# A new brittle star (Ophiuroidea: Ophiodermatina) from the Early Triassic Paris Biota (Bear Lake County, Idaho, USA).

Ben Thuy, Gilles Escarguel, Arnaud Brayard, Kevin G. Bylund, Emmanuel Fara, Nicolas Goudemand, Pierre Guériau, James F. Jenks, Laurel J. Krumenacker, Nicolas Olivier, et al.

## ► To cite this version:

Ben Thuy, Gilles Escarguel, Arnaud Brayard, Kevin G. Bylund, Emmanuel Fara, et al.. A new brittle star (Ophiuroidea: Ophiodermatina) from the Early Triassic Paris Biota (Bear Lake County, Idaho, USA).. Geobios, 2019, 54, pp.55-61. 10.1016/j.geobios.2019.04.004. hal-02168465

HAL Id: hal-02168465

https://hal.science/hal-02168465v1

Submitted on 12 Feb 2025

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A new brittle star (Ophiuroidea: Ophiodermatina) from the Early Triassic
of Idaho (USA)

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Ben Thuy¹, Gilles Escarguel² and the Paris Biota team

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¹Natural History Museum Luxembourg, 25, rue Münster, 2160 Luxembourg-city,
Luxembourg; bthuy@mnhn.lu

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²Université Claude Bernard Lyon 1, Laboratoire d'Ecologie des Hydrosystèmes Naturels et Anthropisés, CNRS, F-69622 Villeurbanne, France

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Key words

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Ophiuroids, evolutionary history, modern clades, morphology, *Shoshonura brayardi*

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Abstract

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Our knowledge of the morphology of Early Triassic ophiuroids is very limited, in spite of the relatively high number of described species. This hampers attempts to use morphology-based phylogenetic analyses in order to explore the early diversification of modern ophiuroid clades between the latest Paleozoic and early Mesozoic. Here, a new ophiuroid from the Early Triassic of southern Idaho (USA) is described and assigned to the new taxon *Shoshonura brayardi* gen. et sp. nov. It is part of an exceptional Early Triassic faunal assemblage recently described as the Paris Biota. Based on the arm morphology, the shape of the arm spines and the arm spine articulations, *Shoshonura brayardi* gen. et sp. nov. is considered a basal member of the suborder Ophiodermatina.

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## 1. Introduction

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29 The Late Permian to Early Triassic interval was a time of global-scale environmental perturbations, including the most devastating mass extinction to ever occur on Earth (e.g.

30 Sepkoski 1981; Erwin 1993). At the same time, however, it is considered a pivotal time in the
31 early evolution of many modern marine invertebrate groups, in particular setting the kick-off
32 for crown-group cladogenesis in echinoderms (e.g. Twitchett & Oji 2005).
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34 With respect to the late Permian to Early Triassic fossil record, ophiuroids are certainly the
35 most prolific group. Chen and McNamara (2006) record as many as 14 ophiuroid species for
36 the boundary interval, which by far outnumbers the coeval fossil record of the other four
37 echinoderm classes (Twitchett & Oji 2005; Villier et al. in press). Regrettably, the majority of
38 the late Permian to Early Triassic ophiuroid occurrences are poorly known. Although
39 commonly named to species level, most records are based on insufficiently preserved
40 specimens and/or superficial descriptions (e.g. Detre & Mihaly 1987; Chen et al. 2004; Chen
41 & McNamara 2006). So far, the only Early Triassic ophiuroid species for which a reasonably
42 exhaustive description is available is Praeaplocoma hessi Broglio Loriga and Bert-Cavicchi,
43 1972 from the Italian Dolomites. Yet, even P. hessi remains poorly known with respect to its
44 higher systematic affinities and its position within ophiuroid evolutionary history.
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46 The latest studies of ophiuroid phylogeny and evolution converge to a mid-Permian origin for
47 the crown group Ophiuroidea followed by a relatively quick radiation of the major extant
48 groups (O'Hara et al. 2014; Thuy & Stöhr 2016). Against this background, the poor state of
49 knowledge of Triassic ophiuroids is particularly unfortunate as it hampers a better
50 understanding of the crucial early cladogenesis of the group.
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The present study describes a remarkable new ophiuroid from the Early Triassic of
southeastern Idaho, USA, previously highlighted as part of the so-called Paris Biota, a highly
diversified fossil assemblage documenting an unexpectedly rapid post-Permian recovery of
complex marine ecosystems (Brayard et al. 2017). The new ophiuroid allows for the
elucidation of at least a small part of the insufficiently known Triassic ophiuroid fossil
record.

