Arthropleura trackway (Diplichnites cuithensis) from the Carboniferous, Serpukhovian, Limestone Coal Formation, Clackmannan Group, Linn Park, Glasgow

The trackways of Arthropleura (Diplichnites cuithensis) are common worldwide from deposits of Carboniferous to Permian age (Ryan 1986; Martino and Greb 2009; Schneider et al. 2010; Davies et al. 2022), and are commonly associated with large Taenidium (Beaconites) barretti burrows (Morrissey and Braddy 2004; Pearson and Gooday 2019). They have previously been noted from a limited number of localities in Scotland, from the Isle of Arran (Upper Carboniferous, Serpukhovian, Limestone Coal Formation, Clackmannan Group) and the coast of East Fife (Lower Carboniferous, Visean, Pittenweem Formation and Anstruther Formation, Strathclyde Group) by Briggs et al. (1979), Pearson (1992) and Whyte (2018), and recently reviewed by Davies et al. (2022). This contribution describes the occurrence of a second Upper Carboniferous trackway from Glasgow, from the Serpukhovian Clackmannan Group (Limestone Coal Formation) (Fig. 1), which was mentioned by Davies et al. (2022), but has not been previously illustrated or described.

The trackway described here was from a small exposure on the east bank of the White Cart Water, at the confluence of a small unnamed tributary (55°48.123'N, 04°15.867'W), in Linn Park, Glasgow (Fig. 2). The trackway occurs within the Carboniferous, Serpukhovian, Limestone Coal Formation, Clackmannan Group (Fig. 3), within a 2–3 m sequence of interbedded sandstones and shales (Fig. 4), dipping at 6–11° to the SW.

The preservation of the Diplichnites trackway is poor and it is difficult to photograph, owing to its occurrence within a heavily shaded river bank. Photographs were taken to test the potential of 3D photogrammetry in recording trackways. Three-dimensional models were created from mosaics of digital photographs taken with a Nikon 3200D SLR camera with a 24 megapixel sensor plate, set to a sensitivity of ISO 200, using an 18‒55 mm aspherical zoom lens. Images were overlapped by about 60%. Three-dimensional point cloud and mesh surfaces were generated from these photographs using Regard3D photogrammetric software (https://www.regard3d.org/), which applies a ‘structure from motion’ method, converting photographs taken from different angles into a 3D model of the object. The reconstructed images were saved as .obj files, and opened using ‘3D Viewer’, where the surface was generated in shaded greyscale. The 3D image was rotated to give optimum lighting and detail and saved as ‘tif’ files.

This large Diplichnites trackway is not well preserved, with one side better preserved than the other. The total width is 46 cm and it is approximately 3 m long (Fig. 5), with a straight to slightly sinuous track, oriented east–west, which is approximately parallel to the crests of asymmetric ripples recorded from the same surface. The trackway occurs on the major bedding surface of the outcrop (Figs 2 and 4), and comprises individual elongate imprints 4–5 cm long perpendicular to the trackway axis on each side of the trackway. Imprints are variably impressed into the surface, with one to two indentations per imprint, although no clear arrangement is easily identified (Fig. 6).

The main sandstone exposed comprises three thin amalgamated tabular beds (Fig. 4), with a complex association of sedimentary structures and ichnofossils that include the D. cuithensis trackway. Structures include trough cross-bedding, indicating a direction of flow 10° N, and asymmetric ripples with a flow direction to the NE (Fig. 4). Although much of the sandstone appears structureless, bedding planes are marked by large irregular horizontal tube-like structures (Fig. 7a and b) with a range of long axis trends from approximately 190 to 302°. Many surfaces are covered by millimetre- to centimetre-width Planolites?, raised in relief on the surface (Fig. 7c), locally branching or cross-cutting. Rare, and apparently paired, open pits of Arenicolites (Fig. 7d) and large (7–13 cm wide) poorly preserved bedding-parallel straight to recurved back-filled burrows of Taenidium barretti (Fig. 7e and f) are also present.

Only one side of the Diplichnites trackway is reasonably well preserved. Nevertheless, the size of the trackway and the details of the imprints, oriented perpendicular to the trackway axis, indicate an affinity to Diplichnites cuithensis, and preservation is similar to that of other Scottish examples (see Briggs et al. 1979; Pearson 1992; Whyte 2018).

Diplichnites and Taenidium barretti (formerly Beaconites barretti; see Keighley and Pickerill 1994) are commonly recorded in the same locations, although not necessarily associated within the same beds (see Morrissey and Braddy 2004). This association has led some researchers to postulate that the two trace fossils were made by the same organism. Beaconites and T. barretti have been attributed to a range of organisms, including polychaetes, myriapods, crustaceans, reptiles, amphibians, ostracoderms and lungfish (Morrissey and Braddy 2004, and references therein), whereas D. cuithensis has been shown to be the trackways of myriapods, and in particular arthropleurids (Morrissey and Braddy 2004). Morrissey and Braddy (2004, fig. 4) illustrated considerable overlap in the width of Diplichnites and T. barretti, although the size relationship illustrated is somewhat inconsistent, with some cases showing trackways larger than burrows, whereas in others the burrows are larger. In the current material, where the two are associated in the same bed, the trackway (D. cuithensis) is considerably larger than the burrow (T. barretti). This size difference suggests that they are not both the product of myriapods. However, given that T. barretti has been interpreted as aestivation burrows (Crowley et al. 2009), resulting from the maker burrowing down to the water table (Morrissey and Braddy 2004), the difference in size between D. cuithensis and T. barretti could reflect the activity of different generations, adult and juvenile forms of Arthropleura, or possibly different species of myriapod.

