The soft tissue fossil record and the diversification of vertebrates.

Abstract

Vertebrates are a highly diverse and disparate group, adapted to a huge variety of habitats and niches. During a rapid diversification in the Early Palaeozoic, they quickly acquired many of the key body plan traits that became central to their success, including bones, paired limbs and eventually jaws. The fossil record of biomineralized taxa has been essential in reconstructing the timing and tempo of the early evolutionary history of vertebrates. Yet they are of limited use for uncovering the origins of biomineralization, or the history of the non-biomineralizing vertebrates. Only the fossils of ‘soft-tissue’ non-biomineralizing taxa can help to answer questions about those events, and produce a more complete reconstruction of the origins of the group. However, soft-bodied early vertebrate fossils are both rare and notoriously problematic, with highly unstable phylogenetic placements that hinder research both into this group, but also into the vertebrates as a whole. The goal of this thesis is to investigate the problems surrounding soft-tissue vertebrate fossils, both in terms of reducing anatomical uncertainty in taxa, and in the controls and influences on their preservation. This was first explored at a taxon level, where I applied modern palaeontological techniques and geo-analytical methods to the problematic, but potentially highly informative, Silurian genus Lasanius. This resulted in robust anatomical reconstruction that allowed for recovery of a statistically supported position as a stem-cyclostome, indicating that biomineralization is a vertebrate, not gnathostome, synapomorphy, that underwent secondary loss in cyclostomes. Next, the drivers of variation in fossil morphology and their subsequent interpretation were considered across taxa. I developed a novel multivariate comparative framework that allowed the simultaneous incorporation of ontogenetic changes, taphonomy derived alterations and phylogenetic constraints on morphology to evaluate fossil anatomy and affinity. Finally, a more global view was taken by investigating the controls on fossilisation in Lagerstätten. Here I used a multivariate comparative approach to identify a strong taphonomic influence on the distribution of soft tissue fossils. Additionally the identification of key Lagerstätten characteristics provides a guide for predicting deposits that have the greatest potential for future success in recovering these rare fossils. Overall, the results presented in this thesis provide guidance for the incorporation of soft tissue vertebrate taxa into analyses. They demonstrate the need for an integrated approach: one that combines reliable morphological data with consideration of the potential biases that are often responsible for the problematic nature of these invaluable fossils.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Roy Wogelius (Supervisor) & Rob Sansom (Supervisor)