Valletia auris Favre - Skelton and Smith 1927

Valletia auris Favre in Favre & Richard, 1927 Fig. 2 * 1927 Valletia auris n. sp.; Favre in Favre and Richard: 26–28, pl. 1, figs 5–10, text-figs 7, 8. 1927 Valletia auris n. sp., var. crassa n. var.; Favre in Favre and Richard: 28, pl. 1, figs 11–16. 1934 Valletia auris Favre 1927 – Kutassy: 133. 1934 Valletia auris var. crassa Favre 1927 – Kutassy: 133. 1973 Heterodiceras aff. luci (Defrance) – Lau: pl. 4, figs b – h. 1985 Valletia ‘ sp. ’ – Skelton: 167. 2000 Valletia auris Favre – Skelton and Smith: 110. 2011 “ Valletia ” auris – Sano and Skelton: 8. 2015 Valletia auris Favre in Favre and Richard, 1927 – Schneider and Skupien: 624. 2021 Valletia auris Favre in Favre et Richard, 1927 – Yanin: S 145, pl. 22, figs 11, 12. Diagnosis. Valletia with quickly expanding subequal valves, both with faint to distinct, bluntly rounded dorsoventral angulation, separating shell into approximately equal-sized anterior and posterior portions. Commissure sub-circular to oblique-D-shaped in outline, its posterior-ventral part rather well rounded. Right (attached) valve with single prominent arcuate tooth 1, curving around deep socket 2 a’ below, which is separated from body cavity by narrow rim. Left (free) valve with massive, columnar anterior tooth 2 a and much smaller to almost indistinct posterior tooth 2 b, separated by deep, arcuate socket 1 ’, curving around the back and top of 2 a. Material. Large left (free) valve; BSPG 2019 IV 3. Left (free) valve; BSPG 2016 XXI 1391. Juvenile left valve; BSPG 2019 IV 2. Measurements and figure numbers. Type locality and stratum. Late Kimmeridgian limestones at Pierre Châtel, Département Ain, France. Description. Left valves from Saal very small to moderately sized (L = 8–32 mm). Shell moderately inflated, very quickly expanding in diameter; oblique-D-shaped in outline, with anterior margin forming the straight line of the D; posterior, dorsal and ventral margins well rounded. Juvenile shell oblique-short-ovate in outline. Umbo strongly prosogyrate, coiled, slightly projecting. Rounded angulation extending from umbo to anterior-ventral corner, separating shell into faintly sloping anterior and more steeply sloping posterior portions of approximately equal size. Ornament of commarginal costellae preserved on posterior portion of juvenile specimen. Somewhat reticulate ornament of irregular radial and commarginal costellae occurring on anterior slope of adult individual. Shell interior only exposed in one specimen but markedly corroded. Columnar, slightly upward bent anterior tooth (2 a) and deep, arcuate central socket (1 ’) well visible. Posterior tooth (2 b) absent, presumably corroded. Nymph also corroded. Adductor muscle attachment scars not discernible, but indicated by downward sloping hinge plate, both anteriorly and posteriorly. No right valves available from Saal. Remarks. The three specimens from Saal are left valves and, to our opinion, represent juvenile to adult individuals. These left valves clearly were not attached to a substrate, thus represent Diceratidae. The larger specimen (Fig. 2 E) has a rather irregularly growing early shell portion, probably due to space restrictions, but has no attachment surface. The medium-sized specimen has a wide hinge plate tapering ventrally along the anterior and posterior shell margins to accommodate the adductor muscles on these shelves (Fig. 2 C); there are no myophores below. Thus, the specimens represent Valletia, not Diceras. With regard to shell shape, the specimens closely resemble Valletia auris as figured by Favre and Richard (1927), particularly the specimens in their plate 1, figures 5 to 10. Favre (in Favre and Richard 1927) remarked that, among other characters, V. auris differed from Valletia antiqua Favre in Joukowsky and Favre 1913 in its non-carinate left valve. In fact, the specimens from Saal do show a marked angulation separating the anterior and posterior shell portions (as does the right valve in pl. 1, fig. 10 of Favre and Richard), but this angulation is blunt and rounded, very unlike the sharp carina in V. antiqua, where flank and posterior slope meet at angles below 90 degrees. As a result, the posterior-ventral part of the shell is sharply pointed in V. antiqua, while it is well rounded in V. auris (see also Schneider and Skupien 2015). The medium-sized left valve, where the hinge is exposed, clearly shows the columnar, upward-bent anterior tooth (2 a) typical of Valletia auris, as well as the deep arcuate socket (1 ’) for the prominent tooth of the right valve; this hinge configuration is distinct from the one in Diceras. The much smaller but usually distinct posterior tooth normally present in V. auris is absent. However, we are rather convinced that this is a result of the poor preservation, where much of the shell surface, in- and outside, also including the posterior adductor muscle scar, is corroded. Like the specimens from Saal, the type material of V. auris comes from upper Kimmeridgian strata and thus is of largely the same age; this also makes it the stratigraphically oldest species in Valletia. As mentioned by Schneider and Skupien (2015), aged individuals of this species were described as variety crassa by Favre (in Favre and Richard 1927). Sano and Skelton (2011) also referred specimens from the Bau Limestone of Sarawak (Borneo Island, Malaysia) to Valletia auris, which had previously been assigned to Heterodiceras (Lau 1973). Yanin (2021) assigned material from Berriasian strata of the Crimean Mountains (Yayla Mountains), Crimea, Ukraine, to Valletia auris. These very well-preserved specimens have a short-ovate commissure and seem to expand slightly less quickly than the specimens of the type series, as well as the material studied by us. However, given the general variability of these characters seen in the type series of Valletia auris, the Crimean records are retained in this species here. A peculiarity of the specimens from Saal is the preservation of external ornament, which to date was not only unknown in Valletia auris, but in Valletia in general. As can be seen in various Diceratidae and Epidiceratidae, this ornament is restricted to the outermost, originally calcitic shell layer, which, somewhat surprisingly, is often not preserved, while the originally aragonitic shell portion below is. Interestingly, the juvenile shell from Saal only has ornament on its posterior slope, but it is unclear whether this is a pristine feature or rather a matter of preservation. This ornament consists of short, more or less commarginal costellae, not the radial / spiral costae commonly seen in Jurassic rudists (Fig. 2 A). In contrast, the adult specimen only preserves ornament on its anterior shell portion, which is more likely to be an artefact, given that the remainder of the shell does have a weathered aspect. This is the first report of Valletia auris from the Kelheim area, and only the fourth one of this species in general. Occurrence. Late Kimmeridgian strata of Pierre Châtel, Département Ain, France (Favre and Richard 1927). Bau Limestone of Sarawak, Borneo, Malaysia (Sano and Skelton 2011). Late Kimmeridgian Kelheim Member, Frankenalb Formation, of Kelheim-Saal, Bavaria, Germany (this study). Early Berriasian strata of the Crimean Mountains (Yayla Mountains), Crimea, Ukraine (Yanin 2021).