Petraster rigidus ( Billings, 1857 )

Blake, Daniel B. & Lefebvre, Bertrand, 2024, Ordovician Petraster Billings, 1858 (Asteroidea: Echinodermata) and early asteroid skeletal differentiation, Comptes Rendus Palevol 23 (17), pp. 217-239 : 222

Petraster rigidus ( Billings, 1857)

( Fig. 1A)

Palasterina rigidus Billings, 1857: 291 .

Petraster rigidus – Billings 1858: 80, pl. 10, fig. 3. — Spencer 1914: 40. — Schuchert 1915: 141, pl. 27, fig. 5. — Spencer 1927: 359. — Spencer & Wright 1966: 43. — Blake 2018: 28, pl. 4, fig. 5.

Palaeaster matutina – Hall 1868: 294.

TYPE MATERIAL. — Holotype. Canada • GSC 1401 a, Palasterina rigidus Billings, 1857 ; an incomplete, weathered specimen, only the oral surface exposed.

DIAGNOSIS. — Petraster species thought to exhibit two axillaries, one triangular and reaching the lateral margin, the second embedded; a single series of robust actinals well developed, it limited to disk.

TYPE LOCALITY AND HORIZON. — Cobourg beds , Ottawa , 45°026’37”N, 75°026’9”W; Trenton Limestone , Sandbianearly Katian .

DESCRIPTION

Petraster in which arms are moderately elongate, R reaching at least c. 20 mm. Inferomarginals robust, rectangular, relatively wide on disk, more nearly equidimensional on arm; some ossicles appear to bear accessory pustules.Two available interbrachia of the holotype differ, one with a well-defined triangular axillary and a possible second small axillary enclosed on disk adjacent to MAO. Actinals on disk in well-defined single series. Second interbrachium, ossicular arrangement poorly preserved and ill-defined. Axillary, actinals not clearly defined. Adaxials rectangular, transversely elongate.

REMARKS The poorly known type species is similar to other species assigned to the genus based on overall form and the limited data available on oral-surface expression, including inferomarginal shape, disk configuration, axillary, and actinal expressions. The diagnosis focuses on expression of the better-preserved interbrachium. The skeleton aboral to inferomarginals is almost unknown, the madreporite, axials, and MAO unknown. Posited presence of two axillaries serves to separate P. rigidus from other Petraster species except from P. kinahani , with specificities of expression differing between these two. The more lateral axillary is similar to the single axillary of P. crozonensis , the adjacent inferomarginals differ in outline. A few small elongate ossicles lateral to the inferomarginals of one arm appear unlike expressions among other species, although these ossicles as preserved appear dissimilar from traces exposed on other arms.