Both forms of borings in BMNH PEI 5339 present problems of classification. At least some of the small borings are reminiscent of the chambers of Entobia ispp., but they are not linked by lateral canals (Fig. 1c, d). Rather, they are small round holes/pits in a hard substrate and are most probably domiciles. The original diagnosis of Oichnus stated: “Circular to subcircular holes of biogenic origin bored into hard substrates. The hole may pass right through the substrate as a penetration, where the substrate is a thin shell; or end within the substrate as a shallow to deep depression or short, subcylindrical pit” (Bromley 1981, p. 60; see also Donovan and Pickerill 2002, p. 87). The borings in the present specimen “… end within the substrate as a … deep depression …” Therefore, these pits are referred to Oichnus isp.; the parallel-sided shafts embolden us to suggest that they are closest to the type species, Oichnus simplex Bromley, 1981. Oichnus simplex is most commonly identified from shelly substrates; here, the producer was boring into rock (mudrock? limestone?) or wood, yet the message of this paper, as amplified below, is that substrate is not important in classification. Most probably, these pits were domiciles.
The identity of the clavate borings is also problematic. The principal ichnotaxobase used for clavate borings has been substrate, either rocky/shelly (Gastrochaenolites Leymerie) or wood (Teredolites Leymerie); the importance of morphology has been at the level of ichnospecies. The specimens in Fig. 1 are close to two ichnospecies, most particularly Gastrochaenolites turbinatus Kelly and Bromley and Teredolites clavatus Leymerie, neither of which has a discernible neck region; that is, the chamber-to-neck transition is recognized in neither ichnospecies (Kelly and Bromley 1984, text-figs. 3G, 8C, 9A, 10). Unfortunately, the diagnosis of T. clavatus is more concerned with substrate than morphological features: “Clavate Teredolites predominantly perpendicular to the grain in woody substrates having length/width ratio usually less than 5″ (Kelly and Bromley 1984, p. 804). On the same page, the diagnosis of the ichnogenus Teredolites is “Clavate borings in woody substrate …” and, thus, that of T. clavatus actually tells us little more. If substrate is dismissed as an ichnotaxobase, as we believe that it should be, it is apparent that G. turbinatus is a junior synonym of T. clavatus. The latter is not worthy of a separate ichnogeneric name and should be considered Gastrochaenolites clavatus (Leymerie). As both ichnogenera were erected in the same publication, this is the option that will involve the least ichnospecific confusion.
Note at this juncture that Teredolites longissimus Kelly and Bromley, 1984, the only other nominal species hitherto attributed to this ichnogenus, is morphologically highly distinct from all Gastrochaenolites ispp. and, particularly, T. clavatus. It has recently been reclassified as the type species of a new ichnogenus, Apectoichnus Donovan, 2018, based on its distinct morphology and not its substrate.
In short, the type (and, currently, only) species of Teredolites, T. clavatus Leymerie, is similar in form to Gastrochaenolites spp. (compare Kelly and Bromley 1984, text-figs. 3, 9A with 9B). Therefore, how much does substrate matter? The schemes of Pickerill (1994, p. 10) followed Bromley (1990) in recognizing general form, branching, burrow fill, and burrow boundaries as recognizable ichnotaxobases, whilst Bertling et al. (2006, Table 2) summarized recommended ichnotaxobases as morphology (overall shape), orientation, ornamentation, and internal structure. Note that none of these experts included substrate as an ichnotaxobase (contra Höpner and Bertling 2017).
If this argument is rejected that Teredolites clavatus is a Gastrochaenolites, then how should we classify the clavate borings in Fig. 1? Are they Gastrochaenolites turbinatus or Teredolites clavatus? Despite the excellent preservation shown by these specimens, because the substrate is unknown, there is no possible way of determining this using existing logic of classification. Yet, if we consider some other sedimentary structure—say, cross bedding—it remains cross bedding whatever the mineralogy of the grains or the substrate. The utility of substrate when analysing a clavate boring is in giving an indication of the nature of the boring organism (Bromley 2004, p. 462), but that is an ancillary consideration. Too often, it is biological determinations that overrule the simple requirement to classify a trace based on its form (e.g., Donovan 2010). The Latinized binomens used to name trace fossils do not make them organisms, yet it is an historic accident (Osgood, 1975) that continues to confuse ichnologists, sedimentologists, and palaeontologists that they are so. They are not.
On the basis of recognized ichnotaxobases, and not of substrate, T. clavatus is a Gastrochaenolites. In consequence, Teredolites is a junior synonym of Gastrochaenolites (Appendix 1). This synonymization is not, we emphasize, just to allow one specimen (Fig. 1) to be comfortably placed within ichnological classification. Rather, the reverse is true; this specimen exposes a persistent flaw in ichnotaxonomic practice. The specimen is an illustration that has helped clarify our thoughts on the value (or otherwise) of substrate as an ichnotaxobase. Our arguments are pertinent whether the specimen in Fig. 1 is referred to or not.