New evidence from exceptionally “well-preserved” specimens sheds light on the structure of the ammonite brachial crown

C. P A Smith (1) , N. H Landman (2) , J. Bardin (3) , I. Kruta (2, 3)

1 BGS - Biogéosciences [UMR 6282]
2 AMNH - American Museum of Natural History [New York, USA] = Musée américain d'histoire naturelle [New York, USA]
3 CR2P - Centre de Recherche en Paléontologie - Paris

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 hooklike 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.

Mots clés: Evolutionary ecology; Palaeoecology; Palaeontology

Domaines: Sciences du Vivant [q-bio]

C. P A Smith, N. H Landman, J. Bardin, I. Kruta. New evidence from exceptionally “well-preserved” specimens sheds light on the structure of the ammonite brachial crown. Scientific Reports, 2021, 11 (1), ⟨10.1038/s41598-021-89998-4⟩. ⟨hal-03251787⟩