Savignaster septemtrionalis Rousseau & Gale & Thuy 2018, sp. nov

Savignaster septemtrionalis Rousseau & Gale sp. nov . urn:lsid:zoobank.org:act: EF6444E7-C91F-478B-9575-F7E67E4C58D2 Fig. 3 Diagnosis

Savignaster in which the ambulacral bases and shafts are very elongated and poorly differentiated.

The specific epithet is from the Latin ‘ septemtrionalis ’, northern, in reference to the high latitudinal position of the locality.

Holotype

NORWAY: central Spitsbergen, Knorringfjellet, 78°18′04.4″ N, 16°16′02.0″ E (PMO 218.000).

Paratypes

NORWAY: same location as for holotype (PMO 217.976, PMO 217.977, PMO 217.978, PMO 218.008). Ten partial specimens were found at the type locality.

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

The holotype PMO 218.000 (Fig. 3 A–B) is an articulated arm and interradial area, which also shows some portions of a second arm belonging to the same individual. Ambulacrals and adambulacrals are clearly exposed in a transverse section of the groove and disc. The arms are triangular and taper rapidly, and the interradius is acutely angled. The major radius of the holotype (R) is 45 mm, the minor radius (r) 15 mm. The base of the arm is 20 mm across. There are over 30 pairs of ambulacrals and adambulacrals present in each half radius and the two plate rows are of approximately equal width. The ambulacral heads have a large triangular proximal extension and are imbricated proximally. The ambulacrals shafts and bases are elongated and narrow and set nearly at right angles to the axis of the arm. Paratypes PMO 217.976 (Fig. 3C) and PMO 217.977 (Fig. 3D) also show the arrangement of the ambulacral ossicles. Ambulacrals and adambulacrals alternate, and the adambulacrals are slanted laterally and distally. The adambulacrals comprise two parts; a thicker adradial region and a narrow, parallel sided elongated adambulacral extension as in Savignaster wardi (Gale 2011 a, 2011b). The adradial portions are thickened where spine bases were present. In the holotype, scattered abactinal ossicles lie alongside the lateral margin of the adambulacrals (Fig. 3 A–B) and, evidently, the individual was obliquely compacted with the abactinal surface pushed over the adambulacrals. The abactinal ossicles are poorly preserved, but include forms with a pedicel and at least some are identifiable as megapaxillae. Interradial chevron ossicles are present in the interradial region. They form a laterally diverging V-arrangement of nine to ten closely spaced plate pairs situated symmetrically across the interradial line (Fig. 4 A–B).

In paratype specimen PMO 217.978, poorly preserved flattened spines are seen attached to the lateral margins of the adambulacrals (Fig. 3E). These have a rounded basal portion and a flattened, trellis-like distal part; they are comparable in shape with spines on the holotype of Savignaster wardi (Gale 2011a: pl. 21 figs 2, 5). This particular spine morphology is also clearly visible in paratype PMO 218.008 (Fig. 3F), a partial arm preserving multiple articulated spines.

The specimens consist of individuals which have split horizontally approximately along the plane of the ambitus, an unusual preservation mode. The presence of interradial chevron ossicles permit placement of the new species in the clade including Korethrasteridae and Pterasteridae (Gale 2011 a, 2011b). It can furthermore be assigned to Savignaster based on the similarity of the shape (long adambulacral extensions) of the adambulacrals to those of S. wardi Gale, 2011. The ambulacrals differ significantly in shape from those of both S. wardi and S. trimbachensis (Gale 2011a: pl. 23) in which club-shaped, quite well-differentiated ambulacral bases are present; the ambulacral shafts and bases are considerably elongated, undifferentiated, and very short with parallel sides in S. septemtrionalis sp. nov. Savignaster was identified by Gale (2011 a, 2011b) as a basal pterasterid, which possesses most, but not all of the synapomorphies of extant members of the family. Savignaster is known from the Bathonian of Kutch, India (A.S. Gale, pers. obs.), the Oxfordian of Switzerland and France (Gale 2011 a, 2011b) and now from the Volgian of Spitsbergen. The Knorringfjellet specimens represent the first articulated material of the genus Savignaster in which the arrangement of the ambulacral groove ossicles can be seen.

In the original description of the genus Savignaster, Gale (2011a) inferred that the type species lacked the abactinal canopy typically present in pterasterids. In Savignaster, the abactinal and lateral adambulacral spines are enlarged and highly muscularized, features which would prevent their enclosure within a canopy membrane. However, as noted by Gale (2011a), Savignaster was probably capable of reducing its coelomic volume by contracting muscles present between abactinal ossicles, thus pushing out the papulae and increasing its respiratory surface. Nance & Braithwaite (1972) discussed a similar contraction capacity in Recent mucus-secreting Pteraster tesselatus and Gale (2011a) suggested improvement of oxygen uptake as a driver of specialized pterasterid evolution. Accordingly, Savignaster septemtrionalis sp. nov. would have been well adapted to the fluctuating oxygen availability in the Late Jurassic PalaeoBarents Sea of Spitsbergen.