BIOGEOGRAPHY IN THE FOSSILIZED BIRTH-DEATH MODEL: EXAMINING PROETID DISPERSAL AND DIVERSIFICATION THROUGH THE LATE PALEOZOIC

The fossilized birth-death (FBD) process has become increasingly utilized to assess evolutionary questions about fossil lineages in a Bayesian context. The total FBD process is a hierarchal model which includes components to evaluate rates of anatomical evolution, shifts in diversification rates, and the tree model itself based on speciation, extinction, and sampling intensity. Although existing Bayesian approaches allow for analyses of biogeographic shifts and possible differences in diversification rates tied to biogeography, FBD analyses have been unable to combine these to assess whether relationships between biogeography and diversification change over time. This is an important consideration because some models of differential survivorship over critical intervals posit elevated relative success for lineages in particular regions or environments.

Here, we modify algorithms used to model viral spread to assess changes in biogeographic dispersal (“migration”) in a fossil lineage over time using BEAST (v2.6.7). We used a multistate dataset of the last trilobites, proetids, which included morphological characters and biogeographic data. Our model shows many proetid lineages originated in what is considered the “Old World” realm at the end-Silurian followed by dispersal throughout the “Old World”, “Eastern Americas”, and “Malvinokaffric” realms throughout the Devonian. Proetid lineages remaining after the end-Devonian extinction event, including members of the new family Phillipsidae, are found mainly in the “Old World” realm. This corresponds to the hypothesis of a northward dispersal and/or survival in some proetid lineages following the mass extinction event. Our study demonstrates how dispersal and biogeographic information can be incorporated into classic FBD models to interpret changes within lineages.