ABSTRACT

Basking sharks are presented in the modern fauna by a single species (Cetorhinus maximus), although they have been much more diverse in the past. This group of sharks first appeared in the fossil record in the middle Eocene of the Antarctic and the U.S.A., but most of the described extinct taxa are known from the Oligocene and lower Miocene of Europe. Gill rakers are the most abundant among basking shark remains and their morphological details play an important role in species diagnostics. The shape of isolated gill rakers from 16 Oligocene localities of Poland was analyzed using various morphological approaches, including geometric morphometrics. Results indicate that descriptive characters have a wide range of variation and low diagnostic value, and they are associated directly with the position of gill rakers on the gill arch. Morphological indices describe proportions by discrete structures and could be effective only in the identification of the stratigraphically most distant taxa. Geometric morphometrics revealed significant differences between all of the species designated earlier except for †Caucasochasma zherikhini and †Keasius parvus. At the same time, considering the obtained results on morphology along with the geographic distance, Oligocene basking sharks from Poland should be assigned to †K. parvus. Geometric morphometrics of gill rakers supports the taxonomic distinctness of both †K. rhenanus and †K. septemtrionalis. Study results indicate that reliable taxonomic attribution of extinct basking sharks by the shape of gill rakers, in the absence of other skeletal elements (teeth and/or vertebrae), should be sample-based using multivariate approaches.

摘要

姥鲨在现代动物群中由单一物种（Cetorhinus maximus)，尽管它们在过去更加多样化。这组鲨鱼首先出现在南极和美国中始新世的化石记录中，但大多数描述的已灭绝分类群来自欧洲的渐新世和下中新世。鳃耙是姥鲨遗骸中最丰富的，它们的形态细节在物种诊断中起着重要作用。使用各种形态学方法（包括几何形态测量学）分析了来自波兰 16 个渐新世地区的孤立鳃耙的形状。结果表明，描述性特征变异范围广，诊断价值低，且与鳃耙在鳃弓上的位置直接相关。形态指数描述了离散结构的比例，并且只能在识别地层最远的分类群中有效。几何形态计量学揭示了所有早先指定的物种之间的显着差异，除了 †Caucasochasma zherikhini和† Keasius parvus。同时，考虑到获得的形态学结果和地理距离，来自波兰的渐新世姥鲨应归入† K. parvus。鳃耙的几何形态测量学支持 † K. rhenanus和 † K. septemtriionalis的分类差异。研究结果表明，在没有其他骨骼元素（牙齿和/或椎骨）的情况下，根据鳃耙形状对已灭绝的姥鲨进行可靠的分类归属，应该使用多变量方法基于样本。