Didacna subcatillus Andrusov, 1910

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 : 1233-1234

publication ID	https://doi.org/ 10.1016/j.jglr.2019.04.001
DOI	https://doi.org/10.5281/zenodo.5019001
persistent identifier	https://treatment.plazi.org/id/5D36A832-FF9C-FFC6-FCE2-BA7D7332F81F
treatment provided by	Carolina
scientific name	Didacna subcatillus Andrusov, 1910
status

Treatment

( Fig. 5 View Fig : 3, Fig 6 View Fig : 1-3).

*1910 Didacna catillus var. subcatillus Andrusov : 78, pl. 9, figs. 22–23.

1953 Didacna subcatillus Andrussov – Fedorov: 127, pl. 14, figs. 16–20.

1958 Didacna subcatillus Andrussov – Nevesskaja: 24, pl. 3, figs. 1–6.

1986 Didacna subcatillus Andrussov, 1910 – Yakhimovich et al.: 84, pl. 4, figs. 3, 4.

2007 Didacna subcatillus subcatillus Andrussoff, 1910 – Nevesskaja: 935, pl. 21, figs. 6–14.

Dimensions – max. L 43.4 mm, H 33.8 mm.

Characterization – Didacna subcatillus is a broad trapezoid highly variable species that does harbor several specific forms that we could link through intermediates forms in the studied material ( Fig. 6 View Fig : 1–3). The posterodorsal keel is usually low, the posterior margin is rounded. The posterior ridge is pronounced and mostly angular in juvenile stages and can become more subrounded in adult stages. The posterior ribs range between 18 and 25. Interspaces are narrow (about 40–60% of the width of the ribs). Towards the anterior margin ribs become often less pronounced and the posterior rib width and prominence increases towards the posterodorsal ridge. The rib cross-section is irregular, usually flat or subrounded. The location of the semidiameter is on the posterior ridge in juveniles and close to the center of the shell in adults. The ventral half of the shell is typically straight and dipping. Juvenile shells have already a trapezoid to low trigonate outline with a barely rounded lower margin and a well-developed posterodorsal keel. We found strong variations in shell shapes and sizes and investigated the possible presence of sibling (sub-)species. Some individuals grow as large as 43.4 mm (L) × 33.8 mm (H), while 95% of individuals show maximum dimensions of 15 mm (L) × 12 mm (H). One particular phenotype is small, has a thick shell that is somewhat convex and has markedly thick hinge plate and teeth and regularly developed ribs that have a more or less rounded cross-section ( Fig. 6 View Fig : 1a, 2a). This form however is connected via intermediates ( Fig. 6 View Fig : 1b, 1c, 2b, 2c) with thin-shelled forms that are less convex and have thinner hinge features ( Fig. 6 View Fig : 1d, 2d).

Taxonomic notes – The various forms of Didacna subcatillus have been attributed to a number of species and forms in the past, to which Nevesskaja (2007) attributed biostratigraphic value. However, we were unable to delimit these forms in our variable material ( Fig. 8 View Fig ) and consider them synonyms.

Ecology – Unknown (extinct). Didacna subcatillus preferred lower saline waters than most Didacna species ( Nevesskaja, 1958; Yanina, 2012).

Kingdom	Animalia
Phylum	Mollusca
Class	Bivalvia
Order	Veneroida
Family	Cardiidae
Genus	Didacna