Amano et al.—New and Mesozoic-relict mollusks from Paleocene wood-fall communities
Type species.—Tellina flexuosa Montagu, 1803 (by original designation); Recent, Britain and Ireland.
Thyasira (Thyasira) oliveri Amano and Jenkins, new species urn:lsid:
zoobank.org:act:1F5DB57D-6A94-45F3-808B- 30551F18F70C
Figure 2.9, 2.10, 2.12, 2.13, Table 3 2015b Thyasira sp.; Amano et al., p. 47, figs. 16–17.
Type specimens.—Holotype, JUE 15993, from 550m upstream of Katsuhira-kita-zawa, Urahoro Town, Hokkaido (Loc. 19); Paratypes, JUE 15994-1–2, from 600m upstream of Katsuhira- kita-zawa (Loc. 20); Paratype, JUE 15995, from 40m upstream of Oppushini-zawa (Loc. 44). Type locality: 550m upstream of Katsuhira-kita-zawa in Urahoro Town, Hokkaido, Japan, 42°59'26''N, 143°38'41''E; Paleocene.
Diagnosis.—Small equilateral-ovate Thyasira with ridged second posterior fold, rather wide posterior sulcus, and narrow auricle.
Occurrence.—Upper Selandian to lowermost Thanetian, Katsuhira Formation, eastern Hokkaido, Japan.
Description.—Shell of moderate size (8.1–12.8mm L), thin, equilateral-ovate, longer than high, rarely higher than long, well inflated (W/L=0.52–0.58). Anterodorsal margin nearly straight; anterior margin subcircular; ventral margin broadly arcuate. Second posterior fold distinct; posterior sulcus rather deep; first posterior fold wide and ridged; submarginal sulcus distinct; auricle narrow, short. No medial flattened area observed. Beak prosogyrate, centrally situated. Surface smooth except for irregular growth lines. Lunule narrow, slightly sunken. Cardinal tooth very weak. Anterior adductor muscle scar quadrate, large; posterior one unknown.
Etymology.—Named after Dr. P. Graham Oliver (National Museum of Wales) who has made major contributions to the taxonomy of Recent thyasirids.
Materials.—One specimen each from Locs. 10, 19, 29, and 44; two specimens from Loc. 20; see Table 3 for dimensions.
Remarks.—Thyasira (Thyasira) oliveri n. sp. closely resembles the Cretaceous species T. (T.) tanabei Kiel, Amano, and Jenkins, 2008 in having a ridged first posterior fold. However, the new species differs in having a wider first posterior fold than T. (T.) tanabei. As discussed by Amano et al. (2015b), both
Table 3. Measurements of Thyasira (Thyasira) oliveri Amano and Jenkins, n. sp. H=height, L=length,W=width, –=not available; + =dimension in slightly imperfect case.
Locality Specimen Loc. 19 JUE 15993, holotype
L (mm) H (mm) W(mm) Valve –
10.3 9.7+ 8.1 9.0
Loc. 20 JUE 15994-1, paratype 12.8+ 10.3+ Loc. 20 JUE 15994-2, paratype 10.8 Loc. 44 JUE 15995, paratype Loc. 10 JUE 15937 Loc. 29 JUE 15936
10.1+
11.2 7.4 9.0 9.4
–
6.0 –
5.0 5.9
left
right both right both both
639
T. (T.) baca Devjatilova in Devjatilova and Volobueva, 1981 from the Paleocene Getkilninskaya Formation of western Kamchatka (Devjatilova and Volobueva, 1981) and T. (T.) mironovi Kalishevich in Kalishevich et al., 1981 from the lower Paleocene Sinegorsk Formation in South Sakhalin are dis- tinguished from the new species in having triangular shells with a narrower posterior area and a wider first posterior fold with an angular ventral end.
Subclass Heterodonta Neumayr, 1884 Order Carditoida Lamarck, 1809
Family Astartidae d’Orbigny, 1844 (1840) Genus Astarte Sowerby, 1816 Subgenus Astarte Sowerby, 1816
Type species.—Venus scotica Maton and Rackett, 1807 (by original designation)=Pectunculus sulcatus Da Costa, 1778 var. scotica (Maton and Rackett, 1807); Recent, Britain and Ireland.
Astarte (Astarte) paleocenica Amano and Jenkins, new species urn:lsid:
zoobank.org:act: 8AAD8CF1-CC70-417C-8B96- 278D39CBE47A
Figure 3.1–3.8, Table 4
Type specimens.—Holotype, JUE 15996, from the bank of the Urahoro River at the mouth of Kokatsuhira-zawa, Katsuhira, Urahoro Town, Hokkaido (Loc.1); paratype, JUE 15997, from Loc. 1; paratype, JUE 15998, from 320m upstream of Oppushini-zawa (Loc. 45). Type locality: river bank of the Urahoro River at the mouth of Kokatsuhira-zawa, Katsuhira in Urahoro Town, Hokkaido, Japan, 42°59’20”N, 143°38’04”E; Paleocene.
Diagnosis.—Very small subquadrate Astarte with distinct fine commarginal ribs and a subtruncated posterior margin. Ventral margin finely crenulated. Lunule asymmetrical, narrow, and well demarcated.
Occurrence.—Upper Selandian to lowermost Thanetian, Kat- suhira Formation, eastern Hokkaido, Japan.
Description.—Shell very small (3.3–4.2mm L), thin, inequi- lateral, nearly equivalve, subquadrate, slightly longer than high, moderately inflated (W/L=0.63). Anterodorsal margin con- cave; anterior margin rounded; ventral margin broadly arcuate. Posterodorsal margin nearly straight, gently inclined; posterior
margin truncated. Beak prosogyrate, anteriorly situated at two- fifths of shell length. Surface sculptured with regular, fine commarginal ribs. Lunule asymmetrical, left one wider than right, deeply sunken, demarcated by sharp ridge. Escutcheon narrow, flat. Anterior cardinal tooth of right valve (3a) indis- tinct; posterior cardinal tooth (3b) rather thick, inclined poster- iorly; anterior lateral tooth thin. Anterior tooth (2a) of left valve rather thick, divided by groove, inclined anteriorly; posterior tooth (2b) also moderately thick, inclined posteriorly. Anterior adductor muscle scar small, semicircular; posterior adductor muscle scar pear-shaped, rather large. Pallial line entire. Inner ventral margin finely crenulated.
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