Hanging on - lucinid bivalve survivors from the Paleocene and Eocene in the western Indian Ocean (Bivalvia: Lucinidae) Author Taylor, John D. Author Glover, Emily A. text Zoosystema 2018 2018-04-10 40 7 123 142 journal article 10.5252/zoosystema2018v40a7 26ac6a51-9d0a-4cb0-8502-230b45cf66e6 1638-9387 3738245 urn:lsid:zoobank.org:pub:7652DEC7-3C6C-414F-AF2C-7C396D78F6F6 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. FIG. 5. — Eomiltha Cossmann, 1912 fossils: 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 contorta dorsal 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 , 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.