As indicated, D. cuithensis has been interpreted as the trackway of Arthropleura (Briggs et al. 1979; Fayers et al. 2010) and the distribution of the Arthropleuridae ranges from theVisean to early Permian, Sakmarian (Fayers et al. 2010; Davies et al. 2022). Therefore older examples of Diplichnites, such as Diplichnites gouldi from the Old Red Sandstone, have been suggested to be the products of other arthropods (Fayers et al. 2010). The example described here is the same age as that originally reported by Briggs et al. (1979) from the Isle of Arran. Although D. cuithensis has been recorded from strata as old as the Holkerian, it is most commonly found world-wide from the late Westphalian to early Permian (Ryan 1986). Scottish D. cuithensis have not been recorded from earlier than the Holkerian (Visean), which is the earliest occurrence world-wide (see Martino and Greb 2009), or later than the Pendleian (Serpukhovian). In theory, further examples may occur within the Scottish coal measures, but have not been observed through lack of natural exposures. It is also possible that D. cuithensis may exist in rocks older than the Holkerian, although, given their apparent absence in the literature (see Ryan 1986; Martino and Greb 2009; Schneider et al. 2010; Davies et al. 2022), this may represent the actual lower range for the species. As noted, T. barretti occurs in close association with Diplichnites, with both D. cuithensis and the older D. gouldi, mainly recorded from the Old Red Sandstone (Trewin and Kneller 1987a, b; Morrissey and Braddy 2004). Examples of T. barretti have been recorded from late Tournaisian to Holkerian (Visean) fluviatile sediments of Donegal (Buckman 1992), indicating that Diplichnites may also be present (although not observed) associated with the Irish T. barretti. Although other pre-Holkerian (early Visean) localities in Scotland and elsewhere may await discovery of D. cuithensis, it is also possible that Taenidium was constructed by a range of arthropods, and does not solely represent the activity of Arthropleura; further investigation into strata of this age may clarify the range of D. cuithensis/D. gouldi and their association with T. barretti.

One interesting aspect of the distribution of the Scottish D. cuithensis is their appearance in the Upper Carboniferous (Serpukhovian) on the west coast, and lower Carboniferous (Visean) on the east coast. However, this is likely to be a function of the small number of recorded localities, lack of exposure and occurrence of relevant facies, rather than representing a mass westward migration over time.

The large irregular structures spreading on bedding planes (Fig. 7a and b) are particularly hard to interpret. They may represent stigmaria, soft-sediment injection structures (injectites), somewhat irregular bioturbation or even preferential mineralization around later fractures. The lack of carbonaceous material in association with these structures, and the absence of typical stigmarian ornamentation, does not provide any firm evidence that these structures are the remnants of root systems. No obvious vertically oriented cross-cutting wall-like structure is observed, suggesting that they do not represent injectites or preferential cementation developed around vertical cracks (joints) within the sandstone. Although, on first impression, these structures appear to have a preferred orientation, their recorded orientations trend from 190 to 303° and are within a range spreading over more than 90°, indicating a more random pattern of orientation. The latter is consistent with these structures having an origin as large irregular bioturbation features (owing to burrowing), although the possibility that they represent disturbance caused by the presence of rootlets cannot be completely ruled out.

Diplichnites cuithensis has been described from a wide range of clastic terrestrial facies, from alluvial fans, alluvial plains, and fluvial, coastal and deltaic environments (Schneider et al. 2010). The Limestone Coal Formation typically comprises repeated upward-coarsening cycles, capped by seat earth and coal, developed in fluviodeltaic embayments of variable salinity (Barron et al. 2005). The occurrence of disturbed ground (Fig. 4), and closely associated explorative coal mine workings (Fig. 2) indicates the presence of a thin coal seam capping the upward coarsening sequence. Diplichnites cuithensis and associated trace fossils therefore appear to be associated with fluvial channel deposits, and are overlain by coal formed in a swampy environment.

The 3D reconstructions provide some details of the trackway (Figs 5c and 6b), although most of this is poorly imaged. The nature of preservation (as observed naturally and in 3D reconstructions) suggests that the trackway represents an undertrack, which explains the general lack of detail. Although 3D photogrammetry was not particularly successful in the present case, nevertheless, its use for other Diplichnites trackways may be a valuable tool for future investigations of such material, permitting the construction of 3D surfaces that can be rotated and manipulated to maximize morphological data from sites that are poorly preserved and difficult to assess.

This paper documents a new locality in the Serpukhovian Limestone Coal Formation, in the Greater Glasgow area, that records the occurrence of a D. cuithensis myriapod trackway, associated with a restricted range of ichnofossils that include the commonly associated Taenidium barretti, occurring in a fluviatile sequence passing upwards into coal-bearing deposits. Recognition of this new location increases the number of occurrences of D. cuithensis from Upper Carboniferous rocks, and is the first in the mainland of the west of Scotland, where most previous records are from Lower Carboniferous sites on the east coast. The co-occurrence of D. cuithensis and T. barretti further illustrates the link between these trace fossils previously suggested as the work of the same myriapods. In this case, the trackways are much broader than the burrows, and may represent the activity of separate generations or cohorts, or even different myriapods.

Thanks go to Jackie, Chloe and Josh Polson for help when we first came across this trackway in 1995. Also, we thank the Linn Park rangers (1995) for pointing us in the direction of this interesting location. In addition, we acknowledge the helpful comments and suggestions from the reviewers and editor.

JOB: conceptualization (lead), formal analysis (lead), investigation (lead), methodology (lead), writing – original draft (lead), writing – review & editing (lead); SJC: investigation (supporting), methodology (supporting), writing – original draft (supporting), writing – review & editing (supporting); PGP: investigation (supporting)

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

All data generated or analysed during this study are included in this published article. Raw data for digital reconstructions can be requested from the corresponding author.