Caspiconcha Kelly

Jenkins, Robert G., Kaim, Andrzej, Little, Crispin T. S., Iba, Yasuhiro, Tanabe, Kazushige & Campbell, Kathleen A., 2013, Worldwide distribution of the modiomorphid bivalve genus Caspiconcha in late Mesozoic hydrocarbon seeps, Acta Palaeontologica Polonica 58 (2), pp. 357-382 : 359-362

Genus Caspiconcha Kelly in Kelly et al., 2000

Type species: Caspiconcha whithami Kelly in Kelly et al., 2000; Upper Barremian (Lower Cretaceous), NE Greenland .

Emended diagnosis.—Shell subtrapezoidal to cuneiform, or even modioliform in shape with extreme reduction of its anterior margin; triangular depression running from the umbonal area widening towards the mid flank; umbones located anteriorly but not reaching the anterior end; exterior ornament smooth apart from commarginal growth lines; edentulous hinge structure; anterior adductor muscle scar deeply set

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within myophoric buttress; caspiconchid process present, partially covering anterior adductor muscle scar; entire shell aragonitic.

Species included.— Caspiconcha whitmani Kelly, 2000 from the Barremian of Greenland ( Kelly et al. 2000), C. rubani Kiel, Campbell, and Gaillard, 2010 from the Hauterivian of the Crimean Peninsula, Ukraine ( Kiel and Peckmann 2008; Kiel et al. 2010), and C. major ( Gabb, 1869) of northern California. These Californian localities largely constitute seep carbonates within the Buchia bivalve−bearing mudstone of the “Knoxville Beds”, lower Great Valley Group, which is Late Jurassic to Early Cretaceous in age. They include Tithonian (Paskenta), Valanginian (Bear Creek), Hauterivian (Wilbur Springs), Albian (Cold Fork of Cottonwood Creek), and some poorly dated, probably Lower Cretaceous, sites (East of Knoxville, East Berryessa) ( Gabb 1869; Stanton 1895; Sandy and Campbell 1994; Kiel et al. 2008a). In addition, the newly recognized Barremian (Eagle Creek) seep deposit near Ono, northern California, also contains C. major , many lucinid bivalves, but no buchiids.

We also include in Caspiconcha a probable new species, reported below, from the Albian Utagoesawa Creek, Yubari area, central Hokkaido, and a single specimen that is probably a different species from the Campanian Omagari seep, northern Hokkaido. Myoconcha aff. transatlantica (Ascher

7, East Berryessa. CRO: Coast Range Ophiolite. The map is modified from Kiel et al. (2008a). D. Overview photograph of the Eagle Creek site. The hydrocarbon−seep carbonates crop out near the junction between Eagle Creek and North Fork of Cottonwood Creek. E. Photograph of typical seep carbonate bodies at the Eagle Creek site. Right−top inset shows detail of carbonate cements with stromatolite−like laminae. F. Photograph of strata at the Eagle Creek locality yielding Caspiconcha specimens.

1906) from Lower Cretaceous (Hauterivian) strata of the Czech Carpathians could also be a Caspiconcha species, and Kelly et al. (2000) suggested that an undescribed bivalve from the Cretaceous of New Zealand also might be related to Caspiconcha ( Kelly et al. 2000) . The age of the latter is Late Albian to mid−Cenomanian (Kiel et al. in press). Caspiconcha sp. has been also reported recently from the Late Albian Ispaster seep carbonates in northern Spain (Agirrezabala et al. in press). All occurrences listed above are from confirmed or likely hydrocarbon seep deposits, apart from Ascher’s (1906) locality, which seems to be no longer accessible ( Vašíček and Skupien 2004). Caspiconcha and Caspiconcha −like species are listed in Table 2.

Remarks.— Caspiconcha shares a number of characters with Myoconcha (see above), but lacks the external radial ornament of that genus and, unlike it, possesses an edentulous hinge. The shell of the type species ( C. whithami Kelly, 2000 ) was interpreted by Kelly et al. (2000) to be composed of aragonite. We can confirm this feature for the genus in specimens of C. major ( Gabb, 1869) from the Eagle Creek locality. Kelly (in Kelly et al. 2000: 242) suggested that the original inner shell layer of C. whithami was nacreous, although in all Greenland specimens the original shell had been entirely replaced by silica. The Eagle Creek C. major specimens ( Fig. 3 ) clearly show that in this species the shell layers are not nacreous, but have cross−lamellar and homogenous microstructures ( Fig. 4 ). According to Morris et al. (1991: 273), the shell of Myoconcha is composed entirely of homogeneous structure, although Carter (1990: 271) argued that further SEM observations are necessary to fully clarify the microstructural composition of Myoconcha shells. The differing shell microstructure of Myoconcha and Caspicon−

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cha does not necessarily mean that they cannot be closely related, because Morris et al. (1991: 53) inferred that an evolutionary sequence from nacreous through homogenous to cross−lamellar microstructure is commonly observed in bivalves. Therefore, the shell microstructure of Caspiconcha could be interpreted as an end−member of a similar shell macroevolutionary sequence in the family Kalenteridae .

Kelly et al. (2000) incorporated Caspiconcha into the subfamily Myoconchinae , identifying numerous characters of that genus in common with Myoconcha . He also classified the Myoconchinae as a subfamily of the Modiomorphidae within the superfamily Modiomorphoidea . The Modiomorphidae was considered by Morris (1978), Fang and Morris (1997) and Carter et al. (2000) as a family of Anomalodesmata . However, many other ideas for its placement have developed since then (see recent discussion in Griffin and Pastorino 2006). The entirely aragonitic shell of C. major supports placement of the Modiomorphidae among the anomalodesmatans, which have wholly aragonitic shells ( Carter 1990), rather than with the Mytilidae , the majority of genera of which possess both calcitic and aragonitic shell layers ( Carter 1990).

Stratigraphic and geographic range.—Tithonian (Upper Jurassic) to Campanian (Upper Cretaceous). Barremian of NE Greenland, Tithonian to Albian of California, Hauterivian of Crimea and possibly Czech Republic, Albian–Cenomanian of New Zealand, Albian of Spain, and Albian and Campanian of Hokkaido.

Figures

Fig. 3. Modiomorphid bivalve Caspiconcha major (Gabb, 1869) CAS 72527−9 from Eagle Creek, California, USA, Upper Barremian (Lower Cretaceous). Right valve. A. External view. B. Internal view. Details of pedal elevator muscle scar (C) and mantle muscle scars indicated by small arrowheads (D). Location of shell microstructure analysis (Fig. 4) is marked with a dotted white line. Black arrowheads point to the anterior.

Fig. 4. Shell microstructure of modiomorphid bivalve Caspiconcha major (Gabb, 1869) CAS 72527−9 from Eagle Creek, California, USA, Upper Barremian (Lower Cretaceous). External shell surface upwards in all figures. A.Vertical cross section through shell in the pallial region; see Fig. 3B for location. B. Demarcation between middle (cross lamellar) and inner (complex cross lamellar) layers. C. Outer layer homogeneous structure. D. Lower part of inner layer, note diagenetic alteration at base.