A revision of Prolimulus woodwardi Fritsch, 1899 with comparison to other highly paedomorphic belinurids

Introduction

Xiphosurida, are an extant group of euchelicerates with an extensive fossil record spanning most of the Phanerozoic (Van Roy, Briggs & Gaines, 2015). They are often referred to as “living fossils” (Størmer, 1952), considered examples of stabilomorphism (Kin & Błażejowski, 2014) and morphological conservatism (Bicknell & Pates, 2020). Such statements are mostly applicable to the late Mesozoic and Cenozoic forms (Avise, Nelson & Sugita, 1994; Rudkin & Young, 2009; Kin & Błażejowski, 2014; Lamsdell & McKenzie, 2015; Błazejowski, Gieszcz & Tyborowski, 2016; Bicknell & Pates, 2019; Bicknell et al., 2019b; Bicknell & Pates, 2019). Conversely, most Paleozoic and early Mesozoic forms record evolutionary exploration (Bicknell, 2019; Bicknell, Amati & Hernández, 2019; Bicknell et al., 2019a; Bicknell, Naugolnykh & Brougham, 2020; Bicknell et al., in press; Bicknell, Hecker & Heyng , 2021; Bicknell & Pates, 2020). The evolutionary history of these earlier species illustrate morphological plasticity and exploration of different ecological niches (Lamsdell, 2016; Lamsdell, 2020a; Bicknell et al., 2019b). Belinurina—a clade containing Belinuridae—is a particularly diverse group known from the Carboniferous and Permian that successfully colonized freshwater environments. Belinurids have been considered at length (Størmer, 1952; Eldredge, 1974; Anderson & Selden, 1997; Haug et al., 2012; Haug & Haug, 2020) and the presence of hypertrophied genal spines or complete loss of genal spines characterizes the group. Furthermore, phylogenetic analyses illustrated that Belinurina was a monophyletic superfamily traditionally thought to contain Alanops (Racheboeuf, Vannier & Anderson, 2002), Anacontium (Raymond, 1944), Belinurus (Pictet, 1846), Euproops (Meek & Worthen, 1865), Liomesaspis (Raymond, 1944), and Prolimulus (Fritsch, 1899). Despite the interest in belinurids, an array of species described in the early 20th century require revision (see Lamsdell & McKenzie, 2015; Bicknell et al., 2019a: Bicknell, Lustri & Brougham, 2019; Bicknell & Pates, 2020; Bicknell & Smith, in press for revisions of similar historical material). Expanding on the recent pulse in the revision of such historically important species, we reevaluate Prolimulus woodwardi Fritsch, 1899. We present a phylogenetic analysis including P. woodwardi and the morphologically comparable Stilpnocephalus pontebbanus Selden, Simonetto & Marsiglio, 2019, as well as a geometric morphometric analysis of species within Belinurina. These analyses highlight the extreme morphologies exhibited by Prolimulus and its kin, suggesting the requirement for a clade to contain these species.

Geologic and Stratigraphic Context

Upper Paleozoic continental strata in central and western Bohemia are formally subdivided into the Plzeň, Manětín, Žihle, Radnice, Kladno-Rakovník, and Mšeno-Roudnice basins (Pešek, 1994; Opluštil et al., 2013; Pešek, Sivek & Sivek, 2016; Fig. 1A). Sedimentary successions within these basins comprise of four formations—the Kladno Formation (composed of older Radnice and younger Nýřany members), that unconformably overlies the basement rocks; followed by the Týnec Formation, Slaný Formation, and terminated by the Líně Formation (Pešek, 1994; Opluštil et al., 2013; Fig. 1B).

The Nýřany Member of the Kladno Formation is composed of cyclically arranged, predominantly coarse- and medium-grained sediments of fluvial origin (Pešek, 1994; Opluštil, Martínek & Tasáryová, 2005). Fine-grained sediments of floodplain, palustrine, and lacustrine origin are also present (Opluštil, Martínek & Tasáryová, 2005). These cycles are usually terminated by thin coal seams (Pešek, 1994; Opluštil, Martínek & Tasáryová, 2005). From a palaeoenvironmental perspective, the Nýřany Member was deposited in a large alluvial plain with a braided river system with locally developed lakes, wetlands, and peat swamps (Opluštil, Martínek & Tasáryová, 2005), located in a nearly equatorial latitude (Krs, Krsová & Pruner, 1995). Recent U-Pb dating estimated that the Nýřany Member is between 308.3–305.9 ± 0.1 Ma, spanning the late Moscovian to early Kasimovian (Opluštil et al., 2016; Fig. 1B).

