Horseshoe crabs are called ‘living fossils’, but even so, we know surprisingly little about their early evolution. Previously, only two species were described from the Ordovician (485-444 million years ago). Then there was a 80 million year gap in their fossil record, until the late Devonian.
A new paper, published in Royal Society, describes a newly discovered species of horseshoe crab from the Silurian (444-419 mya). Ciurcalimulus discobolus is about 425 million years old, and has morphology that is similar to the two older species. This also establishes the survival of basal horseshoe crabs into the Silurian Period.
A recognisable morphology
Ciurcalimulus discobolus is known from a single specimen, that’s only 4.5cm across. Many of the available details are only visible under laser stimulation. “This once again demonstrates the utility of laser-stimulated fluorescence in revealing details of unmineralized anatomical structures” Dr. Lamsdell notes in his paper.
Ciurcalimulus discobolus gen. et sp. nov., holotype (YPM IP 548961). (a) Imaged dry under polarized light, showing specimen relief. (b) Imaged immersed in ethanol under polarized light, providing greater contrast of the carbonized cuticle. (d) Imaged under ultraviolet light, exhibiting fluorescence of both the carbonized cuticle and to a lesser extent the exfoliated regions of the fossil, affording the clearest view of the segment boundaries in the thoracetron axis. (e) Imaged under 447 nm blue laser, with carbonized cuticle and exfoliated regions again both fluorescing. The outline of the fossil is most readily apparent here as is the morphology of the cardiac lobe.
Ciurcalimulus more closely resembles the Ordovician horseshoe crab Lunataspis, but not so much the later Devonian horseshoe crabs such as Patesia or Pickettia. Ciucalimulus, like Lunatapsis, has a heavily rounded shell covering its upper abdomen and a multisegmented postabdomen, among other common anatomical features.
Ciurcalimulus discobolus, on the other hand, is easily distinguished from later horseshoe crabs that followed 80-million years later, Dr. Lamsdell writes, “through its unique combination of characteristics unknown among other species.”
“The species is clearly distinct from the North American Devonian xiphosurids Patesia and Pickettia”, according to the paper. “Owing to the lack of a well developed ophthalmic ridge, which is present in both Devonian genera.”
“Also in the retention of a multisegmented postabdomen in Ciurcalimulus”, which was also seen in the earlier Ordovician horseshoe crabs, but has now been lost in these younger Devonian horseshoe crabs.
Simplified phylogeny of Xiphosura showing the phylogenetic position of Ciurcalimulus discobolus.
Where did horseshoe crabs come from?
Over the course of their evolutionary history, horseshoe crabs attained a global distribution,ut the oldest described horseshoe crabs were found in ancient North America.
Dr. Lamsdell notes that “the additional discovery of Ciurcalimulus suggests that [North America] was an important site of early horseshoe crab evolution, although it must be recognized that there is a strong historical bias in palaeontological research toward localities in Europe and former European colonies . As such, [North America] is likely to be more heavily sampled than other palaeocontinents such as Gondwana, an important consideration given that the oldest currently known horseshoe crab is an undescribed species from Morocco.”
With such a limited early fossil record we cannot definitively conclude where horseshoe crabs originated. However, Dr. Lamsdell poses two hypotheses. One is that horseshoe crabs originated in what is now North America. The rarity of horseshoe crabs in the Ordovician and Silurian Periods could either be a genuine indication of low diversity, or could perhaps be the result of their inhabiting environments that are less conducive to preservation, such as deeper-water habitats.
The other possibility is that the North American species may represent dispersal from a largely unsampled Gondwanan record. He notes that “More discoveries will be required to further elucidate the early evolution of the group, but the description of Ciurcalimulus, bridging an 80 million-year gap in the horseshoe crab fossil record, is an important step towards this goal.”