SO HAPPY TOGETHER: MULTIPHYLETIC GROUP BEHAVIOR AT THE UNION CHAPEL TRACKSITE (PENNSYLVANIAN, ALABAMA)

Among the thousands of trackways and other trace fossils from the Union Chapel cornucopia are several that show parallel locomotion of different individuals separated by short distances on the same lamina. These include temnospondyl amphibian trackways (Cincosaurus cobbi), xiphosuran trackways (Kouphichnium aspidon), and fish swimming trails (Undichna ispp.). For amphibians and fish, these are the oldest known examples of this kind of group behavior, and among the few reported from the geologic record. Although testing synchroneity of the tracemakers' locomotion is difficult, this circumstance seems likely because they occur on the same intertidal laminae within the shale, and each lamina represents only part of one tidal cycle, i.e., less than a day. Further supporting evidence is that paralleling animals normally proceeded in the same direction. Fish trails include some that overlap such that one fish must have followed another of similar locomotion and size, indicating schooling or courtship rather than predation. Schooling behavior is advantageous in fishes to locate food, confuse predators, and decrease turbulence while swimming. Courtship is likely for paralleling xiphosurans, whose modern relatives (e.g., the horseshoe crab Limulus) display similar behavior on modern beaches. Seasonal mass courtship and egg-laying lessen predation by overwhelming predators with an excess of prey. Amphibians' motives for group behavior are less certain, although predation deterrence is one possibility. A more likely reason, however, is movement from one pool to another to avoid desiccation during low tides. Also, the sameness in track sizes and morphology suggest the same age-group and species of amphibian. The Union Chapel paleoenvironmental setting – the nearly freshwater, but tidally influenced head of an estuary – was shallow, turbid, and rich in nutrients. Today, this kind of setting acts as a courtship ground and nursery for many kinds of animals. The presence of small and large examples of the same ichnospecies at Union Chapel suggests that they already were used for this purpose by animals of many phyla by the Early Pennsylvanian.