Caspiconcha major ( Gabb, 1869 )

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 : 362-371

Caspiconcha major ( Gabb, 1869)

Figs. 2–10 .

1869 Modiola major sp. nov.; Gabb, 1869: 191–192, 246, pl. 31: 88.

1876 Modiola major Gabb, 1869 ; Whiteaves 1876: 74.

1894 Modiola major Gabb, 1869 ; Stanton (in Diller and Stanton 1894): 442.

1885 Modiola major Gabb, 1869 ; White 1885: 20.

1895 Modiola major Gabb, 1869 ; Stanton 1895: 48, pl. 3: 1.

1897 Modiola major Gabb, 1869 ; Cooper 1897: 84.

1902 Modiola major Gabb, 1869 ; Anderson 1902: 45.

1907 Modiola major Gabb, 1869 ; Crandall 1907: 34.

1909 Modiola major Gabb, 1869 ; Grabau and Shimer 1909:521, fig.701. 1914 Modiola major Gabb, 1869 ; Dickerson 1914: 128.

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1914 Modiola major Gabb, 1869 ; Lawson 1914: 8.

1930 Myoconcha major ( Gabb, 1869) ; Stewart 1930: 104, pl. 4: 1.

1993 Modiola major Gabb, 1869 ; Campbell et al. 1993: 39, 42–44, fig. 2.

1993 Modiola major Gabb, 1869 ; Campbell and Bottjer 1993: 334, table 1.

1995 Modiola major Gabb, 1869 ; Campbell and Bottjer 1995b: 470, 474, fig. 5.

2006 Modiola major Gabb, 1869 ; Campbell 2006: 394, Table 1.

2006 “ Modiolus ” major Gabb, 1869 ; Squires and Saul 2006: 121.

2008 Modiola major Gabb, 1869 ; Kiel and Peckmann 2008: 757.

2010 Caspiconcha major ( Gabb, 1869) ; Kiel et al. 2010: 37, 43.

Type material: Gabb (1869) did not designate a holotype in his original description. Here we designate the single specimen ( MCZ 108539) figured by Gabb (1869: pl. 31: 88) to be the lectotype. Thus the four remaining syntypes are paralectotypes. Lectotype: MCZ 108539, Fig. 5 . Almost complete large articulated specimen. Poorly preserved, with the umbonal part of the right valve missing. The specimen label states, “East of Knoxville, Lake County” as the collection locality, the site details of which are further assessed in Kiel et al. 2008a. Paralectotypes: MCZ 108538 and 108540, Fig. 6 . The MCZ 108538 includes three specimens, which we designate as MCZ 108538A, B and C, from Wilbur Springs, Colusa County, California. MCZ 108540 is from “East of Knoxville, Lake County”.

Type locality: East of Knoxville in Napa County, California, USA .

Type horizon: “Knoxville Beds”, lower Great Valley Group .

Emended diagnosis.—Shell strongly elongated cuneiform with nearly straight dorsal and ventral margins; umbo above hinge line; weak internal ridge running from anterior pedal elevator muscle scar towards ventral margin of the posterior adductor muscle scar; rounded anterior adductor muscle scar deeply set; posterior adductor muscle scar rounded with dorsally located narrow posterior pedal retractor muscle scar projected anteriorly; well rounded connected line between ventral part of posterior pedal retractor muscle scar and posterior adductor muscle scar; shell entirely aragonite.

Description.—The description as follows is a composite of features seen in Gabb’s (1869) type material, Stanton’s (1895) material, three new specimens from the Eagle Creek locality, and some other specimens collected from various Cretaceous hydrocarbon seep sites in California ( Table 1). Because of the highly intraspecific and ontogenetic variability of the species we also include detailed descriptions of these specimens separately in Appendix 2, in part to avoid future confusion. A schematic drawing of the species is presented in Fig. 2 showing the most important shell characters.

The shell is large (lectotype 131 mm long, 60 mm high, 34 mm wide) and strongly elongated cuneiform in shape with nearly straight dorsal and ventral margins. The valves are equivalved and strongly inequilateral, and the anterior margins are moderately inflated near the umbones. The umbones are close to the anterior end of the shell and are situated slightly above or almost at the same height as the hinge line. The shell thickness is variable, and is thickest at the myophoric buttress where the anterior muscle scar area is located. The shell is relatively thick in the antero−dorsal to anteroventral area, thinning conspicuously in the posterior−ventral to mid−flank of the ventral area.

The dorsal margin is straight in lateral profile for the full extent of the nymph and ligament area and is then curved ventrally at the postero−dorsal area. The posterior margin is well rounded. The ventral margin is almost straight and is feebly indented at the mid−flank. The anterior margin is short and rounded. The shell is moderately inflated in dorsal aspect, with a wedge shape in both the anterior and posterior margins. A triangular depression runs from the umbonal area, widening towards the mid flank. There is an external ornament of commarginal growth lines, which are more pronounced on the shell anterior.