In the Plzeň Basin, the lower parts of the Nýřany Member contain the locally developed Main Nýřany Coal with intercalated beds of lacustrine sapropelic coal (Fritsch, 1883; Purkyně, 1899; Pešek, 1994; Štamberg & Zajíc, 2008). This sapropelic coal yielded diverse and exceptionally well-preserved fauna. Most of sapropelic coal fossils originated from the Humboldt Mine in Nýřany (near Plzeň, Fig. 1A) and represent various euarthropods (including Prolimulus woodwardi), acanthodians, chondrichthyans, dipnoans, actinopterygians, and early diverging tetrapods (Fritsch, 1883; Fritsch, 1902; Štamberg & Zajíc, 2008). After closure of the Nýřany and Třemošná coalfields, the sapropelic coal was unavailable for sampling, until a recent excavation (Bures & Tichavek, 2012).

Material and Methods

Geometric morphometric methods

Following Bicknell et al. (2019), a morphometric dataset of landmarks and semilandmarks from 91 specimens across 19 species was collected to explore Belinurina morphospace. Landmarking and semilandmarking was conducted using the Thin-Plate Spline (TPS) suite (http://life.bio.sunysb.edu/morph/index.html). The TPS file was constructed using tpsUtil64 (v.1.7). The TPS file was imported into tpsDig2 (v.2.26), which was used to place four landmarks across the prosoma and thoracetron and 40 semi-landmarks along the right prosomal shield (Fig. 2; Table 1). Semilandmarks were placed in a clockwise direction along the most anterior section of the prosomal shield, coinciding with the first landmark, ending at the third landmark: the most lateral prosomal-thoracetronic articulation point. Points were digitised as xy coordinates. When the right side was poorly preserved, the left side was used, and these data were mirrored. These data populated the TPS file (Supplementary Information 2). TPS file was imported into R. The ‘geomorph’ package (Adams & Otárola-Castillo, 2013) was used to conduct a Procrustes Superimposition and Principal Components Analysis (PCA) of the data (Data S3). Only the first two Principal Components (PCs) were considered as they explained 87% of the variation in the data. The examined species were representatives of Alanops, Belinurus, Euproops, Liomesaspis, and Prolimulus. We were unable to include Anacontium and Stilpnocephalus as opisthosomal sections are not known from these genera. We had initially used generic assignment of Bicknell & Pates (2020) for this analysis. However, during the course of peer review, Lamsdell (2020b) presented a revision of Xiphosurida and proposed that Belinurina consisted of 14 genera. To compare, contrast, and explore the distribution of these newly erected groups with the more conservative perspective of Bicknell & Pates (2020), we presented the distribution of genera suggested in Bicknell & Pates (2020) and Lamsdell (2020b).

Table 1:

Description of landmarks.

Landmarks used for the geometric morphometric analysis depicted in Fig. 2.

Landmark number	Description of landmark
Landmark 1	Anterior-most prosomal point along organismal sagittal line
Landmark 2	Distal-most prosomal point along organismal sagittal line
Landmark 3	Lateral-most section of prosomal-thoracetron articulation
Landmark 4	Thoracetron-telson articulation

DOI: 10.7717/peerj.10980/table-1

Systematic Palaeontology

Amended diagnosis: Belinurid with a round prosoma that is slightly wider than long. No eyes, cardiac lobe, or ophthalmic ridges are present. Thoracetron is completely fused, without traces of segmentation, often showing a thoracetronic doublure. Thoracetron-telson articulation is ‘U’-shaped. Telson is keeled.

