Xandarosaster hessi Blake, 1984

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 : 181-182

Xandarosaster hessi Blake, 1984

Fig. 15A–C

Xandarosaster hessi Blake, 1984: 638 , figs 2a–i, 3.

Material examined

The single specimen available ( NMB M9683 ) comprises fragments of three arms and dissociated ossicles, intimately entwined with an isocrinid crinoid on a single slab, from the Bajocian of Reigoldswil (Canton Baselland, Switzerland). The specimen is very fragmentary and is not easy to interpret, because pressure solution has welded scraps of calcite onto most ossicles, making discrimination of features difficult.

Description

The ossicles of the ambulacral groove are the best preserved and most distinctive part of this specimen ( Fig. 15A ). The ambh imbricate strongly proximally, and are elongated and triangular. The ambulacrals are waisted, and the ambb carry short asymmetrical flanges for padam and dadam, and articulation surfaces ada1 and ada2. The adambulacrals are rectangular and very elongated, being approximately three times longer than broad; half to two-thirds of the actinal face is occupied by a V-shaped depression for the adadm ( Fig. 15B–C ). An oblique ridge runs from the proximal part of the V to the distal abradial margin. Proximal adambs carry two large swollen spine bases set obliquely on the proximal face of the ossicle, and more distal adamb have a single spine base.

Several oral ossicles are visible. These are very broad and gently convex on the external surface, and an adambulacral articulation ridge and deep V-shaped concavity for the oradm muscle are present. The abactinal ossicles are of even, small size, have a lobed base and carry a large centrally placed, rounded spine boss. Possible marginals are elongated-rectangular and imbricate distally, with a very large round spine base on the proximal part of the external face. The abactinal and marginal spines are conical and moderately long, and have swollen bases. They are made up of elongated trabeculae of stereom.

Remarks

The ambulacral/adambulacral articulation of X. hessi is utterly dissimilar to that of benthopectinids. The ambh is broad and short in X. hessi , with a strong, short transverse actinal ridge. In benthopectinids, the ambb is triangular and elongated. Xandarosaster hessi has short, oval and nearly symmetrical facets for padam and dadam, which are triangular and strongly asymmetrical in benthopectinids. In X. hessi ada2 and adada are fused and ada3 is absent. In benthopectinids, all three facets are discrete, and ada2 and adada are placed on a ridge adjacent to a concavity on both ambulacrals and adambulacrals.

The construction of the spines, with elongated hyaline trabeculae, is unlike that of benthopectinids in which the spines are made of thorny stereom with distally directed barbs. Moreover, the elongated, strongly imbricating, asymmetrical ambh are not present in benthopectinids, where the heads are short and do not imbricate.

In summary, X. hessi does not possess a single character of the Benthopectinidae , but has features unique to the Spinulosida (sensu Gale 2011a), including spines constructed of elongated hyaline trabeculae and elongated, triangular proximal ambh which strongly imbricate proximally. Its affinity with other spinulosidans is uncertain, although some aspects of the ambulacrals and adambulacrals are broadly comparable with those of solasterids. The very elongated rectangular adambulacrals are unique to X. hessi . Adambulacrals of this type, currently indeterminate ( Fig. 15D ), are also found uncommonly in Jurassic sedimentary rocks such as the lower Oxfordian of Andelot-Morval, France.

Figures

Fig. 15. A–C. Xandarosaster hessi Blake, 1984. A. Holotype slab (NMB M9683), showing articulated ambulacral ossicles and isolated adambulacrals (arrowed); the original of Blake (1984: fig. 2a1–3). B–C. Line drawings of actinal views of adambulacrals; note extreme elongation, long interadambulacral muscle insertion site and boss for a single, large subadambulacral spine. D. Similar adambulacral ossicle of indeterminate form (NHMUK PI EE 18008). Provenance: A–C. Bajocian of Reigoldswil, Canton Baselland, Switzerland. D. Oxfordian, Andelot-Morval, Jura, France (for details of locality, reference is made to Gale 2018: fig. 20). Scale bars: A = 10 mm; B–C = 5 mm; D = 0.5 mm.