It is unfortunate that the miscellaneous holes, pits and depressions produced in wood, rocks and skeletons (bones, shells and tests), both pre- and post-mortem, by a wide range of invertebrates, plants and fungi, are called borings. A less inspiring name for a fascinating suite of structures is hard to imagine. Borings represent a range of activities, although most can be interpreted as feeding – predation or parasitism – or construction of a domicile (=home). Borings may or may not be assignable to a particular species, although shelly borers, such as gastropods, may rarely be preserved in situ (see, for example, Baumiller, 1990, text-fig. 1). And borings are real evidence of ancient organism-organism or organism-substrate interactions that would be impossible to determine based on the evidence of skeletal remains alone. Therefore, borings breathe life into a dead fossil record and, in truth, are exciting.
Small round holes in shells
Borings vary in complexity from the complicated interconnected chambers of clionoid sponges and the trace fossil (ichnogenus) Entobia Bronn (Fig. 1), to the simplicity of small round holes, formerly included in the ichnogenus Oichnus Bromley, although this is now considered a junior synonym of Sedilichnus Müller (Zonneveld and Gingras, 2014). Consider these small round holes. Nothing could be simpler in morphology, although there is sufficient variation that several distinct ichnospecies have been named (Figs 2-4). Yet simplicity in morphology does not mean that their origin, function and palaeoecology are so easy to interpret. For example, consider a shell preserved with a small round hole that penetrates it (Fig. 2B-E). This is likely a mark of a predator, which has bored through the shell in search of a meal. Morphologically similar holes in Palaeozoic crinoid thecae may be parasitic (Baumiller, 1990). Dead crinoid stems may form a hard surface that may be bored into by an invertebrate constructing a home (Fig. 3A). I have just described three essentially identical pits or borings, but they represent three contrasting behaviours, namely predation, parasitism and dwelling. Even with simple borings, it is best not to jump to conclusions until all the available evidence has been examined.
Contrast these conclusions with interpretations of non-penetrative small round holes. In the shells of benthic molluscs, these are commonly construed as evidence of failed predation (Fig. 2A). Some fossil shells may bear more than one such failed predatory borehole. Gaemers and Langeveld (2015) recently described the unexpected occurrence of similar such boreholes in the otoliths (ear ossicles) of Neogene codfish from the Netherlands. Because of the lozenge-like shape of the otoliths, the authors interpreted the pits as failed ‘predation’ by naticid snails, which mistook them for their usual prey – infaunal bivalve molluscs.
READ MORE...To view the rest of this article, you need A subscription. FROM JUST £2.95.
If you are already a subscriber, login here.