Pitfalls, traps, and webs in ichnology: Traces and trace fossils of an understudied behavioral strategy

The trapping of prey, where predators use external resources to help capture prey, is a specialized type of feeding behavior that is identified in the trace fossil record only rarely. Trapping traces that have been reported in the literature include spider webs, ant-lion burrows, scorpion pits, cerianthid tube anemone burrows, echiuran worm burrows, polychaete worm (Paraonis) burrows, and deep-sea graphoglyptids burrows. There is uncertainty, however, if all of these examples actually represent traps. Paraonis burrows, for example, have been represented as trapping traces, but there is a question if they actually represent this kind of behavioral strategy. Previous references and new field work indicate that Paraonis likely employs a selective deposit feeding strategy. In the fossil record, most of the known trapping traces are represented by spider webs, which are preserved in amber, and graphoglyptid burrows. Trace fossils that could represent trapping strategies may exhibit some basic morphological attributes, including (1) a conical depression composed of loose sediment; (2) an open pit; (3) a physical snare composed of a sticky substance; and/or (4) adequate spacing between the burrows, pits, or snare material without much overlapping. The interpretation that at least some graphoglyptids (e.g., Spirorhaphe) represent trapping was based on a trapping model for Paraonis, but since Paraonis does not trap prey, the question arises whether graphoglyptids should be considered trapping at all. The variety of graphoglyptid morphologies supports the idea that graphoglyptids were not all doing the same thing. Previously, the ethological category of agrichnia has been applied to graphoglyptids and has been used to denote both trapping and farming behaviors, although the two behaviors display distinctly different feeding strategies. Some graphoglyptids may represent farming traces, while others may represent trapping traces, but it is unlikely that an individual burrow represented both behaviors. The new behavioral category ‘irretichnia’ is proposed here to encompass trapping trace fossils, due to its unique behavioral significance and also to separate trapping from farming.