Holotype: NM M Me 1031; NM M Me 1032

Syntype: NHMUK PI In 18588

Referred material: MB.A 1989; MCZ 109537; NM Me 39; NM Me 108; NM Me 109; NM Me 138; NM Me 139; NM Me 140; NM Me 141; NM Me 142; NM Me 143; NM Me 144; NM Me 145; NM Me 146; NM M 1038; NM M 1045; NHMUK PI I 3395.

Locality, horizon, and age: Nýřany (Humboldt Mine, active between 1865 –1902), Plzeň Basin; Main Nýřany Coal, Nýřany Member of the Kladno Formation; ∼307–308 Ma, late Moscovian, Pennsylvanian.

Descriptions: NM M 1031 and NM M 1032 (Fig. 3) are part and counterpart originally described and figured by Fritsch (1899), Fritsch (1902) and revised in Prantl & Pribyl (1955). They consist of an articulated prosoma, thoracetron, and telson, preserved as flattened impressions in ventral view. Prosoma is round, slightly wider than long: 20 mm wide and 11.9 mm long. The prosomal doublure is preserved and has a maximum width of 1.9 mm. No genal spines, lateral compound eyes, cardiac lobe, or ophthalmic ridges noted. Possible traces of a labium are preserved in NM Me 1031 (Figs. 3A, 3C). Appendages are preserved and mainly visible on NM Me 1032. Chelicera are present, but chelate podomeres are not preserved (Figs. 3B, 3D). Proximal sections of walking legs are preserved as slight imprints. Furthermore, podomeres of the sixth appendages are noted (Fig. 3). Third and fourth podomeres of two disarticulated appendages preserved outside prosoma. Thoracetron lacks tergal expression, is round and slightly smaller than prosoma: 18.5 mm wide and 10 mm long. No lateral spines noted. A prominent margin—likely thoracetronic doublure—preserved, is wide 2.1 mm (Fig. 3). Poorly preserved opercula are present on posterocentral thoracetron (Figs. 3A, 3C). Telson articulates with thoracetron along on posterior thoracetron margin. Telson 4.7 mm long and fragmentally preserved. Four Spiroglyphus vorax Fritsch, 1895 specimens are attached to thoracetron.

NM Me 146 (Fig. 4A): Articulated prosoma, thoracetron and partial telson, preserved as flattened impression in ventral view. Prosoma round, slightly wider than long: 11.5 mm wide and 8.5 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, appendages, or ophthalmic ridges noted. Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: 10.9 mm wide and 7 mm long. No lateral spines are noted. Thoracetron-telson articulation unclear, but occurs on posterior thoracetron margin. Telson partly preserved and 8.2 mm long. Two specimens of Spiroglyphus vorax attached to prosoma, three to prosoma-thoracetron border, and ten to thoracetron.

NM Me 39 (Fig. 4B): Articulated prosoma and thoracetron, preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 10.5 mm wide and 7.5 mm long with a pronounced prosomal doublure. No genal spines, lateral compound eyes, cardiac lobe, appendages, or ophthalmic ridges noted. Thoracetron lacks tergal expression, is round, and slightly smaller than the prosoma; 9.3 mm wide and 5.6 mm long. No lateral spines noted.

NM Me 142 (Fig. 4C): Articulated prosoma, thoracetron, and partial telson, preserved as a mostly flattened impression in ventral view. Limited relief noted in posterior prosoma, prosoma-thoracetron articulation, and anterior thoracetron. Prosoma round, slightly wider than long: 12.6 mm wide and 6.8 mm long, with a pronounced prosomal doublure. No genal spines, lateral compound eyes, cardiac lobe, or ophthalmic ridges noted. Chelicera, the left set of chelate podomeres, and the proximal sections of walking legs preserved. Thoracetron lacks tergal expression, is round, and slightly larger than prosoma: 11.5 mm wide and 5.9 mm long. Pronounced thoracetronic doublure noted, 1.7 mm wide. No lateral spines noted. Thoracetron-telson articulation unclear, but occurs on posterior thoracetron margin. Telson partly preserved, 6.7 mm long.

