Nearchaster spinosus ( Blake, 1973 ) Gale & Jagt, 2021

Gale, Andy S. & Jagt, John W. M., 2021, The fossil record of the family Benthopectinidae (Echinodermata, Asteroidea), a reappraisal, European Journal of Taxonomy 755, pp. 149-190 : 160-161

Nearchaster spinosus ( Blake, 1973) comb. nov.

Mistia spinosa Blake, 1973: 48 , pl. 16 figs 30–44, pl. 17 figs 1–21, 35–36.

Brisingid (?) – Zullo et al. 1964: 334.

Material examined

UCMP A-5018 (holotype no. 10675) is the type and only specimen; it exposes a partly disarticulated abactinal surface showing the disc and proximal portions of four arms. Large marginal spines and smaller abactinal ones are visible. Marginals, adambulacrals, ambulacrals and abactinal ossicles of the holotype were figured individually by Blake (1973).

Occurrence

Keasey Formation (Lower Oligocene) near Mist, Oregon ( USA).

Description

The abactinal surface of UCMP A-5018, embedded in matrix ( Blake 1973: pl.16 fig. 44), shows part of the disc and four proximal arms. Although the outline is retained, the ossicles are jumbled and largely dissociated, such that adambulacrals and ambulacrals are visible on the actinal surface. The marginal spines, largely in place, are elongated and tapering. The abactinal spines are much smaller, perhaps one-fifth the size of those on the marginals. The adambulacrals are well preserved, subrectangular, with 2–3 large subadambulacral spine bases, and the concave inner (abactinal) surface and ridge bearing ada2 and ada3, characteristic of benthopectinids. The ambulacrals have the typical hourglass shape of benthopectinids, and asymmetry of the interambulacral muscles (P1 small, P2 large) is seen. The marginals are longer than broad with a convex, mound-like outer surface which carries 1–2 large spine bases and a number of sparsely scattered smaller ones. The inner surface of the marginals is flat. The abactinal ossicles are parapaxillae, with centrally placed, single spine bases, surrounded by a ring of smaller spines.

Remarks

As recognised by Blake (1973), the distinctive characters of the ambulacral, adambulacral and marginal ossicles place this form firmly in the Benthopectinidae . Comparison with extant benthopectinid species studied here indicates that Mistia spinosa shares important characters with the Recent Pacific genus Nearchaster , including the following:

1. Adambulacrals are nearly identical in shape to those of Nearchaster aciculosus , and both carry 2–3 bases for subambulacral spines.

2. Marginals are closely similar to those of N. aciculosus in both shape and distribution of spine bases.

The overall form of the body, with large marginal spines, and shorter abactinal spines on the disc is broadly similar to the development in the genera Benthopecten , Nearchaster and Myonotus Fisher, 1911 (see Fisher 1911: pls 22–26). The proportionate sizes and distributions of spines in Mistia spinosa are closest to those in Benthopecten claviger Fisher, 1910 , Myonotus intermedius (Fisher, 1910) and Nearchaster aciculosus (Fisher, 1910) (see Fig. 2E–F herein).

The abactinal parapaxillae of Mistia spinosa are very close in structure to those of N. aciculosus , with a central spine base surrounded by a ring of smaller ones.

In conclusion, Mistia spinosa is a benthopectinid which has remarkably detailed similarities of ossicle morphology to the present-day Pacific species Nearchaster aciculosus and it is therefore provisionally placed in that genus. The genus thus has a history in the Pacific Ocean of at least 33 million years.

Figures

Fig. 2. A–B. Pectinaster filholi Perrier, 1885, Recent, in abactinal (A) and actinal (B) views, Porcupine Trough, NE Atlantic (A.S. Gale collection, unregistered). C–D. Benthopecten simplex (Perrier, 1881), Recent, in abactinal (C) and actinal (D) views, Rockall Trough, NE Atlantic (NHMUK EE 13563). E–F. Nearchaster aciculosus (Fisher, 1910), Recent, in abactinal (E) and actinal (F) views, NE Pacific (USNM, unregistered). Scale bars: A–B = 7 mm; C–D = 6 mm; E–F = 20 mm.