Tilicrassatella sanmartini, Devries, 2016

Devries, Thomas J., 2016, Fossil Cenozoic crassatelline bivalves from Peru: New species and generic insights, Acta Palaeontologica Polonica 61 (3), pp. 661-688 : 673-675

Tilicrassatella sanmartini sp. nov.

Fig. 8H.

Etymology: Named in honor of José de San Martín (1778–1850), a liberator of Argentina and Peru.

Holotype: Adult right valve with damaged hinge and posterioventral portion missing: UWBM 101857 , L (89.0), H (55.7), T 19.8.

Type locality: B8328, near confluence of Quebrada Gramonal and the Río Ica, East Pisco Basin, Peru.

Type horizon: Lower Chilcatay depositional sequence, Chattian or Aquitanian.

Diagnosis.— Tilicrassatella with posterior exceedingly elongate but not constricted. Resilifer directed more posteriorly than ventrally.

Description.—Shell to about 100 mm long, ellipsoidal, elongate, moderately inflated, beak less than one third distance from anterior end, L:H ratio 1.5, T:H ratio 0.2, maximum inflation posterior to beak. Anteriodorsal margin steeply descending, evenly convex; posteriordorsal margin straight. Anterior margin sharply and evenly rounded; posterior margin rounded ventrally, bluntly truncate dorsally. Ventral margin broadly rounded to straight, not constricted posteriorly. Primary posterior ridge weakly rounded, diverging 15−20° from posteriodorsal margin; secondary posterior ridge absent. Lunule elongate, narrow; escutcheon elongate, wider than lunule, about 40% length of posteriodorsal margin. Beak not preserved. Umbo rounded, orthogyrate, with closely spaced commarginal ribs. Remainder of exterior with irregular commarginal growth lines. Resilifer broad, slightly raised above hinge plate, inclined posteriorly 40°, extending nearly to vmHP. Right cardinal tooth thick, wedge-shaped. Right posterior pseudocardinal tooth starting at beak, passing resilifer anteriorly. Inner ventral margin crenulate.

Remarks.—This specimen differs from its sympatric congeners, Tilicrassatella ponderosa and T. torrens , by its extreme posterior elongation and posteriorly directed resilifer, features shared by many taxa attributed to Bathytormus , including the Eocene Argentinian Bathytormus longior ( Ihering, 1897) . One specimen of B. longior in particular, figured by Santelli and del Río (2014: fig. 6.7), greatly resembles the Peruvian specimen, although it is proportionally longer (L: H 1.7) with a less robust escutcheon, lunule, and umbo.

In addition to a full-length resilifer and crenulated inner ventral margin, Chavan (1939) attributed to Bathytormus a rostrate posterior, and Wingard (1993), a pronounced inequilaterality at all growth stages. Neither of the these characters describes the Peruvian specimen.

Chattian-Burdigalian specimens from the Guadal Formation of southern Chile attributed questionably to Bathytormus longior by Frassinetti and Covacevich (1999) are less elongate and more truncate posteriorly than specimens of B. longior described by Ihering (1897) and Santelli and del Río (2014). The Guadal specimens, with so few umbonal commarginal ribs (9−10 per 10 mm radially), are not examples of the contemporaneous Tilicrassatella , all three species of which have 17−25 umbonal ribs in the same radial interval.

Stratigraphic and geographic range.—Chattian to Aquitanian. Chilcatay depositional sequence, East Pisco Basin, south-central Peru.