A New Upper Jurassic Ophthalmosaurid Ichthyosaur from the Slottsmøya Member, Agardhfjellet Formation of Central Spitsbergen

Abundant new ichthyosaur material has recently been documented in the Slottsmøya Member of the Agardhfjellet Formation from the Svalbard archipelago of Norway. Here we describe a partial skeleton of a new taxon, Janusaurus lundi, that includes much of the skull and representative portions of the postcranium. The new taxon is diagnosed by a suite of cranial character states including a very gracile stapedial shaft, the presence of a dorsal process on the prearticular and autapomorphic postcranial features such as the presence of an interclavicular trough and a conspicuous anterodorsal process of the ilium. The peculiar morphology of the ilia indicates a previously unrecognized degree of morphological variation in the pelvic girdle of ophthalmosaurids. We also present a large species level phylogenetic analysis of ophthalmosaurids including new and undescribed ichthyosaur material from the Upper Jurassic of Svalbard. Our results recover all Svalbard taxa in a single unresolved polytomy nested within Ophthalmosaurinae, which considerably increases the taxonomic composition of this clade. The paleobiogeographical implications of this result suggest the presence of a single clade of Boreal ophthalmosaurid ichthyosaurs that existed during the latest Jurassic, a pattern also reflected in the high degree of endemicity among some Boreal invertebrates, particularly ammonoids. Recent and ongoing descriptions of marine reptiles from the Slottsmøya Member Lagerstätte provide important new data to test hypotheses of marine amniote faunal turnover at the Jurassic-Cretaceous boundary.

近日，在挪威斯瓦尔巴群岛（Svalbard archipelago）的阿加德菲耶莱组（Agardhfjellet Formation）斯洛斯莫亚段（Slottsmøya Member）中，报道了大量新发现的鱼龙（ichthyosaur）化石材料。本文记述了一个新属种——扬氏龙（Janusaurus lundi）的部分骨骼化石，其标本包含大部分头骨及具代表性的颅后骨骼（postcranium）区段。该新类群可通过一系列头骨特征组合加以鉴别诊断，包括极纤细的镫骨杆（stapedial shaft）、前关节骨（prearticular）上存在背突，以及具有独有衍征的颅后骨骼特征，如锁骨间槽（interclavicular trough）与髂骨（ilium）显著的前背突。其髂骨的特殊形态表明，大眼鱼龙类（ophthalmosaurids）的骨盆带（pelvic girdle）存在此前未被认知的较高形态变异程度。本文还针对大眼鱼龙类开展了大规模物种级系统发育分析，纳入了来自斯瓦尔巴群岛晚侏罗世（Upper Jurassic）的新发现及未记述鱼龙化石材料。分析结果显示，所有斯瓦尔巴群岛的鱼龙类群均嵌套于大眼鱼龙亚科（Ophthalmosaurinae）内，形成单一的未解多歧支（polytomy），这极大地提升了该演化支的分类学组成丰度。该结果的古生物地理学（paleobiogeographical）意义表明，晚侏罗世末期存在一个单一的北方区系大眼鱼龙类演化支，这一模式也与部分北方区系无脊椎动物（尤其是菊石类（ammonoids））的高度特有性特征相符。近期及正在进行的对斯洛斯莫亚段化石库（Lagerstätte）中海洋爬行动物的研究记述，为检验侏罗纪-白垩纪（Jurassic-Cretaceous）界线处海洋羊膜动物（marine amniote）的群落更替假说提供了重要的新数据。