Salvaster roberti

Saucède, Thomas, Dudicourt, Jean-Christophe & Courville, Philippe, 2012, Description of two new fossil echinoids (Echinodermata: Echinoidea) from the Early Hauterivian (Early Cretaceous) of the Paris Basin (France), Zootaxa 3512, pp. 75-88 : 80-81

Salvaster roberti gen. et sp. nov.

Figures 2 A–E & 3A–B

Holotype. The holotype and only known specimen is UBGD 277467 ( Figs. 2 and 3 ) housed in the geological collections of Université de Bourgogne (Dijon, France).

Type locality. Collected by P. Robert at “Les Rèbles” (47°35’N, 03°13’E) near Saint-Sauveur-en-Puisaye, Yonne, France; Early Cretaceous, Hauterivian, Acanthodiscus radiatus chronozone, Calcaires à Spatangues Formation.

Etymology. After the last name of Mr. P. Robert who collected the specimen in the field in November 1995.

Diagnosis. As for the genus, by monotypy.

Description. Test length is 21.5 mm, test width 19.4 mm and test height 14.2 mm. In apical view, test outline ovate, deeply indented anteriorly ( Fig. 2 A). In lateral view, gross shape rectangular with apical side inflated and convex ( Fig. 2 B). Lateral flanks gently and evenly curved towards the ambitus. Ambitus low, situated at the lowest third of test height. Ambulacrum III deeply sunken, forming a conspicuous and narrow frontal groove running from the apex down to the peristome ( Fig. 2 C–D). Groove vertical at the ambitus, evenly curved below towards the peristome and above towards the apex. Groove adjoining interambulacra 2 and 3 forming sharp keels on each side. Posterior end of test truncated, vertical and narrow, with two weak posterior bulges, visible in oral view ( Fig. 2 C). Oral surface grooved with depressed ambulacra.

Apical system nearly central, the distance between the apical centre and the anterior side is 47% of test length. There are ten apical plates: four genital plates, five ocular plates and one supplementary plate ( Fig. 3A ). Plates arranged in the intercalary pattern, which means that anterior genitals 2 and 3 are separated from posterior genitals 1 and 4 by oculars II and IV ( Fig. 3A ). Supplementary plate located between the anterior ocular pair II and IV, and the posterior genital pair 1 and 4. Each genital perforated by one gonopore. Genital 2, the largest apical plate, is forming the madreporite.

Periproct supramarginal, oval, vertical and flush with the test. It is located at the top of the posterior side ( Fig. 2 E). Peristome circular in outline and anterior in position, the distance to the anterior side is ca. 40% of test length. The peristome slightly faces in anterior direction.

Ambulacra flush with test above the ambitus. Ambulacral plates much smaller than interambulacral plates. Small, simple ambulacral pore-pairs, the two columns equally developed, but pores mostly indistinct. Paired interambulacra composed of rectangular plates, wider than high adorally. Plastron raised and slightly keeled, protosternous with plating behind the labrum biserial and undifferentiated ( Fig. 3B ). Next plates progressively enlarged backwards. Apical side covered with few and evenly scattered tubercles. Inner margin of the anterior groove more densely tuberculated, as well as the oral side, mostly on interambulacral plates. Fascioles are not available.

Remarks. The intercalary apical plating of S. roberti gen. et sp. nov. is a diagnostic feature of the order Holasteroida and the protosternous plastron a plesiomorphic condition present in the stem group Meridosternata ( Smith 2004). Despite a very peculiar test shape, S. roberti gen. et sp. nov. has most affinities with the genus Pseudholaster Pomel (1883) , which is known from the Hauterivian to the Paleocene and its type species Pseudholaster bicarinatus (Agassiz, in Agassiz & Desor 1847) from the Albian and Cenomanian ( Smith & Wright 2003; Smith 2004). Both genera present a deep frontal groove running from the apex to the peristome, although the interambulacra adjoining the frontal sulcus do not form such a sharp keel in Pseudholaster spp. as compared to S. roberti gen. et sp. nov.. In both genera, the peristome is oval, slightly inclined and typically facing to the anterior, and the protosternous plastron is slightly keeled. Pseudholaster spp. differ by their cordate test shape, almost petaloid ambulacra and the absence of supplementary plates in the apical system. Taphraster campicheanus (Cotteau 1858) , a ‘basal holasteroid’ from the Hauterivian of Switzerland is also characterized by a deep frontal groove ( Smith 2004). However, T. campicheanus is different from S. roberti in its slightly depressed paired ambulacra and downward projecting subanal region. Details of the apical and plastron plating of T. campicheanus are insufficiently known to precise its potential affinity with S. roberti gen. et sp. nov..

In S. roberti gen. et sp. nov., the most distinctive feature is the presence of a very deep and sharply rimmed frontal groove. The occurrence of a frontal groove is not that atypical in atelostomates. It is closely related to the way echinoids feed and is a convergent, more or less pronounced character in atelostomates ( Smith 1984, 2004). It appeared independently in the Collyritidae ( Cardiolampas , Cardiopelta and Proholaster ) and the Tithoniidae ( Tithonia and Tetraromania ) as soon as the Middle Jurassic and was present in all species of Collyritidae by the Early Cretaceous ( Cardiopelta and Proholaster ). In the Holasteroida , a deep and vertical frontal groove evolved independently in the Cretaceous and Paleocene representatives of the Cardiasterina Pomel 1883 and ‘stegasterid’ ( Smith 2004).

Figures

FIGURE 2. A-E, Salvaster roberti gen. et sp. nov., holotype, UBGD 277467, Calcaires à Spatangues Fm, Saint-Sauveur; apical (A), lateral (B), oral (C), frontal (D) and posterior (E) views of test, x 2.2. Scale bar: 10 mm.

FIGURE 3. A – B, drawings of plate pattern in Salvaster roberti gen. et sp. nov., holotype, UBGD 277467; apical system (A) and plastron (B).