Substrate is a poor ichnotaxobase: a new demonstration
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Substrate is a poor ichnotaxobase: a new demonstration Stephen K. Donovan1,2 • Timothy A. M. Ewin3 Received: 24 January 2018 / Accepted: 1 March 2018 Akademie der Naturwissenschaften Schweiz (SCNAT) 2018
Abstract Substrate is a poor ichnotaxobase, yet it has been widely used for distinguishing the clavate (club-shaped) borings commonly produced by bivalves. A chert nodule from the Upper Cretaceous of Morocco encloses the external mould of a clast of unknown composition (rock? wood?) which had been extensively bored, producing both small cylindrical and large clavate pits. Numerous small, short borings are referred to Oichnus isp. cf. O. simplex Bromley; Oichnus is otherwise commonly limited to shelly substrates. More significantly, the clavate borings may either be Gastrochaenolites turbinatus Kelly and Bromley, considered confined to rock and shelly substrates, or be Teredolites clavatus Leymerie, thought to be limited to wood. These borings are morphologically indistinguishable, only being differentiated by substrate, and thus, they are considered to be synonymous herein. Gastrochaenolites clavatus (Leymerie) has priority and is the type species of Teredolites Leymerie, now considered a junior synonym of Gastrochaenolites Leymerie. Thus, the clavate borings of this specimen are identified as Gastrochaenolites clavatus (Leymerie). Keywords Ichnosystematics Taphonomy Oichnus Gastrochaenolites Teredolites Apectoichnus
Introduction Size is a poor ichnotaxobase (Pickerill 1994, p. 11), yet size undoubtedly influences, in part, the apportioning of traces between ichnospecies and ichnogenera. Trace fossils are not biological entities per se, but rather are sedimentary structures generated by the activities of plants and animals. Thus, the primary consideration of the systematics of trace fossils is the form or shape; because they are sedimentary structures, not biological species, their size is not really a consideration except to help differentiate between certain ichnospecies within a given ichnogenus (such as Paleodictyon Meneghini; Uchman 1995, text-figs. 21–23). For example, small round holes in shells are assigned to the ichnogenus Oichnus Bromley 1981 (Pickerill and Donovan Handling editor: D. Marty. & Stephen K. Donovan [email protected] 1
Taxonomy and Systematics Group, Naturalis Biodiversity Center, P.O. Box 9517, 2300 RA Leiden, The Netherlands
2
Department of Earth Sciences, University of New Brunswick, Fredericton, NB E3B 5A3, Canada
3
Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, England
1998; Wisshak et al. 2015; Donovan and Pickerill 2017). Microborings of similar morphology should also be assigned to Oichnus (see, for example, Blissett and Pickerill 2007, pp. 88–90). In addition, how large can a small round hole be before it ceases to be Oichnus? In truth, Oichnus is produced by the activities of a variety of small invertebrates, commonly predatory, parasitic, or excava
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