKU P1062; Chen et al., 2007： SPCV 10305, 10306; Shang and Li, 2009： IVPP V 11853; Shang et al., 2012： IGGCAS 2005F001）. Based on the similarity of cranial morphology, Shang et al. （2012） referred all of these specimens to one species Shastasaurus tangae. Two additional large-sized long-snout ichthyosaur specimens （IVPP V 15652, SDM 20090101） are used for further comparison. These specimens can be referred confidently to S. tangae based on the cranial characters, such as long snout; premaxilla without subnarial process; moderately developed anterior terrace of supratemporal fenestra; high parietal sagittal crest bifurcated both anteriorly and posteriorly; relatively short postorbital skull region. However, they are slightly different from the holotype of S. tangae in limb morphology.