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## 2. Geological context

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The described specimens were collected from the Early Triassic outcrops near the city of
Paris, southeastern Idaho, USA, that exposes thin-bedded mudstones of the Lower Shale unit
within the Lower Triassic Thaynes Group. The strata yielding the ophiuroid specimens were
dated to the early Spathian based on ammonoid and conodont evidence. The depositional
setting is interpreted as a distal upper offshore environment on a flat platform above storm-
wave base, originally in a near equatorial position on the western margin of Pangea. For
details of the geological setting of the ophiuroids, see Brayard et al. (2017).

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## 3. Material and Methods

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The described specimens are housed in the collections of the Université de Bourgogne, Dijon,
France (acronym UBGD). Part of the paratype specimen was detached from the slab,
mounted on an aluminium stub and gold-coated for scanning electron microscopy using a
Jeol Neoscope JMC-5000. Terminology follows Stöhr et al. (2012), Thuy and Stöhr (2011)
and Thuy and Stöhr (2016), and higher systematic classification follows O’Hara et al. (in
press).

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4. Systematic paleontology (B. Thuy)

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Class OPHIUROIDEA Gray, 1840

Subclass MYOPHIUROIDEA Matsumoto, 1913

Infraclass METOPHIURIDA Matsumoto, 1913

Superorder OPHINTEGRIDA O’Hara et al., in press

Order OPHIACANTHIDA O’Hara et al., in press

Suborder OPHIODERMATINA Ljungman, 1867

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Genus *Shoshonura* nov. gen.

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**Derivation of the name:** Name formed to honour the Shoshoni Native American tribe who
originally inhabited the southeastern part of Idaho where the fossil ophiuroid was discovered.

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**Type species:** *Shoshonura brayardi* sp. nov., monotypic.

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**Species included:** only the type species.

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**Occurrence:** Paris Canyon, Bear Lake County, Idaho.

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**Diagnosis:** as for species.

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*Shoshonura brayardi* nov. sp.

Figs. 1-4
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**Derivation of the name:** Species named in honour of Arnaud Brayard (Dijon, France) who drew my attention to the new ophiuroid described herein, and who skillfully coordinated the compilation of evidence from various taxonomic groups, including the ophiuroids, to unravel the big picture behind the extraordinary Paris Biota.

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**Type material:** Holotype UBGD 305651 (Fig. X-Y); A single paratype: UBGD 305652 (Fig. X-Y);

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**Type locality and horizon:** Paris Canyon, west of the city of Paris, Bear Lake County, Idaho, United-States of America; Early Triassic, earliest Spathian (after Brayard et al., 2017).

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**Occurrence:** only known as yet from the type locality.

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**Diagnosis:** Large ophiuroid with disc interradii covered by minute spherical granules; two genital slits per interradius, no genital papillae; single apical oral papilla; ventral midline of arms depressed, forming a shallow radial furrow; proximal ventral arm plates with deeply concave lateral edges bordered by an elevated ridge; tentacle pores rapidly decreasing in size from proximal to median segments and reduced to within-plate perforations in distal segments; lateral arm plates thick, with finely meshed stereom devoid of conspicuous ornamentation on outer surface; at least five spine articulations sunken in depressions of distal lateral arm plate portion in proximal arm segments; spine articulations composed of proximally merged dorsal and ventral lobes forming an oblique horse-shoe-shaped ridge

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encompassing small muscle and nerve openings; arm spines thick, conical, as long as half an
arm segment.

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**Description:** Large, articulated specimen exposing the ventral side (Fig. 1), disc diameter of 14 mm; disc round, interradial areas convex and with small, rounded scales covered by minute spherical granules (Fig. 2A-B). Two genital slits per interradius, approximately equaling one third of the disc radius in length, no papillae or modified granules discernible on edge of genital slit. One abradial genital plate visible in ventral view (Fig. 2C), long, sabre-like with widened distal portion and straight to slightly convex adradio-distal edge, no groves or perforations discernible.

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Oral shields small (Fig. 2A-B), equaling one sixth of the disc radius in length, with nearly right, rounded proximal angle and slightly narrower rounded, square-like distal portion.

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Adoral shields (Fig. 2A-B) with a pointed proximal tip and a wider distal portion, densely covered by granules extending on the edges of the oral shields. Jaws (Fig. 2A-B) slender and elongate, covered by granules, bearing a single conical apical oral papillae; at least four lateral oral papillae, the proximal ones conical and slightly larger than apical oral papillae, and the median ones elongate with oblique distal tip and two to three times larger than apical oral papillae; distalmost lateral oral papillae not unambiguously discernible. Second oral tentacle openings and first and second ventral arm plates concealed by matrix.

