Amphorometra atacica

Amphorometra atacica (Doncieux, 1911)

Fig. 18

Synonymy. Antedon atacicus Doncieux, 1911: 160–161, pl. 15, fig. 28a, b; Biese and Sieverts-Doreck, 1939: 79; Glenotremites atacicus Gislén, 1924: 125, 127, 191; Biese and Sieverts-Doreck, 1939: 95; Jagt et al., 2002: 88.

Type material. Holotype, a single centrodorsal belonging to the Bories Collection housed in the palaeontological collections of the Paul Sabatier University of Toulouse under the catalogue number UPS BO-347.

Material examined. 15 centrodorsals in the Martinez Collection, 11 from Le Coucou including 2 here figured (Fig. 18A–F) (MNHN.F. A82017), 2 from Lavade, including 1 here figured (Fig. 18G–I) (MNHN.F. A82018 a), 1 from La Prade (Arquettes-en-Val), 1 from Montauriol; 1 IIBr and 2 cirrals from Le Coucou; 9 brachials including 5 here figured (Fig. 18J–Q) (MNHN.F. A82018 b) from Lavade, 1 brachial from La Commanderie.

Emended diagnosis. Centrodorsal hemispherical to inverted conical (0.62<H/D<1.04); lateral faces with interradial ridges often protruding and converging towards apex, rendering star-like appearance, interradial crests forming conspicuous projections on adoral face; cirri arranged in two columns of 3 or 4 cirrus sockets separated by interradial ridges, cirrus sockets subcircular in shape with marginal radiating crenulations; adoral face with depressed and smooth insertion areas of radials; basal insertions narrow and raised. Axial synarthry at IBr1+2 and IIBr1+2, other free IIBr with muscular synarthries, except for some pairs with IIBr joined by syzygy of radiating crenularium. Radial circlet, IBr2ax and distribution of syzygies along arms unknown.

Type stratum. Lower blue marls of the middle Ilerdian (early Ypresian), NP10–lower NP11.

Type locality. Les Lanes Sud, southwest of the village of Fontcouverte.

Description. Main quantitative characters of centrodorsals and brachials given in Tables 19 and 20, respectively. General shape of highly variable profile, from inverted conical (Fig. 18F) to hemispherical (Fig. 18A), with very prominent (Fig. 18A, C) to discrete (Fig. 18G, I) interradial ridges converging towards apex and rendering starlike appearance when not worn (Fig. 18I). Interradial ridges terminating in marked projection on adoral side (Fig. 18A) corresponding to outer end of basal insertion (Fig. 18B, C). Between two interradial ridges, two rows of 3 to 4 (rarely 5) circular cirrus sockets, wide, deep, and presenting marginal radiating crenulation (Fig. 18H), i.e., total number of 30 to 35 cirri. Cylindrical proximal cirrals wider than high with one of facets having perilumen markedly in relief. Aboral face of centrodorsal with smooth insertion surfaces of radials, depressed between interradial projections; narrow, slightly lanceolate, raised basal insertions bordered by fine symplexial crenularium observable on best-preserved facets (Fig. 18E), central cavity of variable diameter (0.17 <D’/D <0.26) subpentagonal to circular. Radials unknown. Symmetrical muscular synarthry articulating radial and IBr1 with large aboral ligament fossa and moderately developed muscle areas (Fig. 18J). Axial synarthry at IBr1+2 and IIBr1+2 with broad fulcral ridge encompassing neural canal and deep ligament depressions (Fig. 18K, M). IBr2ax unknown. Proximal muscular synarthry of IIBr1 asymmetrical (Fig. 18L). Muscular synarthry articulating two IIBr with fulcral ridge slightly oblique, aboral ligament area wide and deep, adoral ligament areas almost equal to or slightly wider than muscular areas, distal facet of proximal brachial having finely festooned border, making outer surface of arms slightly rough (Fig. 18N–P). Presence of IIBr pairs united by flat syzygy with 20 to 25 radiating crenulae (Fig. 18Q), IIBr with one syzygial facet less frequent than free IIBr with two muscular synarthries.

Remarks. Amphorometra ornata Rasmussen, 1972 from the lower Ypresian London clay is very close to A. atacica. Paul (1992) completed the description of that species on the basis of abudant material including in particular a theca with proximal arm segments still connected. In addition to a highly developed ornamentation, especially on the arms, this species differs from A. atacica in having a lower centrodorsal (0.4 <H/D <07) (Fig. 19) with less differentiated interradial ridges, the proximal synarthry of IBr1 with more elongated muscular areas and the synarthries at IBr1+2 and IIBr1+2 with a narrower fulcral crest and deeper ligament depressions, and syzygies with <20 radiating crenulae. At a comparable centrodorsal diameter, the number of cirri is higher in A. ornata.

Occurrence. Early Ypresian of Corbières (middle Ilerdian, NP10–11).