Gibbolucina Cossmann 1904

Genus Gibbolucina Cossmann, 1904

Phacoides (Gibbolucina) Cossmann in Cossmann & Peyrot, 1904: 13.

TYPE SPECIES. — Venus callosa Lamarck, 1806: Eocene, Lutetian (original designation).

DIAGNOSIS. — Shell small L to 15 mm, subtrigonal, ventrally round- ed, umbones prominent. Shallow sulcus defining posterior dorsal area in most species. Lunule inset, long. Sculpture of irregular low commarginal lamellae. Hinge with two cardinal teeth in left valve and single, often bifid, cardinal in right valve, lateral teeth absent. Ligament short, in shallow resilifer. Anterior adductor muscle scar short, narrow, detached for half to 2/3 of length, pallial line entire, inner shell margin smooth, interior often thickened, with pallial blood vessel scar in deep groove.

GEOLOGICAL RANGE. — Eocene (Lutetian) to Recent. The fossil record of Gibbolucina species extends from the early Eocene to the Miocene.

INCLUDED SPECIES. — Eocene. Lutetian: Gibbolucina callosa (Lamarck, 1806) (Fig. 1A-G) and G. gibbosula (Lamarck, 1806).

Bartonian. Gibbolucina lefevrei (Cossmann, 1887) (Fig. 1J, K), G. axinoides (Dufour, 1881) (Fig. 1H, I), G. profunda (Dufour, 1881). Priabonian (Ludien). Gibbolucina incomposita (von Koenen, 1893) see Pacaud & Ledon 2007).

Miocene Aquitanian. Gibbolucina avitensis (Cossmann & Peyrot, 1912) (1912: 271-273, pl. 27, figs 14-17). Saint Avit, Landes, Aquitaine (MNHN.F.06430).

Burdigalian. Gibbolucina trigonula (Deshayes, 1830) Moulin de Gamachot, Aquitaine (Cossmann & Peyrot 1912: pl. 26, figs 70- 73) (Fig. 1L-P).

Pliocene. A Late Pliocene-early Pleistocene species, Gibbolucina salebrosa (Woods, 1931), was described from southwestern Australia (see Ludbrook 1978: pl. 3, figs 6-9) but this lacks hinge teeth, and the deeply scooped lunule of Gibbolucina and also has widely spaced prominent commarginal lamellae. We regard this species as distinct from Gibbolucina but in a broadly related genus (undescribed). Another Pliocene species, Gibbolucina confirmans (Ludbrook, 1955), was described from two incomplete shells from a borehole near Adelaide, South Australia but the generic assignment is doubtful.

Recent. Gibbolucina zelee n. sp.

COMPARISON WITH OTHER GENERA

Despite Chavan (1969) and others including Eomiltha as a subgenus of Gibbolucina or, alternatively, Gibbolucina as a subgenus of Eomiltha (Ludbrook 1955), there is little similarity of shell characters (see below) and any relation-

ship is highly unlikely. In contrast, several described genera have similar characters to Gibbolucina and are likely broadly related. These are Megaxinus (type species Lucina transversa Bronn, 1831), Rasta Taylor & Glover, 2000 (type species Rastafaria thiophila Taylor & Glover, 1997 [Taylor & Glover 1997, 2000]), and Parvidontia Glover & Taylor, 2007 (type species P. laevis Glover & Taylor, 2007). To date, only Rasta lamyi has been included in molecular analyses (Taylor et al. 2011, 2016) where it groups in the subfamily Lucininae.

Megaxinus species are often similar in shape to Gibbolucina but all lack hinge teeth, although there are sometimes irregular folds on the hinge plate. They also exhibit ontogenetic changes in shape, becoming relatively higher and thicker shelled with age (Glover & Taylor 1997; Cosel & Bouchet 2008). Furthermore, the anterior adductor muscle scar is longer and detached from pallial line for about 2/3 of length rather than about half of length in most Gibbolucina species. Rasta species also similarly lack hinge teeth but have a subtrigonal shape, higher than long, with prominent umbones and the two known species, R. thiophila and R. lamyi, possess in live or fresh shells, distinctive, long periostracal extensions (Glover & Taylor 1997; Taylor & Glover 1997; Taylor et al. 2005). An earlier Rasta- like species is Megaxinus ellipticus var. trigona Sacco, 1901 described from the early Pliocene of northern Italy (MRSN BS154.02.006) and which has a similar higher than long shape, with sharp, curved umbones.

Another similar genus, Parvidontia, was introduced (Glover & Taylor 2007) based on P. laevis from New Caledonia, a small species with sub-circular, thin shells and small cardinal teeth. Subsequently, Glover & Taylor (2016) concluded that the P. laevis specimens were likely juvenile shells and described a second species, P. mutabilis Glover & Taylor, 2016, from the Philippines that, in larger individuals, has the general shell form of Megaxinus but with small cardinal teeth. Parvidontia is similar to Gibbolucina in shape but has a very thin hinge plate, a sculpture of fine commarginal lamellae and lacks a thick periostracum.