Growth, form and function in problematic Cambrian paraconodonts

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. Middle to upper 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.

One reason why paraconodonts are so poorly understood is a scarcity of suitable material. 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 (Fig. 1). The new 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). Several thousand paraconodont specimens have already been extracted from the rock and mounted on glass microscope slides. Undissolved rock samples are available for thin-sectioning and for specimen extraction using a bespoke laboratory procedure (Leicester), with additional material potentially available from drillcore sampling in Saskatchewan, Canada.



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). Specimens are available in museum collections and potentially from additional field sampling in Sweden. 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.