Fossil horseshoe crabs (Chelicerata: Xiphosura) can occasionally be found in huge numbers at specific localities (e.g., the Pony Creek Shale locality in Kansas; Leibach et al. Reference Leibach, Rose, Bader, Mohr, Super and Kimmig2021 and references therein) but, much more commonly, their remains are rare and isolated in occurrence. Indeed, much of the fossil record of horseshoe crabs comes from single specimens or small collections from isolated localities scattered throughout geological time. Hence, the discovery of a single specimen of the synziphosuran Limuloides limuloides (Woodward Reference Woodward1865) in Silurian rocks of the Welsh Borderland is worthy of record. It is one of only a few specimens which show articulated prosoma and opisthosoma, and comes from a locality which has not previously yielded chelicerates, of Wenlock age in the Welsh Borderland. Previously, only two specimens of this species have been recorded from the Wenlock Series, from the Coalbrookdale Formation of Dudley and Ledbury (Howard Reference Howard2024).
Synziphosurina was erected as a suborder of Xiphosura by Packard (Reference Packard1886) for a number of arthropods which could be placed into neither Eurypterida nor Xiphosura, but appeared to be connecting forms between these groups. He included Bunodes von Eichwald, Reference von Eichwald1854, Hemiaspis Woodward, Reference Woodward1865, Pseudoniscus Nieszkowski, Reference Nieszkowski1859, Exapinurus Nieszkowski, Reference Nieszkowski1859 and (with reservation) Neolimulus Woodward, Reference Woodward1868 in the new suborder. The genus name Hemiaspis was preoccupied, so the earlier nomen nudum Limuloides (Salter, Reference Salter1857) was validated by Woodward (Reference Woodward1865); and Exapinurus is in Bunodes (see Howard, Reference Howard2024 for the complex nomenclature surrounding Limuloides). Eldredge (Reference Eldredge1974) and Bergström (Reference Bergström1975) both revised Synziphosurina. Eldredge included in the suborder Weinbergina Richter & Richter, Reference Richter and Richter1929, Legrandella Eldredge, Reference Eldredge1974, Bunodes, and Limuloides, whilst Bergström (who considered it an order) included the same genera except for Legrandella (which was mentioned in an addendum). Both of these authors included Pseudoniscus and Neolimulus within the xiphosurids. Anderson & Selden (Reference Anderson and Selden1997) considered the synziphosurines to form a paraphyletic grade of stem-group horseshoe crabs, rather than monophyletic. More recent phylogenetic analyses which have included synziphosurines within a broader sample of Euchelicerata (Lamsdell Reference Lamsdell2013, Reference Lamsdell2016; Lamsdell et al. Reference Lamsdell, Briggs, Liu, Witzke and McKay2015; Selden et al. Reference Selden, Lamsdell and Qi2015) have shown synziphosurines to be polyphyletic. Half of synziphosurine taxa (e.g., Legrandella and Dibasterium Briggs et al., Reference Briggs, Siveter, Siveter, Sutton, Garwood and Legg2012) resolved in the euchelicerate stem group, while the rest, the more traditional synziphosurines such as Limuloides, Pseudoniscus, and Bunodes, as well as Houia (Lamsdell et al., Reference Lamsdell, Xue and Selden2013) from China, resolved between Xiphosurida and Dekatriata Lamsdell, Reference Lamsdell2013 (Chasmataspida + Eurypterida + Arachnida) (Selden et al. Reference Selden, Lamsdell and Qi2015, fig. 6).
1. Material and methods
The specimen (NHMUK PI In 66591, Figs 1, 2) consists of a single piece showing the connected prosoma and opisthosoma in dorsal relief (Fig. 1). The specimen was found by R. L. Kennedy, and passed on to me by Richard Fortey in 2022; it remained unidentified until 2025. The handwritten note accompanying the specimen reads: ‘Gorstian Ashton Shale Fm. (nilssoni) woodland S of B4385 about 200 m N of Broughton Farm.’ This seems to be the locality mentioned by Cave & Hains (Reference Cave and Hains2001, p. 87) as ‘small quarry and dingle south of main road [SO 3037 9091] Mudstone, blocky, calcareous weathered brown, yields decalcified rugose coral and ‘Clorinda’ dormitzeri? c. 20.0 m’. These authors noted that this dingle contains the only exposure of the base of the Aston Mudstone Formation. However, despite problems with diachroneity and the lack of graptolite biozone markers in the shelly sequences, Cave & Hains (Reference Cave and Hains2001, p. 89) concluded that ‘No part of the Aston Mudstone Formation in the Aston Dingle area is of Ludlow age’ and that the Aston Mudstone Formation is probably entirely within the Wenlock. Hence, the strata from where the specimen described here originated is most likely to fall within the nassa biozone (Cave & Hains Reference Cave and Hains2001, table 6).
