On the appendicular anatomy of the xiphosurid Tachypleus syriacus and the evolution of fossil horseshoe crab appendages

Xiphosurida-crown group horseshoe crabs-are a group of morphologically conservative marine chelicerates (at least since the Jurassic). They represent an idealised example of evolutionary stasis. Unfortunately, body fossils of horseshoe crabs seldom preserve appendages and their associated features; thus, an important aspect of their morphology is absent in explorations of their conservative Bauplan. As such, fossil horseshoe crab appendages are rarely considered within a comparative framework: previous comparisons have focussed almost exclusively on extant taxa to the exclusion of extinct taxa. Here, we examine eight specimens of the xiphosurid Tachypleus syriacus (Woodward, 1879) from the Cenomanian (ca 100 Ma) Konservat-Lagerstätten of Lebanon, five of which preserve the cephalothoracic and thoracetronic appendages in exceptional detail. Comparing these appendages of T. syriacus with other fossil xiphosurids highlights the conserved nature of appendage construction across Xiphosurida, including examples of Austrolimulidae, Paleolimulidae, and Limulidae. Conversely, Belinuridae have more elongate cephalothoracic appendages relative to body length. Differences in appendage sizes are likely related to the freshwater and possible subaerial life modes of belinurids, contrasting with the primarily marine habits of other families. The morphological similarity of T. syriacus to extant members of the genus indicates that the conserved nature of the generic lineage can be extended to ecological adaptations, notably burrowing, swimming, possible diet, and sexual dimorphism.