Didacna cristata Bogachev, 1932

van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, Journal of Great Lakes Research 46 (5), pp. 1227-1239 : 1231-1232

Didacna cristata Bogachev, 1932

( Fig. 5 : 6).

*1932 Didacna praetrigonoides var. cristata Bogachev : 44, pl. 4, figs. 9–14.

1948 Didacna cristata Bogachev – Fedorov: 19, pl. 1, figs. 4–6, pl. 2, figs. 4–8.

1953 Didacna cristata Bogachev – Fedorov: 127, pl. 16, figs. 1– 11.

1983 Didacna cristata Bogachev, 1932 – Popov: 201, pl. 12, figs. 9–22.

Dimensions – max. L 19.8 mm, H 16.3 mm.

Characterization – Medium-sized, relatively high, subtrigonate to subovate convex and robust Didacna . Beak clearly defined. Posterodorsal margin steep, thin, well delimited. Semidiameter located on posterior ridge that is well developed and sharp. In about half of the specimens two elevated ribs form the posterior ridge. Posterior margin rounded angular. Between 15 and 21 posterior ribs that are wide and flat and about three times as wide as the interspaces. Juveniles have a wide trapezoid outline with a rounded lower margin and a subtruncate posterior margin. The hinge plate and cardinal teeth are robust.

Taxonomic notes – Bogachev (1932) distinguished the new variety D. praetrigonoides var. cristata from typical D. praetrigonoides by its smaller size, more triangular outline and sharper keel. Fedorov (1953) showed that the variety not only differs in shape and size from D. praetrigonoides but also in its stratigraphic position. While D. praetrigonoides is characteristic for the upper Khvalynian deposits, D. praetrigonoides var. cristata is an index fossil of the lower Khvalynian of the east coast of the Caspian Sea. Fedorov (1953) mentioned that the juvenile D. praetrigonoides forms are similar to D. cristata . Nevesskaja (1958) disagreed with Fedorov while studying a collection of mollusks from the Khvalynian deposits of West Turkmenistan. She stated that the Khvalynian beds of Turkmenistan yielded relatively small shells (up to 50 mm long), with a well-developed plate like a keel and a small number of ribs (23–27). Nevesskaja (1958) concluded that the variety should not be regarded as a distinct taxon because it only shows the plasticity of the species but cannot be well delineated in time or space (see also Nevesskaja, 2007). In contrast, Popov (1983) supported the opinion of Fedorov (1953) based on a comparative statistical analysis of shell dimensions of D. cristata and D. praetrigonoides (Table 41 in Popov, 1983). He demonstrated that the differences between D. cristata and D. praetrigonides were bigger than between many others Quaternary Didacna species. Here, we follow the view of Fedorov (1953) and Popov (1983).

Ecology – Unknown (extinct). In general trigonoid Didacna species prefer low salinities ( Yanina, 2012), but since D. cristata is extinct the paleosalinities can only be inferred.

Figures

Fig. 5. Overview of the Cardiidae.(1) Didacna subcrassa; (a) RGM-1309816 LV;(b) RGM-1309815 RV.(2) Didacna subpyramidata; (a) RGM-1309797 LV; (b) RGM-1309814 RV. (3) Didacna subcatillus; (a) RMG-1309819 LV; (b) RMG-1309820 RV. (4) Didacna emendata; (a) RGM-1309799 LV; (b) RGM-1309798 RV. (5) Didacna ebersini; A (a) RGM- 1309817 LV; (b) RGM-1309818 RV. (6) Didacna cristata; (a) RGM.1309800 LV; (b) RGM.1309801 RV. Scale bars = 1 cm.