Euaxosida sp. A

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 : 226-228

Euaxosida sp. A

(Fig. 7A1, A2)

MATERIAL EXAMINED. — UCBL-FSL 712909 (coll. Auvray), the oral surface of about one-half of a single specimen consisting of two interbrachia, two arms, and a portion of the disk. Only overall ossicular form survives, the surface texture of remaining ossicles altered and pitted after removal of the secondary rust-colored deposits. Radii of two remaining arms approximately 19 mm and 23 mm; two remaining interradii 11 mm and 13 mm, the distended mouth frame indicating flattening with concomitant increases in radii values.

LOCALITY. — Tarafin Signit, Morocco; middle part of the Lower Ktaoua Formation, early Katian.

DESCRIPTION

Disk large, arms triangular, abruptly tapering; interbrachium thought abruptly rounded. Vaulting probably low. Aboral skeleton might consist of small, spicular ossicles.Any madreporite unknown. Specimen margin appearing marked by an ambital framework series of approximately equidimensional, triangular? ossicles. Actinals rectangular, elongate transverse to ambulacral ossicles. Axials known only from incomplete transverse ridges, axials appearing rectangular, podial basins relatively small. Adaxials appearing proportionately small, rectangular, and transversely elongate.

REMARKS

Ossicular preservation is poor. Arm taper and ossicular arrangement suggest arms are complete. The two remaining arm tips as preserved appear to have been somewhat rounded suggesting some specimen arching in life. Mouth angle ossicles are represented by incomplete ossicular swellings, these dilated as to indicate some specimen flattening with burial.

Both surviving interbrachia exhibit now-disrupted irregular elongate ossicles, at least some directed away from the axials, these suggestive of somasteroid virgals; however, the ossicles do not correspond in number with the axialadaxials, they are not uniform, and orientation of distal ossicles appears irregular (Fig. 7A1). Although uncommon among asteroids, homoplastic ossicular alignment suggestive of virgal series is developed in crown-group Tremaster Verrill, 1879 (Fig. 7B1, B2), and Jurassic occurrences similar to that of extant Tremaster have been described ( Hess 1981; Smith & Tranter 1985). Axial shape, including transverse ridge expression, is asteroid-like. Inferred euaxosidan affinities are based on the apparently weak linkage between the axial and adaxial based on ossicular positioning and presence of a pulled-apart and therefore weakly linked axial/adaxial series. A large disk with peripheral marginal series is unusual but occurs elsewhere (Fig. 9D) among Paleozoic asteroids.