Petraster Billings, 1858

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 : 220-221

Genus Petraster Billings, 1858

Petraster Billings, 1858: 79 .

Uranaster Gregory, 1899: 348 . — Spencer & Wright 1966: 43.

DIAGNOSIS. — Euaxosidans attaining moderate size, arm radius R reaching at least c. 35 mm; disk arched, arms subcylindrical, elongate. Abactinal ossicles small proportional to overall body size, irregular equidimensional to subspherical, bases commonly faceted; aboral ring, superomarginals, intermarginals, carinal series variably and not clearly differentiated, especially not on disk. Inferomarginal series well-defined. Superomarginals dorsal-lateral in position. Intermarginals thought to be limited to disk, in a single or irregular series.

DESCRIPTION

Palasterinid, disk size moderate, interbrachia narrowly rounded; arms elongate, outline triangular and gradually tapering to narrowly rounded tips; disk and arms probably moderately arched in life. Configuration of extraxial ossicles aboral to inferomarginals varied among species. Abactinals small relative to overall body size, somewhat varied in size and form, plate-like, granular, irregular spicular, or rod-like. Aboral ring, centrale not definitively recognized. Carinal series obscure to moderately well-defined. Superomarginals enlarged relative to ossicles of adjacent series, approximately equidimensional, domal; disk superomarginals obscure in some species. Adradialia granular or spicular, variably developed. Intermarginals granular, variably developed.Madreporite where known circular, aboral; on larger specimens, can be partially enclosed by pair of enlarged, crescentic ossicles. Inferomarginals, adaxials well-developed, adaxials rectangular to equidimensional, domal, pustulate; enlarged accessory bases lacking. Inferomarginals on oral surface proximally, becoming marginal distally, abutting adaxials for at least most of arm length.

Axials nearly equidimensional to weakly rectangular, wider than long; radial water-vascular channel relatively broad; transverse ridge well-developed, podial basins approximately equally shared by subsequent axials. Adaxials approximately equidimensional, external surface texture similar to that of inferomarginals; adaxials more numerous than inferomarginals at least on disk, more nearly paired on arms. Oral disk ossicles variably developed. Axillary number where known one or two, actinal series present or absent, variably developed. Accessories where known including relatively small spinelets and granules; large spines not recognized.

REMARKS

Uranaster was synonymized with Petraster ( Spencer & Wright 1966) . The type species of Petraster , P.rigidus Billings ( Fig. 1A1, A2), is incompletely known leading to the use of P. kinahani ( Baily, 1879) as the generic standard of comparison ( Dean Shackleton 2005). These interpretations are followed here.

Species inclusion in Petraster emphasizes irregularity of the skeleton aboral to the inferomarginals together with a fundamental uniformity of oral configuration. Five species are included, P. ramseyensis tentatively. Petraster caidramiensis n. sp., P. crozonensis , and P. kinahani are known from fairly complete specimens, in all three, aboral series (i.e., “above” the inferomarginals) of the arms are more clearly defined than those of the disk, in part because of limitations of preservation but also because disk ossicles appear less clearly differentiated. The subcylindrical proximal arm intervals of Petraster specimens were subject to compaction under sediment load exaggerating water-vascular channel breadth, the proportionately more robust distal arm intervals retaining a more natural water-vascular channel appearance.