BIZARRE BRIMS: THE EVOLUTION AND FUNCTION OF HARPIFORM CEPHALIC BRIMS IN TRILOBITES

In the fossil record we can see the many shapes that life has developed over the long course of Earth’s history. At times these forms are hauntingly familiar, but even more fascinating are those without clear modern analogues. One example of this phenomenon can be found in the trilobites of the order Harpetida and the superfamily Trinucleioidea. During the Ordovician, derived members of both groups evolved a wide flattened and pitted cephalic brim, quite unlike any structure known in modern arthropods. These brimmed trilobites were highly successful by many measures. Brimmed harpetids persisted for over 100 million years, achieving a global distribution and surviving the end-Ordovician mass extinction. This extinction event eliminated the brimmed trinucleids, 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 how 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.

To this end, we have tracked a suite of brim-related morphological characters across a recently developed phylogenetic tree of both harpetid and trinucleid trilobites and are now able to propose a sequence of character acquisition by which harpiform brims evolved in parallel in both groups. In addition, we present a novel three-dimensional morphospace capturing variation in the cephalic brims of these trilobites. By plotting real taxa within this morphospace, we find multiple aspects of brim shape that seem to be covariable despite lacking any inherent constructional relationship. In addition, our two major clades of brimmed trilobites occupy distinct areas within this morphospace. These findings may indicate that brim shape is strongly phylogenetically conserved or that the harpiform brim served a biological function that constrained its outward morphology. Finally, we explore how our morphospace may be used as a tool to test various hypotheses regarding the function of the harpiform brim, which has been previously interpreted as everything from snowshoe for soft sediments to a sieve for filtering food particles from suspension.