TESTING THE LIMIT: SIMULATING TRILOBITE EVOLUTIONARY ORIGINS UNDER THE FOSSILIZED BIRTH-DEATH PROCESS

Trilobites are an icon of the ‘Cambrian Explosion’, and their sudden first-appearance around 521 mya signifies the provisional boundary between the Terreneuvian and the Cambrian Series 2. Yet, a consensus on the timing of their evolutionary origins has remained elusive. Darwin was the first to be troubled by the sudden appearance of trilobites in the fossil record. He posited that the incompleteness of the fossil record can provide for a protracted, cryptic evolutionary history. In the present, two main hypotheses have arisen to reconcile evolutionary theory with the current state of the trilobite fossil record. The first, based on a Bayesian phylogenetic tip-dated analyses (and morphological clock estimates), conservatively estimates the age of trilobite origin near the base of the Cambrian, ca. 541 mya. This also implies that trilobites had homogenous rates of evolution throughout the Cambrian. The second suggests that the fossil record is a relatively accurate representation of trilobite evolutionary history, and that they had a burst of rapid evolution around their first-appearance (ca. 521 mya), before swiftly settling to background rates.

What are the limits of the current state of the trilobite fossil record and what can methodological advancements tell us about the timing of trilobite origin? Here we address this question using simulations under the fossilized birth-death process. We constrain diversification and turnover parameters for the process with estimates from data from the Paleobiology Database for Cambrian trilobites. In altering these parameters and allowing for rate variation, we explore the different hypothesized diversification scenarios. We alter the sampling rate parameter to allow variation through time and explore the effect of potentially sampling Terreneuvian trilobites, as well as the lack of known Early Cambrian Lagerstätten. These simulations provide important insight into the extent with which we can answer questions about trilobite origins and the Cambrian Explosion as a whole with current methods and the fossil record.