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

Overview

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.

Methodology

The project will entail the construction and analysis of an inclusive phylogenetic dataset comprising morphological characters drawn from the literature and observation of extant and fossil cephalopod material. Microstructural and microstructural details will be obtained through detailed revisionist studies of key soft-bodied cephalopod fossils, principally using existing museum collections in the UK and Germany. New morphological observations will be objectively evaluated through the production of a maximally instructive morphological dataset, constructed using a homology-driven approach to character definition.

Phylogenetic characters will be defined atomistically, by decomposing complex variation into a series of individual, biologically independent characters. This approach maximises the information content of data (in the form of codable columns in a phylogenetic matrix) that can be extracted from fossil organisms, whilst minimising the distorting influence of missing data. Character correlation will be mitigated by defining hierarchies of reductively coded characters, which semantically acknowledges logical interdependence between characters and allows the appropriate treatment of inapplicable character states. Biological consistency will be maximised by adopting an explicitly defined framework of homology that will be tested and refined by iterative evaluation.

The resultant dataset will be evaluated alongside molecular sequence data in Bayesian and parsimony frameworks in order to measure the strength of evidence for competing hypotheses. This will deliver the first measured, quantitative evaluation of the implications of Nectocaris for cephalopod evolution, providing an objective perspective on a heated debate and furthering our understanding of the evolutionary processes that gave rise to modern biodiversity.

Project Timeline

Year 1

– Training in phylogenetic methods and techniques. Definition of phylogenetic characters and initial population of dataset from literature sources.

Year 2

Obtaining data from extant and fossil cephalopods via visits to museum collections. Redescription of key taxa and incorporation of data into phylogenetic matrix. Training in palaeontological data analysis.

Year 3

Phylogenetic analysis, exploration and evaluation of results. Writeup.

Year 3.5

Phylogenetic analysis, exploration and evaluation of results. Writeup.

Training
& Skills

Training will be provided in palaeontological data collection and analysis, and the construction of phylogenetic morphological datasets. The candidate will learn to use and apply new models of morphological evolution being constructed by the supervisory team, with the integration of molecular and stratigraphic information.

Further training will be provided in scientific writing, open data archiving, image preparation, and the use of scientific analytical software, including R and MrBayes.

References & further reading

Brazeau, Martin D., Thomas Guillerme, and Martin Ross Smith. 2019. ‘An Algorithm for Morphological Phylogenetic Analysis with Inapplicable Data’. Systematic Biology 68(4):619-631.

Kröger, Björn, Jakob Vinther, and Dirk Fuchs. 2011. ‘Cephalopod Origin and Evolution: A Congruent Picture Emerging from Fossils, Development and Molecules’. BioEssays 33(8):602-613.

Smith, Martin R. 2019. ‘An Ordovician Nectocaridid Hints at an Endocochleate Origin of Cephalopoda’. Journal of Paleontology, online ahead of print.

Smith, Martin Ross and Jean-Bernard Caron. 2010. ‘Primitive Soft-Bodied Cephalopods from the Cambrian’. Nature 465(7297):469-72.

Sutton, Mark D., Catalina Perales-Raya, and Isabel Gilbert. 2016. ‘A Phylogeny of Fossil and Living Neocoleoid Cephalopods’. Cladistics 32(3):297-307.

Further Information

For further information please contact Martin Smith: martin.smith@durham.ac.uk
 

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