Critical re-evaluation of Limulidae uncovers limited Limulus diversity

Abstract

<p>Horseshoe crabs are archetypal marine chelicerates with an exceptionally long fossil record. Due to the historical nature of the genus <i>Limulus</i>, which extends back to Linnaeus' descriptions, many horseshoe crab fossils were traditionally placed in <i>Limulus</i> and the family Limulidae. Despite continued research into the accurate placement of species within Limulidae, a systematic outline of characteristics that define limulid genera, specifically using exclusively dorsal characteristics, does not yet exist. However, such an approach is essential as appendage data is rarely preserved in fossil horseshoe crabs. Here we present a systematic review of Limulidae with a focus on dorsal features, and illustrate all accepted limulid species across the 12 genera. Through this descriptive lens, we consider the validity of supposed <i>Limulus</i> species outlined in a recent xiphosurid review. We find evidence for only one fossil <i>Limulus</i> species: <i>Limulus coffini</i>. This revision therefore excludes <i>Limulus</i> from Jurassic-aged deposits. We refer <i>'Limulus' darwini</i> from the Upper Jurassic (Upper Tithonian) of Poland to <i>Crenatolimulus darwini</i> comb. nov. and <i>‘Limulus’ woodwardi</i> from the Middle Jurassic (Aalenian) of England to <i>Mesolimulus woodwardi</i> comb. nov. This highlights that the <i>Limulus</i> evolutionary record is highly constrained and started as recently as the Late Cretaceous. The rare <i>Limulus</i> fossil record emphasizes the current need for conservation of extant species and the importance of thoroughly scrutinizing the morphology of fossil specimens to uncover all facets of the limited limulid evolutionary record.</p>