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.
Yet not all such boreholes are the product of predators. My favourites are those found in fragments of crinoid, rarely the thecae (Fig. 2H), but commonly lengths of stem called pluricolumnals. These are particularly common from certain Lower Carboniferous (Mississippian) sites such as Salthill Quarry in Clitheroe, Lancashire (Donovan et al., 2014). Such boreholes do not pierce the axial canal of pluricolumnals and may show site selectivity, such as boring at plate triple junctions of thecae (Donovan, 1991, 2015b). They also commonly engendered deformities in the host echinoderm, indicating that it was alive when infested; boreholes that did not produce an associated growth response may have represented the infesting of dead individuals. Where present, these deformities are not part of the trace fossil per se (Donovan, 2015a), but are intimately associated with it. The producer of the pit or boring obviously aggravated the living crinoid, which commonly reacted by swelling in a variety of ways. Andy Tenny, an amateur collector in Lancashire, recently found a crinoid pluricolumnal of circular section with such pits distributed through 360º, but with growth reactions on one side only (Fig. 3; Donovan and Tenny, in press). Our interpretation is that, when the crinoid was alive, one side was infested by pit-formers, each of which caused an associated growth reaction (Fig. 3B-D). Why they congregated on one side may be an indication of a preferred orientation of that part of the stem for suspension feeding by the domicile-building (=pit-forming) organism. After the crinoid died, it fell to the seafloor with the pitted side lowermost. The exposed side of the stem was then invaded by similar pit formers (Fig. 3A, B, D), but there was no growth reaction from the dead crinoid.
Another example of infestation in life is the pit Sedilichnus excavatus (Donovan and Jagt, 2002), which is found most commonly infesting the Maastrichtian (uppermost Cretaceous) irregular echinoid Hemipneustes striatoradiatus (Leske) (Figs 2F, G, 4). This echinoid is locally common in the Cretaceous of the southern province of Limburg in the Netherlands and adjacent areas of Belgium. Hemipneustes striatoradiatus is a large, high-domed holasteroid echinoid, which was poorly adapted to burrowing and probably lived furrowing through the sediment surface, ingesting grains at or just below this level, and digesting organic films and microorganisms found there. Unlike deep burrowing echinoids, this lifestyle left the test of H. striatoradiatus exposed and available for infestation by encrusters and pit-forming invertebrates. Most prominent among these are S. excavatus, which are commonly quite large, with concave sides and a central boss, and may be present as dozens of individual pits on a single echinoid test (Fig. 4A, B). That infested echinoids were alive is indicated by the presence of blisters (=growth deformities) on the insides of tests immediately below each S. excavatus pit (Fig. 4C-G). Sedilichnus excavatus is most common on the upper halves of tests and our original interpretation was that the pit-formers were getting a free ride above the sediment surface; that is, they were positioned where water currents (that they presumably needed for suspension feeding) would be rich in food, but with a reduced sediment content.
This interpretation is still considered valid, but a specimen recognised more recently has added further insight (Donovan and Jagt, 2013; Fig. 2F). This echinoid has only four pits of S. excavatus, the distribution of which is notable: each is closely associated with one of the ambulacral regions, all except that at the anterior, which is different from the others on the apical surface in form and function. The close association of S. excavatus and these ambulacra, adapted principally for respiration, is suggestive that there is an intimate relationship between them. It may be that the organism that made S. excavatus was predatory, eating the fleshy tube feet that arose from the pore pairs, or more likely parasitic, perhaps harvesting plankton from a more dense feeding current as water flowed over the tube feet of the ambulacra.
I have no intention of discussing all known borings in similar detail. However, I do choose to illustrate my assertion that borings are interesting by discussing another favourite of mine, the clavate (club-shaped) borings of, most commonly, shelly or lithic substrates, such as limestone, mudrock or peat, namely Gastrochaenolites Kelly and Bromley, 1984 (Fig. 5). Usually, these structures are the domiciles of boring bivalves, which may be preserved in situ in some specimens (Fig. 5I). However, beware that, after the death of the borer and loss of its shell, the borehole may be invaded by other organisms, like serpulid worms or nestling bivalves, which favour such a protected environment.
The range of forms of the Gastrochaenolites borings is well demonstrated by Kelly and Bromley (1984, text-fig. 3). Apart from the gross morphology of the boring, often there is some form of sculpturing on the wall of Gastrochaenolites. This may either be the result of the boring action of the sculptured shell of the producing bivalve, most commonly in Gastrochaenolites ornatus Kelly and Bromley (Fig. 5D-HE), and termed a bioglyphs, or it can be due to the cut effect through distinctive structures in the substrate, called a xenoglyph. For example, a bored fossil may expose the internal structure of a scleractinian coral as a xenoglyph. In some Gastrochaenolites, quite the reverse has occurred and the boring bivalve lines the boring with a thin, calcareous layer, obscuring any features of the substrate (Donovan et al., 2007).
A bivalve boring into, for example, in situ beds of limestone or mudrock has ample space to grow to its full size without any problems of space. But consider the examples where space is limited, such as boring into a loose cobble on the sea floor (Fig. 5A-H). Then limitations of space become an important consideration. Some borers have dealt with such problems by producing a geniculated (J-shaped) Gastrochaenolites, most easily constructed where there are no other borings in the cobble (Fig. 5F-H). Yet, where there are multiple borings, such changes of direction may be more problematic. A chalk cobble collected by Dr Fiona E Fearnhead (Angela Marmont Centre, the Natural History Museum, London) from the beach at Margate in Kent, demonstrates this space problem (Fig. 5A-C). There are so many Gastrochaenolites borings in this cobble of chalk that the largest specimen had to retract and start boring in a new direction, producing an apparently branched boring.
In conclusion, I hope that I have effectively conveyed my message: borings in rocks and fossils are very interesting, indeed. I chose two common ichnogenera that are particularly well known to me, but similar features and interpretations would have been true of any pair of boring ichnotaxa. Borings, like other trace fossils, are frozen activity and, as such, give a different insight into the fossil record than do body fossils.
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