Interesting borings

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.

figure-1Fig. 1. Entobia cretacea (Portlock, 1843), the Natural History Museum, London (BMNH) S.9015, clionoid sponge boring preserved in flint, chalk drift (=clay-with-flints?), Croydon, Surrey (after Donovan and Fearnhead, 2015, fig. 2). Note the cushion-shaped chambers connected by fine canals; the small tubercle-like structures in the centre of some chambers are the apertural canals. This complex boring was infilled by flint before the host shell was dissolved away. Specimen not coated for photography. Scale bar equals 10mm.

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.

figure-2Fig. 2. Small round holes in shells and tests. (A-E) Sedilichnus ispp. from the Upper Pliocene Bowden shell beds, Bowden Formation, south-east Jamaica (after Donovan and Pickerill, 1999, fig. 5; specimens deposited in the Department of Earth Sciences, University of New Brunswick, Fredericton, Canada). (A) Disarticulated right valve of Crassitellites sp. with two successful (= penetrative) and two unsuccessful (near umbo) Sedilichnus simplex (Bromley, 1981). (B) Natica castrenoides Woodring penetrated by Sedilichnus paraboloides (Bromley, 1981) (possibly evidence of cannibalism). (C) Acetocina lepta Woodring penetrated by S. paraboloides. (D) Disarticulated left valve of Crassitellites sp. penetrated by large S. simplex. (E) Disarticulated left valve of Barbatia sp. penetrated by S. paraboloides. (F, G) Holasteroid echinoid Hemipneustes striatoradiatus (Leske) and the pits Sedilichnus excavatus (Donovan and Jagt, 2002), all Upper Cretaceous (Maastrichtian) of the Netherlands and Belgium. (F) Natuurhistorisch Museum Maastricht, the Netherlands (NHMM) RZ 00162 (after Donovan and Jagt, 2013, fig. 1A). Apical surface with the positions of four pits of S. excavatus marked by asterisks (*). The numerical designation of ambulacra (Roman numerals, I-V) and interambulacra (Arabic numerals, 1-5) is provided as an explanation of the notation used in the text. (G) NHMM MA 0234-1 (after Donovan and Jagt, 2013, fig. 2C), oblique view of the apical surface. Ambulacrum V (left posterior) right of centre with S. excavatus between the columns of pore pairs; other specimens are in interambulacrum 4 (left lateral). An encrusting oyster, situated posteriorly, partly overgrows a S. excavatus (far right). (H) Multiple S. paraboloides infesting the dorsal cup of the Lower Carboniferous crinoid Amphoracrinus gilbertsoni (Miller), the Natural History Museum, London (BMNH) EE8728 (after Donovan et al., 2006, fig. 1C). Enlarged lateral view of (mainly) dorsal cup, E-ray central, showing sub-horizontal arrangement of closely spaced pits. Note absence of pits above the line of the arm facets. Scale bar equals 5mm. All specimens uncoated except (H), which is whitened by ammonium chloride. Scale bars equal 10mm, except (H).

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