NM Me 145 (Fig. 4D): Articulated prosoma, thoracetron, and telson, preserved as flattened impression in ventral view. Prosoma round, slightly wider than long: 13.3 mm wide and 8 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, or ophthalmic ridges noted. No appendages are preserved. Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: 11.8 mm wide and 8.2 mm long. No lateral spines noted. Telson articulates with posterior thoracetron margin. Telson completely preserved and 15 mm long. Five Spiroglyphus vorax specimens attached to thoracetron.

NM M 1038 (Fig. 5): Articulated prosoma, thoracetron, and partial telson, preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 19 mm wide and 10.8 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, or ophthalmic ridges noted. One disarticulated appendage preserved on left prosomal side (Fig. 5B). Thoracetron lacks tergal expression, is round, and slightly smaller than the prosoma: 17.5 mm wide and 10.3 mm long. No lateral spines noted. Thoracetron-telson articulation unclear, but occurs on posterior thoracetron margin. Telson partly preserved and 17.1 mm long. One Spiroglyphus vorax specimen attached to thoracetron.

NM M 1045 (Fig. 6): Articulated prosoma, thoracetron, and telson, preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 11.6 mm wide and 7.5 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, or ophthalmic ridges noted. Chelicera are only preserved appendages (Fig. 6B). Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: 10.5 mm wide and 7.5 mm long. No lateral spines noted. Thoracetron-telson articulation is a ‘U’-shaped indentation in posterior thoracetron margin (Fig. 6C). Telson completely preserved, 13 mm long, and keeled. Four Spiroglyphus vorax specimens attached to thoracetron and all show growth lines (Fig. 6C).

NM Me 108 (Fig. 7): Twelve individuals preserved on a slab. Mean prosomal size is 11.5 mm wide and 8.4 mm long, mean thoracetron size is 10.5 mm wide and 7.4 mm long. Two individuals (F and I) are articulated. In both cases, prosomal sections rotated relative to thoracetron. Both preserve prosomal and thoracetronic doublures. No genal spines, lateral compound eyes, cardiac lobe, or ophthalmic ridges noted for either specimen. Thoracetrons lack tergal expression and lateral spines. Individual I preserved telson insertion (Fig. 7). Four individuals (C, D, E, and L) are thoracetron and telson fragments. Five individuals (A, G, H, J, and K) likely represent disarticulated prosomal sections. Individual G shows a possible appendage pair on anterior prosomal edge. Individual B is fragmentary and may represent a disarticulated prosoma and thoracetron, or parts of different individuals. Cluster lacks any orientation and evidence of epibionts.

NM Me 141 (Fig. 8A): Articulated prosoma and thoracetron, poorly preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 19.4 mm wide and 11 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, appendages, or ophthalmic ridges noted. Only right side of thoracetron preserved. Thoracetron lacks tergal expression and lateral spines. Five Spiroglyphus vorax specimens attached to thoracetron.

NM Me 109 (Fig. 8B): Articulated prosoma, thoracetron, and partial telson, preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 10.3 mm wide and 5.9 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, appendages, or ophthalmic ridges noted. Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: 9.3 mm wide and 5 mm long. No lateral spines noted. Thoracetron-telson articulation unclear, but occurs at posterior thoracetron margin. Telson partly preserved, 11 mm long. Two Spiroglyphus vorax specimens attached to thoracetron.

NM Me 139 (Fig. 8C): Articulated prosoma, thoracetron, and partial telson, poorly preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 20 mm wide and 15 mm long, and preserves pronounced prosomal doublure. No genal spines, lateral compound eyes, a cardiac lobe, appendages, or ophthalmic ridges noted. Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: 17 mm wide and 12 mm long. No lateral spines noted. Thoracetron-telson articulation unclear, but occurs at posterior thoracetron margin. Telson fragmentary and 4.3 mm long. Three Spiroglyphus vorax specimens attached to thoracetron.

NM Me 143 (Fig. 8D): Articulated prosoma and thoracetron, preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 10 mm wide and 6.7 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, or ophthalmic ridges noted. Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: and 9.6 mm wide and 6.4 mm long. No lateral spines noted. Four Spiroglyphus vorax specimens attached to thoracetron (Fig. 8D).

