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Belemnite phylogeny and decline during the mid-Cretaceous

By Kevin Stevens

Posted 12 Oct 2021
bioRxiv DOI: 10.1101/2021.10.11.463885

Belemnites are common fossil coleoid cephalopods of the Mesozoic. They began to diversify in the Triassic-Early Jurassic and maintained this diversity until the early Early Cretaceous. During the mid-Cretaceous, they declined in diversity and distribution, being restricted to only the Boreal and Austral Realm since the Turonian. Here, I present the first cladistic analysis of belemnite phylogeny, spanning taxa representative of the whole diversity and stratigraphic range of the group. This analysis shows that the usually applied subdivision of all belemnites into "Belemnitina" and "Belemnopseina" is not supported. A newly identified clade, the Pseudoalveolata, is suggested here. Pseudoalveolate belemnites represent the last remaining belemnites after the Aptian. Oceanic anoxia and warming are likely the main cause of the mid- Cretaceous belemnite decline, resulting in the Aptian-Albian dominance of the warm-adapted pseudoalveolate genus Neohibolites. The rise of teleost fish diversity during the mid- Cretaceous is discussed and its relevance for belemnite evolution. Some teleosts (e.g., Enchodus) might have taken over the mesopredator niches left by belemnites during the mid- Cretaceous, being better adapted to warming seas. Belemnites were not able to recover their earlier widespread distribution and diversity and the last remaining, disjunctly distributed families, the northern Belemnitellidae and southern Dimitobelidae, became extinct at the K/Pg-boundary.

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