1. Introduction
Xiphosurans are an aquatic group of chelicerate arthropods defined by the possible autapomorphies of ophthalmic ridges and an axial region of the opisthosoma (Dunlop & Selden, Reference Dunlop, Selden, Fortey and Thomas1997). Only four species exist today and are all members of the order Xiphosurida, further united by their large crescentic prosomal shield and the fusion of the opisthosomal tergites into a thoracetron. The xiphosurid fossil record extends back as far as the Ordovician (Rudkin, Young & Nowlan, Reference Rudkin, Young and Nowlan2008; Van Roy et al. Reference Van Roy, Orr, Botting, Muir, Vinther, Lefebvre, El Hariri and Briggs2010), while that of their supposed stem lineage ranges from the Ordovician to the Carboniferous (Moore, McKenzie & Lieberman, Reference Moore, McKenzie and Lieberman2007; Van Roy et al. Reference Van Roy, Orr, Botting, Muir, Vinther, Lefebvre, El Hariri and Briggs2010). This stem lineage has, in the past, been considered a monophyletic clade with subordinal status (Eldredge, Reference Eldredge1974), but is now considered a paraphyletic grade informally termed ‘synziphosurines’ (Anderson & Selden, Reference Anderson and Selden1997). It consists of xiphosurans with freely articulating opisthosomal segments. The analysis of Anderson & Selden (Reference Anderson and Selden1997) retrieved the Australian Late Devonian synziphosurine Kasibelinurus Pickett, Reference Pickett1993 as the sister-taxon to Xiphosurida, united with the order by the reduction of the number of opisthosomal segments to nine (but see Section 5 below).
Here we report the discovery of a new species of Kasibelinurus from the Lochkovian Xiaxishancun Formation of Yunnan, China. This is only the second record of a horseshoe crab from China, the other being Yunnanolimulus luopingensis Zhang, Hu, Zhou, Lv & Bai, Reference Zhang, Hu, Zhou, Lv and Bai2009, a xiphosurid from the Triassic of Luoping, also in Yunnan Province (Zhang et al. Reference Zhang, Hu, Zhou, Lv and Bai2009). The discovery of the Chinese Kasibelinurus not only extends the age of the genus back some 50 million years but also shows that the South China palaeocontinent was accessible to xiphosurans prior to the Mesozoic era. This provides a further link between South China and Australia during the Devonian. These palaeocontinents were considered to be in relatively close proximity to each other in Early Devonian time (Scotese & McKerrow, Reference Scotese, McKerrow, McKerrow and Scotese1990) and shared a number of floral components in their fossil assemblages (Hao & Gensel, Reference Hao and Gensel1998, Reference Hao, Gensel, Gensel and Edwards2001).
2. Material and methods
The specimen described herein was collected from the lower part of the Xiaxishancun Formation at a locality south of the Shangxishan Reservoir near Qujing city, Yunnan, SW China (see Fig. 1 of Xue, Reference Xue2012). It is preserved in a thin bed of greenish grey mudstone, where axes of Uncatoella verticillata Li & Cai, Reference Li and Cai1978, a marine dasycladalean alga, were also found. The specimen was immersed under water prior to photographing in order to enhance the contrast. The photograph was taken with cross-polarized illumination. The specimen is housed at the School of Earth and Space Sciences, Peking University, P. R. China.
Kasibelinurus yueya sp. nov. (a) Photograph of holotype PKU-XH500. (b) Camera lucida of PKU-XH500. Solid lines show the original shape and carapace structures of the specimen, while dashed lines indicate breaks or indistinct margins where the specimen is obscured by the matrix. Scale bars represent 1 mm.
