SMALL CLIMACTICHNITES TRACKWAYS: THEIR ABUNDANCE AND IMPLICATIONS FOR TRACKMAKER PHYSIOLOGY

Climactichnites wilsoni is the trackway of an unknown, soft-bodied late Cambrian animal that inhabited marginal marine settings and might have been among the first animals to venture onto land. The width of these trackways (4- 30 cm) is directly related to the size of the trackmaker. The smallest reported trackways are 4- 8 cm wide and are restricted to one quarry in Wisconsin. It has been suggested that small trackways are rare because the animals were unable to compact the wet sand on which larger trackways are preserved, or that a high rate of predation by arthropods (such as those that produced Protichnites) limited the number of potential small trackmakers.

Here we report on small Climactichnites (~2-3 cm wide) that occur in Québec, New York, Wisconsin, and Missouri. These trackways sometimes occur in abundance on bedding planes, indicating that large numbers of the animal inhabited some localities. The trackways exhibit variable morphologies ranging from those with lateral ridges, transverse bars, and furrows to those missing one or more of these features. Variable morphologies are sometimes exhibited by trackways on the same slab. This variability is interpreted to result from differences in trackmaker behavior, sediment rheology, taphonomy, and/or fossil collection approaches. The trackways occur on fine- to medium-grained sandstones, some of which have intercalcated coarse grains. In some cases the trackways crosscut and modify oscillation ripples, indicating that these small animals were able to modify relatively large sedimentary structures during track formation.

Together, these trackways are important because they demonstrate that small, possibly juvenile Climactichnites-producing animals were abundant and geographically widespread, and that they had the physiological capacity to manipulate sediment to produce trackways like those of larger trackmakers.