Limuloides limuloides, NHMUK PI In 66591, photographs: (a) dorsal view; (b) right dorsolateral view; (c) left dorsolateral view. Scale bars represent 5 mm.
Limuloides limuloides, NHMUK PI In 66591, interpretative drawing. Numerals i–x refer to the opisthosomal tergites. Abbreviations: atn = axial tergite node; atp = axial tergite portion; cl = cardiac lobe; ir = interophthalmic ridge; ll3–4 = left lobe 3–4; lr1–2 = left radial ridge 1–2; ltn = lateral tergite node; ltp = lateral tergite portion; ol = ocular lobe; or = ophthalmic ridge; ps = pleural spine; rl3–4 = right lobe 3–4. Scale bars represent 5 mm.
The specimen is preserved in a greenish-grey siltstone. It is not flattened, which possibly represents the original relief in life. The anterior and lateral sides of the carapace drop away precipitously (Fig. 1b, 1c) from the central area, which is rather flat and featureless. More of the right side of the specimen is preserved than the left, but the margins on both sides are incomplete. Part of the left side of the carapace central area is broken away. The telson is not preserved. Where present, remains of the cuticle are a slightly darker brown than the matrix.
The specimen was studied using a Leica MZ16 stereomicroscope, and photographed using a Canon 5DSR digital camera with a 50 mm macro lens mounted on a copy stand, captured with DSLR Assistant (www.dslrassistant.com). The specimen was photographed dry in low-angle light. The drawing (Fig. 2) was made from the photograph using Affinity Designer (affinity.serif.com), and measurements were made with Graphic (www.graphic.com). All measurements are in mm. Morphological terminology follows Howard (Reference Howard2024).
2.2. Systematic palaeontology
Superclass Euchelicerata Weygoldt & Paulus, Reference Weygoldt and Paulus1979
Family Bunodidae Packard, Reference Packard1886
Included genera. Bunodes von Eichwald, Reference von Eichwald1854, Limuloides (Woodward, Reference Woodward1865), Bembicosoma Laurie, Reference Laurie1899, Pasternakevia Selden & Drygant, Reference Selden and Drygant1987
Remarks. The family Bunodidae is diagnosed on its ten visible opisthosomal tergites, with the first hidden between the posterior margin of the carapace and the hypertrophied second opisthosomal tergite. The axial portion of the opisthosoma is generally wide in relation to the lateral portions (see Howard Reference Howard2024). The specimen described here shows the hypertrophied second opsithosomal tergite and a relatively wide axial region to the opisthosoma. Recent phylogenetic analysis (Lamsdell Reference Lamsdell2025, supplementary fig. S1) resolved Bunodidae as sister to Xiphosura.
Limuloides (Woodward, Reference Woodward1865)
Remarks. Limuloides is diagnosed as a bunodid in which the carapace margin is ornamented with spines and bears a broad field that slopes gently between the ophthalmic ridge and carapace margin; nine radiating ridges scalloped, extending outwards horizontally across the broad field (see Howard Reference Howard2024). The specimen described here can be placed in this genus because, despite lacking carapace anterior and lateral margins, its morphology otherwise conforms to this diagnosis.
Limuloides limuloides (Woodward, Reference Woodward1865)
Remarks. There are two species included in the genus at present: L. limuloides and L. horridus (Woodward, Reference Woodward1872). The latter differs from the type species in lacking the distinctive sloping field and bearing spines around the whole of the carapace margin (see Howard Reference Howard2024). The specimen described here clearly belongs to the type species L. limuloides.
