TESTING FOR CONVERGENT EVOLUTION IN EXTINCT ORGANISMS WITHOUT CLOSE MODERN RELATIVES: PHYLOGENETIC ANALYSIS OF THE TRILOBITE ORDERS HARPETIDA AND TRINUCLEIDA

For researchers studying extinct animals without close living relatives, it can be challenging to recognize and verify instances of convergent evolution. Morphological similarity can mislead phylogenetic analyses when no molecular phylogenies exist to act as independent assessments of relatedness. Yet this challenge also offers an opportunity to better understand the evolutionary process.

The trilobite orders Harpetida and Trinucleida share a common origin within ‘Ptychopariida’ during the Late Cambrian, as well as an unusual morphology marked by a broad, pitted cephalic brim. It is thought that these two groups evolved their striking brims convergently. This hypothesis implies that harpetids and trinucleids are phylogenetically distinct and that their last common ancestor lacked such a brim. When trinucleids were considered part of the order Asaphida, this situation seemed strongly indicated. However, recent analyses showing that the supposed synapomorphies linking Asaphida and Trinucleida arose convergently indicate that it is high time to test this assumption.

Here we present a new phylogenetic analysis of the evolutionary relationship between Harpetida and Trinucleida. This work uses a matrix of discrete morphological characters drawn from Beech & Lamsdell (2021), Bignon et al. (2020), and the reference literature. This matrix was applied to a broad sample of harpetid and trinucleid trilobites. Taxa were coded from fossil specimens at the CMC, YPM, MCZ, and the NHM, as well as high-quality photographs and figures. Outgroup taxa were selected from Asaphida, ‘Ptychopariida’, and ‘Redlichiida’. From the completed matrix, parsimony and Bayesian phylogenies were created. Ancestral state reconstruction was performed for the parsimony tree to reveal how many times the unusual brim configuration arose. Adding functional morphologic data to these phylogenetic analyses will allow for a robust test of whether these two trilobite clades evolved their brims convergently.