Sloveniolimulus rudkini Bicknell et al., 2019b

Bicknell, Russell D. C., Žalohar, Jure, Miklavc, Primož, Celarc, Bogomir, Križnar, Matija & Hitij, Tomaž, 2021, Revisiting horseshoe crab fossils from the Middle Triassic (Anisian) Strelovec Formation Konservat-Lagerstätte of Slovenia, Palaeontologia Electronica (a 36) 24 (3), pp. 1-15 : 4-8

publication ID	https://doi.org/ 10.26879/1168
persistent identifier	https://treatment.plazi.org/id/A57A3B45-FF8E-FFC9-537C-2D07F3C0FDEF
treatment provided by	Felipe
scientific name	Sloveniolimulus rudkini Bicknell et al., 2019b
status

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Figures 2 View FIGURE 2 , 3 View FIGURE 3

2010? Limulitella or? Psammolimulus Križnar and Hitij , p. 96

2019b Sloveniolimulus rudkini Bicknell et al. , figs. 5, 6

2019a Sloveniolimulus rudkini Bicknell et al. , Bicknell, p. 6

2020 Sloveniolimulus rudkini Bicknell et al. , Bicknell and Pates, fig. 32C

2020 Sloveniolimulus rudkini Bicknell et al. , Lamsdell, possible nomen dubium, p. 28.

2021a Sloveniolimulus rudkini Bicknell et al. , Bicknell et al., fig. 7.

2021b Sloveniolimulus rudkini Bicknell et al. , Bicknell et al., p. 1468

Holotype. PMSL T-993.

Referred material. PMSL T-2071; PMSL T-2072.

Distribution. Strelovec Formation, Kamnik-Savinja Alps, Slovenia; Middle Triassic (Anisian).

Type locality and horizon. Unit 2 of the Strelovec Formation in Section S1 of the Slatinski Plaz, Robanov Kot Valley, Kamnik-Savinja Alps, Slovenia.

Referred material locality. Western slopes of the Kalška gora Mountain, within the Kamniška Bistrica Valley. Above Unit 2 of the Strelovec Formation, no exact stratigraphic data.

Amended diagnosis. Limuloid with genal spine splay and indentations along spines. Genal spine indentations occur within last quarter of spine length.

Preservation. PMSL T-993 is preserved as a flattened, external mould in black, thin-bedded limestone. Pustule structures within and around the specimen are present, asymmetric and random. These structures reflect either the precipitation of a mineral phase in bacterial mats, or diagenetic mineral precipitation within the exoskeleton during fossilisation (Briggs and Wilby, 1996; Tetlie and Van Roy, 2006; Iniesto et al., 2016). PMSL T-2071 and PMSL T-2072 are preserved as domed internal moulds on brown-yellow, thin bedded limestones without the pustulose structures observed in PMSL T-993. The preservational mode of all specimens limits the record of delicate features, such as lateral compound eyes, ocelli, and appendages.

Description. The holotype, PMSL T-993, is an articulated prosoma, thoracetron, and partial telson ( Figure 2 View FIGURE 2 ). Specimen 57.8 mm long. Prosoma parabolic in shape, 16.1 mm long at midline and 39.5 mm wide between distal genal spine points. Prosomal rim ~1.0 mm wide. Left ophthalmic ridge present, albeit surrounded by pustules, has slight concavity, 7.7 mm long. Section of prosomal doublure noted. Cone shaped cardiac lobe observed under low angle light, 6.5 mm wide posteriorly, tapering to 3.4 mm. Cardiac lobe medial ridge 6 mm long. Genal spines splay slightly, extend posterolaterally, and terminate within first third of thoracetron length. Both spines have an indentation 4 mm from spine termini. Left genal spine terminus 20.2 mm from midline. Angle between left genal spine and left side of thoracetron 62°. Right genal spine 19.5 mm from midline. Angle between right genal spine and right side of thoracetron 64°. Prosomal-thoracetron hinge pronounced, 20.8 mm wide, 1.5 mm long. Thoracetron trapezoidal, 15.8 mm long, 20 mm wide anteriorly, tapering to 2 mm posteriorly. Thoracetronic flange defined mostly by pustules. Thoracetronic doublure evident along margins and thoracetronic rim ~ 1.5 mm wide. Medial thoracetronic lobe rectangular, 17.1 mm long, 4.5 mm wide, lacks ridge. Posterior margin of thoracetronic doublure imprinted over medial thoracetronic lobe. Pleural lobes lack relief and segmentation. Left pleural lobe 5.7 mm wide anteriorly, tapering to 2.3 mm. Right pleural lobe 7.6 mm wide anteriorly, tapering to 2.2 mm. Moveable spine notches on right side. Thoracetron-telson embayment weakly concave. Telson 23.2 mm long, anterior section of telson keel present.

PMSL T-2071 is an articulated prosoma and partial thoracetron ( Figure 3 View FIGURE 3 A-B, E). Specimen 23.6 mm long. Prosoma parabolic in shape, 16.1 mm long at midline, 35.0 mm wide between left genal spine point and rock edge on right side. Prosomal doublure prominent, a possible ventral perspective. Prosomal rim ~1.0 mm wide. Left genal spine extends posterolaterally, terminates within first third of thoracetron length. Right genal spine absent. Left spine has an indentation 6 mm from spine terminus. Left genal spine terminus 18.8 mm from midline. Possible occipital lobe section extends along spine. Angle between left genal spine and left side of thoracetron 44.7°. A white to light brown section on right side of prosoma noted ( Figure 3E View FIGURE 3 ). Within this region, minute, ~30 µm wide spheres observed. Thoracetron broken at rock edge, measurements are therefore not indicative of true size. Thoracetron trapezoidal, 7.5 mm long, 24.7 mm wide anteriorly. Thoracetronic rim 0.8 mm wide.

