Dadocrinus montellonis, Saucède & Smith & Olivier & Durlet & Gueriau & Thoury & Fara & Escarguel & Brayard, 2023

Saucède, Thomas, Smith, Christopher, Olivier, Nicolas, Durlet, Christophe, Gueriau, Pierre, Thoury, Mathieu, Fara, Emmanuel, Escarguel, Gilles & Brayard, Arnaud, 2023, A new Early Triassic crinoid from Nevada questions the origin and palaeobiogeographical history of dadocrinids, Acta Palaeontologica Polonica 68 (1), pp. 155-166 : 159-161

Dadocrinus montellonis sp. nov.

Figs. 2–5 .

ZooBank LSID: urn:lsid:zoobank.org:act:ECA27511-2CA8-4BA7-A689-CE2CEEE0EFD9

Etymology: From the geographical name Montello Canyon, the place where the specimen originates (genitive singular case).

Holotype: One near complete and articulated specimen UBGD 292410 preserved on a fine sandstone slab along with holdfasts and distal columnals of two other specimens.

Type locality: Montello Canyon, Elko County, northeastern Nevada, USA ( Fig. 1B, C ).

Type horizon: Middle–upper Spathian (upper Olenekian, Lower Triassic) of the Thaynes Group (sensu Lucas et al. 2007) ( Fig. 1C, D ).

Material.— Holotype only.

Diagnosis.—Small dicyclic species of Dadocrinus with a contiguous circle of connected and well-developed infrabasals, clearly visible in lateral view. Primibrachials 1

and 2 very thin, higher than wide, only half as wide as radials. Pinnules stem from every third brachial.

Description.—The almost complete specimen consists of articulated arms, cup and stem. The total length of the specimen, from the holdfast to the preserved extremity of arms is 46.5 mm. The crown itself is 16.5 mm long (but entire arms are not preserved) and has 3 radials, 3 basals and 3 infrabasals visible ( Figs. 2 , 3 ).

Crown: Dicyclic cup low, conical in shape ( Fig. 4 ). Circle of infrabasals contiguous, infrabasals very well developed and clearly visible in lateral view. They are wider than long, 0.6 mm long, and 0.8 mm wide, almost rectangular in shape and articulate in between basals with a curved, convex suture. Basals and radials high and thin. Basals are twice as long as wide and insert with a roof-shaped suture between radials. Radials are as long as wide. From the lateral view only, it can be deduced that radial articular facets with primibrachials are muscular and sloping outward. Infrabasals, basals and radials seem to be connected by synostoses with deep ligament pits visible on basals. Surface of infrabasals, basals, and radials smooth.

Arms: Ten arms, uniserial throughout, branching at the second primibrachials ( Fig. 4A 2 ). Brachials relatively high and rectangular in shape (not wedge-shaped). Primibrachials 1 and 2 very thin, higher than wide, only half as wide as radials. First secondibrachials thin and higher than wide, then following ones as high as wide, progressively thinner and narrower distally. At least 28 articulated secondibrachials can be counted on one arm but arms are very likely longer. Dorsal side of all brachials rounded and even, but not flat, sharp-edged or keeled. Pinnulars smooth and thin, about three times as long as wide, pinnules widely spaced, branching on every third brachial. First pinnular twice shorter than the following ones. Terminal pinnulars not visible.

Stem: Total length is 30 mm. Column cylindrical with no cirri. No differentiation in size between nodals and internodals is visible, but proximal and proxistele are only partially preserved and the shape of proximal columnars is hardly discernible. Typically, the proximal however appears a bit enlarged but not higher as compared to the following columnals. In the mesistele, all columnals are equally sized and circular, about twice as wide as high. Columnals become barrel-shaped in the dististele; they are about three times as high as medial columnals. Articulations between columnars symplexial with multiradiate crenulation visible in the mesistele. Terminal columnal with discoid, encrusting holdfast.

Remarks.— Dadocrinus montellonis sp. nov. shows the presence of a conical dicyclic to cryptodicyclic cup with a circle of contiguous basals, radial articular facets sloping outward, and a cylindrical column with no cirri ending in a terminal disk, which are diagnostic features of the family Dadocrinidae ( Hagdorn 2011; Hess and Messing 2011). Unfortunately, the preservation of the specimen, only visible in lateral view, and surficial weathering hinder observation of articulations between skeletal elements. Three nominal species of Dadocrinus have been described so far, all reported from the Anisian (Middle Triassic) of Central Europe ( Poland, Germany, Switzerland, Austria, Hungary, northern Italy), Turkey, and New Zealand: Dadocrinus gracilis (Buch, 1845) , Dadocrinus grundeyi Langenhan, 1903 , and Dadocrinus kunischi Wachsmuth and Springer, 1887 ( Lefeld 1958; Eagle 2003, 2004; Hagdorn 1996, 2011, 2020; Hess and Messing 2011).

