In the last few decades, hook‐like structures have been reported in the Mesozoic ammonite family Scaphitidae. Despite their exceptional preservation and debates about their function, no detailed reconstruction has been available until now. For the first time, we describe the composition and details of the morphology of these structures found in the body chambers of six specimens of the Campanian ammonite Rhaeboceras halli (Meek & Hayden) using high resolution x‐ray imaging. The hook‐like structures are composed of a thin layer of brushite. The base of each hook is open on one side forming an internal cavity, now filled with sediment. The tips of the hooks end in one or two cusps or, rarely, exhibit a blunt end. We used geometric morphometrics to capture the morphological disparity of the bicuspidate morphotypes comprising 98% of the hooks. Principal component analysis revealed chirality among the hooks and a cluster analysis recognized five morphologies. Contrary to the previous interpretation of these structures, we conclude that they are not radular teeth. They are much larger and more variable in size and shape than any known ammonite radulae and completely out of proportion with respect to the size of the jaw. The chirality, the hook‐like shape, and the absence of a size relationship between the hooks and the body chambers in which they occur, lead us to propose that these hooks could represent elements of the brachial crown related to copulatory behaviour. If so, these would be the first reported remnants of brachial crowns in ammonites.

中文翻译：

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白垩纪白mon中的神秘的钩状结构（Scaphitidae）

在过去的几十年中，中生亚mon族Scaphitidae报道了钩状结构。尽管对其进行了特殊的保存并对其功能进行了辩论，但迄今为止尚无详细的重建信息。首次，我们描述了在Campanian mon石Rhaeboceras halli的六个标本的体腔中发现的这些结构的组成和形态学细节。（Meek＆Hayden）使用高分辨率X射线成像。钩状结构由薄层透钙磷石组成。每个挂钩的底部在一侧开口，形成一个内部空腔，现在充满了沉积物。钩的尖端以一或两个尖瓣结束，或很少有钝的末端。我们使用几何形态计量学来捕获占98％钩的二尖瓣形态型的形态差异。主成分分析显示出钩子之间的手性，聚类分析识别出五种形态。与这些结构的先前解释相反，我们得出的结论是它们不是球状齿。它们比任何已知的am石锯齿形大得多，并且在大小和形状上更加可变，并且相对于颚的大小完全不成比例。手性，钩状形状 并且钩子与它们所处的体腔之间不存在尺寸关系，导致我们提出这些钩子可以代表与交配行为有关的肱冠的元素。如果是这样的话，这将是最早报道的铵盐中的肱冠残留物。