3. Stratigraphy
The Pridoli–Lower Devonian deposits are well developed in the Qujing area (see Fig. 1 in Xue, Reference Xue2012 for locations and details of the geology). In ascending order, the Xiaxishancun Formation, the Xitun Formation, the Guijiatun Formation and the Xujiachong Formation constitute the Cuifengshan Group (Liu et al. Reference Liu, Hao, Wang and Liu2004; and see Fig. 2 in Dupret & Zhu, Reference Dupret and Zhu2008). These four formations are generally considered to be Early Devonian in age (Li & Cai, Reference Li and Cai1978; Cai et al. Reference Cai, Fang, Li, Wang, Geng, Gao, Wang, Li and Liu1994), but their exact age is still a matter of controversy (Rong & Chen, Reference Rong and Chen2000; Tian et al. Reference Tian, Zhu, Huang and Liu2011; Xue, Reference Xue2012). A Pridoli age was suggested for the Xiaxishancun Formation based on thelodont microfossil assemblages (Wang, Reference Wang1997; Rong & Chen, Reference Rong and Chen2000). The palynological study by Tian et al. (Reference Tian, Zhu, Huang and Liu2011) also suggested a Pridoli age for this formation. However, the Siluro-Devonian boundary was placed at the base of the Xiaxishancun Formation, based on other comprehensive studies of spores, conodonts (from the underlying Yulongsi Formation), ostracods, vertebrate assemblages, geochemical stratigraphy and others (Wang, Liu & Li, Reference Wang, Liu and Li1992; Fang et al. Reference Fang, Cai, Wang, Li, Wang, Geng, Wang, Gao, Wang and Li1994; Cai et al. Reference Cai, Fang, Li, Wang, Geng, Gao, Wang, Li and Liu1994; Zhao et al. Reference Zhao, Wang, Zhu, Mann, Herten and Lücke2011), and these studies indicate that the Xiaxishancun Formation is early Lochkovian in age. Regarding our new fossil, we tentatively suggest an earliest Lochkovian age, but a Pridoli age is also possible.
The Xiaxishancun Formation is mainly composed of greyish and greenish grey siltstone, mudstone and fine sandstone in lithology, and may represent the deposits of marine tidal flats (Cai & Wang, Reference Cai, Wang and Li1995). This formation yields abundant fossils, including land plants: Zosterophyllum sp. and Z. xishanense Hao, Xue, Liu & Wang, Reference Hao, Xue, Liu and Wang2007 (Hao et al. Reference Hao, Xue, Liu and Wang2007); algae: Uncatoella verticillata, Discinella cuifengshanensis Li & Cai, Reference Li and Cai1978 and Chovanella sp.; invertebrates: bivalves (Dysodonata), ostracods (leperditiids, beyrichiids and Cryptophyllus) (Wang, Liu & Li, Reference Wang, Liu and Li1992), gastropods (Platystroma and Straparollus); and fish: galeaspids (Yunnanogaleaspis, Polybranchiaspis, Dongfangaspis, Laxaspis, Diandongaspis, Nochelaspis, Damaspis and Stephaspis) and antiarchs (Yunnanolepis, Heteroyunnanolepis, Chuchinolepis and Minicrania) (Zhao & Zhu, Reference Zhao and Zhu2010).
4. Systematic palaeontology
Subphylum CHELICERATA Heymons, Reference Heymons1901
Class XIPHOSURA Latreille, Reference Latreille1802
STEM XIPHOSURIDA
Family Kasibelinuridae Pickett, Reference Pickett1993
Genus Kasibelinurus Pickett, Reference Pickett1993
Type species. Kasibelinurus amicorum Pickett, Reference Pickett1993, by original designation.
Included species. Kasibelinurus yueya sp. nov.
Stratigraphical range and distribution. Devonian (Lochkovian to Famennian) of China and Australia.
Emended diagnosis. Xiphosurans with ophthalmic ridges effaced; precardiac field defined by anterior prolongations of the cardiac furrows; opisthosomal pleurae with distinct inner and outer zones; pretelson elongate (emended from Pickett, Reference Pickett1993).
