The taxonomic utility of forefin morphology in Lower Jurassic ichthyosaurs: Protoichthyosaurus and Ichthyosaurus

The validity of the parvipelvian ichthyosaur genus Protoichthyosaurus was evaluated by examining the type material and recently recognized specimens. Protoichthyosaurus has a wide forefin with at least five primary digits, an anterior digital bifurcation in the forefin, a humerus that is nearly equal in width distally and proximally, and a coracoid with well-developed anterior and posterior notches, features it shares with the more common genus Ichthyosaurus. Previously, Protoichthyosaurus was synonymized with Ichthyosaurus because of an incomplete understanding of forefin variation in the latter taxon. However, Protoichthyosaurus has several features in the forefin that distinguishes it from Ichthyosaurus, including the presence of three elements in the distal carpal row, a contact between distal carpal 3 and the ulnare, a bifurcation of distal carpal 2 to form two elements in the metacarpal row, the proximal element of the bifurcation nearly separating distal carpal 2 from distal carpal 3, distal carpal 3 separating the intermedium from contact with distal carpal 4, and the absence of digit V. A phylogenetic analysis confirms that Protoichthyosaurus is the sister taxon of Ichthyosaurus and is nested in Ichthyosauridae. We recognize two species, P. prostaxalis and P. applebyi, sp. nov., but not P. prosostealis. Protoichthyosaurus is unequivocally from the Hettangian but might be present in the Rhaetian to lower Sinemurian. The genus is geographically widespread across the U.K. http://www.zoobank.org/urn:lsid:zoobank.org:pub:23CA73CE-BB9A-4EAD-81BA-A6D27ABC82CB SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP Citation for this article: Lomax, D. R., J. A. Massare, and R. T. Mistry. 2017. The taxonomic utility of forefin morphology in Lower Jurassic ichthyosaurs: Protoichthyosaurus and Ichthyosaurus. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1361433.本研究通过检视正模标本（type material）与新近确认的标本，对狭盆鱼龙类（parvipelvian ichthyosaur）原鱼龙属（Protoichthyosaurus）的分类有效性展开评估。原鱼龙属具备至少五枚主要指骨的宽大前鳍，前鳍存在前部指骨分叉结构；其肱骨远端与近端宽度近乎相等，喙骨则具有发育良好的前切迹与后切迹——这些特征均为其与更为常见的鱼龙属（Ichthyosaurus）所共享。此前由于对鱼龙属前鳍的形态变异认知不足，原鱼龙属曾被归为鱼龙属的同物异名。但原鱼龙属的前鳍存在多项可将其与鱼龙属区分开的独有特征，包括远端腕骨列（distal carpal row）包含三块骨块、远端腕骨3与尺骨腕骨（ulnare）存在关节连接、远端腕骨2发生分叉并在掌骨列中形成两块骨块、该分叉的近端骨块近乎将远端腕骨2与远端腕骨3分隔开、远端腕骨3阻止中间腕骨（intermedium）与远端腕骨4形成关节连接，以及不存在第V指骨。系统发育分析（phylogenetic analysis）结果证实，原鱼龙属为鱼龙属的姊妹群，且隶属于鱼龙科（Ichthyosauridae）。本研究确认该属包含两个有效物种：原鱼龙（P. prostaxalis）与阿普比原鱼龙（P. applebyi，sp. nov.），但不包含P. prosostealis。原鱼龙属的化石明确发现于赫塘阶（Hettangian），但也可能存在于瑞替阶（Rhaetian）至下西涅缪尔阶（lower Sinemurian）地层中。该属的地理分布广泛覆盖英国全境。http://www.zoobank.org/urn:lsid:zoobank.org:pub:23CA73CE-BB9A-4EAD-81BA-A6D27ABC82CB 补充数据：本文的补充材料可于www.tandfonline.com/UJVP免费获取。本文引用格式：Lomax, D. R., J. A. Massare 及 R. T. Mistry. 2017. 《下侏罗统鱼龙前鳍形态的分类学应用：原鱼龙属与鱼龙属》. 《脊椎动物古生物学杂志》. DOI: 10.1080/02724634.2017.1361433.