Data from: Iterative ontogenetic development of ammonoid conch shapes from the Devonian through to the Jurassic

We measured longitudinal growth in conch cross-sections of 177 Devonian to
Jurassic ammonoid species to test whether conch ontogenetic development
parallels the iterative evolution of pachyconic or globular conch shapes.
Ontogenetic trajectories of two cardinal conch parameters, conch width
index and umbilical width index, show a few common recurring ontogenetic
pathways in terms of the number of ontogenetic phases. The most common,
with three phases in the conch width index (decrease–increase–decrease)
and umbilical width index (increase–decrease–increase), is termed here
C-mode ontogeny (after the Carboniferous genus Cravenoceras). Many of the
studied globular Palaeozoic and Triassic species (of the latter,
particularly the arcestid ammonoids) share principal patterns in the
triphasic C-mode conch ontogeny in closely related groups but also between
unrelated groups as well. The repetition of conch growth patterns is an
example of convergent evolution of the entire life history of globular
ammonoids. The studied Jurassic globular shaped ammonoids deviate from the
growth patterns seen in earlier groups showing less pronounced ontogenetic
trajectories with nearly isometric or weakly asymmetric growth without
distinct phases. This trajectory is termed here M-mode ontogeny (after the
Jurassic genus Macrocephalites). No major change in the ontogenetic modes
of pachyconic and globular ammonoids occurred moving from the Palaeozoic
into the Mesozoic; the survivors of the end-Permian extinction event
iteratively developed conch ontogenies similar to those of Palaeozoic
forms. In contrast, the Triassic–Jurassic boundary marks the major event
with the evolution of some cardinal conch parameters relating to globular
ammonoid ontogeny.