Thyasira Lamarck, 1818

Kiel, Steffen & Goedert, James L., 2007, New mollusks associated with biogenic substrates in Cenozoic deep-water sediments of Washington State, Acta Palaeontologica Polonica 52 (1), pp. 41-52 : 48-49

Genus Thyasira Lamarck, 1818

Type species: Tellina flexuosa Montagu, 1803 ; Recent, North Sea. Thyasira xylodia sp. nov.

Fig. 7 .

1995 Thyasira peruviana? Olsson ; Goedert et al. 1995: 154, table 1, figs. 4, 5.

2006 Thyasira n. sp.; Kiel and Goedert 2006a: 2626, fig. 2e, f.

Derivation of the name: From Greek xylon, wood; after its association with wood.

Holotype: LACMIP 13370 . Large, articulate specimen with damaged posterior margin, right valve largely concealed by matrix.

Paratypes: USNM 532043 and 532857, both from USGS loc. 26898−B; LACMIP 12336 , from USGS loc. 26898.

Type locality: USGS locality 26898, Clallam County, Washington State, USA .

Type horizon: Upper lowermost Oligocene, Pysht Formation.

Material.— The type material and specimens were found at the wood−fall sites USGS loc. 26896−C, 26898−B, 26901, and with the whale specimen USNM 314593 from USGS loc. 26896. These specimens are deposited in the USNM. Further specimens were found with whale bones at CSUN loc. 1578 (= USGS loc. 26898), the site from which (Goedert et al. 1995) described Thyasira peruviana? Olsson, 1931 . These specimens are deposited in the UWBM.

Diagnosis.—Large Thyasira with equilateral obliquely−ovate outline; posterior margin truncate, posterior fold concave, sub−marginal sulcus very narrow, no auricle; lunule deeply excavated, broad, but not reaching the end of the anterodorsal margin.

Description.—Prodissoconch 132 ̊m wide, and 46 ̊m thick, smooth apart from very fine radial and concentric wrinkles. Dissoconch prosogyrate, 2 nd posterior fold concave, forming a sharp edge with the narrow posterior sulcus; 1 st posterior fold also with sharp inner edge, sub−marginal sulcus smooth and very narrow; length of lunule approx. 70% of anterodorsal margin; ventral margin in some specimens undulating, corresponding to a weak depression along the midline. Surface smooth apart from fine commarginal growth lines. The holotype is 21 mm high and long. The terminology used here follows that of Oliver and Killeen (2002).

Discussion.—Goedert et al. (1995) reported Thyasira peruviana ? from Oligocene whale−falls in the Makah and Pysht Formations in western Washington, a species which had previously only been reported from suspected cold−seeps in northwestern Peru ( Olsson 1931). Goedert et al. (1995) noted that some of the paratypes figured by Olsson (1931) do not show the sharp ridge in the posterodorsal area that characterizes the type specimen. Examination of Peruvian voucher material of T. peruviana in the USNM showed that this sharp ridge is in fact present in all examined specimens. The holotype figured by Olsson (1931) appears to lack the large posterodorsal area that is present in some of Olsson’s paratypes. This was also observed among the USNM specimens, however, this lack can in all cases be attributed to corrosion.

Whereas the thyasirid Conchocele bisecta ( Conrad, 1849) occurs throughout the siltstones and cold−seeps of the Lincoln Creek, Makah, and Pysht formations, Thyasira xylodia has so far only been found associated with either wood or whale bones. It is distinguished from Conchocele bisecta by its truncate posterior margin with a very narrow sub−marginal sulcus. Goedert and Squires (1990) reported a large thyasirid from a upper middle to upper Eocene seep in the Humptulips Formation in Washington as Thyasira folgeri Wagner and Schilling, 1923 , a species originally reported from the Wagonwheel and San Emigdio formations in California (Wagner and Schilling 1923; Squires and Gring 1996). Thyasira xylodia differs from these specimens by its truncate posterior margin with a very narrow sub−marginal sulcus. Although T. folgeri was not listed by Squires and Gring (1996) in their synonymy of C. bisecta , they mentioned that Smith (1956) and Jenkins (1931) both thought that T. folgeri was based on juvenile specimens of C. bisecta . However, observations by the senior author of material from the Humptulips and Wagonwheel seeps, and Recent specimens of C. bisecta in the USNM and LACM zoology collection indicate that the Humptulips specimens are consistently more inflated and proportionately shorter than C. bisecta . We thus consider the Humptulips species to be distinct from C. bisecta . Whether the Humptulips species does indeed belong to T. folgeri as initially proposed is not entirely certain, and is under study.

Stratigraphic and geographic range.—Uppermost Eocene to lower Oligocene, Makah, Lincoln Creek, and Pysht formations, Washington State, USA. Associated with wood and whale bones.

Figures

Fig. 7. Thyasirid bivalve Thyasira xylodia sp. nov. from Eocene to Miocene whale− and wood−falls in Washington State. A. Holotype LACMIP 13370, from a upper lower Oligocene wood−fall at Murdock Creek (USGS loc. 26898−J) in lateral (A1) and umbonal (A2) views. B. Paratype USNM 532043, juvenile specimen from a upper lower Oligocene wood−fall at Murdock Creek (USGS loc. 26898−B) in umbonal (B1) and lateral (B2) views, and close−up on microsculpture at ventral shell margin (B3). C. Juvenile specimen USNM 532857, showing prodissoconch, from same locality as B.