Davidonia and incipient speciation

During my PhD I worked with a model system composed of three assemblages of the helcionelloid molluscs Davidonia (referred to as Mackinnonia during my work with them). Helcionelloids’ precise phylogenetic position remains unclear in part due to the typically poor preservation of these organisms, which usually survive in the fossil record only as internal moulds also known as steinkerns; internal imprints of their single valve.

Davidonia is a morphologically variable genus of these molluscs with a global distribution. In my work with this group I studied three assemblages of steinkerns, two of which then described as D. rostrata and one as D. taconica. These likely closely related species had proven difficult to delimit morphologically using traditional cladistic methods due to there existing few qualitative characters to distinguish them.

I used an outline-based morphometric approach that translated their morphology into continuous values rather than discrete characters in order to resolve this difficulty. This method proved capable of detecting subtle variation in the outline of these organisms’ shells and consequently I was able to conclude that the two alleged conspecific assemblages of D. rostrata were as morphologically distinct from one another as they were from D. taconica, suggesting that these assemblages together represent three species and not two.

I also developed a method of analysing variation in ontogenetic trajectories between these species on a per-specimen basis, which also significantly separated the three assemblages into three ontogenetically distinct species. This work demonstrates the utility of studying morphological variation in the fossil record with methodological approaches that refrain from discretizing variation and thus losing the resolution necessary to detect fine-scale patterns.

Finally, I interpreted the uncovered patterns of variation in terms of a speciation process and defined the newly determined morphologically distinct rostrata assemblages as incipient species; “populations (or assemblages) of a species that exhibit minor or no morphological differences in terms of characters but significantly different morphology in quantitative terms.” This is an example of the paleobiological approach I employ in my work; using palaeontological material to clarify biological processes and drive theoretical innovation.

These results are published here [pdf].