The concept of ‘heteromorph ammonoids’

• Published in: Lethaia Vol. 54; no. 5; pp. 595 - 602
• Main Authors: Landman, Neil H., Machalski, Marcin, Whalen, Christopher D.
• Format: Journal Article
• Language: English; Norwegian
• Published: Oslo Wiley 01.12.2021; Scandinavian University Press
• Subjects: Ammonoidea; Ammonoids; Ancyloceratina; Ecological distribution; heteromorphs; morphotype; Niches; Phylogenetics; Phylogeny; Scaphitidae
• ISSN: 0024-1164; 1502-3931
• DOI: 10.1111/let.12443

‘Heteromorph ammonoids’ encompass all ammonoid species whose shapes do not conform to a closely coiled planispiral shell. The term is useful as a broad description for such ammonoids. However, as a concept, ‘heteromorph ammonoids’ no longer has any scientific value or explanatory power. Although such ammonoids have traditionally been considered aberrant forms, they represent instead an integral part of the evolutionary history of the Ammonoidea. ‘Heteromorph ammonoids’, as a whole, are a polyphyletic group, consisting of a heterogeneous mixture of taxa without any phylogenetic, morphological or ecological coherence. Their treatment as a single entity risks conflating convergences and phylogenetic affinities. It also vastly oversimplifies the stunning array of morphologies and ecological niches occupied by these animals. Investigation into the uncoiling (and recoiling) of ammonoids is a legitimate and worthwhile enterprise, especially in view of the realization that this phenomenon occurred several times in the history of the Ammonoidea. However, few insights can be gained by treating ‘heteromorph ammonoids’ as a single entity. Studies of such ammonoids should focus on monophyletic groups within a well‐constrained phylogenetic and stratigraphical framework to yield meaningful results.