SHADOWS OF THE PAST: THE DISJUNCT DISTRIBUTION OF EXTANT HORSESHOE CRABS EXPLAINED THROUGH BIOGEOGRAPHIC ANALYSIS OF FOSSIL TAXA
In order to test these hypotheses and reconstruct historical distribution patterns of Limulina, the clade of horseshoe crabs including the extant species, we conducted a biogeographic analysis in BioGeoBEARS of 41 species using an existing Bayesian phylogenetic topology. Limulina originate in the Carboniferous with Limuloidea, the clade including crown group horseshoe crabs, originating in the late Carboniferous but undergoing a major radiation in the Triassic. As such our analysis spans over 250 million years of geologic time during which continental configurations changed dramatically. In order to accommodate this we conducted analyses with 9 geographic regions defined, separating Europe into a number of smaller continental regions, as well as analyses uniting Europe into a single region. Both sets of analyses produce concordant results indicating a European origin for Limuloidea with repeated dispersal events to North America and Asia. Critically, the modern Asian species are the result of separate dispersal events while the disparate modern distribution of Xiphosura is due to the loss of their ancestral European range.