Echinosphaeraster scutatus
Creators: name: Gale, Andrew Scott
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Animalia
Echinodermata
Asteroidea
Valvatida
Sphaerasteridae
Echinosphaeraster
Echinosphaeraster scutatus

description: Echinosphaeraster scutatus (Goldfuss, 1833)
(Figs 10C, D, 14A–C, 15A–K, 16A–D)
$1833 Asterias scutatus Goldfuss: 210, pl. 63, fig. 8a–g.
1875 Spherites scutatus (Goldfuss); Quenstedt: 725, pl. 88, figs 35–47.
1906b Sphaeraster scutatus (Goldfuss); Schondorf: 266, figs 8–13, 22, 24, 25, 27, 32; pl. 24, figs 8–11.
1975 Sphaeraster scutatus (Goldfuss); Hess: 37, pl. 8, fig. 17.
1984 Sphaeraster scutatus (Goldfuss); Blake: 76, figs 1C, E, F, 2D, 4A, 7, 8E.
2003 Sphaeraster scutatus (Goldfuss); Radwanska: 67, pl. 1, figs 1–16.
Types. The abactinal ossicle figured by Goldfuss (1833, pl. 63, fig. 8a–c) is here selected as lectotype. The present whereabouts of this specimen are unknown, but according to Schondorf (1906b), it is in either the Munich or Berlin collections.
Diagnosis. As for the genus.
Material. Abundant ossicles from the Kimmeridgian of south-west Germany and Switzerland are present in the collections of the University of Tubingen (GPIT /80/94- 12; GPIT/A5/51; GPIT/A5/52, south-west Germany), the Naturhistorisches Museum Basel (Switzerland, unreg.) and the Natural History Museum, London (NHMUK EE 17699–17701, material from the lower Kimmeridgian of Plettenberg, Germany). The present description is largely based on photographs and a highquality cast (GMHL 1929) of an almost complete, articulated individual in the collection of the late Mr J. Buijs of Huizen, the Netherlands, collected from the Kimmeridgian of the Swiss Jura, which has not been described previously (Buijs 2011). This specimen forms the basis for the first accurate reconstruction of E. scutatus.
Description. The well-preserved individual (GMHL 1929, Fig. 14) is approximately 9 cm in diameter, and displays almost the entire abactinal surface, a large part of which shows the ossicles in original position, as well as scattered, disarticulated spines. The base of the specimen is crushed, and obscured by matrix. The primary abactinal ossicles (radials, interradials and centrale) are seen in place, although partly obscured by matrix. The undistorted portion of the abactinal surface is gently domed, but less strongly than in S. tabulatus (see above). The arrangement of primary radial and interradial ossicles is essentially similar to that in S. tabulatus (compare Fig. 11C with Fig. 16A).
A circlet made up of five pr and five pir (Fig. 16A) occupies over half the diameter of the adapical surface. Within this, 11 ico surround the small, pentagonal ce. The ce articulates with four similarly sized ico in all interradii except B-C, in which a pair of elongated plates are present. These possess proximal semicircular notches that define a triangular periproct; the third side of the periproct is formed by the notched distal side of the ce. All ico and the ce lack spine bases.
The pir are heptagonal and distally articulate with two similarly sized interradial hexagonal ossicles (Fig. 16A), which in turn articulate with three similar plates. The next row is made up of four plates, of which the more radially positioned pair are smaller than the interradially positioned ossicles and lack central spine attachments. Thus, only seven ossicles in each interradius carry large spines. The pr are hexagonal (Fig. 15C), and more distal radials form a single column of hexagonal plates. The primary radial and the two adjacent plates are of nearly equal size, and each carries a large central, circular spine base. The radials and interradials that carry large spines are present on the distal part of the abactinal face of S. scutatus and are similarly sized and shaped. They bore an outwardly (abactinolaterally) directed array of 50 large conical spines, with five smaller spines on the ossicles. The fifth row of interradial ossicles and the fourth to fifth rows of radials form the ambitus (Fig. 16A, C).
