Cyrtodonta staffordae Pojeta and Gilbert-Tomlinson, 1977

Jakobsen, Kristian G., Brock, Glenn A., Nielsen, Arne T. & Harper, David A. T., 2016, A Darriwilian (Middle Ordovician) bivalve-dominated molluscan fauna from the Stairway Sandstone, Amadeus Basin, central Australia, Acta Palaeontologica Polonica 61 (4), pp. 897-924 : 914-915

Cyrtodonta staffordae Pojeta and Gilbert-Tomlinson, 1977

Fig. 11H–K .

1977 Cyrtodonta staffordae sp. nov.; Pojeta and Gilbert-Tomlinson

1977: 20, pl. 14: 1–3.

Type material: Holotype ( CPC 15541 ) is illustrated by Pojeta and Gilbert-Tomlinson (1977: pl. 14: 1, 2) . Paratype ( CPC 15540 ) figures by Pojeta and Gilbert-Tomlinson (1977: pl. 14: 3). Both specimens are sandstone moulds .

Type locality: Deering Creek , Amadeus Basin, southern Northern Territory, Australia .

Type horizon: Stairway Sandstone, Darriwilian (Middle Ordovician).

Material.― 17 specimens from the Petermann Creek section and 13 specimens from the Areyonga Gorge section, Middle Ordovician of Australia. We illustrate three internal moulds ( CPC 41475–41478 ) .

Description.―Shell subquadrate with straight dorsal margin; anterior margin rounded and projecting dorsally, ventral and posterior margins broadly rounded. Umbo subdued and projecting only slightly above dorsal commissure. A single posterior lateral tooth is present in the right valve ( Fig. 11K ).

Remarks.― Cyrtodonta staffordae strongly resembles C. wattii . The shape of C. wattii , however, appears slightly more elongate as compared to C. staffordae which broadens in a posterior direction (compare Pojeta and Gilbert-Tomlinson 1977: pl. 14: 3 vs. pl. 15: 3). The left valve has three elongate posterior lateral teeth and at least three anterior teeth according to Pojeta and Gilbert-Tomlinson (1977: 21). Anterior teeth are not observed in the material at hand and most specimens only preserve one lateral posterior tooth. A single fragmentary specimen, however, shows three posterior lateral teeth ( Fig. 11J ) as well as a duplivincular ligament very similar to that of C. wattii (see Pojeta and Gilbert-Tomlinson 1977: pl. 15: 3, 5). Due to the fragmentary state of the specimen it is uncertain whether it belongs to C. wattii , as other cyrtodontids also possess three posterior lateral teeth (e.g., Cyrtodonta hazeli Pojeta and Gilbert-Tomlinson 1977 : pl. 16).

Pojeta and Gilbert-Tomlinson (1977) noted that C. staffordae has a subdued umbo whereas C. wattii possesses a prominent umbo. This is not always clear from their illustrations (compare Pojeta and Gilbert-Tomlinson 1977: pl. 14: 3 vs. pl. 15: 2), further complicating discrimination of the two species. Separation can be based on the posterior lateral tooth or teeth. Cyrtodonta staffordae is further distinguished from the equally subquadrate C. carberryi by having broadly rounded anterior and posterior margins rather than straight oblique margins.

Cyrtodonta staffordae can be separated from the C. carberryi and Cyrtodonta sp. B by a less elongated profile or more symmetric shape as well as a slightly lower shell profile. Cyrtodonta staffordae is separated from Modiolopsis pojetai sp. nov. by possessing a less elongated profile (or a more symmetric shape).

Stratigraphic and geographic range.― Cyrtodonta staffordae ranges over 55 m through most fossiliferous beds in the Petermann Creek section (PC -5–PC +11; Fig. 5 ), whereas it has a shorter range over 18 m in the Areyonga Gorge section (bed A -1–A +5; Fig. 4 ). Pojeta and Gilbert-Tomlinson (1977: 20 and 53) reported C. staffordae from the Stairway Sandstone at two localities (based on two specimens) in the Gardiner Range.

Cope (1996) reported C. cf. staffordae from the lower Floian of South Wales based on one specimen. The specimen is thus somewhat older, smaller, and there are indications of a left lateral tooth, which is not present in typical C. staffordae . It seems unlikely the two species are conspecific. Cope (1996) also reported two specimens of C. cf. wattii from the lower Floian of South Wales. Cyrtodonta wattii was reported by Pojeta and Gilbert-Tomlinson (1977: 21) from the Stairway Sandstone at 12 localities in the Amadeus Basin, not including the Petermann Creek or Areyonga Gorge. The range of the species is within the Stairway Sandstone Formation, Darriwilian (Middle Ordovician). The region is in the southern Northern Territory, corresponding to the central part of the Amadeus Basin ( Fig. 1).

Figures

Fig. 11. Nucularcid and pteriomorphian bivalves from the Stairway Sandstone, Middle Ordovician, Australia. Internal moulds (B–K) and latex casts of external moulds (A, L, M). A. Sthenodonta sp. B (CPC 41468) from Petermann Creek, left-lateral view showing well marked growth lines. B. Johnmartinia cordata Pojeta and Gilbert-Tomlinson, 1977 (CPC 41469) from Areyonga Gorge; left-lateral view with dorsal border (B1), anterior view showing the pointy beak (B2). C–F. Nuculites wattii Tate, 1896 from Areyonga Gorge. C–E. Specimens in left (C, E) and right-lateral (D) dorsal views, showing well-defined anterior myophoric buttress, D additionally shows dorsal teeth (C, CPC 41470; D, CPC 41472; E, CPC 41472). F. Right-lateral view of fragmentary specimen with chevron shaped posterior teeth pointing anteriorly (CPC 41473). G. Cyrtodonta carberryi Pojeta and Gilbert-Tomlinson, 1977 (CPC 41474) from Areyonga Gorge; in right-lateral view with growth lines and at least one dorsal lateral tooth (G1), dorsal view illustrating the height of shell profile (G2). H–K. Cyrtodonta staffordae Pojeta and Gilbert-Tomlinson, 1977 from Petermann Creek (H–I, K) and Areyonga Gorge (J). H. Specimen in right-lateral view showing distinct growth lines (CPC 41475). I. Specimen in left-lateral view with three posterior lateral teeth (CPC 41476). J. Fragmentary posterior margin with duplivincular ligament and indications of three posterior lateral teeth (CPC 41478). K. Specimen in right-lateral view with a posterior lateral tooth (CPC 41477). L, M. Cyrtodonta sp. B. L. Anterior (L1) and left-lateral (L2) views of sub-ovate shell-profile (CPC 41479) from Petermann Creek. M. Sub-ovate shell-profile with growth lines (CPC 41480) from Areyonga Gorge. Scale bars 10 mm.

Fig. 5. Composite log of the Stairway Sandstone and abundance-charts showing distribution of bivalves and rostroconchs in the Petermann Creek section.

Fig. 4. Composite log of the Stairway Sandstone and abundance-charts showing distribution of bivalves and rostroconchs in the Areyonga Gorge section.