PMSL T-2072 is an articulated prosoma and partial thoracetron ( Figure 3 View FIGURE 3 C-D). Specimen 32.5 mm long. Prosoma parabolic in shape, 19.2 mm long at midline, and 33.9 mm wide between genal spine points. Prosomal rim 0.9 mm wide. Coneshaped cardiac lobe observed under low angle light, 6.3 mm wide posteriorly, tapering to 2.8 mm. Genal spines extend posterolaterally and terminate at halfway along thoracetron. Left spine has indentation 7.4 mm from spine terminus. Spine terminus 20.2 mm from midline. Angle between left genal spine and left side of thoracetron 54.2°. Right genal spine stunted; likely damaged during life and a possible record of predation. Posteriormost section of right genal spine 16.7 mm from midline. Angle between right genal spine and right side of thoracetron 56.1°. Thoracetron trapezoidal, 13.3 mm long, 20.8 mm wide anteriorly, terminating at rock edge. Thoracetron flange evident along right margin.

Remarks. Re-examining Sloveniolimulus rudkini , we agree with Lamsdell’s observation that there are few diagnostic features present on these fossils. However, indentations along S. rudkini genal spines are a cardinal morphological feature that differentiates this material from other Triassic xiphosurid taxa. Such genal spine indentations (or kinks) are known from other taxa (e.g., Dubbolimulus peetae Pickett, 1984 , Franconiolimulus pochankei Bicknell , et al. 2021b, Panduralimulus babcocki Allen and Feldmann, 2005 , Tasmaniolimulus patersoni Bicknell, 2019 ; Figure 4 View FIGURE 4 ). The presence of this feature in all specimens considered here, as well as multiple other taxa, illustrates that it is not a taphonomic artefact (sensu Lamsdell, 2020). Furthermore, genal spine morphologies are a useful feature for determining genera (see Bicknell, 2019; Bicknell et al., 2021b; Figure 4 View FIGURE 4 ). The presence of this morphology, coupled with the genal spine splay and position of the ophthalmic ridge (when compared to T. patersoni and F. pochankei , the most morphologically similar taxa) are sufficient evidence for maintaining S. rudkini .

The new material presents an interesting taxonomic puzzle: PMSL T-2072 has genal spines terminating circa halfway along the thoracetron. This hypertrophy of genal aligns the Strelovec Formation material with Austrolimulidae , rather than Limulidae (contra Bicknell et al., 2019b, 2021a). Indeed, many Permo-Triassic austrolimulids have overdeveloped genal spines (Riek, 1955; Meischner, 1962; Allen and Feldmann, 2005; Lerner et al., 2017; Bicknell, 2019). However, austrolimulids are typically considered marginal- to non-marine organisms (Lamsdell, 2016; Bicknell and Pates, 2020) and, despite limited plant remains, the Strelovec Formation represents a marine palaeoenvironment (Miklavc et al., 2016); an observation that contradicting an austrolimulid assignment. Nonetheless, given this uncertainty, we currently place Sloveniolimulus rudkini within the Superfamily Limuloidea , but not within a family.

Two other possible explanations for the morphological variants should be discussed. The first is that there are two horseshoe crab groups preserved within the Strelovec Formation, and the second is that the hypertrophied genal spines reflect sexual dimorphism. The first alternative is plausible, especially as multiple taxa from different families have been documented in the same deposit (see Raymond, 1944; Bicknell et al., in press). However, to avoid the potential over-splitting of limited material, we presently do not support this thesis. The concept of sexual dimorphism is intriguing, especially as reduced moveable spines in females of extant taxa are known—consider female Tachypleus tridentatus (Leach, 1819) (Shuster Jr., 1982; Botton et al., 1996). However, hypertrophied genal spines have not previously been considered sexually selected characteristics. Furthermore, suggesting genal spine hypotrophy reflected such selection would require many assumptions regarding structures that can be sexually selected and is not an overly parsimonious explanation. As such, at this point, we do not support this second idea. In sum, the Strelovec Formation xiphosurids presented here likely represent the morphological extremes of the same taxon.

In re-examining the material, we identified three points that differ from the original Bicknell et al. (2019b) description. We illustrate that (1) Sloveniolimulus rudkini ophthalmic ridges are concave, not straight, (2) the supposed operculum observed by Bicknell et al. (2019b) likely reflects a thoracetronic doublure, and (3) the telson of PMSL T-993 is slightly longer than previously documented.

The spheres in PMSL T-2071 ( Figure 3E View FIGURE 3 ) resemble phosphatised coccoid microbes observed on the Late Jurassic-aged Nusplingen Mesolimulus walchi (Briggs et al., 2005) . However, the spheres in PMSL T-2071 are approximately three times larger than those documented by Briggs et al. (2005). Despite this size, they fall well within the size limits of microbes (sensu Levin and Angert, 2015), indicating that the spheres are indeed evidence of microbial activity.