Abstract
The invertebrate trace fossils Selenichnites rossendalensis and Crescentichnus tesiltus are recorded and described from the Middle Jurassic Gristhorpe Member of the Cloughton Formation of the Cleveland Basin. This is the first record of these ichnospecies from the basin and now completes the occurrence of these and other traces assumed to have been made by limulids from all three non-marine formations of the Ravenscar Group.
Trace fossils made by invertebrates are common in the non-marine terrestrial deposits of the Middle Jurassic of the Cleveland Basin, Yorkshire. Of these, traces attributed to a xiphosurid (limulid) maker have been regularly documented since Selenichnus (Romano & Whyte 1987), later emended to Selenichnites (Romano & Whyte 1990), was first described from the marine Scarborough Formation. The type ichnospecies Selenichnites hundalensis from this unit was interpreted as a limulid resting trace and shallow excavation by the maker. Since then, records of Selenichnites have been described from the Scalby Formation of the Cleveland Basin (Romano & Whyte 2013). Subsequent records of traces also assigned to limulids from the Cleveland Basin include trackways of the ichnogenus Kouphichnium, which have been described from the Saltwick Formation (Romano & Whyte 2003; Romano & Taylor 2016).
This note describes specimens of Selenichnites rossendalensis and Crescentichnus tesiltus found by one of us (RT) from the Gristhorpe Member of the Cloughton Formation of the Cleveland Basin (Figs 1 and 2). This is the first record of these ichnospecies from the Cleveland Basin and now means that traces assigned to limulids occur in all three non-marine formations of the Middle Jurassic of the Cleveland Basin of Yorkshire, as well as in the marine Scarborough Formation.
(A) Map showing the location of Cloughton Wyke. (B) Map showing the locality (asterisk) in Cloughton Wyke from which the specimens described here were found [National Grid Reference TA 0201 9517].
Stratigraphic log measured by one of the authors (MR) and Martin Whyte through the upper part of the Gristhorpe Member (Cloughton Formation) and lower part of the Scarborough Formation outcropping between Cloughton Wyke and Hundale Point, showing the approximate horizon from which the specimens described here came (marked by a bar and asterisk). The gutter bed in the Gristhorpe Member (listed in this paper) and the convoluted sandstone in the Scarborough Formation (see Rawson & Wright 2000, fig. 21) are indicated. Scale in metres.
Systematic ichnology
Ichnogenus Selenichnites Romano & Whyte, 1990
(=Selenichnus Romano & Whyte, 1987)
Type ichnospecies
Selenichnus hundalensis Romano & Whyte, 1987, originally described from the Scarborough Formation, Middle Jurassic (Bajocian), Yorkshire.
Selenichnites rossendalensis (Hardy, 1970) (Fig. 3)
- 1970
- Kouphichnium rossendalensis sp. nov.; Hardy, p. 188.
- 2015
- Selenichnites rossendalensis (Hardy, 1970); Romano & Whyte, p. 276.
- 2015
- Selenichnites rossendalensis (Hardy, 1970); Lerner & Lucas, p. 143.
Material
A single specimen of a trackway preserved as a positive hypichnion on the underside of slab of fine grained sandstone approximately (before trimming) 84 × 80 cm across and 6 cm thick. The specimen occurs on a surface with low-amplitude ?current ripples with a wavelength of c. 6 cm. In section the dominantly sandstone host rock has few, very thin, mud grade laminae that show evidence of disruption. Subhorizontal burrows and fine vertical rootlets are present. The specimen is deposited in the collections of the Scarborough Museums Trust, Woodend, and numbered SCARB:2018.022.
Undersurface of slab with Selenichnites rossendalensis from the Gristhorpe Member of the Cloughton Formation showing positive hypichnion preservation. Scale bar is 10 cm.
Stratigraphy and locality
The specimen was collected from the Gristhorpe Member of the Cloughton Formation in Cloughton Wyke, just south of the route down to the beach from Salt Pans Road (Fig. 1), c. 1.0 m above the gutter bed (Fig. 2) and c. 9 m below the Scarborough Formation (Rawson & Wright 2000, fig. 21).
Description
The specimen consists of three overlapping traces preserved in a series. The most complete trace is the first formed at the beginning of the series. This shows a prominent crescent-shaped trace, 50–55 mm across and 45 mm long, with indistinct markings within the crescent trace. A long posteriorly directed and narrowing ridge 70 mm in length ends with a sharp point. This trace overlaps a seemingly similar trace which in turn overlaps the third more indistinct anterior trace. Neither of the latter (later) two traces show the posteriorly directed ridge. The best preserved (posterior trace) is deepest at the anterior end where it projects just over 10 mm below the surface of the bedding plane on which it is preserved. The third, most anterior trace is composite and less well-defined with indistinct margins.
