Sachs Sven, Hornung Jahn J, Kear Benjamin P
Naturkundemuseum Bielefeld, Abteilung Geowissenschaften, Bielefeld, Germany; Engelskirchen, Germany.
Hamburg , Germany.
PeerJ. 2016 Dec 22;4:e2813. doi: 10.7717/peerj.2813. eCollection 2016.
The holotype of is one of the most historically famous and anatomically complete Early Cretaceous plesiosaurian fossils. It derived from the Gerdemann & Co. brickworks clay pit near Gronau (Westfalen) in North Rhine-Westphalia, northwestern Germany. Stratigraphically this locality formed part of the classic European "Wealden facies," but is now more formally attributed to the upper-most strata of the Bückeberg Group (upper Berriasian). Since its initial description in 1914, the type skeleton of has suffered damage both during, and after WWII. Sadly, these mishaps have resulted in the loss of substantial information, in particular many structures of the cranium and limb girdles, which are today only evidenced from published text and/or illustrations. This non-confirmable data has, however, proven crucial for determining the relationships of within Plesiosauria: either as an early long-necked elasmosaurid, or a member of the controversial Early Cretaceous leptocleidid radiation. To evaluate these competing hypotheses and compile an updated osteological compendium, we undertook a comprehensive examination of the holotype as it is now preserved, and also assessed other Bückeberg Group plesiosaurian fossils to establish a morphological hypodigm. Phylogenetic simulations using the most species-rich datasets of Early Cretaceous plesiosaurians incorporating revised scores for , together with a second recently named Bückeberg Group plesiosaurian (Hampe, 2013), demonstrated that referral of these taxa to Leptocleididae was not unanimous, and that the topological stability of this clade is tenuous. In addition, the trait combinations manifested by and were virtually identical. We therefore conclude that these monotypic individuals are ontogenetic morphs and is a junior synonym of . Finally, anomalies detected in the diagnostic features for other "Wealden" plesiosaurians have prompted reconsiderations of interspecies homology versus intraspecific variability. We therefore propose that the still unresolved taxonomy of should emphasize only those character states evident in the examinable fossil material, and specifically accommodate for growth-related modifications delimited via osteologically mature referred specimens.
该标本是历史上最著名且解剖结构完整的早白垩世蛇颈龙化石之一。它采自德国西北部北莱茵 - 威斯特法伦州格罗瑙（威斯特法伦）附近的格德曼公司砖厂粘土矿坑。在地层学上，该地点是欧洲经典的“威尔德相”的一部分，但现在更正式地归属于比克贝格组（上贝里亚阶）的最上层。自1914年首次描述以来，该标本的骨架在二战期间及战后都遭受了损坏。遗憾的是，这些不幸事件导致了大量信息的丢失，尤其是颅骨和肢带的许多结构，如今仅能从已发表的文字和/或插图中得以证实。然而，这些无法确证的数据对于确定该标本在蛇颈龙目中的关系至关重要：它要么是早期的长颈薄板龙科成员，要么是有争议的早白垩世瘦头龙类辐射的一员。为了评估这些相互竞争的假说并编纂一份更新的骨骼学纲要，我们对现存的标本进行了全面检查，并评估了比克贝格组的其他蛇颈龙化石以建立形态学模式标本。使用包含该标本修订分数的、物种最丰富的早白垩世蛇颈龙数据集进行系统发育模拟，以及与另一种最近命名的比克贝格组蛇颈龙（Hampe，2013）一起分析，结果表明将这些分类单元归入瘦头龙科并不一致，并且该类群的拓扑稳定性很脆弱。此外，该标本和另一标本所表现出的性状组合几乎相同。因此，我们得出结论，这些单型个体是个体发育形态，该标本是另一标本的次同物异名。最后，在其他“威尔德”蛇颈龙的诊断特征中检测到的异常促使人们重新考虑种间同源性与种内变异性。因此，我们建议，对于该标本仍未解决的分类，应仅强调在可检查的化石材料中明显的那些特征状态，并特别考虑通过骨骼成熟的参考标本界定的与生长相关的变化。
