TREPTICHNUS: AN ANTI-PREDATOR MAZE?

Treptichnus is a well-known fossil burrow consisting of a series of conjoined segments that often exhibits a zigzagging appearance. Treptichnus is known from marine paleoenvironments of the lower Paleozoic, and from continental paleoenvironments from the Carboniferous to Recent. Many marine occurrences have a similar geometrical pattern to trackways made by modern predatory priapulids (Vannier et al., 2010, Geology), suggesting that similar predatory animals may have been responsible for the early Paleozoic forms. We have been examining Treptichnus from continental paleoenvironments of the Jurassic of Massachusetts, which are similar to modern traces in soft-sediment that are attributed to insect larvae. Interestingly, borings with similar zigzagging geometry are also made by root-mining activities of insect larvae (e.g., Skelly and Kearby, 1970, Annals of the Entomological Society of America), so the Treptichnus morphology is not unique to predators or to burrows. Here, we argue that the branching morphology of Treptichnus-like burrows serves an anti-predatory function. Whereas burrowing animals are protected from surficial predators, they are vulnerable to predators that invade their burrow. An animal in an unbranched burrow is particularly vulnerable: the burrow provides the predator with a direct route to its prey, which is then cornered. A branched burrow effectively functions as a maze to a burrow-invading predator—given a series of multiple branch points, the predator must choose correctly each time in order to find the prey, or spend time backtracking. Given the modern analog used here, this model applies most directly to continental Treptichnus, but there is no reason that other branched burrows would not serve similar functions. Burrowing predators might even employ this tactic as a means of avoiding other predators while searching for prey. If the anti-predation hypothesis is correct, then the occurrence of Treptichnus at the base of the Cambrian might reflect the evolution of anti-predatory behaviors in burrowing Metazoa in response to rising predation pressures during the Ediacaran-Cambrian transition.