Ostrea jibananandai, Halder & Mitra, 2021

Halder, Kalyan & Mitra, Aniket, 2021, Facultative monogamy in an early Eocene brooding oyster and its evolutionary implications, Acta Palaeontologica Polonica 66 (3), pp. 647-662 : 650-655

Subclass Autobranchia Grobben, 1894 Infraclass Pteriomorphia Beurlen, 1944

Fig. 3. Ostreid bivalve Ostrea jibananandai sp. nov. from Mangrol open pit mine, Surat, western India, Ypresian Cambay Basin. A. PG/CB/Os 47, holo- → type, left valve of a carrying female, internal view (A 1); detail showing left valve of an attached male (A 2); external view showing large attachment scar on plant matter demarcated by arrow (A 3). B. PG/CB/Os 46, paratype, left valve of a carrying female with auricles, internal view (B 1); detail showing left valve of an attached male (B 2); external view of dorsal part showing attachment on plant matter indicated by arrow (B 3). C. PG/CB/Os 3, paratype, both valves in place, external view of right valve (C 1); external view of left valve (C 2); internal view of left valve (C 3), inset marks two attached males; detail showing left valve of two males (C 4), arrows point to their hinges, the specimen indicated by white arrow is with its length greater than height .

Order Ostreida Férussac, 1822

Superfamily Ostreoidea Rafinesque, 1815

Family Ostreidae Rafinesque, 1815

Subfamily Ostreinae Rafinesque, 1815

Genus Ostrea Linnaeus, 1758

Type species: Ostrea edulis Linnaeus, 1758 , by subsequent designation ICZN opinion 94, 1926); Recent, northeastern Atlantic.

Ostrea jibananandai sp. nov.

Figs. 3–5.

Zoobank LCID: urn:lsid:zoobank.org:act:0E347615-3E71-4D17-A357-935B8C930BB4

Etymology: In honour of the late Jibanananda Das (1899–1954), one of the most prominent Bengali poets.

Type material: Holotype: PG/CB/Os 47, well preserved left valve of carrying female. Paratypes: PG/CB/Os 1, a female with both valves preserved; PG/CB/Os 1a, LV of a female attached on RV of PG/CB/ Os 1; PG/CB/Os 3, a female shell with both valves preserved; PG/ CB/Os 5, RV of a female; PG/CB/Os 25, LV of a specimen with indeterminate sex; PG/CB/Os 30, RV of a female; PG/CB/Os 36, LV of specimen with indeterminate sex; PG/CB/Os 41, RV of a specimen with indeterminate sex; PG/CB/Os 45, LV of a male; PG/CB/Os 46, LV of a female. All with preserved shell material. All from the type locality and horizon .

Type locality: Mangrol open pit mine, Surat, Gujarat , western India.

Type horizon: 1 m below the most prominent white shell bed, Mangrol open pit mine section, Cambay Shale , Ypresian (early Eocene), Cambay Basin .

Material.—Eighty five specimens, including the holotype, the paratypes, and 19 attached males from the Mangrol open pit mine section. Several juveniles attached on large shells are also assigned to this species.

Diagnosis.—Shells showing pronounced sexual dimorphism. Female shells large, attaining approximately 85 mm in height, relatively thin; distinctly higher than long; oblique. Strong shape modification occurring during ontogeny, from rounded in early juvenile stage to rather high in adulthood. Prominent umbonal cavity present in both valves. Left valve with anterior flange-like extension in dorsal part of the shell, providing settling space for dwarf male(s). Right valve with corresponding extension overarching dwarf male(s). Shell surface almost smooth, sometimes with blunt radial ridges on left valve and radial gashes on right valve. Dwarf male shells stunted, attaining approximately 20 mm in height, distinctly higher than long, attaching on inner surface of flange-like extension of left valves of females.

Measurements (in mm).—The largest specimen (♀), H = 86.5, L = 72.3; the smallest specimen (♂), H = 4.9, L = 3.87. For details see table S 1 in SOM, Supplementary Online Material available at http://app.pan.pl/SOM/app66-Halder_ Mitra_SOM.pdf).

Description.—Female shell large, maximum observed height 86.5 mm, higher than long, shape variable from subquadrate to subtrigonal; umbo opisthogyrous, incurved; juvenile shell up to a height approximately 5 mm almost circular, bulged in right valve, often rotated towards posterior nearly at right angles with respect to adult shell; left valve weakly convex, relatively thin; right valve thinner and flat to concave, except for convex umbonal region; anterior shell margin broadly rounded, posterior margin concave, ventral margin slightly rounded; surface of left valve commonly with large attachment scar, remaining part almost smooth, only with comarginal growth threads, sometimes with low blunt radial ridges; outside of right valve smooth with fine radial gashes in some specimens; hinge short, sometimes with auricles; ligamental area longer than high, base of ligament more or less straight, resilifer shallow, wider than flat bourrelets, surface of ligament longitudinally striated; chomata simple, observed mainly in juvenile stage, restricted near hinge; depression housing male(s) in flange-like anterior extension of hinge area in left valve narrow and high, with growth striae on its surface; similarly-shaped low striated platform at corresponding position in right valve; umbonal cavity prominent in left valve, relatively weak in right valve; adductor muscle scar large, high, reniform, oblique, both ends rounded, positioned in posterior half of the shell, slightly above mid-height, position variable to a certain extent with respect to shell height; muscle scar in juveniles suboval and narrow; Quenstedt muscle scar small, rounded, almost central and slightly below resilifer; commissural shelf narrow, slightly concave, proximal margin obscured on ventral side, prominent only in dorso-lateral part.

