MEASURING MORPHOLOGICAL CHANGE: EVOLUTIONARY RATE OF DISCRETE CHARACTERS IN XIPHOSURA

Horseshoe crabs, aquatic chelicerate arthropods of the class Xiphosura, are a quintessential group of “living fossils.” The term is colloquially used to refer to clades that display little apparent morphological change through time, which is often attributed to a consistently low evolutionary rate. The term living fossil is heavily critiqued and regularly criticized for being applied uncritically without any quantitative testing. Here, we evaluate the supposed conservatism of living fossils directly by calculating evolutionary rates of Xiphosura from morphological characters. Xiphosura are a model group for this type of investigative study because they exhibit a dynamic evolutionary history despite their reputation as living fossils. Within the past 480 million years, Xiphosura are known to have transitioned to nonmarine environments multiple times and displayed both paedo- and peramorphic trends. Despite this apparent complexity, a comprehensive study of evolutionary rate in horseshoe crabs using fossil data has not yet been conducted.

Using the R package Claddis, the rate of discrete character evolution is Xiphosura was calculated for 55 taxa ranging from the Ordovician Lunataspis aurora to all four modern representatives. The resulting rates were temporally and phylogenetically heterogeneous. Specifically, high rates of evolution cluster within the two clades that display heterochronic trends and affinities for nonmarine environments. Additionally, these high evolutionary rates occur after the transition to nonmarine habitats, indicating a driving environmental pull behind shifts to heterochronic morphologies. Conversely, the clades that show no concerted heterochronic trends produced varied high and low rates, with a tendency towards lower rates of discrete character evolution. Xiphosura also show variable rates of evolution through time, with higher rate clusters corresponding to mass extinction events and radiations. Average evolutionary rate in Xiphosura has also decreased through time, though there is still variability present. Overall, horseshoe crabs show variable and dynamic evolutionary patterns through time, indicating that the group as a whole are not living fossils.