Retrolucina, Taylor & Glover, 2018

Taylor, John D. & Glover, Emily A., 2018, Hanging on - lucinid bivalve survivors from the Paleocene and Eocene in the western Indian Ocean (Bivalvia: Lucinidae), Zoosystema 40 (7), pp. 123-142 : 132-133

Genus Retrolucina n. gen.

urn:lsid:zoobank.org:act:DAFC3EBA-0C19-4D63-8248-65A6F761670A

TYPE SPECIES. — Lucina voorhoevei Deshayes, 1857 . Recent, Western Indian Ocean

DIAGNOSIS. — Shell large L to 80 mm, thin-shelled, ovoid, laterally compressed, longer than high, tapering to anterior and posterior. Umbones low. Posterior dorsal area faintly delineated by shallow sulcus. Sculpture generally smooth with growth lines, sometimes with irregular short anterior and posterior marginal folds. Lunule small, narrow, triangular. Hinge line thin, two small bifurcate cardinal teeth in both valves, lateral teeth absent. Anterior adductor muscle scar long, narrow, curved, extends ventrally to past mid-line of shell, detached and widely separated from pallial line for nearly all of length. Pallial line narrow, entire. Shell interior with fine radial ridges. Inner shell margin smooth.

ETYMOLOGY. — Latin ‘retro’ – backwards and lucina in reference to similarity of Eocene species to the sole living species. Feminine.

DISTRIBUTION. — Western Indian Ocean particularly Mozambique.

GEOLOGICAL RANGE. — Lucina defrancei Deshayes, 1857 , Eocene, Lutetian, Paris Basin ( Fig. 6 I-N) is very similar to R. voorhoevei n. comb. in shape, external sculpture, hinge teeth, and characters of anterior adductor muscle scar.

REMARKS

Previously included in Eomiltha , R. voorhoevei n. comb. differs from the type species by a number of characters. In shell outline, R. voorhoevei n. comb. tapers posteriorly compared with E. contorta that is posteriorly truncate with a shallow sinus. The shell exterior is smooth compared with the more rugose Eomiltha , the anterior adductor scar is thinner and longer and the cardinal teeth smaller. In Retrolucina n. gen. the pallial line lacks the posterior angle towards the posterior adductor scar of Eomiltha contorta . Although Retrolucina n. gen. and Eomiltha are clearly related such morphological differences within living lucinids would suggest different generic placement. This is demonstrated by the various laterally compressed lucinids (e.g. Gloverina , Taylorina , Dulcina , Elliptiolucina ) described from Indonesia and Philippines by Cosel & Bouchet (2008) with several of these later corroborated by molecular analyses ( Taylor et al. 2011, 2014, 2016).

An Eocene (Lutetian) species usually referred to Eomiltha is Lucina defrancei Deshayes, 1857 (synonym L. cuvieri Bayan, 1870 ) from the Paris Basin ( Figs 6 I-N; 7C, D) it is very similar to Retrolucina voorhoevei n. comb. in shell characters and we regard it as congeneric and an antecedent. Despite an extensive literature and collection search we failed to find any species resembling R. defrancei and R. voorhoevei n. comb. recorded from deposits between the Eocene and present day.

Further back in geological time we previously ( Taylor & Glover 2000, 2006) compared Illiona prisca (Hisinger, 1837) from the Silurian of Gotland, Sweden with R. voorhoevei n. comb., it has a similar flat-shelled, elongate-ovate shape and internally has a very long anterior adductor muscle scar that extends posteriorly to the midline of the shell. The resemblance in shape is remarkable but, in the absence of any fossil record of similar forms from the later Palaeozoic through the Mesozoic, likely results from morphological convergence rather than phylogenetic continuity.

Figures

FIG. 6. — A-H, Retrolucina voorhoevei n. comb.(Deshayes,1857),Recent;and I-N, R.defrancei (Deshayes,1857),Eocene;A-C, exterior of right and interiors of right and left valves, Mozambique (ANSP 234103),L 70 mm; D, E, exterior and interior of left valve, Mozambique (USNM 628930), L 78 mm; F, dorsal view (NHMUK 20170373), L 78 mm; G, H, details of hinge of right and left valves (NHMUK 20170373),scale bar,10 mm; I, J, Retrolucina defrancei (Deshayes,1857) exterior and interior of left valve Eocene,Lutetian,Chaumont-en-Vexin,Oise,France (MNHN.F.J07396), L 35 mm;K, L, Retrolucina defrancei (Deshayes,1857),Eocene,Lutetian,Chaussy,Seine et Oise, France, (RBINS IG10591), L 71 mm;M, N, Retrolucina defrancei (Deshayes,1857), Eocene,Lutetian,Parnes,France,Deshayes collection (NHMUK 33283a),L 50.4 mm.