Myrteinae Chavan, 1969

Gill, Fiona L. & Little, Crispin T. S., 2013, A new genus of lucinid bivalve from hydrocarbon seeps, Acta Palaeontologica Polonica 58 (3), pp. 573-578 : 573-574

?Subfamily Myrteinae Chavan, 1969

Remarks.—The supra−generic level systematic classification of the family Lucinidae has been in a state of flux, with conflicting classification schemes based on modern and fossil shell morphology (e.g., Chavan 1969; Bretsky 1976). However, recent lar−shaped depression underneath the umbo, which extends further to the posterior than the anterior, and causes a deflection in ventral margin of hinge plate. Anterior margin of depression bounded by sharp ridge. Exterior shell ornament with commarginal growth lines and faint radial striae in anterior portion of well−preserved shells. Anterior adductor muscle scar elongate, diverging from pallial line at an angle of approximately 40 °, with divergent part approximately twice as long as non−divergent part. Posterior adductor muscle scar kidney−shaped to rounded with notch in anterior margin visible in some specimens. Both muscle scars crossed by fine, irregular striations.

Remarks.— Elongatolucina is characterised by an elongate, elliptical shell shape with umbones positioned one third of the total length from the anterior. It possesses a straight ventral margin, and an elongated anterior adductor muscle scar which diverges from the pallial line at an angle of 40 °. The sub−umbonal depression on the hinge plate could be interpreted as a socket but does not correspond to a projecting tooth in the other valve and instead corresponds to a similar depression. Therefore, we consider the hinge to be edentulous. Elliptiolucina species have a similar hinge morphology.

Elongatolucina is similar to Cryptolucina Saul, Squires, and Goedert, 1996 , the genus to which the type species was originally assigned, in that it is a large fossil lucinid with an edentulous hinge. However, Elongatolucina is distinguished from Cryptolucina (now considered to contain only C. megadyseides Saul, Squires, and Goedert, 1996 and C. kuhnpassetensis Kelly in Kelly et al., 2000) by its thinner shell, considerably greater degree of elongation (height to length ratio 0.59–0.66 for Elongatolucina compared to 0.76 for Cryptolucina ), elliptical rather than circular to sub−triangular outline, much lower and less prominent umbones, which are situated well anterior of the mid−line rather than centrally, and a straight rather than convex ventral margin. These morphological features are, in our opinion, sufficiently different to the warrant separation of C. elassodyseides from the remaining species of Cryptolucina at the generic level. The existing genus diagnosis for Cryptolucina remains accurate for the remaining species without amendment.

Nipponothracia Kanie and Sakai, 1997 is another large, somewhat elongate, edentulous fossil bivalve genus, which although originally described as a thraciid ( Kanie and Sakai 1997), was recognised as a lucinid by Kase et al. (2007). However, Nipponothracia differs from Elongatolucina in being less elongated and more rounded in shell outline, with more centrally positioned umbones and a conspicuously rounded ventral margin.

Elongatolucina also closely resembles the modern lucinid genus Elliptiolucina von Cosel and Bouchet, 2008 in its elongate outline and edentulous hinge. However, Elongatolucina differs in being generally more elongate than Elliptiolucina ( Fig. 2 ), with evenly rounded anterior and posterior margins and a straight ventral margin, in contrast to the truncated anterior and posterior margins of Elliptiolucina . Elliptiolucina also has a convex ventral margin, which results in a somewhat trapezoid shell outline. In addition, the angle of divergence of the anterior adductor muscle scar is consistently greater in Elongatolucina (40–43 °) than in Elliptiolucina (22–30 °).

A further modern species that bears a superficial resemblance to Elongatolucina is Jorgenia Taylor and Glover, 2009 , which is an elongate lucinid with a fairly straight dorsal margin. Elongatolucina can be easily distinguished from this genus, however, by being more elongate and having a straight ventral margin. Jorgenia has truncated posterior and anterior margins and a distinctly rounded ventral margin. In addition, Elongatolucina is completely edentulous, whereas Jorgenia has one or two small cardinal teeth in each valve.

Geographic and stratigraphic range.—Eocene Humptulips Formation, Washington State, USA (CSUN localities 1582 and 1583), Miocene Huso Formation, Venezuela.

Figures

Fig. 2. Plot of length versus height data for Elongatolucina elassodyseides, E. sp. aff. Elassodyseides, and Elliptiolucina magnifica holotype.