Trypanites sozialis Eisenack, 1934

Diagnosis. Relatively short, straight or inclined boring with a relatively low length/diameter ratio (c. 0.5–15).

Differential diagnosis. Trypanites sozialis differs from T. solitarius (Hagenow, 1840), T. weisei Mägdefrau, 1932 and T. fosteryeomani Cole & Palmer, 1999 by its relatively small length/ diameter ratio and the high variation in length and diameter. Trypanites mobilis Neumann et al., 2008 differs from T. sozialis by its emplacement in a calcareous bioclast (Øhlenschlæger et al. 2022; Fig. 6). Very shallow T. sozialis with a length/diameter ratio of c. 1 or even less were tentatively assigned to Circolites Mikulas, 1992, by Dronov et al. (2002), or may be confused with Oichnus simplex Bromley, 1981 if occurring in calcareous skeletal substrate (e.g. Vinn et al. 2021a). Circolites and incomplete Oichnus are wider than deep, whereas Trypanites is defined by a length/diameter ratio larger than 1 (Wisshak et al. 2015), although our data series includes incipient borings with a length/diameter ratio less than 1. Likewise, Tremichnus paraboloides Brett, 1985 consists of circular pits (wider than deep) in echinoderm skeletons (e.g. Thomkaet al. 2022). Another transition field exists with Palaeosabella prisca (McCoy, 1855), which are distally slightly extended borings (e.g. Hecker 1983), and Sanctum laurentiensis Erickson & Bouchard, 2003, which occurs in bryozoan skeletons and has a shaft and a cavity or chamber (e.g. Wyse Jackson & Key 2007). Borings identical to T. sozialis but preserving an elevated apertural rim were assigned to Anoigmaichnus odinsholmensis Vinn et al., 2014 without the designation of a holotype (for further specimens, see Vinn et al. 2021b). Subsequently, A. zapalskii and A. bretti Vinn et al., 2018 were erected, based on the elevation of the apertural rim, shape of boring and its size. The rim feature is not uncommon in Trypanites (e.g. T. weisei; see Mägdefrau 1932; Bertling 1999) and therefore, A. odinsholmensis and A. zapalskii are regarded as junior synonyms of T. sozialis (see Wisshak et al. 2019), whereas the holotype of A. bretti appears with an irregular shape and is probably better accommodated in the ichnogenus Sanctum Erickson & Bouchard, 2003.