All species of Dadocrinus are characterised by the presence of high and thin basals and radials, ten arms uniserial throughout branching at the second primibrachials, with brachials relatively high and rectangular, second primibrachials axillary with two almost equally wide muscular facets, and smooth pinnulars ( Hess and Messing 2011; Hagdorn 2020). Species of Dadocrinus differ with each other mainly in overall body size, in the position, development, and visibility of infrabasals in the cup, and in the morphology of proximal columnals ( Hess and Messing 2011; Hagdorn 2011, 2020). However, these characters may vary within a single fossil assemblage, and the three nominal species have been regarded as distinct ontogenetic stages or ecophenotypes of one single species ( Głuchowski 1986; Hagdorn et al. 1996; Eagle 2003; Hess and Messing 2011). D. montellonis sp. nov. differs from the three nominal species of Dadocrinus by its smaller size, the circle of contiguous infrabasals, the thin primibrachials, and pinnules stemming from every third brachial and not on alternate consecutive brachials starting from the second secondibrachials ( Eagle 2003).

In D. gracilis , infrabasals are rarely visible, the proximal part of arms is slightly rounded then V-shaped, and in the column, proximal nodals are a bit thicker and higher than internodals.

Juveniles of D. grundeyi show a dicyclic cup but infrabasals are not contiguous and are partly covered by the proximal columnal ( Lefeld 1958). Moreover, in D. grundeyi , the dorsal side of proximal arms is weakly rounded, then more rounded distally, pinnules articulate on alternate side of brachials (Hagdorn 1996: pl. 3), and axillary primibrachials are the same size as radials.

In D. kunischi , individuals grow larger (up to 7 cm for the crown, 20 cm in total), infrabasals are not visible, and pinnules articulate on every brachial (Hagdorn 1996: pl. 2). In the proxistele, nodals and first internodals are wider and higher.

Hagdorn (2020) recently described a new genus and species of the family Dadocrinidae, Aszulcicrinus pentebrachiatus Hagdorn, 2020, from the lower Anisian (Middle Triassic) of Upper Silesia ( Poland). Dadocrinus montellonis sp. nov. differs from A. pentebrachiatus by a complete circle of contiguous infrabasals, ten arms branching at the second primibrachial (against five in A. pentebrachiatus), the dorsal side of radials and brachials rounded (not sharp-edged, keeled or V-shaped) and the cup is low cone-shaped.

Based on small isolated columnals, Oji and Twitchett (2015) described a new genus and species from the lower Induan (Griesbachian, Lower Triassic) of Oman, Baudicrinus krystyni Oji and Twitchett, 2015 , which they attributed to the family Dadocrinidae , thereby pushing back the origin of the family to the very early Triassic. However, new material from the middle Griesbachian of Oman was revisited by Brosse et al. (2019) who re-assigned B. krystyni to the family Holocrinidae Jackel, 1918 , based on the presence of sub-pentagonal proximal columnals, bifurcated culmina on columnals, and cylindrical cirri. These new interpretations support the hypothesis of early representatives of Holocrinidae as the oldest post-Palaeozoic crinoids ( Hagdorn 2020).

Figures

Fig. 2. Overall view of the holotype of the encrinid crinoid Dadocrinus montellonis sp. nov. (UBGD 292410), Spathian, Lower Triassic, Montello Canyon, Nevada, USA. Natural light photography (A1) and camera lucida drawing (A2). The dashed line shows the outline of a shell imprint.

Fig. 3. Advanced imaging of the holotype of the encrinid crinoid Dadocrinus montellonis sp. nov. (UBGD 292410), Spathian, Lower Triassic, Montello Canyon, Nevada, USA.A1, Ca map; the colour scale goes from dark to light yellow for low to higher relative concentration of calcium (Ca is abundant in the calcite crinoid pieces, but rarer in the fine clastic sediment hosting the fossil).A2, UV-excited luminescence composite image; ilumination/detection couples: red 385/732 nm; green 385/571 nm; blue 385/835 nm.

Fig. 4. Close-up views of the crown of the encrinid crinoid Dadocrinus montellonis sp. nov. (UBGD 292410), Spathian, Lower Triassic, Montello Canyon, Nevada, USA. Natural light photography (A1), camera lucida drawing (A2), Ca map (A3), and X-ray and visible excited luminescence composite image (A4). Illumination/detection couples: red, natural light/650 nm; green, X-ray/571 nm; blue, X-ray/650 nm.

Fig. 5. Close-up view of holdfast of the encrinid crinoid Dadocrinus montellonis sp. nov. (UBGD 292410), Spathian, Lower Triassic, Montello Canyon, Nevada, USA, also showing the outline of a shell imprint (dashed line) for possible crinoid anchorage. Natural light photograph (A1) and camera lucida drawing (A2).

Fig. 1. Location, geological and stratigraphic setting of the Thaynes Group where Dadocrinus montellonis sp. nov. was found. A. Palaeogeographic map of the late Early/early Middle Triassic showing the occurrence record of Dadocrinus species. B. Present-day map showing the location of the Western USA Basin. C. Simplified geological map of north-eastern Nevada with location of Montello Canyon.D. Stratigraphic position of Dadocrinus montellonis sp. nov. Maps modified after Brayard et al. (2017) and Smith et al. (2021).