PhD: Growth, form and function in problematic Cambrian paraconodonts

The Cambrian evolutionary ‘explosion’ is recorded by the proliferation of biomineralized structures in the fossil record. However, many ‘small shelly fossils’, remain difficult to interpret in anatomical, phylogenetic and functional terms, limiting their utility for reconstructing evolutionary patterns and processes. Cambrian assemblages of ‘paraconodont’ microfossils are suggested to include the evolutionary precursors to euconodont feeding elements, which are tooth-like structures that evolved independently of teeth in other vertebrates. Simple, conical paraconodont elements strongly support this evolutionary scenario (Murdock et al. 2013), but other paraconodont elements are more structurally complex, with unresolved growth modes and functional roles. Were the earliest conodonts more ecologically disparate than has been appreciated, or are ‘complex’ paraconodont elements anatomically or phylogenetically unrelated to conodonts? Resolution of this question will either constrain the origins of an extraordinary convergent radiation of ‘toothy’ vertebrates, or reveal previously unknown Cambrian bodyplans.

This project will take advantage of a recently discovered assemblage of diverse, abundant and exceptionally well-preserved paraconodont elements from the upper Cambrian Deadwood Formation of western Canada. The specimens exhibit a unique, entirely carbonaceous mode of preservation which renders internal growth lines visible in transmitted light (Butterfield and Harvey 2012). A principal aim is to resolve the seemingly paradoxical growth patterns in Westergaardodina, Proacodus and Serratocambria, in order to test possible homologies with coniform paraconodonts, euconodonts, and extant relatives (in particular, lampreys).
For a three-dimensional appreciation of complex paraconodont-element growth, comparative specimens from phosphatic ‘small shelly fossil’ assemblages will be analysed via thin sectioning (Leicester) and synchrotron radiation X-ray tomographic microscopy (data processing in Oxford). The collection of comparative data on growth and decay patterns in modern taxa including lampreys would be a useful addition, though not critical to the success of the project.

Aside from palaeobiological aspects, the project would benefit from a systematic taxonomic and taphonomic study of the new assemblage. Initial analysis suggests that many of the Westergaardodina morphotypes are new to science. A corollary of this work would be a refined biostratigraphic scheme for use in non-trilobite-bearing Cambrian strata.

This project will require a multi-faceted approach to solving palaeobiological problems. Large collections of unpublished specimens are already available for study, but additional material will be obtained via laboratory processing of rock samples for both ‘small carbonaceous fossils’ (SCFs) and ‘small shelly fossils’ (SSFs). Additional sampling from drillcore and/or field localities could be pursued. You will be trained in the delicate art of microfossil picking and mounting for analysis. Resolving growth mode and element outlines will require advanced microscopy techniques, including scanning electron microscopy (SEM, Leicester), synchrotron radiation X-ray tomographic microscopy (SRXTM, probably at the Swiss Light Source), and transmitted light microscopy (TLM) with differential interference contrast (using Leicester’s newly acquired Zeiss AxioImager microscope). Morphological data on growth mode will be analysed using graphical and/or morphometric techniques. Testing taphonomic hypotheses will require petrographic thin-sectioning and/or experimental element dissolution, combined with basic geochemical analysis (BSEM/EDX). The project could include the dissection and microscopic examination of modern lampreys, both fresh and decayed, in Leicester’s state-of-the-art taphonomy lab.