BURROW CASTING ON A TIDAL FLAT: TRACES FOR THE RECORD

Burrow casting is an important technique for establishing the link between trace makers and the traces they produce. Here we document experimental in situ burrow casting on an intertidal sand flat at Bodega Bay, California. Casts were made with both Plaster of Paris and marine resin when the sand flat was exposed at low tide. Though resin can yield a fine level of detail in casts, we prefer plaster because it is easily seen against the substrate, non-toxic, more economical, and still produces faithful replicas of burrow structures. A third option of observing burrowing in narrow aquaria is not favored because it limits the burrower’s motion and does not yield structures identical to those found under natural conditions, thus limiting its potential utility for trace fossil studies. Trenching, another common technique, is not favored because it destroys the three-dimensionality of the burrow and yields no permanent record of burrow morphology.

We found that plaster yielded the best results when 1) mixed very thin, 2) poured in a thin stream into burrow openings as the tide was going out, 3) allowed to set for at least one hour, 4) excavated slowly and carefully with trowels and brushes, and 5) consolidated after cleaning in the lab. Burrows successfully cast included those of callianassid and upogebid shrimp, errant and sessile polychaete worms, and crabs. Burrow shapes successfully cast included L, J, U, vertical, and complex branching morphologies. Plaster casts were ideal for counting number of burrow entrances, chambers and passageways, and for measuring burrow diameter, total length and volume. Plaster casts can also be embedded and sectioned to mimic the outcrop view of potential traces. Plaster of Paris is an ideal casting medium for recording gross burrow morphology in an economical and environmentally-friendly manner.