Sellaea stryx (Di Stefano, 1889)

(Fig. 8 A-F)

Caprotina stryx Di Stefano, 1889: XI, 23, 24, 40, pl. 7, fig. 1a-f. — Parona 1899: 381. — Parona et al. 1909: 186, pl. XXII, figs 2-3. — Kutassy 1934: 140, 141.

Caprotina cfr. strix – Parona 1897: 14.

Sellaea stryx – Rineau & Masse 2022: 2, 5, 9, 12, 13.

MATERIAL EXAMINED. — Sellaea is found in dense accumulation of both connected and isolated shells in Mazo Chico (Soba Valley) (Fig. 8G). Only outcrop photographs of natural sections have been studied due to the hardness of the limestone beds that prevent sampling specimens.

STUDY INTERVAL. — Mazo Chico limestones, Ramales Formation (upper middle Albian).

DESCRIPTION

Shell inequivalve; left valve conical and coiled (less than one whorl), right valve conical and straight, triangular in longitudinal section (Fig. 8A, B; mean height from umbo to commissure approximately 130 mm). Valves ovoid in transversal section (mean section 80 × 60 mm) with a ventral part straight to slightly depressed. Some right valves deformed in transversal section due to growing encrusted to other Sellaea individuals in bouquets (Fig. 8C). Outer calcitic shell layer very thin (mean thickness 0.5mm), generally not preserved. Inner aragonitic shell layer thick (mean thickness in ventral part 4 mm). Ligament invaginated in a reniform posterodorsal ligamentary cavity.

Left valve anatomy

Body cavity wide, triangular in transverse section due to ventral flexure. CTS slightly curved to the AT, deep, elongated and narrow, separated from the BC by a very thin blade (thickness <0.5 mm). PAC well developed posteriorly to the PM (Fig. 8A), divided by a thick wall (mean thickness 4 mm) in two ovoid cavities (10 × 18 mm). A single row of six millimetric circular cavities extend the PAC ventrally on a single specimen (Fig. 8D). PM-LV vertical, inwardly directed, and rooted on a strong wall (mean thickness 5 mm), which separates CTS and PAC-LV, protruding on the RV. PT and AT centimetric, straight and conical.AAC-LV present anterodorsally (Fig. 8B). AM-LV vertical on a thick AM-LV/BC wall.

Right valve anatomy

Body cavity wide, triangular in transverse section due to ventral flexure. ATS and PTS circular, separated by a very large CT that can reach up to 40 mm in height. CT crescentic, surrounding the ATS and tilted towards the posterior side. PMC lying posteriorly, in which lies a vertical PM-RV facing the protruding myophore of the opposite valve. AM-RV rooted anteriorly from the BC on a horizontal shelf; shelf entirely devoid of canals (Fig. 8B).

DISCUSSION

The description of these shells can only match those of Sellaea stryx due to the AM-RV located on a horizontal shelf that lacks pallial canals (Fig. 8B). The absence of pallial canals is common to Sellaea stryx and Sellaea sicula (Di Stefano, 1889), formerly placed in Caprotina d’Orbigny, 1842 due to this absence and relocated in Sellaea after cladistic analysis (Rineau & Masse 2022). Sellaea stryx differs from S. sicula by its horizontal AM-RV, whereas it is vertical and protruding into the LV in S. sicula as in Caprinulidae Yanin, 1990. The row of pallial canals on the LV observed in a single shell (Fig. 8D) has never been observed in other S. stryx specimens. It is likely to be homologous to the PAC, which extends ventrally and become partially canaliculated. It is however not sufficient to raise a new species at the present time, and we consider it here as part of the taxon’s intraspecific variability.

Sellaea, formerly understood to be close to Caprotina, was placed in the Caprinulidae by Skelton (2013a) following Steuber & Bachmann (2002), who proposed the existence of a parallelism concerning the acquisition of pallial canals probably due to overall similarity, but without specifying synapomorphies. However, Rineau et al. (2020) tested the phylogenetic position of a Sellaea species for the first time within rudists at the family scale and found it to be a sister group to all canaliculated rudists, with Hippuritidae and Radiolitidae. The position of the genus Sellaea will require a more exhaustive cladistic study to precisely assess its position relative to other genera of Monopleuridae Munier-Chalmas, 1873, Caprinidae, and Caprinulidae in order to test the homology of accessory cavities and pallial canals. Furthermore, the monophyly of Sellaea has not yet been formally tested, particularly with regard to Pachytraga. On the other hand, Rineau & Masse (2022) have clearly demonstrated the existence of two clades within Sellaea, one restricted to Texas, and the other present in the Mediterranean province and Tibet. With the exception of Sellaea quadripartita (Orbigny, 1842), all Sellaea are considered to be of Albian age. Sellaea quadripartita was formerly described as a Caprotina species by d’Orbigny and reassigned to Sellaea by Rineau & Masse (2022). This species has all diagnostic features of Sellaea, with two accessory cavities in the LV and a row of canals in the RV. It is a Cenomanian species known from Western France, and it probably belongs to the Mediterranean Sellaea clade sensu Rineau & Masse (2022).