Conchocele conradii ( Rosenkrantz, 1942 )

Hryniewicz, Krzysztof, Amano, Kazutaka, Jenkins, Robert G. & Kiel, Steffen, 2017, Thyasirid bivalves from Cretaceous and Paleogene cold seeps, Acta Palaeontologica Polonica 62 (4), pp. 705-728 : 710-711

Conchocele conradii ( Rosenkrantz, 1942)

Fig. 5 .

1925 Thyasira bisecta Conrad, 1849 ; Hägg 1925: 46–48, pl. 4: 14–17, pl. 5: 18, 19, non 20.

1942 Thyasira conradii n. sp.; Rosenkrantz 1942: 277–278.

1970 Thyasira (Conchocele) conradi Rosenkrantz, 1942 (sic!); Rosenkrantz 1970: 427, fig. 7.1, non fig. 7.2.

1970 Conchocele conradii ( Rosenkrantz, 1942) ; Vonderbank 1970: pl. 10: 2a, b.

1980 Conchocele conradii ( Rosenkrantz, 1942) ; Thiedig et al. 1980: 143, pl. 1: 1, 3, non 4.

2016 Conchocele conradii ( Rosenkrantz, 1942) ; Hryniewicz et al. 2016: fig. 12A.

Type material: Holotype unknown; lectotype MMH 10792 ( Rosenkrantz 1970).

Type locality: South-western side of Colesbukta, Spitsbergen, Svalbard .

Type horizon: Paleocene, Basilika Formation.

Material.—All of the material studied comes from Paleocene Basilika Formation, Colesbukta area, Spitsbergen, Svalbard. Two specimens from GPIBo, including one figured by Vonderbank( GPIBo Nr.159)(1970); 14unnumbered specimens from PMO; 37 specimens from NRM, including one figured ( NRM PZ Mo 182204; Fig. 5A ), this comprises five specimens (Mo PZ 149148–152) illustrated by Hägg (1925) and one ( NRM PZ Mo 182204) by Hryniewicz et al. (2016); 121 unnumbered specimens from the collections of ZPAL, and one numbered and figured specimen ( ZPAL L.16/1). The material from GPIBo contains only specimens with sandstone matrix which are unlikely to be of seep origin. The specimens from PMO are all of seep origin; the material from NRM and ZPAL contains a mixture of seep and non-seep specimens, with seep specimens prevailing.

Remarks.— Conchocele conradii from the south-western side of Colesbukta on Spitsbergen, Svalbard, was first described as Thyasira bisecta Conrad, 1849 , by Hägg (1925), who noticed that the specimens from Svalbard are larger than the largest specimens of C. bisecta from the Miocene Astoria Formation type locality in Oregon, USA ( Moore 1963). He also noticed that the Svalbard specimens are closely related to C. townsendi from the Cretaceous of the James Ross Basin, Antarctica, discussed above. A new species name, Thyasira conradii , was thereafter proposed for the Svalbard species by Rosenkrantz (1942). Its generic affinity was later revisited by Rosenkrantz (1970) and by Vonderbank (1970), who formally moved it to Conchocele .

An allegedly similar species, Conchocele aff. conradii , was mentioned to occur in the lower Paleocene Kangilia Formation in West Greenland ( Rosenkrantz 1970). The illustration provided therein ( Rosenkrantz 1970: fig. 7.2) is insufficient for a more detailed comparison; the overall shell outline presented there is somewhat different from that of C. conradii , and is more reminiscent of C. townsendi . Therefore, until more material is available for study, we do not include the specimens from the Kangilia Formation in C. conradii . Conchocele conradii differs from C. townsendi by its somewhat shorter shell, more rounded ventral margin, broader posterior sulcus, and the lack of two ridges on the anterior margin of the shell. The specific significance of the latter character is unknown, however, as not all specimens of C. townsendi have the two ridges mentioned. All other Paleogene species of Conchocele that we have investigated have more quadrangular shells than C. conradii .

Stratigraphic and geographic range. —Paleocene cold seep carbonates and associated sunken driftwood environments of the Basilika Formation, Colesbukta, Spitsbergen, Svalbard. Possibly also upper Eocene–lower Oligocene strata of the Renardodden Formation, Renardoden, Belsund area, Spitsbergen, Svalbard ( Thiedig et al. 1980). A similar species Conchocele aff. conradii was reported from lower Paleocene Kangilia Formation from West Greenland ( Rosenkrantz 1942, 1970).

Figures

Fig. 5. Thyasirid bivalve Conchocele conradii (Rosenkrantz, 1942) from Paleocene strata of the Basilika Formation, Colesbukta, Spitsbergen, Svalbard. A. NRM PZ Mo 182204; a medium sized internal mold in left lateral view with no clear outline of an anterior adductor muscle scar visible. B. ZPAL L.16/1; an internal mold in right (B1) and left (B2) views with fragments of the shell adhering (no clear anterior adductor muscle scar visible); in dorsal view (B3) showing fragments of a posterior sulcus; anterior fragments of the shell (B4) showing flat anterior margin without ridges.