A mineralised skeleton is one of the most important and familiar vertebrate characteristics. Despite this, however, surprisingly little is known about the origin and early evolution of the vertebrate skeleton.  This is largely because the mineralised skeleton arose in lineages which are now completely extinct; thus in order to understand the developmental and evolutionary origins of the skeleton our only recourse is to study fossils. Fortunately there exists an extensive record of Palaeozoic jawless and jawed vertebrates with skeletons; owing to the high preservation potential of mineralised tissues.  Furthermore, because new mineralised tissue is added to the skeleton during life, a lasting record of previous growth stages is preserved.

Fig. 1. Vertebrate evolutionary tree based on the published phylogenies of Donoghue et al. (2000, 2006), Sansom et al. (2010) & Davis et al. (2012).

The research project will primarily focus on the development and evolution of the dermal skeleton, which demonstrates the stratigraphically earliest occurring and phylogentically most primitive mineralised tissues (Donoghue & Sansom 2002).  Rudimentary understanding of the evolutionary origin and developmental patterning of vertebrate dermal tissue can be addressed using histological studies of discrete ontogenetic sequences of extinct jawless and jawed taxa occupying progressively crownward positions within the gnathostome total-group.  Data will be collected through conventional thin sectioning and micro CT scanning, however the research will also necessitate state-of-the-art synchrotron X-ray tomography.  This latter technique is used to produce high resolution 3-dimensional models which can be dissected in much the same way as tissue from living vertebrates.  This will permit detailed sub-micrometre scale comparison between the dermal skeletal tissues of living and extinct vertebrates. 

In order to constrain the step-wise acquisition of skeletal characters within evolutionary time, I will also conduct molecular clock-based analysis.  This study will utilise Bayesian total-evidence analysis combining genetic and phenotypic matrices in order to estimate divergence times.  This technique has a number of advantages over traditional molecular clock-based analyses.  Firstly, the inclusion of extinct lineages within analyses utilising data restricted to extant taxa (e.g. molecular data) can potentially affect resultant tree topology (Near 2009).  Secondly, and most importantly, the molecular data is calibrated by placement of stratigraphically defined fossil taxa within the tree, rather than potentially non phylogenetic post hoc calibration of internal nodes (Pyron 2011, Ronquist et al. In Press). 

The results of this research project will contribute greatly to our understanding of the timing and sequence of dermal bone evolution and will be relevant to palaeontologists and developmental biologists alike.  Using total-evidence analysis, it will be possible to accurately date genome duplication events and consider how they relate to the acquisition of phenotypic characters within evolutionary time, thus resolving the long standing debate as to whether such duplication events were responsible for driving vertebrate macroevolution (Sidow 1996, Wagner et al. 2003, Prohaska and Stadler 2004, Donoghue and Purnell 2009).   

References

Davis, P., Finarelli, J. A. & Coates, M. I. 2012.  Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes, Nature, 486, 247-250.

Donoghue, P. C. J., Forey, P. L. & Aldrdge, R. J.  2000.  Conodont affinity and chordate phylogeny, Biological Reviews of the Cambridge Philosophical Society, 75, 191-251.

Donoghue, P. C. J. & Purnell, M. A.  2009.  The evolutionary emergence of vertebrates from among their spineless relatives.  Evolution: Education and Outreach, 2, 204-212.

Donoghue, P. C. J. & Sansom, I. J. 2002.  Origin and early evolution of vertebrate skeletonization.  Microscopy Research and Technology, 59, 352-372.

Donoghue, P. C. J., Sansom, I. J. & Downs, J. P.  2006.  Early evolution of vertebrate skeletal tissues and cellular interactions, and the canalization of skeletal development.  Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 306B, 278–294.

Near, T. J.  2009.  Conflict and resolution between phylogenies inferred from molecular and phenotypic data sets for hagfish, lampreys, and gnathostomes.  Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 312B, 749–761.

Prohaska, S. J. & Stadler, P. F.  2004.  The duplication of the Hox gene clusters in teleost fishes. Theory in Biosciences, 123, 89–110.

Pyron, R. A.  2011.  Divergence time estimation Using fossils as terminal taxa and the origins of Lissamphibia.  Systematic Biology, doi: 10.1093/sysbio/syr047.

Ronquist, F., Klopfstein, S., Vilhelmsen, L., Schulmeister, S., Murray, D. L. & Rasnitsyn, A. P.  In Press.  A Total-Evidence Approach to Dating with Fossils, Applied to the Early Radiation of the Hymenoptera.  Systematic Biology, doi: 10.1093/sysbio/sys058.

Sansom, R. S., Freedman, K., Gabbott, S. E,, Aldridge, R. J. & Purnell, M. A. 2010.  Taphonomy and affinity of an enigmatic Silurian vertebrate, Jamoytius kerwoodi White, Palaeontology, 53, 1393-1409.

Sidow, A.  1996.  Gen(om)e duplications in the evolution of early vertebrates. Current Opinion in Genetics & Development, 6, 715-722.

Wagner, G. P., Amemiya, C. & Ruddle, F. 2003.  Hox cluster duplications and the opportunity for evolutionary novelties.  Proceedings of the National Academy of Sciences, 100,14603–14606.