Parallel evolution controlled by adaptation and covariation in ammonoid cephalopods

Area of Science:

• Paleontology
• Evolutionary Biology
• Marine Biology

Background:

• Understanding evolutionary trajectories of clades is a major goal in evolutionary biology.
• Parallel evolution, common in lineages, is often attributed to natural selection (functional constraints), but developmental constraints are also possible but harder to identify.
• Further research is needed to understand the underlying processes of parallel evolution.

Purpose of the Study:

• To investigate a potential case of parallel evolution in two distinct ammonoid lineages (Auguritidae and Pinacitidae) during the Early-Middle Devonian.
• To analyze the morphological shifts in shell coiling, body size, suture complexity, and umbilical lid development within these lineages.
• To determine the driving factors behind the observed parallel evolution, distinguishing between adaptation and developmental constraints.

Main Methods:

• Phylogenetic analysis of ammonoid lineages.
• Morphological analysis of shell characters including coiling, adult body size, suture lines, and umbilical lids.
• Comparative study of evolutionary trends in Auguritidae and Pinacitidae from the Early-Middle Devonian (405-395 Ma).

Main Results:

• Two ammonoid lineages, Auguritidae and Pinacitidae, exhibited parallel morphological evolution over several million years.
• Key evolutionary trends included increased shell involution, larger adult body size, more complex suture lines, and the development of an umbilical lid.
• Increased involution and umbilical lid development likely resulted from natural selection for improved hydrodynamic properties.
• Sutural complexity appears to have evolved due to covariation with increasing adult size and whorl overlap, influenced by morphogenetic properties.

Conclusions:

• The parallel evolution in these Devonian ammonoid lineages was driven by a combination of factors.
• Adaptation for improved shell hydrodynamics played a role in the evolution of involution and umbilical lids.
• Covariation, or constructional constraints, influenced the evolution of sutural complexity.
• This case demonstrates that parallel evolution can be simultaneously driven by adaptation (natural selection) and covariation (developmental constraints).