XIPHOSURAN BEHAVIOR AND POPULATION DATA REPRESENTED BY TRACE FOSSILS FROM THE POTTSVILLE FORMATION (WESTPHALIAN A), UNION CHAPEL MINE

Xiphosuran (horseshoe crab) trace fossils are much more abundant in the fossil record than their body fossils. Xiphosuran trace fossils from the Union Chapel Mine (Westphalian A=Langsettian, about 310 million years ago) of northwestern Alabama, USA, provided sufficient data to study population data and infer sexual dimorphism and sexual maturity in several specimens.

The trace fossils have two general forms. The standard Kouphichnium (as undertracks) is the dominant form. These ancient trackways are remarkably similar to traces made by the modern horseshoe crab Limulus polyphemus, which is often referred to as a living fossil. Also, a trace consisting of a repeating body print is common. This form has been interpreted as an undulatory burrowing pattern. This behavior is contrary to the habits of L. polyphemus. Previous studies have demonstrated that undulation is not uncommon in trace fossils. In addition, one positive-negative specimen shows this behavior as continuous in significant depths of mud, which would result in suffocation in modern forms of horseshoe crabs, which live in sand. This fossil evidence suggests that the gills of horseshoe crabs have been modified, or specialized to sandy conditions, or have been more diverse than in the present. The population data shows a distinct difference in body size. Twenty-six trackways ranged from 1.5 to 3.9 cm wide, and two specimens were larger, 5.8 and 8.4 cm wide. One possible explanation is that the adults would primarily live in deeper water, only coming to the shallows or shore to mate and lay eggs, as modern horseshoe crabs do.

Sexual dimorphism and maturity are expressed in modern horseshoe crabs as a difference in the first pair of walking legs. The normal walking legs resemble the pincers of crabs or scorpions, but the males have graspers, adapted to hold onto the female during fertilization of the eggs. The morphological difference can be expressed in their trackways. One of the large trackways clearly shows a difference among several individual tracks, so the sex of the tracemaker can be inferred to be male. Also, the largest trackway clearly shows a lack of morphological difference in tracks, so it can be inferred that the tracemaker was female. This adds a new dimension to future studies, allowing identification of horseshoe crab sex from the fossil record for the first time.