Agnostus and genetic canalization

The Cambrian Series 3 Agnostus pisiformis is an arthropod-like trilobite, the precise phylogenetic position of which is still not fully determined. It inhabited oxygen-deficient shallow seas and is found in abundance in the Alum Shales of Scandinavia, in particular Sweden as well as in Wales and Canada.

Due to this abundance and geographical range, as well as its relatively simple morphology featuring an isopygous and almost featureless exoskeleton, Agnostus pisiformis was the ideal model organism for my early work during my PhD, in which I explored morphometric methods capable of mapping fine-scale patterns of continuous variation in the fossil record.

I employed elliptical Fourier analysis to study the shape of the head and tail regions of these organisms and uncovered significant intraspecific variation in the shape of the tail between geographically disparate but temporally equivalent assemblages.

It has previously been established that these organisms lived during a period of deepening sea levels known as the Steptoean Positive Carbon Isotope Excursion (SPICE) event, which would introduce varying levels of increasing dysoxic stress to these organisms.

The morphological variation detected between localities was therefore interpreted as a plastic phenotypic response to environmental stress; an indication of the relative measure of genetic canalization found in these different assemblages. These results are published here [pdf].

Using palaeontological material to investigate biological mechanisms in this manner is the aim of paleobiological work, and in my continued and current work with Agnostus pisiformis this has been made more explicit.

In my ongoing work with this model organism, I have broadened the outlook to also consider its lineage creating a system composed of Agnostus pisiformis and its later forms Homagnostus obesus and Trilobagnostus holmi. These later forms existed during the peak of the SPICE event and after it, respectively.

Early results indicate that these different forms of the lineage together capture the process of plasticity-led evolution, although this is theoretically challenging to investigate in the fossil record. This page will be updated as this project progresses.