Gigantism among Late Jurassic limulids: New ichnological evidence from the Causses Basin (Lozère, France) and comments on body-size evolution among horseshoe crabs

An abundant ichnological material composed of xiphosuran trackways and isolated traces was discovered in Upper Jurassic limestones from the Causses Basin (Causse Méjean, Lozère, France). The morphology of the imprints supports their identification as Kouphichnium isp. In contrast to the most frequent case, the trackways are composed of omnipresent pusher imprints sometime associated with leg traces, but with no telson mark. We argue that this pattern reflects actual surface traces rather than an incomplete set of undertracks. The size distribution of the sampled ichnites is broadly bimodal. This is best explained by sexual dimorphism, a phenomenon frequently observed in modern xiphosurans. Analysis of the trace fossils further suggests that several growth stages are recorded and that the horseshoe crabs were walking in a protected and flat environment like a lagoon. This area, certainly close to a mating ground, was occasionally affected by a continental influence. The biometric study of the tracks suggests a gigantic size for the trackmakers whose body may have reached 84 cm in length. This discovery complements the few reports on other gigantic horseshoe crabs in the Jurassic of Western Europe, thus casting doubt on the postulated increase in body size from the Palaeozoic to the Recent. Furthermore, a literature review shows that there are still major gaps in the record of limulid body-fossils and tracks. Thus, neither of these archives can be taken at face value for quantifying the body-size evolution of horseshoe crabs.