Ophiosuperstes praeparvus Thuy & Maxwell & Pruss 2021, gen. et sp. nov.
Creators: name: Thuy, Ben
name: Maxwell, Vivienne
name: Pruss, Sara B.
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Animalia
Echinodermata
Ophiuroidea
Ophiosuperstes
Ophiosuperstes praeparvus

description: Ophiosuperstes praeparvus gen. et sp. nov.
Figs. 4–5
Etymology. praeparvus is Latin for very small or minute, as these microfossils were extracted from a small shelly fossil-style assemblage.
Holotype. MnhnL OPH177
Paratypes. MnhnL OPH178 – 190
Type locality. Lost Cabin Springs locality, southern Nevada (Maxwell et al. 2020).
Type stratum. Bed LC–18–34 within the Virgin Limestone Member of the Moenkopi Formation, Spathian, lower Triassic.
Diagnosis. Ophintegrid ophiuroid with stout lateral arm plates of rounded outline, very weak oblique elongated spur on ventro-proximal tip of outer surface, otherwise devoid of spurs or conspicuous outer surface ornamentation; with up to six small, vertical spine articulations slightly sunken into distal edge of lateral arm plate, composed of arched, shifted dorsal and ventral lobes merged at their proximal tips and encompassing the muscle and nerve openings; no sigmoidal fold; tentacle openings developed as small notches lined by a shallow groove in proximal to median lateral arm plates, and reduced to a within-plate perforation in distal lateral arm plates; ventral arm plates trapezoid; radial shields rounded isosceles triangular, without extensions or incisions.
Description of holotype. MnhnL OPH177 is a dissociated proximal lateral arm plate of stout, rounded outline, approximately two times higher than long, with rounded dorsal and distal edges and protruding ventro-proximal portion; outer proximal edge evenly concave, lined by a relatively broad but poorly defined band of slightly more coarsely meshed stereom with a faint horizontal striation but without spurs other than a very weak oblique elongated spur on the ventro-proximal tip of the outer surface (Fig. 4A); outer surface devoid of constriction, covered by a relatively finely meshed stereom with trabecular intersections transformed into very small tubercles; five relatively small, equal-sized spine articulations along the distal edge, with a slight dorsalward increase in the size of the gaps between the spine articulations, at the same level as the outer surface stereom; spine articulations (Fig. 4B) almost vertical, composed of arched and slightly shifted dorsal and ventral lobes encompassing a small muscle opening and a slightly smaller nerve opening; ventral edge with a small but well developed tentacle notch, lined by a shallow, poorly defined groove; inner side of lateral arm plate (not figured) with a large, well-defined, single vertebral articular ridge composed of the same stereom as the remaining inner side; inner distal edge without spurs; small but clearly defined tentacle notch; no perforations discernible.
Paratype supplements and variation.
Lateral arm plates: median lateral arm plates (MnhnL OPH178 – OPH180) similar to holotype with respect to general outline, outer surface stereom (Fig. 4C), outer proximal edge, shape and position of spine articulations (Fig. 4D) and morphology of inner side (Fig. 4E, G) but with lower height/length ratio, fewer spine articulations (three to four) and smaller tentacle notch. Distal lateral arm plate (MnhnL OPH181) longer than high, of rounded rectangular outline with straight dorsal and ventral edges, with three closely-spaced spine articulations and with tentacle openings developed as within-plate perforation close to ventralmost spine articulation (Fig. 4F).
Vertebrae (MnhnL OPH184 – 187) roughly disc-shaped (proximal vertebrae, Fig. 4J, K) to cylindrical (median and distal vertebrae, Fig. 4L, M), with concave lateral saddle showing a single, tongue-shaped articular surface (Fig. 4L) with the lateral arm plate; large, straight dorsal muscle fossae and much smaller, straight ventral ones; distal face with dorsalwards converging zygocondyles and with a small zygosphene protruding only very little beyond the ventral edge of the zygocondyles (Fig. 4J); deep and broad ventral furrow with small podial basins (Fig. 4K).
Ventral arm plates (MnhnL OPH182 – 183) trapezoid, with weakly convex distal edge, concave lateral edges and narrower, rectangular proximal portion (Fig. 4H, I); no spurs or conspicuous outer surface ornamentation.
Oral plate (MnhnL OPH188) longer than high, slender, fragile, with abradial muscle fossa developed as central depression (Fig. 5A) and small adradial muscle attachment area lining ventral edge of adradio-distal articular surface.
Radial shield (MnhnL OPH189) with proximal portion broken, rounded isosceles triangular in extrapolated outline (Fig. 5B), no signs of extensions or incisions.
Adradial genital plate (MnhnL OPH190) bar-like, with a longitudinal groove and a swollen distal end (Fig. 5C).
