PhD: The completeness of the early shark fossil record

The fossil record offers a rich resource with which to assess the timing and tempo of evolution among a wide variety of organisms. However, there is a question that underlies all of this work: just how complete is the fossil record? This question is one of the most fundamental posed by palaeontologists. Its answers have major ramifications for our understanding of deep time evolution, biogeography, ecology and diversity.

The early history of sharks (and indeed all vertebrates with jaws) is undergoing a major revision at the present time, with character acquisition and evolution, taxonomic interrelationships and timing of divergence all receiving a renewed scrutiny. Each of these depend on an understanding of the quality of their fossil record. There are a number of curious aspects of the early shark record, notably the c.50 Mya gap between the appearance of shark-like scales in the Ordovician before the first widely accepted shark teeth and, slightly younger, body fossils in the Lower Devonian. Recent analyses recover acanthodians stem-group sharks, and this result fills part of the apparent stratigraphic gap, but there remain considerable uncertainties about how the completeness of these, often poorly preserved, fossils impacts on inferences of diversity and relationships. Published reviews treat these materials indecisively at best (e.g. Brazeau & Friedman 2015).

New approaches to estimating fossil record completeness have recently been developed, and applied to the vertebrate record in both terrestrial (Mannion & Upchurch 2010) and marine (Tutin & Butler 2017) realms.

This project will seek to develop novel character completeness metric datasets for stem- and crown- group sharks through the Palaeozoic, covering the full diversity of body sizes, ecological strategies, and habitat preferences. Statistical analysis of these datasets will be used to address a series of key questions, including: (1) How complete is the early shark fossil record in comparison with other groups? (2) Is completeness impacted by ecological categories, habitat preferences, and/or body sizes? (3) Are changes in completeness correlated with major changes and shifts in global marine diversity, including evolutionary radiations and mass extinctions? (4) How do changes in completeness correlate with broader estimates of fossil record sampling through time and space?

The student will initially carry out a comprehensive literature review to extract and compile existing completeness datasets. Subsequently, new completeness estimates will be generated for a broad range of Palaeozoic clades, from both marine and freshwater environments. These estimates will primarily utilise an adapted version of the ‘character completeness metric’ (CCM). A diverse range of statistical approaches will be applied to the resulting datasets in order to answer the project’s key questions.