Description. Cuticle finely tuberculate. Central area of carapace subrectangular, about twice as wide as long, with slightly recurved anterior and posterior margins; surface relatively featureless, gentle interophthalmic ridges demarcate cardiac area; anterior margin bears row of broad lobes (ll1–4 and rl1–4 on Fig. 2). The median tubercle is interpreted as the ocular lobe, because of its position. Beyond the central area, demarcated by the ophthalmic ridges, the surface falls away to the marginal rim. Two radial ridges (lr1 and lr2 on Fig. 2) can be seen here on the left side. Tergite i is concealed beneath the posterior border of the carapace, as is the anterior border of tergite ii. Tergites are approximately the same length (mean ∼2.25 mm), becoming slightly shorter posteriorly, except for tergite ii which is somewhat hypertrophied relative to the others (visible length 3.0 mm), and tergite x (the pretelsonic segment) which is much longer (visible length 5.0 mm). The axial tergite portion of tergites ii to ix is gently arched, with a median node and a lateral lobe on either side (Fig. 2). The lateral boundary of the axial tergite portion has a rounded shape. Beyond that, the lateral tergite portion is flatter, and bears a lateral tergite node; beyond this node, the tergites curve backwards, increasingly more so posteriorly, terminating in an acute angle (the pleural spine). The more posterior tergites (viii–x) form the postabdomen.
Measurements (total preserved, hence shorter than actual measurements; to nearest 0.5 mm): total width 31.0, length 39.0; carapace width 31.0, length 12.0; central area of carapace width 24.0, length 12.0; sagittal length of tergites: ii 3.0, iii 2.2, iv 2.2, v 2.2, vi 2.2, vii 2.3, viii 2.0, ix 2.0, x 5.0; opisthosomal width 23.0; axial width of tergites: ii 11.0, iii 11.0, iv 11.0, v 11.0, vi 10.5 vii 9.5, viii 8.0, ix 6.5, x 5.0 mm. Maximum width from axis to right lateral most preserved part =14.0, hence total width of opisthosoma ∼28.0. The width of the prosoma would be wider than that of the opisthosoma.
Remarks. The new specimen most closely resembles NHMUK PI In 60018 (Howard Reference Howard2024, text-fig. 5B, pl. 3, fig. 1) in consisting of an attached prosoma and opisthosoma lacking a telson and parts of the lateral edges of the opisthosoma. The central carapace region appears rather featureless compared with other specimens (e.g., Howard Reference Howard2024, pl. 2). In this specimen, the part (NHMUK PI In 60018) lacks spines on the posterior margin of the carapace, but they are present on its counterpart BGS GSM 89612 (Howard Reference Howard2024, text-fig. 5C, pl. 3, fig. 3).
3.3. Discussion
The specimen described here clearly belongs to the species Limuloides limuloides (Woodward, Reference Woodward1865), on account of its possession of the diagnostic features of the genus and species. It shows no additional morphological features not seen already in other specimens, but is one of only a few in which the articulated prosoma and opisthosoma are preserved. The relatively strong relief on the specimen suggests it is close to its shape in life. It comes from a locality which has not previously yielded Chelicerata, of Wenlock (Homerian) age in the Welsh Borderland. Previously, only two specimens of this species have been recorded from the Wenlock Series, one from the Coalbrookdale Formation of Dudley and another from the same strata at Ledbury (Howard Reference Howard2024). These other two Wenlock specimens are from localities which lack both geographical and stratigraphic precision (Howard Reference Howard2024), so the new specimen places L. limuloides precisely in the Homerian. Also, the find extends the geographical range of the species to the northwest of previous occurrences in the Welsh Borders.
The Aston Mudstone Formation is a bioturbated calcareous mudstone, pale to medium grey in colour, decalcifying on weathering to buff and olive-green colours. The fauna consists of pervasive burrows and benthic and pelagic faunas including brachiopods of a comparatively deep-water community, trilobites and graptolites, including the retiolilid Gothograptus (Cave & Hains Reference Cave and Hains2001). Thus, the Limuloides specimen occurred within a relatively deep-water facies, offshore from the contemporaneous Wenlock Limestone of Wenlock Edge to the southeast, and beyond the effects of turbidite flows. The environment was a quiet, distal shelf, with associated fauna suggestive of the Visbyella Community of Hancock et al. (Reference Hancock, Hurst and Fürsich1974).
Acknowledgements
I thank Bob Kennedy for donating this specimen for research purposes, the late, and greatly missed, Richard Fortey for passing it on to me for study, Richard Howard (Natural History Museum, London) for accessioning the specimen to the collections and both Richard Howard and Jason Dunlop (Naturkundemuseum, Berlin) for their constructive comments on the manuscript.
Competing interests
The author declares no competing interests.
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