Nuculites wattii ( Tate, 1896 )

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 : 912

Nuculites wattii ( Tate, 1896)

Fig. 11C–F .

1896 Isoarca wattii sp. nov.; Tate 1896: 108, pl. 2: 17a, b.

1977 Nuculites wattii ( Tate, 1896) ; Pojeta and Gilbert-Tomlinson 1977: 16, pl. 6: 6, 7.

Type material: Lectotype ( SAM T 1220 a) designated by Pojeta and Gilbert-Tomlinson (1977: pl. 6: 6, 7) . Syntype figured by Tate (1896: pl. 2: 17a).

Type locality: Locality between Tempe Downs and Petermann Creek (about 5 km apart) in the Amadeus Basin ( Tate 1896) .

Type horizon: Judging from the quartzitic lithology of the type suite of Nuculites wattii , the taxon is probably from the Stairway Sandstone, probably Darriwilian (Middle Ordovician).

Material.― 90 specimens, all from the Areyonga Gorge section, Middle Ordovician of Australia. The material is rather fragmentary and some of the non-illustrated specimens are tentatively assigned. The four ( CPC 41470–41473 ) best preserved internal moulds are illustrated ( Fig. 11C–F ) .

Description.―Shell tumid and shape ovate with prominent anterior myophoric buttress and highly prosogyral umbo. Posterior tooth row consists of about 20 chevron or half-chevron shaped teeth with apex pointing towards umbo ( Fig. 11F ) and constitute about 75% of the total length (transverse) of the valve.

Remarks.― Nuculites is a designation typically used for early Palaeozoic palaeotaxodonts with a prominent anterior myophoric buttress, and the group is in need of taxonomic revision. Tate (1896: 108) discussed the dentition and noted that there are a few large anterior teeth but numerous smaller teeth located posteriorly. There may be space for a few teeth anteriorly in the specimens but they appear very indistinct (see Fig. 11F ). Tate (1896) counted about 20 teeth posteriorly, which is the same number observed on specimens collected for this study.

When a cast of the species is viewed from above the umbonal ventricosity hides the hinge-line posteriorly and this is a way of separating Nuculites wattii from rather similar Sthenodonta eastii if the myophoric buttress is not preserved. Stratigraphic and geographic range.― Nuculites wattii is endemic to central Australia. The exact type locality is unknown but seems to be close to the Petermann Creek section studied here (see type material). However, not a single specimen assigned to Nuculites wattii was recovered from this section and all specimens derive from bed A -9 ( Fig. 4 ) in the Areyonga Gorge section located some 40 km further north (Fig. 3). 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. 4. Composite log of the Stairway Sandstone and abundance-charts showing distribution of bivalves and rostroconchs in the Areyonga Gorge section.