COLLINS, A. G., LIPPS, J. H., and VALENTINE, J. W., Museum of Paleontology and Dept. of Integrative Biology, University of California, Berkeley, CA 94720-4780
The oldest known metazoan, a sponge, is 580 million years old, while the oldest trace of a crawling metazoan is roughly 565 million years old. The fossil record has yet to reveal direct evidence linking any crawling trace to the metazoan that made it. The work of prior authors on the earliest traces usually suggests that the trace makers are Bilateria with a complete gut (with an anus). We sought to determine whether living representatives of some of the earliest branching metazoan clades, which do not have a complete gut, are capable of leaving traces of their movement. Our observations were made on animals living near the UC Berkeley Gump Marine Station on the island of Moorea, French Polynesia. We found that several flatworms of the clade Polycladida leave a readily discernible trail in soft sediments. Flatworm crawling trails have two ridges, widely spaced relative to their height, made up of sediment aggregated by mucus. These trails resemble the fossil traces Sellaulichnus and Aulichnites, both known from the Vendian. Further, we documented that the ceriantharian Pachycerianthus sp. is capable of "crawling", and that this locomotion also leaves a trail. The ceriantharian trail consists of a single string of sediment bound by mucus, and closely resembles the most ancient animal trace, Planolites.
We conclude: 1) modern traces can be made by metazoans with a body-plan that lacks an anus or a coelom, 2) some of the earliest known traces (from the Vendian) may represent locomotion by such animals; and 3) mucus may play an important, but unexplored role, in the preservation of trails.