REINTERPRETING PSAMMICHNITES: CHEMOKLINOTAXIS RECORDED IN AN EARLY CAMBRIAN TRACE FOSSIL

Psammichnites is a looping, Early Cambrian trace fossil which often crosses over itself. On some specimens a meandering median furrow is present along the top of the burrow. The animal apparently burrowed horizontally through the sediment, a few cm below the sediment-water interface, and a rodlike structure (sometimes inferred to be a siphon or snorkel) projected upward into the water column. This rodlike structure wagged from side to side as the animal advanced through the sediment, creating the meandering median furrow.

In his 1997 book Fossil Art, Dolf Seilacher included a photograph of a spectacular specimen of Psammichnites gigas, which he interpreted to represent a primitive, inefficient, grazing pattern. Dolf suggested that the snorkel “acted like a vacuum, inhaling suspended nutrient particles.” The methodical side-to-side sweeping of the snorkel was, he suggested, partial compensation for the animal's inefficient foraging strategy.

It seems improbable that the Psammichnites animal would evolve an elaborate, side-to-side-sweeping, sediment-collecting organ, yet not be able to evolve an efficient foraging strategy. I propose here an alternative interpretation. Rather than representing the trace of a sediment feeder with an inefficient, genetically programmed, foraging strategy, I suggest that Psammichnites is the pursuit trace of a hunting or scavenging animal whose looping pattern records a directed response to a point-source, chemical stimulus. In this interpretation the rodlike structure is a rhinophore (nose on a stalk).

Animal behavior researchers recognize two categories of elementary behavioral responses: kineses and taxes. Kineses are nondirected responses to stimulus, while taxes are directed responses. Side-to-side motion of a sensory receptor, as recorded in Psammichnites, is precisely the behavior that is observed in single-sensory-receptor animals that navigate by way of klinotaxis, in which the animal turns toward or away from the direction of the stimulus. No other elementary behavioral response characteristically involves this type of motion. Symmetry analysis of the Psammichnites gigas trace figured in Fossil Art further supports this interpretation.