Polarasterias janusensis Rousseau & Gale, 2018

Rousseau, Julie, Gale, Andrew Scott & Thuy, Ben, 2018, New articulated asteroids (Echinodermata, Asteroidea) and ophiuroids (Echinodermata, Ophiuroidea) from the Late Jurassic (Volgian / Tithonian) of central Spitsbergen, European Journal of Taxonomy 411, pp. 1-26 : 6-8

Polarasterias janusensis Rousseau & Gale gen. et sp. nov.

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Fig. 2

Diagnosis

As for genus.

Etymology

The specific epithet refers to the Janus mountain (Janusfjellet) locality.

Material examined

Holotype

NORWAY: central Spitsbergen , Janusfjellet, 78°20′35.4″ N, 15°49′85.2″ E ( PMO 218.011 a).

Paratypes

NORWAY: same location as for holotype ( PMO 217.936 , PMO 217.982 , PMO 218.069 ). More than 50 partial specimens were collected .

Type stratum

Lower Middle Volgian (Middle Tithonian), Slottsmøya Member, Agardhfjellet Formation.

Description

The holotype, PMO 218.011a ( Fig. 2 A–C), is a relatively complete specimen showing an articulated disc with five arms, two of which extend to the tip, preserved in a cut-through manner (i.e., split horizontally approximately along the ambitus) and clearly showing the form and arrangement of the ambulacral groove ossicles. The arms are elongated and the disc is very small. The arms taper slowly and the interbrachial angle is acute (major radius = 65 mm, minor radius = 8 mm). The ambulacral groove is very broad (width = 8 mm) and the ambulacrals occupy roughly 80% of the arm width. The inferomarginals are barely visible on the actinal surfaces of the arms. The presence of actinal ossicles cannot be confirmed from the material available, but the very acute interradii cannot have had significant actinal interradial areas and if actinals were present, they must have been restricted to the proximal arms. There are about 70 ambulacrals and adambulacrals in each half radius. The podial openings for tube feet are strongly quadriserial along the total length of the arms ( Fig. 2A, C ). The ambulacrals have well demarcated heads, shaft and bases and are not very foreshortened ( Fig. 2C ) as in many asteriids ( Gale 2011a). The ambulacral head is angled to the shaft and the base, and strong dentition is present. The ambulacrals are rectangular in actinal view, short and broad, and have a strongly concave proximal surface for a large adambulacral muscle. The adambulacrals have a prominent distal heel contacting the distal ambulacral base. Each adambulacral carried two spines (diplacantid condition), one of which, probably the outer, is significantly larger than the other ( Fig. 2C ). The conical, elongated, adambulacral spines at the base of the arm are over 4 mm in length and form a distally directed fringe to the arm. Abundant straight pedicellariae are seen in cross section scattered among the adambulacral spines ( Fig. 4 B–C). These include ε shaped basal pieces which possess tall median ridges for insertion of the inner longitudinal adductor muscle ( Gale 2011a).

A partly disarticulated mouth frame seen from the actinal surface is visible on paratype PMO 217.982 ( Fig. 2D ) and shows the presence of an adoral carina involving two or three adambulacrals. The oral ossicles are well preserved and have a short, rectangular oral surface which bears two spine articulations.

The terminal ossicle is well preserved in paratype specimen PMO 217.936 ( Fig. 2E ), where it is triangular and as broad as long with a strongly convex distal margin.

Paratype specimen PMO 218.069 ( Fig. 2 F–G) shows the abactinal surface of an arm. The abactinal ossicles are cruciform, with four processes which contacted adjacent rows and columns, and are arranged in well-marked imbricate transverse columns, with up to seven ossicles in each half radius. The largest two abactinals are present adjacent to the adambulacral, and are identified as the inferoand superomarginals ( Fig. 2G ). Up to five slightly smaller abactinals are also present in each column. However, it is not possible to identify radials and adradials, so the exact numbers of plates present cannot be discerned. The abactinal ossicles formed a rectilinear reticulate structure, with the spaces occupied by papular openings, and the centre of each abactinal plate carried a long spine.