Isolated large abactinal ossicles are the best-known fossils of E. scutatus, and these have been well illustrated (see synonymy list). They are hexagonal or rarely heptagonal (primary interradials), slightly broader than tall, and on the external face carry a large spine base, which is set on a centrally placed mound (Fig. 15C, D, F, H). The spine base has a crater-like circular rim and a depressed central region. The margins of the abactinal plates are deeply and evenly notched by papular pores (four to six per side); vertical articular ridges run between the pores and each carries a centrally placed, single circular pit. The spines that articulate with the bases have been well described (Blake 1984, fig. 2D) (Fig. 15E, K). They are conical and taper rather irregularly to a sharp point, and the base is slightly flared for articulation with the crater-like rim of the spine base (Fig. 15E, K). The centre of the base of the spine is hollow, presumably for the insertion of connective tissue or muscle fibres.
Fragments of the actinal surface and isolated ossicles described and figured by Sch¨ondorf (1906b) provide some information on the lower surface of the species. The actinal ossicles are small, numerous and equidimensional to slightly elongated (Fig. 15A, B). The external surfaces are very irregular in shape, and the embayments and protuberances interlock to form a strongly articulated, imbricate tesselation of aligned ossicles. The flat external surfaces of the plates bear four to eight small crater-like spine bases, and the articulated portions of actinal surface show that this was flat. The adambulacral ossicles are 2–3 times as tall as long or broad, and the adradial surface is concave, the abradial gently convex. The abactinal surface, which contacts the ambulacral, is square and typically valvatidan. Ambulacrals are very similar to those of S. tabulatus (see Schondorf 1906a).
An oral ossicle of E. scutatus was figured by Schondorf (1906b, fig. 28). The oral ossicle is taller than long, with a nearly square junction between the actinal and distal margins and a short, pointed pb. The oradm is tall and vertically oriented. The inner (interradial) face displays small rounded surfaces for articulation with the corresponding oral ossicle across the interradial line. The oral ossicle differs from those of most valvatid families, which typically are low and have a long pb (see Gale 2011), but more closely resembles those of the stauranderasterids Manfredaster bulbiferus (see Fig. 8N) and Hadranderaster simplex (Geinitz, 1871) in important respects. Firstly, all possess a short pb and an actinal margin set at right angles to the proximal margin. Secondly, the enlarged, tall, vertical adambulacral articulation structure is found in all three genera.
Comparison with Sphaeraster tabulatus. The new material enables a detailed comparison to be made with Sphaeraster tabulatus. The major differences are:
1. Sphaeraster tabulatus has a very tall, domed form of the abactinal surface compared to the much lower dome of E. scutatus.
2. Echinosphaeraster scutatus has a very broad circlet formed by the pr and pir extending across half of the abactinal surface (Fig. 16A); this is proportionately smaller in S. tabulatus, representing one-third of the abactinal diameter (Fig. 11C).
3. Echinosphaeraster scutatus has a small number (approximatey 50) of large abactinal plates, of similar breadth, between the ambitus and the primary circlet, set in four rows per radius (Fig. 16A); S. tabulatus has a much larger number of abactinal plates (about 450), which are set in four interradial chevrons per radius and taper in breadth towards the ambitus (Fig. 11C).
4. Echinosphaeraster scutatus has large, centrally placed, conical spines on the large abactinal ossicles whilst S. tabulatus has six smaller spine bases placed on each angle of the hexagonal plates.
5. The adambulacrals of E. scutatus are much taller than those of S. tabulatus.
Although the arrangement, size and shape of the ossicles of the central circlet is identical in the two taxa, they differ very significantly in the proportionate breadth of the primary circlet, the number of abactinal plates, their arrangement, spination and size, and the way they taper towards the ambitus.
description: Published as part of Gale, Andrew Scott, 2021, Taxonomy and phylogeny of the ' football stars' (Asteroidea, Sphaerasteridae), pp. 691-741 in Journal of Systematic Palaeontology 19 (10) on pages 710-714, DOI: 10.1080/14772019.2021.1960911, http://zenodo.org/record/10949919