Remarks. The ophiuroid remains described in the present paper include various types of skeletal components, i.e. lateral and ventral arm plates, vertebrae, oral plates, genital plates and fragmentary radial shields. The ossicles of every component type show very little variation other than the changes depending on the position within the arm (e.g. Thuy & Stöhr 2011). In particular, the lateral arm plates all belong to the same species, as indicated by the similarities in plate proportions, outer surface ornamentation, spine articulation morphology and position, and vertebral articular structure morphology (Thuy & Stöhr 2011). Furthermore, the various types of ossicles are of compatible size and, in some cases, show corresponding articulation surfaces, e.g. the vertebral articular ridge on the inner side of the lateral arm plates matching the lateral articular ridge of the vertebrae. We therefore conclude that all the ophiuroid remains belong to the same species.
The spine articulations of the lateral arm plates described in the present paper are composed of small muscle and nerve openings encompassed by a ventral and dorsal lobe. This configuration precludes assignment to the Euryophiurida, whose spine articulations lack dorsal and ventral lobes and have the muscle and nerve openings separated by a vertical ridge (O’Hara et al, 2018), and instead suggests assignment to the Ophintegrida. Within that superorder, affinities are not clear-cut because the lateral arm plates and the other skeletal parts lack characters that are uniquely found in one of the orders. This observation is in line with our phylogenetic estimate in which the ophiuroid described herein holds a basalmost position at the stem of the Ophintegrida.
Comparison with other fossil ophiuroids is hampered by the lack of detailed morphological analyses. Most descriptions of Triassic ophiuroids published to date are superficial and poorly illustrated, omitting important characters such as lateral arm plate microstructures that have been identified as systematically relevant (e.g. Thuy & Stöhr 2016; O’Hara et al. 2018). Praeaplocoma hessi Broglio Loriga & Berti Cavicchi, 1972 from the lower Triassic of Italy shares some superficial similarities with the ophiuroid described herein, in particular regarding the outline of the radial shields and the shape of the ventral arm plates. The lateral arm plates of P. hessi, however, are bulging. The most important difference, however, was revealed by scanning electron microscopy of newly collected specimens from the Dolomites, close to the type area of the species (Twitchett et al. 2005), showing that the spine articulations of P. hessi lack dorsal and ventral lobes and instead have their muscle and nerve openings separated by a vertical ridge (Fig. 6). This observation places Praeaplocoma in the superorder Euryophiurida and allows a clear distinction from the ophiuroid described herein.
Shoshonura brayardi Thuy, 2019 from the lower Triassic of Idaho is another confirmed basal member of the Ophintegrida. It differs from the ophiuroid described herein in the more bulging lateral arm plates and the deeper tentacle notches of the ventral arm plates. Furthermore, the spine articulations are typical of the ophintegrid order Ophiacanthida. Arenorbis squamosus (Picard, 1858) from the middle Triassic of central Europe has lateral arm plates with several spurs along the outer proximal edge, and a slightly raised distal portion with larger, unambiguously ophiodermatid-like spine articulations.
Closest similarities are shared with the middle to upper Triassic genus Aplocoma d’Orbigny, 1852, in particular with respect to the shape of the lateral arm plates. In contrast to the ophiuroid described herein, however, Aplocoma has parallel rather than dorsalward converging zygocondyles on the distal vertebral articulations, ventral arm plates with a pointed rather than straight proximal edge, lateral arm plates with a ventralwards protruding ventro-distal tip and an outer proximal edge with spurs but lacking a fine horizontal striation, and spine articulations that are oblique rather than vertical and that have a weakly developed sigmoidal fold. These characters not only differentiate Aplocoma from the ophiuroid described herein but also suggest a more derived position of that genus within the Ophintegrida. Our phylogeny corroborates this assumption and places the type species of Aplocoma, A. agassizi, at the stem of the order Ophiodermatida, albeit with low support.
Because the ophiuroid described herein differs from all other unambiguously diagnosed fossil ophiuroids, we assign it to the new genus and species Ophiosuperstes praeparvus. For the time being, we prefer to eclipse the numerous poorly diagnosed lower Triassic ophiuroid taxa (e.g. Detre & Mihály 1987; Chen et al. 2004; Chen & McNamara 2006), because the morphological information available for these taxa is too superficial for a meaningful comparison. A detailed re-evaluation taking into account recent progress in ophiuroid systematics is necessary in order to determine whether the respective type specimens yield sufficient morphological information for an unambiguous diagnosis. Therefore, rather than extrapolating or perpetuating these questionable taxa, we prefer to introduce a new, unambiguously diagnosed taxon as a basis for future comparison.
description: Published as part of Thuy, Ben, Maxwell, Vivienne & Pruss, Sara B., 2021, A new phosphatized ophiuroid from the lower Triassic of Nevada and its position in the evolutionary history of the Ophiuroidea (Echinodermata), pp. 369-383 in Zootaxa 5071 (3) on pages 376-380, DOI: 10.11646/zootaxa.5071.3.4, http://zenodo.org/record/5723744