Remarks. Pickett (Reference Pickett1993) suggested that ‘Bellinurus’ alleghenyensis Eller, Reference Eller1938 and ‘Bellinurus’ carterae Eller, Reference Eller1940 may have affinity with Kasibelinurus and may be kasibelinurids, if not directly assignable to the genus. Babcock et al. (Reference Babcock, Wegweiser, Wegweiser, Stanley and McKenzie1995) reassigned Paleolimulus randalli (Beecher, Reference Beecher1902) to Kasibelinurus without comment based on new specimens from the Chadakoin Formation of Allegany County, New York (the original material consisted solely of carapaces). This material, however, was never fully described and, while in general outline they do resemble Kasibelinurus in having a precardiac field and opisthosomal pleurae with distinct inner and outer zones, they lack the elongate pretelson and possess ophthalmic ridges that form a double arch configuration (J. C. Lamsdell, pers. obs.), a characteristic absent in other Kasibelinurus species, and so is herein excluded from the genus.
Kasibelinurus yueya sp. nov.
Material. PKU-XH500 (holotype).
Horizon and locality. Lower part of the Xiaxishancun Formation (early Lochkovian); south of the Shangxishan Reservoir near Qujing city, Yunnan, southwestern China.
Diagnosis. Kasibelinurus with pleural regions of posterior carapace margin angled posteriorly; third tergite enlarged.
Etymology. Anglicized from the Chinese 月 牙 (Yue Ya), meaning crescent moon, in reference to the crescentic outline of the carapace.
Description. Carapace semicircular, almost crescentic. 14 mm wide at broadest point, 8 mm long at centre. Posterior margin straight for 4 mm at axis, then curves posteriorly at 155° before forming genal spines at the posterolateral corners of carapace. Carapace dorsal structures only faintly preserved and the lateral eye position unclear; however, diverging striations on the anterior half of the carapace (4 mm long, 2 mm apart at origin diverging to 3 mm apart distally) may represent the forward portion (precardiac field) of an H-shaped cardiac lobe. Radiating ridge structures on the left side of the carapace axial region occur where the carapace has broken away and may represent cross-sections of prosomal appendage podomeres. Only the axial region of the opisthosoma is preserved; ten tergites and telson present. First tergite partially covered by carapace, visible length 0.3 mm; second tergite 0.5 mm, third 1 mm, fourth 0.3 mm, fifth 0.5 mm, sixth 0.5 mm, seventh 0.6 mm, eighth 0.6 mm, ninth 0.6 mm and tenth (pretelson) 2 mm. Telson lanceolate, 4.5 mm long, 1 mm wide at base.
5. Discussion
Kasibelinurus yueya sp. nov. (Fig. 1) shares a number of characteristics with Kasibelinurus amicorum that strongly attest to their congeneric nature, namely the enlarged pretelson, precardiac field resulting in an H-shaped cardiac lobe and the crescentic xiphosurid-shaped carapace. The Chinese species differs in the angle of the lateral portions of the carapace posterior relative to the opisthosoma and in the slight enlargement of the third tergite. ‘Kasibelunurus’ randalli (Beecher, Reference Beecher1902) shares with both true Kasibelinurus species the possession of the crescentic carapace and H-shaped cardiac lobe but lacks the enlarged pretelson, while it also possesses ophthalmic ridges that meet medially to form a double arch configuration, which is the plesiomorphic condition in xiphosurids (Selden & Siveter, Reference Selden and Siveter1987). Xiphosurids also possess a crescentic carapace and precardiac field, which serves to unite both Kasibelinurus species and ‘Kasibelinurus’ randalli with Xiphosurida to the exclusion of all other synziphosurines. ‘Kasibelinurus’ randalli is further linked to xiphosurids in the shared possession of medially converging ophthalmic ridges, converging ophthalmic ridges being a characteristic absent from the two Kasibelinurus species and all other synziphosurines. Kasibelinurus amicorum and Kasibelinurus yueya would therefore form the sister-clade to a group comprising ‘Kasibelinurus’ randalli and xiphosurids. A full redescription of the available material of ‘Kasibelinurus’ randalli is needed before its position relative to xiphosurids can be properly ascertained, however. The two true Kasibelinurus species meanwhile are clearly differentiated from xiphosurids by their freely articulating thoracic segments. Kasibelinurus was originally linked to xiphosurids through the supposed characteristic of having only nine opisthosomal segments (Anderson & Selden, Reference Anderson and Selden1997); however, Kasibelinurus yueya has an opisthosoma consisting of ten segments, with the anterior-most being partly obscured by the carapace posterior margin. An anterior tergite in this position can also be seen in the holotype of Kasibelinurus amicorum (Pickett, Reference Pickett1993, Fig. 3), resulting in this species also having an opisthosomal count of ten segments. This does not contradict the proposed link to xiphosurids, however, as the earliest xiphosurids have been shown to also possess ten opisthosomal segments (Rudkin, Young & Nowlan, Reference Rudkin, Young and Nowlan2008).
