EARLIEST EVIDENCE OF INVERTEBRATE SEXUAL BEHAVIOR, OR A TIDAL FLAT TRAFFIC JAM IN THE POTSDAM FM. (LATE CAMBRIAN)?

Arthropod trackways are rare ichnofaunal features of intertidal and brackish water sandstone facies throughout the Paleozoic. Because they are made by digits that are potentially identifiable and thus offer the prospect of identification of the maker, these traces stimulate much conjecture about the emergence of earliest land-dwelling animals. But, perhaps we should be extracting other information from these occurrences. They appear to be eurypterid in origin demonstrated by a paired gait with telson drag representative of an aquatic swimming style which resulted in octopodous or hexapodous trackways when the organism crawled in very shallow water or stranded on tidal flats. Makers were not terrestrial organisms in the Late Cambrian, nor had their gait, or habitat preference changed significantly by the Permian when they seem to have become extinct.

Bedding-plane exposures on the Late Cambrian Potsdam Sandstone of northern New York preserve oscillation-rippled surfaces with multiple trackways crossing them on event horizons of Spring-tide-like intervals. Trackways are of two widths, roughly 6 cm and 13 cm suggesting bimodality among the makers. In one instance among the congestion of crossing trails, there appears to have been an encounter, or rather a departure, of a smaller trackway from a larger trackway suggesting the dismount of a smaller (male) individual from the larger (female) individual it had been clasping and riding to that point.

The modern merostome Limulus polyphemus congregates on beaches to breed. Larger females are mounted by the smaller males which are carried to the sight of ovoposition. After egg deposition, the couple seeks the sea, disengaging at some point in the process. These are seasonal, tidally controlled events during which hundreds or thousands of individuals partake in the spawning. The eggs become a sought-after meal by predators.

This unusual site records a eurypterid mating episode on a Late Cambrian tidal flat in Laurentia. It suggests that intertidal sequestering of eggs was an advantage to the track makers, implying that egg predation was already an issue among Cambrian marine invertebrates and provides a mechanism favoring migration of Xiphosura into brackish and freshwater habitats free of predators. This is the earliest evidence of mating behavior known in invertebrates.