Maorithyas humptulipsensis, Hryniewicz & Amano & Jenkins & Kiel, 2017

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 : 717-720

publication ID	https://doi.org/ 10.4202/app.00390.2017
persistent identifier	https://treatment.plazi.org/id/03BA831C-DF2C-CE3F-FF56-578CFD17FA81
treatment provided by	Felipe
scientific name	Maorithyas humptulipsensis
status	sp. nov.

Treatment

Maorithyas humptulipsensis sp. nov.

Figs. 11 View Fig , 12 View Fig .

pallial line posterior sulcus umbo

anterior pedal retractor scar adductor scar

1990 Thyasira (Conchocele) folgeri Wagner and Schilling, 1923 ; Goedert and Squires 1990: fig 2a.

Etymology: Named after the Humptulips River.

Type material: Holotype: ZPAL L.16/10 ( Fig. 12A View Fig ); an articulated specimen with most of the shell preserved. Other figured ( Fig. 12B–D View Fig ) specimens designated as paratypes (NRM Mo 182388, ZPAL L.16/11, ZPAL L.16/12), all from the type locality.

Type locality: LACMIP loc. 12385, Humptulips River , Washington State, USA .

Type horizon: Seep carbonates of the Humptulips Formation, middle Eocene.

Material.— From the type locality: type material and 5 unnumbered specimens housed at ZPAL , 32 housed at NRM from CSUN loc. 1583 (middle Eocene , Humptulips River, western Washington State, USA): 10 unnumbered specimens, the largest of which reaches 45 mm in length, housed at NRM .

Dimensions.— The holotype is 35 mm long and 35 mm high.

Diagnosis. — Bulbous species of Maorithyas with pointed anteroventral angle and weak posterior fold not extending beyond outline of shell.

Description. — Shell up to 62 mm long, thin, equivalve, very inflated, subrounded to pentagonal. External ornament composed of very fine, commarginal growth lines. Umbones subcentral, anterior of midline of the shell, prosogyrous, strongly incurved. Lunule present, heart-shaped, shallow. Anterior margin straight and short, bound by two indistinct ridges running from umbo, anteroventral angle obtuse. Ventral margin rounded, with deepest point around mid-line. Posterior margin straight with very weak sinus. Posterior fold not projecting beyond posterior margin of the shell, posterior sulcus shallow and broad, with weak secondary sulcus sometimes visible amidst fold. Posterodorsal margin weakly concave. Escutcheon absent. Ligament narrow, sunken. Interior of shell covered with thinly spaced, radial striae, more common in anterior. Anterior adductor muscle scar weak and small, visible only in its anterior part. Anterior pedal retractor muscle scar shallow, small and oval. Posterior adductor muscle scar small and weak, restricted to posterior fold.

Remarks.— Maorithyas humptulipsensis differs from the Recent Maorithyas marama Fleming, 1950 , and Maorithyas flemingi Powell, 1955 , by its more rhomboidal shell, more anteriorly situated umbo and less pronounced posterior fold, which does not extend beyond the posterior margin of the shell (cf. Fleming 1950: pl. 25: 2; Powell 1955: pl. 1: 1). However, the degree of extension of the posterior fold seems to have some range of variation in the type species. For example, three specimens identified as M. marama , collected at Stewart Island and stored at the Museum of New Zealand Te Papa Tongarewa (M 26537) show a narrower posterior fold than the type. The Recent “ Maorithyas ” hadalis from the Japan Trench figured by Fujikura et al. (1999: fig. 3) and Okutani et al. (1999) differs in characters of the hinge plate, nymph and anterior adductor muscle scar from Maorithyas humptulipsensis , and may not even belong to Maorithyas ( Valentich-Scott et al. 2014: 17) .

Maorithyas View in CoL is a poorly known genus, comprising only two named and one unnamed extant species ( Fleming 1950; Powell 1955; Spencer 2009: 199). In addition to a single fossil species M. humptulipsensis , it is possible that “ Thyasira ” folgeri Wagner and Schilling, 1923 , from upper Eocene seep carbonates in the Wagonwheel Mountains in central California ( Squires and Gring 1996; Kiel and Peckmann 2007) belongs to Maorithyas View in CoL . The holotype of “ Thyasira ” folgeri (UCMP 11434, UCMP loc. 3195; Oligocene, San Emigdio Formation, Devil’s Kitchen, Kern County, southern California, USA; Moore 1988: pl. 7: 9; Fig. 13 View Fig ) is similarly rounded and inflated as M. humptulipsensis , and also has a very weak to no distinct posterior sinus. It has, however, a less pronounced posterior fold different from that of M. humptulipsensis , and a less pronounced anteroventral angle. Thyasira folgeri from the Wagonwheel Mountain seep ( Squires and Gring 1996: fig. 4.8) is very similar to the holotype in general outline and the lack of a posterior sinus, and is also unlikely to belong to Conchocele View in CoL as classified by Squires and Gring (1996). In our opinion, “ Thyasira ” folgeri from both mentioned localities in Kern County should be considered a species of Maorithyas View in CoL , thus being Maorithyas folgeri ( Wagner and Schilling, 1923) . The holotype of T. folgeri was found together with lucinids in “limy concretions in the black shale member” ( Wagner and Schilling 1923: 243) and might hence also be from a seep deposit.

Stratigraphic and geographic range. — Middle Eocene seep deposits (LACMIP loc. 12385 and CSUN loc. 1583) from the Humptulips Formation, Washington State, USA.