ORGANISM-SEDIMENT INTERACTIONS AND THE EVOLUTION OF A UNIQUE TRILOBITE MORPHOLOGY

Some of the most fascinating fossil taxa are those lacking clear modern analogs. One example of this can be seen in the trilobites of the order Harpetida and the superfamily Trinucleioidea. During the Ordovician, derived members of both groups evolved wide, pitted cephalic brims, quite unlike any structure known in modern arthropods. These brimmed trilobites were successful by many measures. Brimmed harpetids persisted for over 100 million years, achieved a global distribution, and survived the end-Ordovician mass extinction. Although morphologically similar, brimmed trinucleids did not survive beyond the Ordovician, but by then they had already diversified into hundreds of different species and at times been abundant enough to dominate fossil assemblages. Despite this, we still do not fully understand why the harpiform brim evolved or what role it played in the success of these trilobites. Many authors have speculated about the possible functions of a harpiform brim, but to move beyond speculation, we must be able to test competing hypotheses.

We first tracked brim-related characters across a recent phylogenetic tree of both harpetid and trinucleid trilobites, and propose a sequence of character acquisition by which harpiform brims evolved in parallel in both groups. We also explored the hypothesis that the harpiform brim evolved to prevent sinking in soft sediments and demonstrated this idea to be untenable. We now turn to the hypothesis that the brim acted as a plow for displacing sediment. For this we designed a brim morphospace of five continuous variables. We also created a tool in the software package Blender that allows us to populate our morphospace, turning any specified combination of our variables into a realized, three-dimensional trilobite cephalon. By 3D printing a selection of cephalons and moving them through natural sediments in a controlled manner, we can assess how the different shape variables impact the brim’s performance as a plow, and thus whether this function may have controlled the evolution of the harpiform brim. To this end, we have designed an apparatus for moving our model cephalon through in situ sediments and collecting data on the forces involved. This work can both help us to understand how these strange trilobites lived, and show us how best to approach questions of function and convergence when studying fossil taxa without living analogs.