Little can be deduced about the palaeoecology of Kasibelinurus yueya. The palaeoenvironment of synziphosurines is almost exclusively marginal marine (Moore, McKenzie & Lieberman, Reference Moore, McKenzie and Lieberman2007), and the marine tidal flat habitat of the Xiaxishancun Formation concurs with this. Synziphosurines have been shown to leave distinctive trace fossils produced by shallow furrowing in the sediment (Babcock et al. Reference Babcock, Wegweiser, Wegweiser, Stanley and McKenzie1995). Anderson, Poschmann & Brauckmann (Reference Anderson, Poschmann and Brauckmann1998) mentioned an undescribed specimen of ‘Kasibelinurus’ randalli from the Upper Devonian of Pennsylvania that showed enrollment. The morphology of Kasibelinurus yueya is not greatly different from ‘Kasibelinurus’ randalli and basal xiphosurids, so it is likely that it, too, furrowed in the sediment for food, and could enroll when physical or biotic dangers threatened.
Kasibelinurids are now known from the Upper Devonian of Australia (palaeocontinent), the Lower Devonian of the South China Block (SCB, herein), and possibly the Upper Devonian of Pennsylvania (Babcock et al. Reference Babcock, Wegweiser, Wegweiser, Stanley and McKenzie1995). Regarding the palaeobiogeographic connection between the two confirmed members of the family, the proximity of Australia and the SCB during the Devonian has been debated (Torsvik & Cocks, Reference Torsvik, Cocks and Bassett2009). In palaeogeographic reconstructions, the SCB is usually placed off NW Australia, but whether it was attached to Gondwana or separated in the early Palaeozoic is unclear. On the most recent reconstructions (e.g. Torsvik & Cocks, Reference Torsvik, Cocks and Bassett2009; Young et al. Reference Young, Burrow, Long, Turner and Choo2010), the SCB is separated from Gondwana by ocean throughout the early Palaeozoic up to the Devonian. On the contrary, Yang et al. (Reference Yang, Sun, Yang and Pei2004) placed the two palaeocontinents in close proximity, primarily based on palaeomagnetic data. Of course, kasibelinurids are likely to have shown planktotrophic development as in modern xiphosurids, so a narrow ocean would have provided no barrier to dispersal between these landmasses. The similarity, however, of terrestrial floras between the SCB and Australia in the mid-Palaeozoic indicates that these two areas had much in common floristically (Hao & Gensel, Reference Hao and Gensel1998, Reference Hao, Gensel, Gensel and Edwards2001).
Acknowledgements
PAS thanks an Alexander von Humboldt Foundation Research Award. JZX was supported by the National Natural Science Foundation of China (No. 41272018). JCL thanks Amanda Falk (University of Kansas) for assisting with Chinese translations. The authors thank Prof. Shougang Hao (Peking University), with whom JZX collected the fossil in 2006.
