Earlpackardia methowensis, Halligan, 2023

Halligan, William Keith, 2023, Washington State (USA) trigoniids (Bivalvia) from the conglomerate of Patterson Lake (Early Cretaceous), PaleoBios 40 (8), pp. 1-15 : 12-13

EARLPACKARDIA METHOWENSIS n. sp.

Fig. 10 A-D

Zoobank LSID— urn:lsid:zoobank.org:pub:BF86936D-74C7-4D43-A078-08588EA93614

Diagnosis— Small, subequilateral (umbo anterior), oblong; prominent,incurved umbo elevated above dorsal margin, sharply pointed beak, prominent marginal carina extending from the beak toward the posterior margin. Area/escutcheon sunken and unornamented except for fine, oblique growth lines. Posterodorsal margin nearly straight. Umbo and beak ornamented with 16-18 fine, commarginal costellae which are abruptly replaced by 9 coarse, slightly nodular, slightly wavy, subcommarginal, nearly horizontal costae on the anterior flank; costae fade at mid-flank and are replaced by fine growth lines posteriorly.

Holotype — UWBM 112559 from UWBM loc. 9486. A nearly complete LV that is missing the posterior terminus and some of the ventral margin, especially posteriorly. The posterior and ventral shell margins are extrapolated using growth lines as a guide.

Occurrence— Type locality: UWBM loc. B-9486. Upper part of the cPL, Okanogan County, Washington State ( USA). Age : middle Albian .

Etymology— Named after the Methow Valley and the sedimentary basin from which it was collected.

Description— Small (L ≈ 41 mm, H ≈ 28 mm), subequilateral (umbo anterior), oblong, inflated (W/H = 0.3), longer than high (H/L = 0.68); prominent, incurved umbo and pointed beak elevated above dorsal margin. Anterodorsal margin nearly straight and slopes toward truncated anterior end. Posterodorsal margin nearly straight as it courses posteriorly. Posterior terminus unknown. Ventral margin (as extrapolated from growth lines) almost straight anteriorly becoming progressively convex posteriorly.

Sunken area/escutcheon unornamented except for fine, oblique growth lines; no median carina or groove. Area gradually narrows posteriorly. Area rises abruptly at almost 90° to form the posterodorsal margin. Prominent, rounded marginal carina gently concave as it courses from the beak toward the posterior terminus.

Umbo and beak ornamented with 16-18 fine, closely spaced, commarginal costellae that are replaced abruptly by 9 coarse, rounded, slightly nodular, slightly wavy, nearly horizontal, subcommarginal flank costae that extend from the anterior margin to mid-flank where they fade and are replaced by fine growth lines posteriorly. Width of the flank costae about equal to their interspaces. Internal features unknown.

Discussion— There are three other species in the genus, Earlpackardia Cooper 2015b . From the late Barremian-early Aptian of Japan, E. yeharai ( Kobayashi, 1954) is morphologically the most similar to E. methowensis . (Description of E. yeharai from Kobayashi 1957b.) They are both oblong with straight posterodorsal margins and prominent, elevated umbos with pointed beaks ornamented with fine, commarginal costellae. The flank costellae fade at mid-flank in both species and are replaced by fine growth lines posteriorly. Unlike E. yeharai , however, the area and escutcheon in E. methowensis are sunken and lack the costellae seen in the early growth of E. yeharai ; the marginal carina is more prominent in E. methowensis and takes a less steep course posteriorly; the subcommarginal flank costellae are more horizontal, coarse, and nodular than the fine costae in E. yeharai .

Earlpackardia jacksonensis ( Packard, 1921) was found in the Osburger Gulch Sandstone of the Hornbrook Formation near Jacksonville, Oregon ( USA) and is geographically the closest species of the genus to E. methowensis . Although Packard originally gave it the imprecise age of Upper Cretaceous, Sliter et al (1984) dated the Osburger Gulch Sandstone in the Jacksonville, OR area as late Albian(?) to middle Cenomanian. This also makes it chronologically the closest species of the genus to E. methowensis . There are some morphological similarities between the two species. (Description of E. jacksonensis from Packard 1921.) Both are elongate with a prominent, elevated umbo, straight and sloping posterodorsal margin, unornamented area/escutcheon, and coarse, slightly nodose flank costae that fade at mid-flank, being replaced by fine growth lines posteriorly. Unlike E. methowensis , however, E. jacksonensis is moderately large, more posteriorly produced, and has a tumid umbo, blunt beak, and flank costae that are more wavy, oblique, and discontinuous. The marginal carina is less prominent than in E. methowensis .

Earlpackardia amagensis ( Kobayashi 1957a) is from the late Oxfordian ( Andal et al. 1968) Amaga River of Mindoro ( Philippines). In contrast to E. amagensis , E. methowensis is less posteriorly produced, more elongate with a more prominent umbo, and a sunken area/ escutcheon; its marginal carina is more pronounced. The flank costae are coarser and fewer in number, but like E. amagensis , they fade near mid-flank and are replaced by fine growth lines posteriorly.

Figures

Figure 10. Earlpackardia methowensis n. sp., holotype UWBM 112559 from UWBM loc. 9486. A. Flank view. Dotted line indicates proposed reconstruction of missing shell based on growth lines. B. Oblique view demonstrating the umbo, marginal carina, unornamented area, and escutcheon. C. Dorsal view demonstrating the pointed beak and degree of inflation. D. Oblique dorsal view showing the incurved umbo, unornamented area, and escutcheon. Scale bar = 1 cm.