Remarks
The present material from the Gristhorpe Member conforms very closely to the emended diagnosis of S. rossendalensis (Hardy 1970, pl. 40; Romano & Whyte 2015, p. 277). This ichnospecies was first described by Hardy (1970) as Kouphichnium rossendalensis sp. nov. for a curvilinear series of traces with additional isolated traces on the same slab from the Namurian Upper Haslingden Flags, Upper Carboniferous, of northern England. It is from one of these latter traces that a lectotype was designated (Romano & Whyte 2015, p. 277, fig. 2A, B). The range in width of the crescent-shaped trace of the type ichnospecies is between 9 and 17 mm, though more commonly between 16 and 17 mm. This is considerably (just over 40%) smaller than in the present material.
Type ichnospecies
Crescentichnus antarcticus (Weber & Braddy, 2004) from the ?Lower Ordovician Blaiklock Glacier group of W. Antarctica.
Crescentichnus tesiltus (Gibb et al. 2011)
(Fig. 4)
- 2011
- Selenichnites tesiltus Gibb et al., p. 159.
- 2015
- Crescentichnus tesiltus Romano & Whyte, p. 282.
Material
Two complete specimens and traces of others preserved as positive hypichnia on the underside of a loose slab of fine-grained sandstone. The slab was buried by a rock fall before it could be collected or examined in detail, but the slab was c. 75 cm across, and the specimens occur on a surface with low-amplitude ?current ripples with sinuous crests and invertebrate traces. The host rock appeared to be similar to that described above for Selenichnites rossendalensis.
Undersurface of slab with Crescentichnus tesiltus from the Gristhorpe Member of the Cloughton Formation showing positive hypichnion preservation. For scale see text.
Stratigraphy and locality
The specimen was found in a rock fall within the Gristhorpe Member of the Cloughton Formation in Cloughton Wyke, just south of the route down to the beach from Salt Pans Road. Although it has not been possible to prove, it was probably from, or close to, the same horizon that yielded Selenichnites rossendalensis described above.
Description
The specimens consist of isolated traces, two of which show the same orientation. These two traces are simple, gently curved, crescent-shaped hypichnion ridges approximately the same size (c. 60 mm across) as those assigned to S. rossendalensis. Although it is now not possible to determine from the photograph the current direction from the ripple morphology, the curved ridges are orthogonal to the current, pointing either towards or away from the current flow.
Remarks
In the original description (Gibb et al. 2011) this ichnospecies was assigned to Selenichnites. However, in a review of ichnospecies assigned to this ichnogenus, Romano & Whyte (2015) excluded it from Selenichnites and included it with others in their new ichnogenus, Crescentichnus. At the time these authors questioned whether specimens assigned to C. tesiltus do indeed represent a distinct ichnospecies (Romano & Whyte 2015, p. 282) but the open gentle curve of the holotype and specimens described here is quite distinct, and so is provisionally retained here.
Discussion
While all the present ichnospecies assigned to either Selenichnites and Crescentichnus are regarded as having a limulid maker, it should not be assumed that all such, or similar, traces had the same maker (see discussion in Romano & Whyte 2015, p. 287). However, the crescent-shaped or lunate outline and posteriorly directed pointed ridge of this ichnospecies leave little doubt that both correspond respectively to the outline of the prosoma and telson of limulids.
Previous workers (Eldredge 1970) have discussed the probable behaviour of the animals that resulted in traces assigned to Selenichnites (see also Ethological considerations in Romano & Whyte 2015, p. 285). The relatively short composite trail recorded here as S. rossendalensis is unlike that for S. hundalensis, where the single outline of the maker involved both appendage reworking of the sediment, resulting in strongly convex and lunate paired lobes (Romano & Whyte 1987, p. 89), along with minimal rotation in order to achieve shallow burrowing/burial. The trail from the Gristhorpe Member records a relatively short excursion by the limulid maker onto the substrate surface, during which it temporarily underwent shallow burrowing (either from the impact of landing on the sediment floor, or by limited excavation by the appendages, or both). This activity resulted in the best-preserved and most posterior of the three imprints of the animal. Subsequent forward motion in the form of ‘jumps’ or ‘steps’ produced in this case two further imprints, each partially covered by the previous trace in the hypichnial preservation before the animal took off from contact with the sediment. The behaviour deduced from this trace suggests that the animal responsible was searching for food either on the sediment surface or just below, and may be described as a ‘prod trace’ as suggested by Gibb et al. (2011). It is assumed the animal was swimming just prior to making contact with the sediment surface as there is no trace of appendage markings behind the first-formed print. It is, however, possible that such traces may be preserved at a slightly higher level within the slab.
