THOSE TROUBLED TRILOBITES: CLARIFYING EVOLUTIONARY RELATIONSHIPS, DIVERGENCE TIMES AND RATES OF EVOLUTION IN TURBULENT TRILOBITE ORDERS

Despite their exception fossil record and long history of study, disparate classification schemes and disputes over higher-level groupings plague the study of trilobites at present; an issue that deeply hampers research into evolutionary questions. The most recent comprehensive Trilobita classification (sensu Adrain 2011) places Aulacopleurida as an order distinct from Proetida, and includes families formerly assigned to Ptychopariida by Fortey (1997 Treatise). Subsequent parsimony analyses based on a broad sampling of trilobite taxa across multiple orders challenged this conception (i.e. Lamsdell and Selden 2015). Still, group support was low in this analysis and divergence times remain unknown. The apparent appearance of the Proetida in the Ordovician is troubling due to the their cryptic Cambrian history and unknown sister group. Additionally, despite attempts at cladistic analyses of certain groups within trilobites, divergence times and evolutionary rates have rarely been quantitatively analysed.

Here we present phylogenetic analyses using the largest and most comprehensive character matrix to date, which comprises >340 characters, including dependencies and continuous characters. The analyses include 57 exemplar species from all families assigned to either Proetida (sensu Fortey 1997), Aulacopleurida (sensu Adrain 2011), and those previously assigned to Ptychopariida (sensu Fortey 1997) that were excluded from Aulacopleurida. We conducted Bayesian phylogenetic analyses using the fossilized birth-death model to produce trees that would co-estimate topology, divergence times and evolutionary rates. We also assess leaf stability, character influence and stratigraphic fit. We found that the Aulacopleurida forms a non-monophyletic group, and suggest a new taxonomic paradigm incorporating some ‘aulacopleurid’ families back into a monophyletic Proetida with a ptychopariid sister group that has a divergence time in the early Cambrian. Rates of morphological evolution are largely constant through the Cambrian except for some specific taxa with elevated rates, and there is a distinct shift in evolutionary rates around the Cambrian-Ordovician boundary. This wholistic phylogenetic approach provides much needed insight into the evolutionary history of troubled trilobite groups.