PhD: Convergence or cryptic evolution? The origins of the cephalopod body plan

500 million years ago, life on earth was fundamentally transformed by the geologically rapid emergence of complex animal-dominated ecosystems. This ‘Cambrian Explosion’ permanently altered the dynamics of biology and geology on a planetary scale – but what evolutionary processes led to the sudden dominance of macroscopic organisms, and their concomitant interactions with the oceans, sediment and biosphere?

Cephalopods – a charismatic group of molluscs that includes cuttlefish, octopus, and the extinct ammonites and belemnites – offer an illuminating perspective on the dynamics of Cambrian evolution. The conventional view interprets a suite of snail-like Cambrian fossils as representing the gradually evolving roots of the cephalopod lineage. This view has recently been challenged by Nectocaris, an early Cambrian fossil that strikingly resembles a modern squid. If Nectocaris is a cephalopod, then cephalopods evolved from a non-mineralized ancestor in the height of the Cambrian explosion, with an early evolutionary history that largely escaped the fossil record.

This controversial interpretation supports an interpretation that sees anatomical blueprints of the major animal lineages becoming ‘fixed’ in the earliest Cambrian, and overturns the core tenets of cephalopod evolution: reconstructing the ancestral cephalopod as internally shelled, physiologically energetic, and jet propelled contradicts the conventional viewpoint of evolutionary ‘progress’ from sluggish forebears to ‘advanced’ modern taxa.

If Nectocaris is not a cephalopod, however, then a squid-like body plan must have arisen twice, with today’s squid ‘reinventing’ the blueprint established by Nectocaris. This remarkable degree of convergence implies that physical processes constrain biological possibility, with evolution only able to arrive at a finite number of discrete ‘body plans’.

This PhD proposal will conduct the first quantitative test of the relationships of Nectocaris. This objective evaluation will evaluate its implications for cephalopod origins, and for the nature of evolutionary innovation more generally.