The shell interior is smooth apart from muscle scars and the pallial line. The pallial line runs commarginally from the anterior adductor muscle scar towards the posterior and then turns dorsally towards the posterior of the posterior adductor muscle scar without a pallial sinus. The anterior adductor muscle scar is rounded with pedal retractor muscle scars lo−

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cated dorsally. The posterior adductor muscle scar is rounded with an antero−dorsally located narrow posterior pedal retractor muscle scar, which projects towards the anterior. The ventral connection between the posterior pedal retractor muscle scar and posterior adductor muscle scar is depressed posteriorly with well rounded line. A weak internal ridge runs from anterior pedal elevator muscle scar towards the ventral margin of the posterior adductor muscle scar, paralleled by an oblique groove on its ventral side.

The myophoric buttress is moderately steep but varies in shape among individuals. The hinge is stout and edentulous. The nymph and ligament groove is very long and straight. The external ligament is sturdy and triangular in cross−section, with a well−mineralized C−spring−type aragonite fibrous sublayer.

There are at least three shell layers. The outermost layer is homogeneous, the middle layer is of cross lamellar structure, and the innermost layer is of complex cross lamellar structure. The shell is composed entirely of aragonite.

Ontogenetic variation.—The shell anterior thickens significantly as the individual grows and occurs mostly by growth in the inner surface of the shell. As a result the pedal elevator muscle scar becomes much deeper and stronger during growth, although we observed some variation in this character, with some juvenile specimens having deep and some adults having shallow pedal elevator muscle scars.

Remarks.— Caspiconcha major was first described as Modiola major by Gabb (1869) based on poorly preserved specimens. The species was later transferred to Myoconcha by Stewart (1930) who stated that its shell is “… heavily buttressed and with a long posterior lateral [tooth]” ( Stewart 1930: 104). He most likely confused the lateral [tooth] with the ligamental groove, and, notwithstanding, his nomencla−

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possible muscle scar 10 mm A triangular depression edge of myophoric buttress B?u mbo 10 mm

tural change for Modiola major was not followed by later authors (e.g., Campbell 2006) exclusive of Squires and Saul (2006) who referred to this species as “ Modiolus ” major .

The shell shape of the lectotype is apparently much more cuneiform than the shape of the Eagle Creek specimens. However, the elongation of the lectotype is apparently due to the diagenetic deformation of the specimen.

Caspiconcha major differs from C. whithami in general shell shape that is cuneiform rather than subtrapezoidal, and possesses a much thinner shell, particularly in the posterior area. The anterior adductor muscle scar of C. major is rounded rather than elongated, and much smaller than that of C. whithami . The caspiconchid process is short and narrow and possesses a small process−like undulation protruding in−between the two anterior pedal retractor muscle scars. The resilifer of C. major is short and narrow in contrast to that of C. whithami , which is long and wide. The umbo of C. major projects well above the hinge line, while in C. whithami it lies below or at least level with the hinge line.

Kiel et al. (2010) stated that C. rubani can easily be distinguished from C. major by the presence of a strong internal ridge in the former species which, according to Kiel et al. (2010), is absent in C. major . However, the shells of C. major from Eagle and Bear Creeks display a weak, but clearly visible internal ridge. Thus, C. major differs from C. rubani by its much weaker internal ridge and general shell shape that is more cuneiform in C. major rather than subtrapezoidal as in C. rubani . Furthermore, according to Kiel et al. (2010: fig.

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7F), the pallial line of C. rubani converges with the ventral edge in its posterior part. In contrast, the pallial line of C. major is almost parallel to the ventral edge.

Stratigraphic and geographic range.—Upper Jurassic (Tithonian) to Lower Cretaceous (Albian) of California. The ca. 50 myr range of C. major matches some other seep−restricted molluscs, e.g., Conchocele bisecta (Conrad, 1849) , which ranges from the Eocene to the Recent (e.g., Goedert et al. 2003; Amano and Jenkins 2007). However, further investigations of the morphology and ontogeny of additional C. major specimens may reveal that the characters now considered to be of an interspecific variation actually reflect a presence of different species.

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Figures

Fig. 2. Schematic illustration of the right valve internal features of Caspiconcha major (Gabb, 1869).

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.

Fig. 5 Lectotype of modiomorphid bivalve Caspiconcha major (Gabb, 1869) MCZ 108539 from east of Knoxville, California, USA, Lower Cretaceous. The pairs A and B, C and D, and E and F are the same views, respectively with and without morphological interpretations. A, B. Left valve. C, D. Right valve. The shell of the specimen is missing forward of the anterior adductor muscle scar, leaving an internal mould. E, F. Dorsal view. G. Detail of ligament area of left valve. White arrowheads point to the anterior.

Fig. 6. Paralectotypes of modiomorphid bivalve Caspiconcha major (Gabb, 1869). A. MCZ108540 from east of Knoxville, California, USA, Lower Cretaceous. Internal mould of left valve (A1), dorsal view (A2). B–D. Three specimens of Caspiconcha major (Gabb, 1869) from Wilbur Springs, California, USA, Hauterivian (Lower Cretaceous). B. Left valve of MCZ 108538A. C. Internal mould of right valve of MCZ 108538B. D. Right valve of MCZ 108538C. White arrowheads point to the anterior.