Morphometric evolution and phylogeny of Palaeozoic ammonoids. Early and Middle Devonian

Abstract

A combination of methods from cladistics and stratophenetic analyses is used for a reconstruction of Early and Middle Devonian ammonoid phylogeny. The analyses are based mainly on principal characters such as conch geometry (coiling form, whorl expansion rate, whorl cross-section shape), septal geometry (form of septa, number, position, and shapes of lobes), and ornament (growth lines and ribs); a new classification scheme of the ancient ammonoid order Agoniatitida is proposed. It is subdivided into four suborders: Agoniatitina (paraphyletic), Gephuroceratina (monophyletic), Anarcestina (paraphyletic), and Pharciceratina (monophyletic). Morphometric analysis shows the unfolding of several morphological trends, such as the modification of coiling parameters, among the Early and Middle Devonian ammonoids. Two major independent lineages can be recognised in the phylogeny of the Middle Devonian ammonoids, the first represents the agoniatids in which the Gephuroceratina is nested, the second are the anarcestids which gave rise to the Pharciceratina. The new families Latanarcestidae, Tamaritidae, Atlantoceratidae, and Taouzitidae, as well as the new genera Taouzites, Croyites and Meragoniatites are introduced.