All other previous records of Selenichnites from the Cleveland Basin have occurred as single traces (Romano & Whyte 1987, 2013). It was suggested that the single trace of S. hundalensis that showed evidence of digging-in may have been an attempt to ‘escape’ or ‘avoid daylight’ (Romano & Whyte 1987, p. 94) while the single ‘prod traces’ of Selenichnites isp. and Selenichnites isp. indet. (Romano & Whyte 2013) may represent searching for food. The repetitive trails such as in S. rossendalenisis (this paper and Hardy 1970) indicate continuing searching for food, perhaps resulting from an initial success or failure. The most anterior (last) trace of the series in S. rossendalensis from the Gristhorpe Member is the most ill-defined and largest (widest). This indicates more extensive reworking of the sediment, perhaps as a result of finding food. There is more mud grade sediment associated with this last trace of the series which shows that the mud laminae at depth had been reached and reworked, possibly as a result of the maker finding food. Other disturbed, and muddy, areas on the surface of the slab show similar features. The absence of any evidence to suggest that the surface of the sediment was subaerial at the time of the animal making the trace, argues against the shallow trace being a spawning burrow.
The simple traces represented here by C. tesiltus are also interpreted as single ‘prod traces’ but do not show the more composite traces as those described by Romano & Whyte (2013), which show evidence of appendage reworking of the sediment. This may either indicate that the traces are simply the result of only the prosoma impinging on the sediment surface, or that any appendage traces occur at a higher level and are not recorded on the surface now showing the prosoma imprints. It may also be the case that the more arcuate, open trace of C. tesiltus is the result of the limulid's prosoma striking the surface at a higher angle and so not leaving imprints of the posterolateral parts of the prosoma, nor those of the appendages which would not have made contact with the surface.
It is of interest, if perhaps not significant in such a small sample, to note the sizes of the animals that produced these tracks in the Middle Jurassic of the Cleveland Basin (Fig. 5). The smallest animals with the prosoma reaching 40–100 mm across were recovered from the non-marine units, while the larger animal (with prosoma width reaching 170 mm across) was from the marine Scarborough Formation. Lamsdell (2016) has recently discussed the ecological shifts in horeshoe crabs, and noted that modern xiphosurids have not reinvaded the non-marine environmenmt. Although the similarly large example of an animal with a prosoma c. 190 mm across was from the non-marine Saltwick Formation, this was based on an estimate from the width of the appendage imprints of the trackway. Recently one of us (RT) found a single specimen, here assigned to S. cf. hundalensis from the Saltwick Formation in Hayburn Wyke, 2 km north of Cloughton Wyke, though the source horizon could not be located. This specimen was not collected and the width of the prosoma was c. 66 m. This is not included in Figure 5. No firm conclusions may be drawn from this small sample, apart from populations consisting of individuals of limulids of various sizes (presumably representing adults and juveniles) occupied non-marine coastal bay environments during the Middle Jurassic of the Cleveland Basin. The single large specimen from the marine Scarborough Formation remains an enigma. Finally, an explanation offered by one of the referees suggests that the possible reason for size discrepancy in trackways is because juveniles are spending longer periods of time in the non-marine environment than the adults.
Simplified stratigraphic column of the Middle Jurassic of the Cleveland Basin showing horizons or approximate levels at which specimens assigned to either Selenichnites or Kouphichnium occur, and the width of the prosoma of the limulid that left Selenichnites traces at those levels. Note that the width of the prosoma for C. tesiltus is estimated, and that for K. aff. variabilis was calculated from the width of the trackway left by the appendages (see text). The 60–100 mm range for Selenichnites isp. indet. from the Long Nab Member is for four specimens on the same bedding surface. The marine lithostratigraphic units are shaded.
Acknowledgements
We thank Mr Paul Coles who drew the figures, and the two anonymous referees who made useful suggestions to improve the paper. Stewart Molyneux is thanked for his valuable editorial advice and assistance.
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Scientific editing by Stewart Molyneux
- © 2019 The Author(s). Published by The Geological Society of London for the Yorkshire Geological Society. All rights reserved
References
Two previously unrecorded xiphosurid trace fossils, Selenichnites rossendalensis and Crescentichnus tesiltus, from the Middle Jurassic of Yorkshire, UK
