Plesiastropecten hallovensis Peyer, 1944

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 : 178-179

Plesiastropecten hallovensis Peyer, 1944

Fig. 14

Plesiastropecten hallovensis Peyer, 1944: 320 , pls 7–8.

Plesiastropecten hallovensis – Blake 1984: 633, fig. 1a–h.

Diagnosis

As for genus.

Material examined

The holotype ( MZA L13a ) is a complete asteroid in a claystone matrix, comprising part and counterpart, from the Lower Jurassic (Hettangian, liasicus ammonite Zone) of Hallau, Canton Schaffhausen, Switzerland ( Peyer1944). It is contained in the collections of the Museum zu Aller Heiligen, Schaffhausen, and has suffered from considerable over-preparation. Additional material includes a suite of specimens from the same locality and horizon, most notably, specimen MZA L 13b/32a-e, which is a partially dissociated individual of which one arm is well preserved ( Blake 1984: fig. 1a–g). This specimen is coated with a glue-like substance which obscures much detail and requires specialist preparation. The present description is based on these two individuals.

Description

The overall form is well shown by the type specimen ( Fig. 14A ); the arms are moderately long and tapering, the interbrachial angles acute (R:r = approximately 6:1). The arms bear an even fringe of closely spaced marginal spines. The marginals are numerous, narrow and short with a three-tiered structure, resembling tiny ‘cottage loaves’ of bread ( Fig. 14F ). They are clearly visible in MZA L 13b/32a-e, and are robustly paxilliform, bearing a single large spine pit centrally. The abactinal ossicles are conspicuous and relatively large, convex to flat and carry four to six, lobe-like lateral projections. Each abactinal has a single, large and centrally placed crater-like spine pit ( Fig. 14B, D ). The ossicles imbricate, and the lobes are notched on their inside (actinal) surfaces for contact with adjacent ossicles. These ossicles are more or less radially symmetrical in the disc, but elongated in the arm, where the abactinal ossicles are arranged in three rows comprising radials and two adradials ( Fig. 14B ). In the distal arm, the large radial and adradials are separated from adjacent ossicles of the same row by smaller inset ossicles. The adambulacrals were prepared in a small part of specimen MZA L 13b/32b. They are very broad and short (3:1), and carry 5–6 large, horseshoe-shaped spine bases arranged in a single transverse row ( Fig. 14E ). The adambulacrals of opposing rows are slightly angled (150°) in a proximal direction. The ambulacrals are only seen in abactinal view, and the ambh forms a conspicuous, elongated triangular proximal wing which overlaps the more proximal adjacent ambulacral ( Fig. 14D ). The ambb is oval and symmetrical. The marginal spines are conspicuous, forming an even comb-like fringe to the starfish. Each marginal plate carries a single tapering spine with a unique construction. The cross section is a shallow U-shape, and the abactinal surface bears a groove. The rounded actinal surface is made up of 4–6, length-parallel coalescing rods of trabecular stereom. The lateral margins of the spines carry outwardly directed short barbs, probably lost on the holotype through over-preparation. The base of the spine is swollen and rounded, and a simple socket on the base articulates with a boss on the marginal. The abactinal spines are shorter, and round in cross section; these are also composed of elongated trabeculae. The 5–6 adambulacral spines articulate with horseshoe-shaped bases, and are long and gently curved.

Remarks

The robust paxilliform construction of the marginals is not found in any benthopectinid asteroid, but is characteristic of modern solasterids and the Jurassic genus Plumaster , and each marginal carries only a single spine. We cannot agree with Blake (1984) that these resemble marginals of benthopectinids, other than in that they carry large spines. The large, oval or rounded, imbricating abactinal ossicles which are alternately large and spine-bearing and small and spineless in the arm are quite different from the parapaxillae of benthopectinids. The construction of the marginal and abactinal spines, with elongated trabeculae running along the length of the spines and a semicircular, concavo-convex cross section bearing two rows of lateral thorns, are dissimilar to benthopectinid spines, which are conical, cylindrical and carry numerous, irregular, distally directed thorns. The concavo-convex construction is otherwise seen only in the multi-armed Early Jurassic Plumaster ophiuroides (Wright, 1863) . The transversely broad, short adambulacrals, carrying 5–6 large curved adambulacral spines, are quite unlike adambulacrals of benthopectinids which are narrow and rather elongated, but are similar to those of Plumaster ( Gale 2011b: figs 13–14). The elongated, imbricating proximal flanges of the ambulacral heads are not found in any benthopectinid, where the ambulacral heads are short, upright and do not imbricate, as in all paxillosidans ( Gale 2011a), but are similar to those of Plumaster ( Gale 2011b: fig. 14b–c). Other than a superficial similarity in shape and the presence of elongated marginal and abactinal spines (also found in other asteroids), Plesiastropecten hallovensis does not show any of the characteristics of the Benthopectinidae , but bears a close similarity to the Pliensbachian–Aalenian multiarmed genus Plumaster , with which it shares broad, short adambulacrals with 5–6 large transversely arranged, hyaline spines and the flanged, imbricating abactinal ossicles ( Gale 2011b).

Figures

Fig. 14. Plesiastropecten hallovensis Peyer, 1944. A. Holotype (MZA L13a), the original of Peyer (1944: pl. 7) and Blake (1984: fig. 1a–h). B. Enlargement of distal arm. C–F. Line drawings of MZA L 13b/32a–e. C. Ossicle distribution of entire individual; note large, stellate abactinal ossicles which each carry a large boss for attachment of a single spine. D. Ambulacral ossicle, in abactinal view, and two abactinal ossicles. E. Adambulacral ossicle with spines attached. F. Two marginal ossicles, with serrated spines attached. Provenance: Lower Jurassic (Hettangian, liasicus ammonite Zone) of Hallau, Canton Schaffhausen, Switzerland. Scale bars: A = 10 mm; B–C = 5 mm; D–G = 2 mm.