32 managed to capture the morphology of the structures in several specimens. Using high resolution X-ray imaging, Kruta et al. The study of these structures is complicated, however, by the fact that most of them are embedded in the sedimentary matrix filling the body chamber. These hook-like structures have now been documented in approximately 50 specimens of Rhaeboceras halli and closely related species. 2C) and was completely unlike the hook-like structures previously described. The morphology of the radular teeth reported was consistent with that of radular teeth known from other aptychophoran ammonites (Fig. 31 rejected the radular interpretation after discovering evidence of radulae in three specimens of Rhaeboceras halli. Their concerns were justified, as Kruta et al. They did, however, express reservations about such an interpretation because of the unusually large size of the structures (approximately 50% of the length of the upper jaws) and the important morphological differences with other known radular elements. Based on the location of the structures (in the body chamber), they argued that these structures belonged to the ammonites and interpreted them as radular elements. 30 later described similar structures in Rhaeboceras, another member of the Scaphitidae, from the Campanian Bearpaw Shale of Montana. At the time, the authors raised several questions regarding the nature of these structures: (1) Do they belong to the ammonite or are they the remains of some other organism? (2) If they belong to the ammonite, are they radular elements? (3) If not, what are they? They were first described by Landman and Waage 29 who reported them in numerous specimens of Hoploscaphites from the Maastrichtian Fox Hills Formation of South Dakota (Fig. In the last few decades, enigmatic hook-like structures have been discovered in multiple specimens of Late Cretaceous ammonites of the family Scaphitidae, a large group of heteromorph ammonites. Therefore, it is essential to compare any fossilized structures in ammonites to those in both fossil and modern cephalopods. As a result, they are considered convergent acquisitions 23, 26, 27, 28. The hooks in these coleoids (only present today in a few families of the order Oegopsida) differ in morphology, possibly implying that cephalopod hook-like structures appeared multiple times during the history of the group 25. Indeed, coleoid hook-like structures are reported in extant as well as in fossil coleoids since the Carboniferous 23, 24. On the other hand, arm crowns are well documented in fossil coleoids through the presence of sclerotized arm structures such as hooks, most often isolated 11, 12, 13, but occasionally still articulated 14, 15, 16, 17, 18, 19, 20 and/or associated with soft tissue remains 21, 22. Additionally, ammonites are thought to have been preyed upon by many predators 6, 7, 8, 9, and even possibly by other ammonites 10, further reducing the probability of preserving soft tissues. This is most probably due to the retraction of the arms into the body chamber post-mortem 1, and/or the poor preservation potential of the arms’ soft tissue 4, 5. However, no remains of arms or arm structures have ever been discovered in ammonites, not even when internal organs are preserved 3. According to phylogenetic bracketing, it is generally assumed that they had ten arms 1, 2. One of the biggest uncertainties involves the morphology of their brachial crown. Although they are ubiquitous in the fossil record, the anatomy of their soft body is unfortunately very poorly known, hindering our knowledge of their paleoecology and paleobiology. Finally, we propose for the first time a hypothetical reconstruction of an ammonite brachial crown.Īmmonites are an abundant and iconic group of extinct marine organisms. Therefore, it appears that there are at least three independent evolutionary origins of hooks: in belemnoids, oegospids, and now in ammonites. 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. Here, we exploit tomographic data to study their arrangement in space in order to clarify this matter. A previous examination of these structures revealed that they belong to the ammonites. Recently, mysterious hook-like structures have been reported in multiple specimens of Scaphitidae, a large family of heteromorph Late Cretaceous ammonites. One of the biggest enigmas is the morphology of the ammonite brachial crown that has, up till now, never been recovered. Ammonite soft body remains are rarely preserved.
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