Acharax stantoni ( Vokes, 1955 )

Kaim, Andrzej, Jenkins, Robert G., Tanabe, Kazushige & Kiel, Steffen, 2014, Mollusks from late Mesozoic seep deposits, chiefly in California, Zootaxa 3861 (5), pp. 401-440 : 423-424

Acharax stantoni ( Vokes, 1955)

( Fig. 10 )

Solemya occidentalis Stanton, 1895 , p. 55–56, pl. 10, figs. 2–3.

Solemya stantoni Vokes, 1955 , p. 538 [note: Vokes misspelled Stanton’s name as “ Solen occidentalis ”] Supplementary description. The anterior adductor muscle scar is weak and appears to be broadly wedge-shaped with the wider side dorsally. A narrow band is situated posterior of the scar at a distance about as wide as the adductor scar; it extends in a dorsal-ventral direction and is undulating, has about the same height as the adductor scar, and may connect with the posteroventral side of the scar as in other solemyids, but this area is not preserved. The shell microstructure consists of thin inner and outer prismatic layers with a thick granular layer in between that shows some crossed structure.

Material and occurrence: 35 specimens from Paskenta, many only fragmentary, up to 47 mm long; 17 specimens from Eagle Creek (Late Barremian, Early Cretaceous), up to 54 mm; two specimens from Rocky Creek (probably Valanginian, Early Cretaceous), up to 56 mm long; two specimens from Cold Fork of Cottonwood Creek (Albian, Early Cretaceous), up to 35 mm long; two specimens from Bear Creek (Valanginian, Early Cretaceous), up to 60 mm long. Stanton’s type material from Paskenta (Tithonian, Late Jurassic) was also examined ( USNM 23051).

Remarks. Stanton (1895) wrote that the species is widest in the anterior third, as on the illustration on his pl. 10, figs. 2–3, but the newly collected material from Paskenta shows this feature to be variable; several specimens have nearly parallel dorsal and ventral margins (see Fig. B1 herein). The umbo of Acharax stantoni appears to be in a more posterior position than in A. cretacea and A. mikasaensis from the Cretaceous of Japan ( Kiel et al. 2008a). The narrow band extending from the anterior adductor muscle scar toward the dorsal shell margin typically runs obliquely to the dorsal shell margin in extant solemyids (cf. Taylor et al. 2008; Kiel et al. 2008a) in contrast to its almost vertical course in A. stantoni . In this respect, it resembles the narrow band of A. mikasaensis , which, however, is set much closer to the anterior adductor muscle scar than in A. stantoni . An extant species with a very similar anterior adductor scar and band is Acharax alinae Métivier & Cosel, 1993 from the Lau Basin.

Figures

FIGURE 10. Acharax stantoni (Vokes, 1955) from Late Jurassic to Early Cretaceous seep deposits in California. A, B. Stanton’s (1895) type specimens (USNM 23051). C. Specimen with straight anterodorsal margin from the Tithonian of Paskenta (GZG. INV. 84994). D. Specimen with straight anterodorsal margin and original shell material from Late Barremian of Eagle Creek (CAS- 73137). E. Specimen showing posterior adductor muscle scar, from Paskenta (GZG. INV. 84995). F. Internal mold showing outline and radial sculpture, from Paskenta (GZG. INV. 84996). G, H. Specimen showing anterior adductor muscle scar (white arrow in G), from Paskenta (GZG. INV. 84997). I. Shell microstructure, unregistered partial specimen from Eagle Creek.