NM Me 140 (Fig. 9): Articulated prosoma, thoracetron, and telson, preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 14.3 mm wide and nine mm long. Prosomal doublure present. No genal spines, lateral compound eyes, cardiac lobe, or ophthalmic ridges noted. Two disarticulated prosomal appendages are preserved outside left prosomal side. Anterior appendage consists of at least a trochanter, femur, and patella, while posterior appendage possesses possible apotele and pretarsus (Fig. 9B). Two appendage sets preserved exclusively within prosomal shield. The fifth appendage on right side preserves fully articulated femoral, patellar, and trochanteral sections (Fig. 9B). Coxal sections of the sixth appendage pair also noted (Fig. 9B). Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: 11.3 mm wide and 6.5 mm long. No lateral spines noted. Thoracetron-telson articulation unclear, but occurs at posterior thoracetron margin. Telson fragmentary, 9.6 mm long.

NM Me 144 (Fig. 10A): Articulated prosoma and thoracetron, poorly preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 8.8 mm wide and 6.5 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, ophthalmic ridges, or appendages noted. Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: 7.7 mm wide and 7.5 mm long. No lateral spines noted.

NM Me 138 (Fig. 10B): Articulated prosoma and thoracetron, poorly preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 10.4 mm wide and 6.9 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, ophthalmic ridges, or appendages noted. Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: 8.8 mm wide and 6.7 mm long. No lateral spines noted.

NHMUK PI In 18588; syntype (Fig. 11A): Articulated prosoma, thoracetron and telson, preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 14 mm wide and 7 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, ophthalmic ridges, or appendages noted. Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: 13.2 mm wide and 6.7 mm long. No lateral spines noted. Thoracetron-telson articulation unclear, but occurs at posterior thoracetron margin. Telson fragmentary, 12.5 mm long.

NHMUK PI I 3395 (Fig. 11B): Articulated prosoma, thoracetron and telson, preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 15.1 mm wide and 10 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, ophthalmic ridges, or appendages noted. Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: 13.1 mm wide and eight mm long. No lateral spines are. Thoracetron-telson articulation unclear, but occurs at posterior thoracetron margin. Telson fragmentary, 9.5 mm long.

MCZ 109537 (Fig. 11C): Articulated prosoma, thoracetron and partial telson, preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 10.8 mm wide and 8 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, or ophthalmic ridges noted. Proximal sections of prosomal appendages are preserved outside prosoma. Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: 10 mm wide and 7 mm long. No lateral spines noted. Thoracetron-telson articulation unclear, but occurs at posterior thoracetron margin. Telson fragmentary, only articulation point preserved.

MBA. 1989 (Fig. 11D) Articulated prosoma, thoracetron and telson, preserved as a flattened impression in ventral view. Prosoma round, slightly wider than long: 13.9 mm wide and 9.9 mm long. No prosomal doublure, genal spines, lateral compound eyes, cardiac lobe, or ophthalmic ridges noted. A disarticulated appendage preserved on left side of prosoma. Thoracetron lacks tergal expression, is round, and slightly smaller than prosoma: 11.7 mm wide and 7.9 mm long. No lateral spines noted. Thoracetron-telson articulation unclear, but occurs at posterior thoracetron margin. Telson fragmentary, 6.5 mm long.

Remarks: The prosoma and thoracetron shape of Prolimulus woodwardi is morphologically comparable to other belinurids without genal spines (Figs. 12 and 13). However, P. woodwardi has a unique telson insertion morphology. In P. woodwardi the insertion is a ‘U’-shaped indentation in the thoracetron, while Alanops and Liomesaspis lack this feature (Fig. 14). Furthermore, Alanops and Liomesaspis (Fig. 13) possess a ‘thoracetronic boss’, a bulge present on thoracetron over the insert of the telson. In P. woodwardi there is no evidence of this morphology. Finally, Stilpnocephalus has two notable grooves along the prosoma, not observed in P. woodwardi.

