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Abstract：
Impressions of vertebrate bodies or their parts, such as trace fossils and natural molds of bones, are a valuable source of information about ancient faunas which may supplement the standard fossil record based on skeletal elements. Whereas trace fossils of animal activity are relatively common and actively studied within the field of ichnology, and natural impressions of internal or external surfaces are a frequent preservation mode in fossil invertebrates, natural molds of bones are comparatively rare and less extensively documented and discussed. Among them, internal molds (steinkerns) of turtle shells are a relatively well-known form of preservation, but the mechanisms and taphonomic prerequisites leading to their formation are poorly studied. External shell molds are even less represented in the literature. Herein, we describe a historic specimen of a natural external turtle plastron mold from the Triassic (Norian) Löwenstein Formation of Germany-a formation which also yielded a number of turtle steinkerns. The specimen is significant not only because it represents an unusual form of preservation, but also due to its remarkably large size and the presence of a potential shell pathology. Although it was initially interpreted as Proterochersis sp., the recent progress in the knowledge of proterochersid turtles leading to an increase in the number of known taxa within that group allows us to verify that assessment. We confirm that the specimen is morphologically consistent with the genus and tentatively identify it as Proterochersis robusta, the only representative of that genus from the Löwenstein Formation. We note, however, that its size exceeds the size observed thus far in Proterochersis robusta and fits within the range of Proterochersis porebensis from the Grabowa Formation of Poland. The marks interpreted as shell pathology are morphologically consistent with Karethraichnus lakkos-an ichnotaxon interpreted as a trace of ectoparasites, such as leeches. This may support the previously proposed interpretation of Proterochersis spp. as a semiaquatic turtle. Moreover, if the identification is correct, the specimen may represent a very rare case of a negative preservation of a named ichnotaxon. Finally, we discuss the taphonomy of the Löwenstein Formation turtles in comparison with other Triassic turtle-yielding formations which show no potential for the preservation of internal or external shell molds and propose a taphonomic model for the formation of such fossils.
摘要：
脊椎动物身体或其部分的印象，比如化石和骨头的天然霉菌，是有关古代动物区系的宝贵信息来源，可以补充基于骨骼元素的标准化石记录。尽管动物活动的痕迹化石在物学领域相对普遍且研究活跃，和内部或外部表面的自然印象是化石无脊椎动物的常见保存模式，骨头的自然霉菌比较罕见，文献记载和讨论较少。其中,龟壳的内部模具（Steinkerns）是一种相对众所周知的保存形式，但是导致其形成的机制和植物学先决条件研究甚少。外壳体模具在文献中的代表性更低。在这里，我们描述了来自德国三叠纪（Norian）Löwenstein地层的天然外部海龟plastron霉菌的历史标本，该地层也产生了许多海龟steinkerns。标本之所以重要，不仅是因为它代表了一种不寻常的保存形式，但也由于其非常大的尺寸和潜在的外壳病理的存在。尽管最初被解释为Proterochersissp。,最近对海龟的认识取得了进展，导致该群体中已知分类群的数量增加，这使我们能够验证这一评估。我们确认该标本在形态上与该属一致，并初步将其鉴定为Proterochersisrobusta，该属的唯一代表来自Löwenstein组。我们注意到,然而,它的大小超过了迄今为止在Proterochersisrobusta中观察到的大小，并且符合波兰Grabowa组的Proterochersisporebeensis的范围。被解释为外壳病理学的标记在形态上与Karethraichhnuslakkos一致-一种被解释为外寄生虫的ichnotaxon，比如水蛭。这可能支持先前提出的对Proterochersisspp的解释。像半水生乌龟。此外，如果鉴定正确，该标本可能代表一个非常罕见的病例，即一个命名的ichnotaxon阴性保存。最后,我们讨论了Löwenstein地层海龟与其他三叠纪海龟产生的地层相比，这些地层没有显示出保存内部或外部壳霉菌的潜力，并提出了形成此类化石的解剖学模型。
