AN ABERRANT LATE ORDOVICIAN HORSESHOE CRAB REVEALS EARLY MORPHOLOGICAL EXPERIMENTATION WITHIN XIPHOSURA

Horseshoe crabs (Chelicerata: Xiphosura) are generally considered to exhibit a highly conserved morphology throughout their evolutionary history and are one of the archetypal ‘living fossil’ groups. This narrative has been challenged in recent years, with numerous lines of evidence indicate that horseshoe crabs have been an evolutionarily dynamic lineage, exhibiting several shifts into non-marine environments and associated peaks in rates of evolutionary change. Nevertheless, marine forms are still characterized by a relatively limited morphological variability for most of their evolutionary history, as evidenced by a consistent developmental trajectory shared between species over 250 million years. Attempts to ascertain when horseshoe crabs adopted this ontogenetic trajectory are hindered by the sparse early Paleozoic record of the group; only two species, both assigned to the genus Lunataspis, have been described from the Ordovician, and no Silurian species are known.

A new, highly aberrant horseshoe crab from the Late Ordovician Big Hill Lagerstätte, Michigan, provides evidence of early morphological experimentation within the group, indicating that even marine lineages were variable early on in their evolutionary history. The new species represents a distinct genus characterized by a greatly elongated prosomal carapace and is represented by two available specimens (with a third held in a private collection), all of which preserve the same highly unusual carapace shape, indicating the unusual morphology to be a genuine characteristic of the species. Geometric morphometric analysis places the new species in an unoccupied region of morphospace distinct to that of other horseshoe crabs, confirming early morphological experimentation within the clade. Interestingly, while the prosoma is markedly different to any other horseshoe crab species known, the thoracetron is similar to that of Lunataspis. Taken in combination with the known ontogeny of Lunataspis borealis, which exhibits the characteristic xiphosurid development of the thoracetron but a more eurypterid-like ontogenetic trajectory of the prosoma, the new species indicates that developmental canalization occurred within the horseshoe crab lineage, with the thoracetron canalizing prior to the prosoma.