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    Recent borings in limestone cobbles from Marloes Bay, southwest Wales

    Donovan, S.K. and Fearnhead, F.E. and Underwood, Charlie J. (2007) Recent borings in limestone cobbles from Marloes Bay, southwest Wales. Lethaia 40 (3), pp. 233-236. ISSN 0024-1164.

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    Limestone clasts from the beach at Marloes Sands, southwest Wales, contain slender, straight to sinuous borings cross‐cut by younger, clavate borings. The former were probably produced by sipunculids or polychaetes; the latter preserve shells of the boring bivalve Gastrochaena dubia (Pennant). Unusually, the calcareous linings of the clavate bivalve borings extend into many of the slender worm borings. Such linings are considered part of the hard parts of the producing bivalve, but the chance association of the two morphologies of borings has led to the lining becoming intimately associated with both of them. The modified linings of the bivalve borings have a similar morphology to the crypt of certain clavagellid bivalves, perhaps presenting an analogue for the morphology of a pre‐clavagellid, boring ancestor. Walls and linings in burrows and borings are of importance to the systematic ichnologist (e.g., Keighley & Pickerill 1994) and the functional morphology of the producing organism (e.g., Savrda & Smith 1996; Monaco et al. 2005). Fossil clavate borings in limestone (e.g., Gastrochaenolites Leymerie, 1842) and wood (e.g., Teredolites Leymerie, 1842) commonly have calcareous linings, which are ‘part of the hard part of the shell’ (Kelly & Bromley 1984, p. 795). The lining of the boring is secreted by the mantle and hence is part of the body fossil (Evans 1999), although such a structure preserved free of the boring can be treated as an organically generated mould of the trace fossil (table 2 in Donovan 2002). In the extreme example of crypt‐dwelling bivalves, the lining has become secondarily readapted to a life within unlithified sediment (Pojeta & Sohl 1987; Savazzi 1994, 1999). Such calcareous linings have a morphology determined by the boring. The boring/lining relationship can be modified either by the boring organism (e.g., Gastrochaenolites pickerilli Donovan, 2002) or by features of the substrate. It is the latter phenomenon that is the subject of the present paper. Interactions between invertebrates and the surrounding substrate, and modifications of the substrate by organisms forming traces are probably better known from soft sediments and related sedimentary rocks (Bromley & Ekdale 1986; Pickerill 1994), but analogous interplays are recognized from borings (e.g., Donovan 2002). Interference between borings in a hard substrate that has been repeatedly infested can produce structures of unusual topography that complicate interpretation. Herein, such associations are explored by reference to an example of calcareous linings encroaching on adjacent borings that would have been otherwise unlined. While there are probably good functional reasons for such behaviour, the resultant structure is convergent with that of a notable, and related, group of bivalves, begging the question, what evolutionary influence (if any) might be read into such accidental structures? Descriptive terminology of borings follows Häntzschel (1975). All specimens are deposited in the Nationaal Natuurhistorisch Museum Naturalis, Leiden (RGM).


    Item Type: Article
    School: Birkbeck Faculties and Schools > Faculty of Science > School of Natural Sciences
    Depositing User: Sarah Hall
    Date Deposited: 21 May 2019 16:32
    Last Modified: 02 Aug 2023 17:51


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