Cochlea Archimedea: Hitchcock's Fish Trail

摘要

A specimen figured by Hitchcock (1858) and named Cochlea archimedea presents interesting ichnotaxonomic and paleoecologic issues. The ichnogenus Cochlea is considered to be a preoccupied taxon and therefore invalid. The specimen in question is given its appropriate name, Undichna isp., based on its apparent morphology and is reinterpreted as a trail made by a fish. Two working hypotheses for the construction of the trail are: (A) as a single epichnial groove with side levees made by one fin in contact with the substrate and (B) as a hypichnial trail with two grooves made by two fins in contact with the substrate. A more specific approach cannot be taken without knowledge of the true toponomy of the trace fossil, as none is indicated on the original specimen. Regardless, the identification of the first trails made by fish in the Early Jurassic of the Newark Supergroup of Massachusetts indicates the need for more nondinosaurian trace fossil research in the Mesozoic rift basins of eastern North America. The most likely tracemaker, depending on the correct constructional hypothesis, is either a semionotid with a deeply set anal fin (hypothesis B) or Redfieldius (hypothesis A). Considering the abundance of fish body fossils preserved in rocks of this age in eastern North America, it is likely that once recognized, more Undichna will be discovered in future field explorations.