Remarks

This new asteroid is clearly a forcipulatid neoasteroid as forcipulate pedicellariae are present. It could potentially belong to the Terminasteridae Gale, 2011a , Zoroasteridae Sladen, 1889 or Asteriidae Gray, 1840 which can be difficult to separate in fossil material, because the families have many morphological features in common ( Blake 1990). The single most diagnostic feature of the Asteriidae , the presence of both straight and crossed pedicellariae, unfortunately cannot be confirmed in this material. However, various lines of morphological evidence permit assignation to the Asteriidae . These include the form of the terminal ossicle, which is relatively short and broad as in asteriids while markedly elongated in terminasterids and zoroasterids. Moreover, the morphology of the oral ossicle is similar to that in extant asteriids ( Gale 2011a), and the shape of the basal piece of the straight pedicellariae, with a tall median ridge, is also typically asteriid. The strongly quadriserial arrangement of the tube feet along the entire length of the groove is also an asteriid character ( Blake 1990). The arrangement of the abactinal ossicles in up to seven transverse rectilinear columns is not typical of extant asteriids, in which a more or less irregular network of plates extends between the superomarginals and the radials (e.g., Fisher 1928: pls 67, 44, 61; Gale & Villier 2013).

Polarasterias janusensis gen. et sp. nov. differs significantly from other described Jurassic and Cretaceous asteriid genera. Blake (1990) described two new genera and species from the Hettangian (Early Jurassic) of Germany, Germanasterias amplipapularia Blake, 1990 and Hystrixasterias hettangiurnus Blake, 1990 , which he assigned to the Asteriidae . These differ from the described species in the presence of relatively broad actinal interareas, the relatively closed ambulacral groove and the more rapidly tapering arms. Additionally, Germanasterias Blake, 1990 is characterized by having very coarse, numerous, adambulacral spines which are not seen in the Janusfjellet material. Argoviaster occultus Hess, 1972 and Dermaster boehmi de Loriol, 1899 were described from the Bajocian of Switzerland ( Hess 1972). Both have stout, relatively short, rapidly tapering arms and narrow ambulacral grooves. The species from Spitsbergen also differs from Cretasterias reticulatus Gale & Villier, 2013 in its more strongly quadriserial tube feet, and the more numerous columns of abactinal ossicles, but agrees in the overall shape of the arms of and the rectilinear arrangement of the abactinal ossicles.

Modern members of the Asteriidae family are slow-moving, epifaunal carnivores living on the sea bottom. The relatively large body size, small disc size, multi-element arms, quadriserial tube feet and adoral carina present in Polarasterias janusensis gen. et sp. nov. are all features which favor a predatory mode of feeding ( Blake 1987, 1990). The wide ambulacral groove, covered with numerous tube feet, provided a strong adhesion force which could have helped Polarasterias janusensis gen. et sp. nov. to grab and hold its prey. Modern predatory asteroids feed mostly on bivalves, gastropods and other echinoderms, including other asteroids, ophiuroids and echinoids, and are known to form large feeding groups ( Spencer & Wright 1966). Similarly, it is likely that Polarasterias janusensis gen. et sp. nov. lived gregariously as epifauna on the Spitsbergen muddy seafloor, where it hunted for bivalves and echinoderms.

Figures

Fig. 2. Polarasterias janusensis Rousseau & Gale gen. et sp. nov. A–C. Holotype PMO 218.011a. A. Nearly complete specimen preserved in a cut-through fashion. B. Drawing detail from ambulacral groove showing ambulacrals, adambulacrals, adambulacral spines and scattered valves of forcipulate pedicellariae. C. Portion of arm illustrated in B, basal pieces of straight pedicellariae visible in cross section. D. Paratype PMO 217.982, oral region in actinal view. E. Paratype PMO 217.936, Arm tip showing terminal ossicle. F–G. Paratype PMO 218.069. F. Obliquely compressed radius showing adambulacrals, marginals, and columns of abactinal ossicles. G. Drawing detail from F. Abbreviations: abact = abactinal ossicle; adamb sp = adambulacral spine; adamb = adambulacral; amb = ambulacral; im = inferomarginal; ped bp = basal piece of pedicellariae; sm = superomarginal; sp = spine.

Fig. 4. Ophiogaleus sp. PMO 217.899a. A. Specimen preserving the ventral side of the disc area, two nearly complete arms and proximal area of a third arm. B. Detail view of the disc area. C. Drawing detail of the area shown in B. D. Detail view of a proximal section of arm. Abbreviations: abgp = abradial genital plate; adgp = adradial genital plate; ds = disc spine; lopa = lateral oral papillae; op = oral plate; sart = spine articulation; v = vertebra.