Final year DPhil student, funded by the Leverhulme Trust, studying the evolutionary relationships of stem teleosts from the Mesozoic era.

Introduction
Teleosts are the largest group of vertebrates, comprising over 30,000 species, but we know surprisingly little about their origin and early relationships. My research focusses on Pachycormiformes, a group of early diverging stem teleosts that are known from Jurassic and Cretaceous marine deposits all around the world. Most notably, this group shows wide eco-morphological variation in their feeding mechanisms: from large, sharp teeth for carnivory, to elaborate gill rakers for suspension-feeding. This group of suspension-feeders also includes the largest fish to have ever lived: Leedsichthys.

However, despite this ecological disparity, they have received very little detailed treatment of their morphology and phylogenetic relationships. While there are a wealth of pachycormiform fossils available for study, most are flattened or disarticulated, removing characters that could be used in phylogenetic and comparative analyses or restricting studies to just the external bones. This limits the inferences we can make not just about their ecology and evolution, but about the origin and evolution of the teleost lineage as a whole.

Methods & Work to Date
During my DPhil, I have used CT scanning to analyse rare three-dimensional fossils of pachycormiforms, which allows me to view their internal and external morphology as it would have been arranged in life. This method reveals structures that hold a huge array of morphological characters, such as the braincase, to use in a phylogenetic analysis. From these scans, I have redescribed the morphology of two pachycormiforms: the tuna-like, carnivorous Pachycormus sp., and the giant suspension-feeder Martillichthys renwickae. These descriptions were then used in a phylogenetic analysis to reveal the interrelationships of pachycormiforms and their placement on within teleosts.

Current Research & Impact
My current work is looking at what happened to the giant suspension-feeding pachycormiforms at the end of the Cretaceous, when it is presumed they fell victim to the same mass extinction event that wiped out the dinosaurs. I will be using Bayesian statistical methods to apply confidence intervals to the fossil record of the giant suspension-feeding pachycormiforms to estimate when they went extinct and if this correlates with the end-Cretaceous mass extinction. I will also do this with the fossil record of contemporary giant suspension-feeding chondrichthyans – such as whale sharks, basking sharks, megamouth sharks and manta rays – to estimate when these modern lineages emerged. The results will provided an estimate of when these chondrichthyans, the ecological predecessors to the giant suspension-feeding pachycormiforms, evolved and if suspension-feeding chondrichtyans and pachycormiforms were ever in competition with each other. This will build a clearer picture of when giant suspension-feeding pachycormiforms disappeared, when these modern lineages emerged to replace them, and tell us more about how events of the past have shaped the biodiversity we see today.

Dobson, C., Giles, S., Johnson, Z., Liston, J., and Friedman, M. [In Press]. Cranial osteology of the Middle Jurassic (Callovian) Martillichthys renwickae (Neopterygii: Pachycormiformes), with comments on the evolution and ecology of edentuolous pachycormiforms. Papers in Palaeontology.

 

Ball, S., Petrovan, S., Hand, N., Willman, F., Durrant, C., Uller, T., Ashe-Jepson, E., Dobson, C., Lawson, B., Garner, T. W. J. [In Review]. Demographic vulnerability of adders in the UK: results of a genetic study. Conservation Genetics.