The concept of ‘heteromorph ammonoids’

The ammonoids are extinct cephalopods with predominantly planispirally-coiled shells. The term ‘heteromorph ammonoids’ is deeply rooted in literature to ecompass ‘aberrant’ ammonoids (for instance scaphitids) characterized by shells uncoiled in various ways. At one time such forms were considered to be a sign of degeneration foreshadowing the extinction of these cephalopods. Today we know that this was not the case. Actually, ‘heteromorph ammonoids’ are a heterogeneous mixture of taxa without any phylogenetic, morphological or ecological coherence. The term no longer has any explanatory power. For this reason, we think it makes no sense any further to consider the ‘heteromorphs’ as a single entity in palaeobiological studies.

Work partially financed by the National Science Centre, Poland (Grant 2015/19/B/ST10/02033).

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Scaphitid ammonoids are the well-known group of Late Cretaceous ‘heteromorphs’. The illustration shows evolution of a latest Cretaceous scaphitid lineage; note a tendency to ‘recoil’ the shells with time (after Landman et al., 2021, fig. 2).