Results

Discussion

Evolutionary framework of Prolimulus and kin

Belinurids represent the most successful Carboniferous and Permian xiphosurid group that explored freshwater niches (Lamsdell, 2016; Shpinev & Vasilenko, 2018; Shpinev, 2018; Bicknell, Pates & Botton, 2019). The group also has an exceptional diversity and disparity, which is unusual when compared to the Late Mesozoic and Cenozoic forms (Bicknell, 2019; Bicknell et al., 2019a; Bicknell et al., in press-a). Several attempts to colonize freshwater environments likely drove the Belinurina to evolve features that contrast the ‘typical’ xiphosurid morphology, and added to their extreme diversity and disparity (Lamsdell, 2016; Lamsdell, 2020a). Furthermore, freshwater environments can only sustain small populations (Wang et al., 2019) compared to marine conditions and are susceptible to isolating small populations (see DeWoody & Avise, 2000). Allopatric speciation clearly played a central role in the belinurid radiation (Lamsdell, 2016; Lamsdell, 2020a) and permitted innovative characters to be fixed within newly established populations.

The importance of heterochrony during xiphosurid evolution has recently been considered by coding such characters in a phylogenetic framework (Lamsdell, 2020a). This work demonstrated (among other points) that belinurid evolution generally reflects paedomorphosis. While Lamsdell (2020a) did not assess Prolimulus, paedomorphic evolution no doubt drove the development of species that are located in the same tree space in Fig. 15. The reduced body size, short or vestigial genal spines, rounded thoracetron, and absence of pre-telson epimera (terminal thoracetronic spines) are considered paedomorphic characters in Belinurina (Lamsdell, 2020a). The reduction or absence of genal spines and a prosoma:thoracetron ratio of ∼1:1 observed in Prolimulus, Alanops, and Liomesaspis are also comparable to Limulus polyphemus (Linnaeus, 1758) trilobite stages (Fritsch, 1899; Prantl & Pribyl, 1955; Haug & Rötzer, 2018a). However, the fossil genera display fully developed telson spines, unknown to early postembryonic L. polyphemus stages (Haug & Rötzer, 2018a). As such, the lack of genal spines and prosoma:thoracetron ratio are likely phylogenetically significant anatomical similarities, and not aspects of ontogeny. This hypothesis is supported by the Racheboeuf, Vannier & Anderson (2002) dataset that illustrated that the main ontogenetic modification in A. magnificus is increased size. The presence of juvenile characters in adult individuals of Prolimulus and its kin therefore represent a heterochronic event (Gould, 1977; Klingenberg, 1998). Given this unique combination of characteristics, one might consider erecting a clade to house these notably paedomorphic species. Indeed, Raymond (1944) had erected Liomesaspidae to contain these forms; however, this group is not used anymore. Furthermore, given the convoluted relationships between members of Belinurina, it seems unwise to re-introduce terminology. When phylogenetic and taxonomic relationships within the Belinurina are organized, it may then be pertinent to re-erect a higher order group.

Lamsdell (2020b, p. 17) suggested that Prolimulus “strong[s] affinity to Alanops and Pringlia, and there could be an argument for synonymizing Prolimulus with one of these genera”. We disagree with this suggestion based on our observations here. The morphology of the Prolimulus thoracetron-telson articulation differs from Alanops and Pringlia (here considered synonymous with Liomesaspis, following the more conservative Anderson & Selden, 1997, and amount of overlap in morphospace; Figs. 13, 17). Furthermore, a ‘thoracetronic boss’ is not observed in Prolimulus and the telson likely inserted in the thoracetronic doublure, through a telson notch (Figs. 6, 12 and 14). Regardless, if any synonymy were valid, Alanops or Liomesaspis would be synonymized with Prolimulus (not vice versa) as the Czech material has taxonomic priority.

Conclusion

Revision of Prolimulus woodwardi, coupled with phylogenetic and geometric morphometric analyses of belinurids, highlighted a diverse clade within Belinurina. These species without genal spines all share highly accentuated paedomorphic characters, such as vestigial genal spines, and are representative of paedomorphic evolution. A slab of multiple P. woodwardi individuals demonstrates new evidence for Carboniferous horseshoe crab ecology, revealing possible gregarious behavior, and further data on the deep origin xiphosurid clustering. Taken together, the examination presented here demonstrates the morphological variation and ecological conditions that permitted successful colonization of freshwater environments by Carboniferous horseshoe crabs.

Supplemental Information