On the Carboniferous shark Tristychius arcuatus Agassiz from Scotland

J. R. F. Dick
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引用次数: 79

Abstract

Synopsis Skeletal remains of Tristychius arcuatus commonly occur in ironstone nodules from the Scottish Upper and Lower Oil Shale Groups. This material is clearly distinguishable from the small shark from Glencartholm described by Traquair, Woodward and Moy-Thomas under this name. Study of the latter shows that its finspines are distinctly different from the holotype of T. arcuatus and, therefore, it has been renamed. Tristychius arcuatus sensu stricto is a medium-sized hybodont shark with a short gape, a functionally heterocercal tail and narrow-based, tribasal pectoral fins. Its most unusual feature is a well developed opercular gill cover composed of long hyoid rays. Evidence suggests that this character was present in several Palaeozoic sharks, although it is absent in all Recent elasmobranchs. It is not clear whether it was primitively present in chondrichthyans or evolved separately in several lineages. Hybodonts and ctenacanths are recognised as separate, specialised shark radiations, neither of which can be directly ancestral to Recent sharks. Of the two, hybodonts appear to be more closely related to Recent forms, although the presence of typical hybodont finspines in Tristychius arcuatus indicates that they had diverged from ancestral euselachians before the beginning of the Carboniferous.
苏格兰石炭纪鲨鱼Tristychius arcuatus Agassiz
在苏格兰上油页岩群和下油页岩群的铁石结核中,通常会发现弓形三齿龙的骨骼残骸。这种材料与特拉奎尔、伍德沃德和梅-托马斯以这个名字描述的格伦卡瑟尔姆的小鲨鱼明显不同。对后者的研究表明,它的鳍刺与原型弓形龙明显不同,因此它被重新命名。狭口三齿鲨是一种中等大小的下颌鲨鱼,具有短的开口,一条功能性的异角尾巴和窄基的三基胸鳍。它最不寻常的特征是发育良好的由长舌骨射线组成的眼鳃盖。有证据表明,这种特征在一些古生代鲨鱼中存在,尽管在所有最近的板鳃科中都没有。目前尚不清楚它是最初存在于软骨鱼中,还是在几个谱系中分别进化而来。Hybodonts和十六棘鱼被认为是单独的,专门的鲨鱼辐射,它们都不可能是现代鲨鱼的直接祖先。在这两种动物中,下肢动物似乎与最近的形式更接近,尽管在弓形三齿兽身上典型的下肢动物鳍刺的存在表明它们在石炭纪开始之前就已经与祖先真肢动物分道扬镳了。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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