Hanging on - lucinid bivalve survivors from the Paleocene and Eocene in the western Indian Ocean (Bivalvia: Lucinidae) Taylor, John D. Glover, Emily A. Zoosystema 2018 2018-04-10 40 7 123 142 7PFPB Taylor & Glover, 2018 Taylor & Glover 2018 [1073,1192,632,658] Bivalvia Lucinidae Retrolucina GBIF,CoL Animalia Lucinida 11 132 Mollusca genus gen. nov.   urn:lsid:zoobank.org:act: DAFC3EBA-0C19-4D63-8248-65A6F761670A   TYPESPECIES. —  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. 6I-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 typespecies by a number of characters. In shell outline,  R. voorhoevei n. comb.tapers posteriorly compared with  E. contortathat 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  Eomilthaare 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 Indonesiaand Philippinesby 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  Eomilthais  Lucina defrancei Deshayes, 1857(synonym  L. cuvieriBayan, 1870) from the Paris Basin ( Figs 6I-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. defranceiand  R. voorhoevei n. comb.recorded from deposits between the Eocene and present day.  FIG. 5. —  Eomiltha Cossmann, 1912fossils: A,  Eomiltha alburgensis(Vincent, 1930)exterior of left valve, Paleocene, Danian, Calcaire de Mons, Mons, Belgium, (RBINS IG 5496), L 32 mm; B, C, Eomitha  contorta(Defrance, 1825), exterior and interior of right valve, Paleocene, Thanetian, Abbecourt, Oise, France (MNHN), L 52 mm; D -F, Eomitha  contorta(Defrance, 1825)interior and exterior of right valve and exterior of left valve, Paleocene, Thanetian, Sables de Bracheux, Beau- vais, France, (RBINS IG 8260), L 43 mm; G,  Eomiltha contortadorsal view, Abbecourt, (MNHN), L 54 mm; H, I, ‘  Eomiltha’  pandata(Conrad, 1833), Eocene (mid.), Gosport Sand, Claiborne Formation, Alabama, USA (PRI 34183), L 34 mm; J, K, ‘  Eomiltha’  scolaroi Vokes 1969b, holotype, (USNM 646423), early Miocene, Chipola Formation, Farley Creek, Calhoun Co. Florida, USA, L 32.6 mm. Further back in geological time we previously ( Taylor & Glover 2000, 2006) compared Illiona prisca(Hisinger, 1837) from the Silurian of Gotland, Swedenwith  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.