Biogéosciences, UMR 6282, Université Bourgogne Franche-Comté-CNRS-EPHE, 6 boulevard Gabriel, 21000, Dijon, France.
Division of Paleontology (Invertebrates), American Museum of Natural History, Central Park West at 79th Street, New York, NY, 10024, USA.
Sci Rep. 2021 Jun 4;11(1):11862. doi: 10.1038/s41598-021-89998-4.
Ammonite soft body remains are rarely preserved. One of the biggest enigmas is the morphology of the ammonite brachial crown that has, up till now, never been recovered. Recently, mysterious hook-like structures have been reported in multiple specimens of Scaphitidae, a large family of heteromorph Late Cretaceous ammonites. A previous examination of these structures revealed that they belong to the ammonites. Their nature, however, remained elusive. Here, we exploit tomographic data to study their arrangement in space in order to clarify this matter. After using topological data analyses and comparing their morphology, number, and distribution to other known cephalopod structures, in both extant and extinct taxa, we conclude that these hook-like structures represent part of the brachial crown armature. Therefore, it appears that there are at least three independent evolutionary origins of hooks: in belemnoids, oegospids, and now in ammonites. Finally, we propose for the first time a hypothetical reconstruction of an ammonite brachial crown.
菊石软体动物遗体很少被保存下来。其中一个最大的谜团是菊石的腕冠状部的形态，到目前为止，还从未有过恢复。最近，在多个箭石科（Scaphitidae）的标本中报告了神秘的钩状结构，箭石科是一种大型的晚白垩世异形菊石。之前对这些结构的检查表明，它们属于菊石。然而，它们的性质仍然难以捉摸。在这里，我们利用层析数据来研究它们在空间中的排列，以澄清这个问题。在使用拓扑数据分析并将其形态、数量和分布与其他已知的头足类动物结构进行比较后，无论是现存的还是已灭绝的分类群，我们得出的结论是，这些钩状结构代表了腕冠状部装甲的一部分。因此，似乎至少有三个独立的钩状结构进化起源：在箭石、Oegospids 和现在的菊石中。最后，我们首次提出了一个菊石腕冠状部的假设重建。
