ICHNOGENY: GROWTH AND CHANGE IN TRACE FOSSILS

Change through time in species’ burrows, tracks, or other traces (ichnogeny) lends opportunities for understanding ancient behavior and for demonstrating how ichnodiversity compares to tracemaker biodiversity. Ichnogeny is common in resting traces, whose size corresponds to their makers’ (bivalvian Lockeia, arthropodan Rusophycus). Long-established dwelling burrows are reamed out over time, erasing the record, but borings of sponges (Entobia) and bivalves (Teredolites) often preserve younger stages. Tunnels of modern wood-boring beetle larvae also widen along their lengths, corresponding to their makers’ growth. U-shaped dwelling traces likewise show adjustments to growth. Some are extended by forming a spreite as in Diplocraterion. Chaetopterus cuts the organic lining of its U-burrow to dig a new branch, changing a U to a W, and Carboniferous Arenicolites carbonarius represents similar fossil behavior. A shallow U-burrow may be extended into a zigzag, as in Pennsylvanian Arenicolites longistriatus and Treptichnus apsorum. Some become complexly branched systems: Ordovician Palaeophycus alternatus becomes Trichophycus, and modern fiddler crabs Uca extend burrows from I to J to U to W forms over time, raising apparent ichnodiversity.

Tracemakers often show distinct behaviors in each part of their life cycles. Juvenile Limulus make traces like Nereites, and adults Kouphichnium. The larval, pupal, and adult stages of holometabolous insects all leave distinctive traces, as do amphibians with aquatic larval phases succeeded by terrestrial adults. Some species never stop extending structures (roots, ghost shrimp), while others burrow only at one stage (cicadas). Deposit-feeders burrow throughout life. The burrow form may change (Chondrites, Zoophycos), yet most individuals show no growth in tunnel diameter. The food in utilized sediment cannot account for the body mass of their makers; deposit-feeders must re-site themselves. Ichnogeny also poses new challenges in ethology, e.g., most U-burrows probably begin as J-burrows, which cannot be irrigated in the same way. Neoichnologists have not yet investigated the bioirrigation of burrows at different life stages. Such questions encourage collaboration between neoichnologists and biologists on tracemakers with distinctive changes in growth.