Implications of the ‘hemihoplitid-like’ ammonites iterative morphology in the context of the late Tethyan Barremian (Early Cretaceous)

Didier Bert (1, 2) , Stéphane Bersac (2) , Léon Canut (2)

1 GR - Géosciences Rennes
2 GPA - Laboratoire du Groupe de recherche en Paléobiologie et biostratigraphie des Ammonites

Hemihoplitidae evolution is well documented in the north-west Tethyan margin at the lower upper Barremian. In this context, the genus Lenicostites gen. nov. (L. rusticus), with ‘hemihoplitid-like’ morphology, is unexpected because of its age older than the earliest Hemihoplites and its evolutionary stasis that contrasts with the rapid changes of the Hemihoplitidae. Data show no connection between Hemihoplitidae and Lenicostitidae fam. nov. and the resemblance is homeomorphy. The numerous reported occurrences of Hemihoplitinae over the world are reviewed. Their critical revision shows that some of them are contradictory with the evolutionary history of the group. The ‘hemihoplitid-like’ morphology is iterative in several taxa without any phyletic links (homeomorphy). Reasons could be linked to the morphospace occupation in the zone of maximum equilibrium between different constraints (a Pareto-optimal solution), making such morphology effortless to reproduce. The Austral Homeomorphitinae subfam. nov. (with Homeomorphites aguirreurretae gen. nov. et sp. nov.) are assigned to the Neocomitidae. A phyletic link between Shasticrioceras and Antarcticoceras is suggested (Shasticrioceratidae fam. nov.). Until proven otherwise, there is no Hemihoplitinae outside the north and west margins of the Tethys (including the Essaouira Basin). Homeomorphy between Hemihoplitinae and Lenicostitidae fam. nov. is explored and convergence seems the most convincing hypothesis. The further appearance of Camereiceras (Hemihoplitinae) could establish favorable conditions for interspecific competition, and the disappearance of Lenicostites gen. nov. could be interpreted as a complete competitive replacement. In this hypothesis Lenicostites gen. nov. is a victim of the Gause Principle as it lost the Red Queen race.

Keywords: Pareto-optimal solution; ammonites; homeomorphy; Early Cretaceous; systematics; competition

Domains: Paleontology

Didier Bert, Stéphane Bersac, Léon Canut. Implications of the ‘hemihoplitid-like’ ammonites iterative morphology in the context of the late Tethyan Barremian (Early Cretaceous). Cretaceous Research, 2020, 106, pp.104239. ⟨10.1016/j.cretres.2019.104239⟩. ⟨insu-02283637⟩