IS CLIMACTICHNITES REALLY A TRACE FOSSIL?

Climactichnites, an unusual, late Cambrian band-like fossil, was described as a trace found in sandstone formations throughout the northeastern and north-central US and southeastern Canada. Later detailed studies accepted this hypothesis for these very wide (up to 15 cm) and long (up to at least 6 m), ridged and furrowed, curvilinear impressions. The fossils have always been considered trace fossils, even in the titles and first paragraphs of most previous papers. The most recent interpretation is that some very large marginal marine, slug-like organism made the motorcycle-like tracks by secreting copious amounts of mucus and moving across it over the shallow, maybe exposed, sand flats. No other known trace fossil closely resembles Climactichnites. Alternative hypotheses have not been rigorously proposed nor tested. However, some observations of the fossils suggest that alternative hypotheses need further consideration. Based on new fossils from central Wisconsin and a review of the literature, we propose that the evidence shown on numerous sandstone slabs is equally compatible with the hypothesis that these fossils are body impressions of a gelatinous zooplankter that floated into shallow water where they were deposited gently across the extensive tidal sand flats. Although we do not expect morphologic comparisons of a Cambrian organism to be identical to modern ones, the terminal oval structure could be a float followed by a long chain of colonial polyps, as in Recent siphonophores and salps. This hypothesis is supported by several observations: the way the impressions sometimes overlap without disturbing one another, the sharp overlay of the fossils across ripple marks still seen below the specimens, the flattening of sand volcanoes underneath the organism that made the fossils, the lack of the oval terminations visible on many specimens, the massive numbers of them in some places, the alignment of many of them on certain slabs, the taphonomy of hydrated gelatinous animals, and the paleogeography of the Cambrian embayments. We find no clear negative evidence for this hypothesis, and suggest that specimens be restudied to attempt to disprove both the trace and the zooplankton body impression hypotheses.