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Five arms completely preserved from the base to the tip, evenly tapering, composed of up to 53 segments each. Ventral midline of arms (Fig. 2D) slightly sunken and concealed by matrix in most proximal to median segments; ventral arm plates separating lateral arm plates in arm segments covered by disc and in the proximalmost quarter of the remaining arm segments; proximal ventral arm plates widest distally (Fig. 2D), with slightly convex distal edge, deeply concave lateral edges bordered by an elevated ridge, and obtuse proximal angle; median

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149 ventral arm plates devoid of ridge bordering lateral edges. Ventral arm plates rapidly decreasing in size from proximal to median arm segments, reduced to minute, round plates in distal segments. Tentacle openings within disc and in proximalmost free arm segments very large, covered by at least three small, rounded, elongate tentacle scales (Fig. 2D) on tentacle notch of lateral arm plate and at least one slightly smaller, round tentacle scale at lateral edge of ventral arm plate; tentacle pores reduced to small between-plate openings (Fig. 3D) in median arm segments, and to within-plate perforations in distal segments.
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156 Lateral arm plates thick (Fig. 3A), with ventro-distal tip prominent with respect to the ventral arm surface; proximal edge lined by shallow dorso-ventral groove, presence of spurs unknown; outer surface of lateral arm plates with finely-meshed stereom devoid of conspicuous ornamentation; distal portion of lateral arm plates bulging; spine articulations (Fig. 3B) sunken in depressions of distal lateral arm plate portion, evenly spaced, at least five in proximal segments and four in median ones; spine articulations composed of slender, evenly arched and proximally merged dorsal and ventral lobes forming an oblique horse-shoe-shaped ridge encompassing small muscle and nerve openings; arm spines approximately equal-sized, conspicuously thick, conical, pointed, as long as half an arm segment. Proximal lateral arm plates with deeply concave tentacle notch lined by a shallow groove (Fig. 2D); tentacle notch rapidly decreasing in size from proximal to median arm segments, and replaced by within-plate perforation in distal arm segments.
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169 **Paratype supplements:** UBGD 305652 is an articulated arm portion (Fig. 4A) associated with external molds of two further arms, all preserving median to distalmost segments, situated on the edge of the slab with the holotype specimen. The physically preserved articulated arm portion exposing the ventral side and showing a median to distal arm morphology similar to that seen in holotype specimen. Scanning electron microscopy of
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median arm segments corroborating the absence of conspicuous outer stereom ornamentation
on the outer surface of the lateral arm plates and the arm spines. Arm spine articulations (Fig.
4B) forming an oblique horse-shoe-shaped ridge composed of slender, evenly arched dorsal
and ventral lobes proximally merged by a lower, less sharply defined ridge, encompassing
small muscle and nerve openings. Single, large, leaf-shaped tentacle scale.

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## 5. Discussion

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The specimens described herein add to the rather long list of ophiuroid occurrences known
from the Early Triassic (e.g. Chen & McNamara 2006; Twitchett et al. 2005). The new
material stands out, however, when it comes to body size; with a disc diameter of 14 mm, the
holotype specimen by far exceeds all other currently known Lower Triassic ophiuroids
(Twitchett et al. 2005). With respect to skeletal morphology, exhaustive comparison with
previously described species is hampered by the superficial state of knowledge of most Early
Triassic ophiuroids (e.g. Chen & McNamara 2006). Nevertheless, judging from the amount
of information available, none of the currently known species shows a radial depression on
the ventral side of the arms combined with swollen lateral arm plates bearing conspicuously
thick, short arm spines. The specimens at hand are thus described as the new genus and
species *Shoshonura brayardi* nov. gen. et nov. sp.

The higher systematic position of *Shoshonura brayardi* nov. gen. et nov. sp. was briefly
outlined in the supplementary materials of Brayard et al. (2017). The most compelling
evidence of systematic value is provided by the arm spine morphology, unequivocally
placing *Shoshonura* nov. gen. in the order Ophiacanthida. Unfortunately, the holotype and
only known complete specimen of *Shoshonura* nov. gen. does not expose the dorsal side,
making many characters of diagnostic relevance for the position within the Ophiacanthida

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inaccessible, at least for the time being. Novel non-destructive techniques, in particular
micro-computed tomography, might provide insights into these hidden structures.

