A speciational history of "living fossils': molecular evolutionary patterns in horseshoe crabs

Abstract

Horseshoe crabs' exceptional morphological conservatism over the past 150 My has led to their reputation as "living fossils,' but also has served to obscure phylogenetic relationships within the complex. Nucleotide sequences from two mitochondrial genes are employed to assess molecular evolutionary rates and patterns among all extant horseshoe crab species. The American species Limulus polyphemus proved to be the sister taxon to a clade composed of the Asiatic species Tachypleus gigas, T. tridentatus, and Carcinoscorpius rotundicauda, whose relationships inter se were not resolved definitively. Both absolute and relative rate tests suggest a moderate slowdown in sequence evolution in horseshoe crabs. Nonetheless, dates of the lineage separations remain uncertain. -from Authors