Conchocele tessaria ( Olsson, 1931 )

Kiel, Steffen, Hybertsen, Frida, Hyžný, Matúš & Klompmaker, Adiël A., 2020, Mollusks and a crustacean from early Oligocene methane-seep deposits in the Talara Basin, northern Peru, Acta Palaeontologica Polonica 65 (1), pp. 109-138 : 114-115

Conchocele tessaria ( Olsson, 1931)

Fig. 4 .

1931 Thyasira tessaria sp. nov.; Olsson 1931: 53–54, pl. 6: 10, 14.

Material.— Six specimens ( NRM Mo187005–187010, Fig. 4 ) and several further unnumbered specimens (often fragmentary) from Cerro La Salina blocks 1, 4, 6–9, upper Oligocene, Talara Basin, Peru .

Description.—Small to medium-sized for the genus (L max. = 35 mm), inflation moderate (L/W ratio ca. 1.9), rounded-pentagonal in outline; beak prosogyrate, elevated and pointed, umbonal angle 90–100°; posterior fold deep but not sharp, increasingly curved in larger specimens, with resulting in shallow posterior sinus; a weak, secondary sulcus also present in some specimens; anterior margin narrow, demarcated by blunt ridge; lunule symmetrical and well-demarcated; escutcheon lanceolate; external surface sculptured by irregular, commarginal growth lines only. Hinge plate narrow, edentulous, ligament nymph broad and elongate; internal surface with fine radial striations developing tubercles near ventral margin.

Remarks.— Olsson (1931) described three thyasirid species from northern Peru: T. peruviana and T. staufti from the Lomitos cherts, and T. tessaria from the Heath Formation at Pajarabobo, near Belén. Examination of Olsson’s (1931) type material at PRI showed that the specimens of all three species are more inflated than the material we collected; strongly so in the cases of T. peruviana and T. staufti , and less so in the case of T. tessaria . Based on the overall similarity of our specimens to T. tessaria , especially their pentagonal outline, the broad posterodorsal area and the blunt posterior ridge, we considered the material reported here as belonging to T. tessaria . Thyasira peruviana differs from T. tessaria by its much more prominent umbo, the much broader anterior margin, and the much smaller posterodorsal area. Thyasira staufti differs from T. tessaria by its deeper and sharper posterior sulcus, its narrower posterodorsal area, and its more oval outline. Thyasira montanita ( Olsson, 1931) from allegedly coeval strata in southern Ecuador has finer growth lines, a more rounded outline, a broader anterior margin, and a broader and shorter anterior adductor muscle scar (personal observations SK).

Thyasira tessaria is here transferred to Conchocele based on its anteriorly situated beaks, the steeply sloping anterior margin, the weakly impressed lunule, and the broadly rounded posterior margin bearing a deep sulcus, characteristics of Conchocele that are not present in this combination in Thyasira . Conchocele tessaria shows the following differences to other Conchocele species: C. townsendi from Late Cretaceous strata on Seymour Island, Antarctica, has a similar pentagonal shape and narrow anterior margin, but has a deeper and sharper posterior fold ( Hryniewicz et al. 2017). The Paleocene C. conradii from Svalbard is more inflated and has a longer anterior margin than C. tessaria ( Hryniewicz et al. 2017) . Also C. taylori Hickman, 2015 , known from late Eocene to Oligocene strata of the North Pacific region, has a longer anterior margin, but its posterior fold is similar to that of C. tessaria ( Hickman 2015; Hryniewicz et al. 2017). Another late Eocene to Oligocene species is Conchocele bathyaulax Hickman, 2015 from Oregon and Washington state, USA; compared to C. tessaria , it is much more inflated and has a more pointed, terminal umbo ( Hickman 2015; Hryniewicz et al. 2017). The late Eocene C. kiritachiensis Hryniewicz, Amano, Jenkins, and Kiel, 2017 , from a seep carbonate in Hokkaido, northern Japan, differs from Conchocele tessaria by its rhomboidal shape, very broad anterior margin, and pointed, terminal umbo ( Hryniewicz et al. 2017). Finally, Conchocele adoccasa ( Van Winkle, 1919) from a Miocene seep deposit in Trinidad has a broader anterior margin and a more distinct (sharper) posterior fold. This applies also to the specimens assigned to C. adoccasa from presumably Oligocene seep deposits of Colombia ( Kiel and Hansen 2015). Overall, most similar to C. tessaria is the late Eocene to Oligocene C. taylori that is widespread along the North Pacific margin ( Hickman 2015; Hryniewicz et al. 2017).

One specimen (NRM Mo187008) shows two indentations on LV, which might represent a failed attack by a predator such as a durophagous crustacean. Damage to fossil seep mollusks attributed crustaceans has been noted previously (e.g., Kiel et al. 2016, and references therein, Klompmaker et al. 2018).

Stratigraphic and geographic range.—Early Oligocene of the Talara Basin, northern Peru.

Figures

Fig. 4. The thyasirid Conchocele tessaria (Olsson, 1931) from early Oligocene seep deposits at Cerro La Salina (block 1, E; block 7, A–C; block 8, D), Talara Basin, northern Peru. A, B. Left valve of large specimen (A, NRM Mo187005; B, NRM Mo187006). C. Medium-sized specimen (NRM Mo187007), right valve showing fine growth increments (C1), dorsal view showing lunule (C2), left valve showing dorsal sulcus (C3). D. Small specimen (NRM Mo187008), left valve showing a healed shell injury (D1), right valve showing dorsal sulcus (D2) and dorsal view showing lunule posterior area (D3). E. Small specimen (NRM Mo187010) showing hinge of right valve.