In spite of these limitations, the morphological evidence available effectively excludes the
living members of the suborder Ophiacanthina, especially on account of the short, non-erect
arm spines, the thick, non-constricted lateral arm plates and the spine articulations sunken
into shallow depressions of the distal lateral arm plate edge. These same characters, in
contrast, strongly favour assignment to the suborder Ophiodermatina. However, because
crucial features of the dorsal disc skeleton are unknown, the family-level position of
*Shoshonura* is left open for the time being. The radial furrow along the ventral midline of the
arms and the tentacle openings developed as within-plate perforations in distal arm segments
seem to represent plesiomorphic character states (Thuy et al. 2015), suggesting a basal
position of *Shoshonura* nov. gen. within the Ophiodermatina. These character states are
unknown from any living member of the Ophiodermatina, and instead favour assignment to a
new, extinct family.

The existing knowledge of the late Paleozoic and early Mesozoic fossil record of the
ophiuroids is too sparse to assess the phylogeny of the early diversification of modern
ophiuroid clades. Many more studies, particularly on Early Triassic ophiuroids, are necessary
to assess this crucial interval of the ophiuroid evolutionary history. Future works should
specifically aim at an exhaustive analysis of the skeletal features in order to allow for an
inclusion in morphology-based phylogenetic estimates.

## Acknowledgments

B. Thuy thanks Arnaud Brayard (Dijon, France) for providing the opportunity to study the
Paris Biota ophiuroids and to include this article in this special volume. Roby Weis
(Luxembourg-city, Luxembourg) helped us with the preparation of the type specimen.
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300 **Figure captions**
<figure><img src="image_3.png" /><figcaption>**Figure 1:** *Shoshonura brayardi* nov. gen. et nov. sp., articulated ophiuroid in ventral view, UBGD 30565 (holotype); Lower Shale unit, Thaynes Group, earliest Spathian, Early Triassic; Paris Canyon, Bear Lake County, Idaho, USA. Scale bar equals 1 cm.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 2:** *Shoshonura brayardi* nov. gen. et nov. sp., details of ophiuroid shown in figure 1, UBGD 30565 (holotype); Lower Shale unit, Thaynes Group, earliest Spathian, Early Triassic; Paris Canyon, Bear Lake County, Idaho, USA. A: detail of mouth plating; B: interpretative drawing of detail shown in A; C: detail of basal arm segments with disc margin; D: detail of basal arm segments. Abbreviations: AbGP: abradial genital plate; AOP: apical oral papilla; AOS: adoral shield; AS: arm spine; G: granule; LAP: lateral arm plate; LOP: lateral oral papilla; OS: oral shield; TS: tentacle scale; VAP: ventral arm plate. Scale bars equal 1 mm.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure. 3.** *Shoshonura brayardi* Thuy nov. gen. et nov. sp., UBGD 30565 (holotype): details of the ophiuroid specimen shown in Fig. 1; Lower Shale unit, Thaynes Group, early Spathian, Early Triassic; Paris Canyon, Bear Lake county, Idaho, USA. A. Detail of basal arm segments. B. 980 × 980-pixel area from a 1024 × 1024-pixel false color RGB composite of the mouth area and basal arm segments. Image was spatially registered and each color corresponds to the following illumination/detection spectral ranges: blue: 435 nm/435 nm, 70 nm FWHM; green: 385 nm/650 nm, 60 nm FWHM; red: 365 nm/650 nm, 60 nm FWHM. Image intensity levels in each channel were adjusted using a Gaussian stretch (ENVI software, EXELIS). Abbreviations: AS: arm spine; LAP: lateral arm plate; OS: oral shield; TS: tentacle scale; VAP: ventral arm plate. Scale bars: 1 mm.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 4:** *Shoshonura brayardi* nov. gen. et nov. sp., details of ophiuroid shown in figure 1, UBGD 30565 (holotype); Lower Shale unit, Thaynes Group, early Spathian, Early Triassic; Paris Canyon, Bear Lake county, Idaho, USA. A: proximal arm segments in lateral view; B: proximal arm segments in lateral view; C: median arm segments in ventral view; D: distal arm segments in ventral view. Abbreviations: ASA: arm spine articulations; TO: tentacle opening. Scale bars equal 1 mm.</figcaption></figure>
<figure><img src="image_7.png" /></figure>

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<caption>**Figure 5:** *Shoshonura brayardi* nov. gen. et nov. sp., ophiuroid arm fragment, UBGD 30566 (paratype); Lower Shale unit, Thaynes Group, earliest Spathian, Early Triassic; Paris Canyon, Bear Lake County, Idaho, USA. A: arm fragment in ventral view; B: detail of A showing arm spine articulation in latero-ventral view. Abbreviations: AS: arm spine; DL: dorsal lobe; LAP: lateral arm plate; VAP: ventral arm plate; VL: ventral lobe. Scale bars: 500 µm (A), 200 µm (B).</caption>