Description

Description 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. 3 C) and PMO 217.977 (Fig. 3 D) 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, 2011 b). 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. 3 E). 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 2011 a: pl. 21 figs 2, 5). This particular spine morphology is also clearly visible in paratype PMO 218.008 (Fig. 3 F), a partial arm preserving multiple articulated spines.

Common Names

No common names available.

Taxonomic Hierarchy

Kingdom: Animalia

Phylum: Echinodermata

Class: Asteroidea

Order: Velatida

Family: Pterasteridae

Genus: Savignaster

Species: Savignaster septemtrionalis Rousseau & Gale, 2018