Hypelasma fritzlangi sp. nov.

Figs 12 B, 14, 15

2017 Hypelasma sp. – Schneider et al.: 58, pl. 22, fig. D.

Diagnosis.

Hypelasma with bluntly angled rather than distinctly carinate left valve and non-carinate right valve, and with sub-circular to sub-ovate commissure. Right valve cardinal tooth 3 a small in juveniles, possibly reduced in adults; corresponding left valve cardinal socket 3 a’ absent.

Etymology.

In honour of Fritz Lang, amateur fossil collector from Hirschaid, Upper Franconia, Germany, who collected part of the type series.

Material.

Holotype, left valve; BSPG 2016 XXI 1408. Paratype 1, left valve; BSPG 2016 XXI 1405. Paratype 2, specimen with contiguous valves; BSPG 2019 IV 5. Paratype 3, left valve; BSPG 2016 XXI 1397. Paratype 4, juvenile specimen with contiguous valves; BSPG 2019 IV 9. Paratype 5, juvenile left valve; BSPG 2019 IV 6. Paratype 6, juvenile right valve; BSPG 2019 IV 7. Paratype 7, juvenile right valve; BSPG 2019 IV 8. Paratype 8, double-valved specimen; BSPG 2016 XXI 1396. Two more left valves (not figured); BSPG 2016 XXI 1404, BSPG 2020 CIII 14. Three more right valves (not figured); BSPG 2016 XXI 1406, BSPG 2016 XXI 1407, BSPG 2019 IV 10.

Measurements and figure numbers.

Type locality and stratum.

Frankenalb Formation, Kelheim Member; late Kimmeridgian; Saal an der Donau, Lower Bavaria, Germany.

Description.

Shell up to 50 mm high. Commissure sub-circular in outline in juveniles; higher than long, sub-ovate, with a blunt ventral angulation in adults. Left valve expanding rather slowly during ontogeny, with a coiled-cylindrical rather than conical adult portion, its anterior, poorly inflated flank portion separated from the more distinctly rounded posterior one by a broad, rounded bulge. Right valve operculum-shaped, with umbo not extending above commissure. Both valves coiled, with generally less than 1.5 whorls clearly traceable. Ornament of dense, very fine radial striae preserved in some specimens (Fig. 15 C, D). Right valve hinge with small, short-tongue-shaped anterior cardinal 3 a, and prominent long-tongue-shaped posterior cardinal 3 b extending well above commissure, often for nearly half its length. Left valve hinge with distinct but small nodular cardinal tooth 2, situated anterior to median of shell, at anterior end of oblique hinge plate emerging from body cavity. Socket 3 b’ wide and deep, extending behind cardinal tooth 2. Socket 3 a’ not developed. Adductor muscle scars shallow. Left valve anterior adductor muscle scar placed on sloping inner shell wall. Left valve posterior adductor muscle scar supported by weak to distinct buttress. Right valve anterior adductor muscle scar placed on shell wall; faintly rimmed posterior-ventrally. Right valve posterior adductor muscle scar situated on shell wall, distinctly below onset of cardinal 3 b.

Remarks.

Hypelasma fritzlangi sp. nov. most significantly differs from both Hypelasma colloti Paquier, 1898 and Hypelasma salevensis (Favre in Joukowsky and Favre 1913) in the absence of acute carinae. In its left valve, a broad, rounded, rather central bulge is developed, which separates a rather poorly rounded anterior shell portion from a better-rounded posterior one. Correspondingly, the angle between the coiling axis and the commissural plane in H. fritzlangi is often larger than in the other two species, and its growth habit is less pronouncedly requieniid. Its right valve is non-carinate.

Cardinal tooth 2 is less prominent in H. fritzlangi than in the other two species and socket 3 a’ is not developed in adults. In the juvenile left valve of Fig. 14 A, a minute recess directly behind tooth 2 potentially represents 3 a’ (not labeled). As a result, tooth 3 a must either have been reduced in adults, or its ventral side must have directly bordered the body cavity. Unfortunately, no adult isolated right valves are available to assess this condition.

From Plesiodiceras valfinense, Hypelasma fritzlangi is distinguished by its much more slowly expanding whorls and its generally requieniid growth habit. Radial ornament, when preserved, is finer in H. fritzlangi than in P. valfinense. Tooth 2 is relatively prominent and arcuate in P. valfinense, while rather small and nodular in H. fritzlangi. Tooth 3 b is obliquely triangular and not or barely projecting beyond the commissure in P. valfinense; in contrast, it is elongate tongue-shaped in H. fritzlangi and extends well above the commissure.

Both adductor muscles in P. valfinense insert on downward extensions of the hinge plate in the right valve and on raised, inwardly inclined buttresses in the left valve. In H. fritzlangi, the separation of hinge plate and inner shell wall in the right valve is less distinct than in P. valfinense, and while both adductor muscles insert in a similar position, the posterior one is in its entirety positioned below tooth 3 b. In the left valve of H. fritzlangi, the posterior adductor muscle inserts on a more or less prominent buttress, while the anterior one sits rather directly on the inner shell wall.

Occurrence.

Only known from the type locality.