CAMBRIAN ORIGIN FOR MODERN ARTHROPOD MOLTING BEHAVIOR

Molting (ecdysis), the periodic casting off of the exoskeleton, is a behavior mandated by the physiology of arthropods. Two major extant marine arthropod groups, the decapod crustaceans and the chelicerates, are characterized by distinctive molt behaviors that are consistent within each group. This raises questions about the timing of the origin for these different behaviors and the implications that these behaviors have for the evolutionary history of arthropods. Despite the physiological and clade-defining importance of molting, ecdysis is only sparsely documented among non-trilobite fossil arthropods. A taphonomic census of six non-trilobite arthropod genera from the Burgess Shale revealed recurrent exoskeletal configurations that can be interpreted as molt ensembles. Both modern molt motifs, crustacean (shrimp- and crab-like) and chelicerate (limulid- and scorpion-like), are represented among the Burgess taxa. These molt motifs have been highly conserved over more than 500 million years of arthropod evolution. The small number of molt motifs demonstrated by the non-trilobite Burgess Shale arthropods stands in marked contrast to the exuvial habit of trilobites, which were also present in the Burgess Shale fauna. Trilobites did not exhibit a stereotyped molt motif; rather, trilobite molting was non-systematized and variable within a species. The difference between fossil arthropod taxa that exhibited stereotyped, extant molt motifs and those, like the trilobites, that did not, may help to explain the evolutionary patterns among the Arthropoda.