Male shell small, largest height observed 23.18 mm; commonly quite slender and flat, shape variable; left valve relatively thick, right valve thin; umbo low, opisthogyrous; entire left valve attached to depressed flange-like anterior extension of female shell, surface smooth; posterior margin slightly concave, anterior margin slightly convex, ventral margin nearly straight; hinge narrow, ligamental area large compared to shell size, length greater than height, resilifer shallow, in some specimens relatively deep, wider than bourrelets, sometimes only to a small extent; umbonal cavity shallow, sometimes relatively pronounced; adductor muscle scar relatively large, reniform with rounded ends,

Fig. 4. Ostreid bivalve Ostrea jibananandai sp. nov. from Mangrol open pit mine, Surat, western India, Ypresian Cambay Basin. A. PG/CB/Os 27, a large → female (not shown in full) carrying left valve of two dwarf males together, arrows point to their hinges. B. PG/CB/Os 1a, paratype, a left valve attached on the right valve of an articulated specimen, internal view (B 1); close-up showing details of the ligamental area and an attached male (B 2), arrow indicating its hinge. C. PG/CB/Os 30, paratype, adult right valve of a female, internal view (C 1); detail showing depressed platform for accommodation of dwarf male (C 2). D. PG/CB/Os 45, paratype, left valve of a detached male, internal (D 1) and external (D 2) views. E. PG/CB/Os 31, left valve of a large, slender female carrying a dwarf male at its anterior flange, internal view (E 1); detail showing left valve of the attached male of similar shape (E 2). F. PG/CB/Os 32, right valve of an adult female, internal view (F 1); detail showing depressed platform for accommodation of dwarf male (F 2).

placed at mid-height; commissural shelf slightly concave, demarcated by proximal margin only in dorsal part.

Remarks.—The species bears characteristics typical of the genus, such as a relatively flat and higher than long shell, a reniform adductor muscle impression, an elongated ligamental area, weakly developed chomata and a large attachment scar. The genus, however, is commonly characterized by no or only a shallow umbonal cavity, which is well-developed in this species. The representatives of Ostrea are also often characterized by discrepant surface ornamentation of the valves, having a radially ornamented left valve and a more or less smooth right valve ( Stenzel 1971). Both valves of the new Eocene species from India are commonly smooth. The species exhibits considerable variation in several morphological features such as the shell shape, the shape of the ligamental area, and the shape and position of the adductor muscle impression. However, a large intraspecific variability is common in oysters (e.g., Amaral and Simone 2014), and is linked to their gregarious habit and varied substrates ( Del Rio et al. 2001 and references therein).

The new species, having a relatively thin, flat, quite high and almost smooth shell does not resemble any known Paleogene species from the Indian subcontinent. Most Paleogene ostreid species from this part of the world belong to other genera, such as Crassostrea Sacco, 1897 and Cubitostrea Sacco, 1897 (e.g., Cox 1931; Eames 1951; Iqbal 1969a, b, 1972). This also holds true for the basins from western part of the Tethys Realm such as the Paris Basin ( Cossmann and Pissarro 1906; Le Renard and Pacaud 1995). Most of the contemporaneous Tethyan species are characterized by an ornamented left valve, a smooth right valve and by a less slender shell. For instance, Ostrea (Turkostrea) multicostata Deshayes, 1832 , well-known from the Eocene of both the western and eastern Tethyan areas, is characterized by strong radial costae on the left valve. It is also very variable and bears a prominent umbonal cavity ( Pacaud and Lebrun 2019). Ostrea rakhiensis Eames, 1951 , from the lower Eocene of Pakistan is known only by a left valve. It is very slender, and has an almost circular and strongly rotated juvenile shell portion like O. jibananandai sp. nov. but has a radial carina in the middle of the valve, the anterior part of which is characterized by blunt commarginal undulations whereas the posterior part is smooth (see Eames 1951: pl. 11: 50).

Cubitostrea alvarezii ( d’Orbigny, 1842) , the only fossil oyster species known so far to exhibit the carriage of dwarf males by large females, is not slender and bears strong radial costae on the surface of the left valve, whereas the right valve is smooth ( Romero et al. 2018). The female specimen carrying a dwarf male, illustrated by Iribarne et al. (1990), is characterized by a pronounced umbonal cavity. It is smaller and thicker than O. jibananandai sp. nov., and has a high and narrow ligamental area with deeper resilifer. Ostrea puelchana d’Orbigny, 1842 , the living oyster with similar male–female association as in O. jibananandai sp. nov., is also less slender than the latter, and has a thicker and radially ornamented left valve ( Romero et al. 2013; Pascual et al. 1989). Its right valve bears only feeble radial ornaments ( Doldan et al. 2018). It has a similar adductor muscle scar and ligamental area to those of O. jibananandai sp. nov. However, its ligamental area is not as strongly rotated towards posterior as in O. jibananandai sp. nov. The adductor muscle impression in the male of O. puelchana is below the mid-height of the shell interior ( Pascual et al. 1989), in contrast to the mid-height position in O. jibananandai sp. nov.. In all the three oyster species with dwarf male and large host female, the males are generally very slender and bear an adult-looking, large ligamental area with numerous growth increments.

Stratigraphic and geographic range.— Type locality and horizon only.