Hybolophus disenum, 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 : 675-676

Hybolophus disenum sp. nov.

Fig. 9A− D .

Etymology: From Latin di, two, and senex, old man; referring to Axel A. Olsson, then 69, and Wendell P. Woodring, then 67, who together collected the type specimens in 1958. Olsson had invited Woodring to Peru to show him evidence of incorrect foram-based Paleogene ages assigned by Robert M. Stainforth (Linda Hindall, personal communication 2015, citing a letter written by A.A. Olsson to Katherine Palmer). Type material: Holotype, adult right valve with hinge mostly obscured and ventral margin missing: USNM 618239, L (33.9), T 12.6. Paratypes: USNM 618238, H (38.4), T 17.1; USNM 618240, H (25.7); USNM 618241, H (24.1), T (9.3).

Type locality: WP 28, minor tributary of Quebrada Mancora, 700 m S of bridge across Quebrada Mancora and about 100 m E of highway, 15 m higher in section that WP 27. Northern Peru .

Type horizon: Base of Heath Formation , lower Oligocene or lower Miocene (sensu Woodring 1958) .

Diagnosis.—Shell veneriform, inflated. Beak situated nearly medially. Exterior with commarginal growth lines.

Description.—Shell to at least 40 mm, possibly 50−60 mm long, ovate, veneriform, nearly equilateral, L:H ratio 1.25;

→ Fig. 8. Crassatellid bivalves Tilicrassatella from Peru and Chile. A −C. Tilicrassatella ponderosa ( Philippi, 1887) . UWBM 101843, early Miocene, Chilcatay depositional sequence, south-central Peru; exterior (A 1) and interior (A 2) of right valve. B. SGO. PI. 468, lectotype, early to middle Miocene, Navidad Formation, Chile; exterior of right valve. C. UWBM 101844, early Miocene, Chilcatay depositional sequence, south-central Peru; dorsal margin of paired valves, anterior at right. D −G, I. Tilicrassatella torrens sp. nov., early to middle Miocene, Chilcatay or basal Pisco depositional sequences, south-central Peru. D. UWBM 101846, holotype, exterior (D 1) and interior (D 2) of left valve. E. UWBM 101853, paratype, hinge plate of left valve (E 1), umbo (E 2). F. UWBM 101855, dorsal margin of paired valves (F 1), anterior at right, exterior of left valve (F 2). G. UWBM 101852, interior of left valve (G 1), anterioventral inner margin with crenulations (G 2). I. MUSM INV 230, paratype, hinge plate of right valve. H. Tilicrassatella sanmartini sp. nov., UWBM 101857, latest Oligocene to early Miocene, basal Chilcatay depositional sequence, south-central Peru; exterior (H 1) and interior (H 2) of right valve; dorsal margin (H 3), anterior at right. Scale bars 10 mm.

T:H ratio 0.3−0.35, maximum inflation posterior to beak. Anteriodorsal and posteriodorsal margins nearly straight, not steeply descending ventrally. Anterior margin sharply rounded, ventral margin rounded with medial angulation, posterior margin very broadly truncate. Weakly angular primary posterior ridge diverging 20−25°, secondary posterior ridge less than 5° from posteriodorsal margin. Lunule narrowly cordate, escutcheon narrow. Beak prosogyrate. Umbo flattened, orthogyrate to opisthogyrate, with widely spaced commarginal ribs. Remainder of exterior with irregularly spaced commarginal growth lines. Resilifer narrow, extending to vmHP. Left anterior cardinal tooth narrow, wedge-shaped, inclined anteriorly 40°, dorsal intersection with lunule damaged. Left posterior cardinal tooth narrower, inclined anteriorly 10°, extending to beak. Right anterior pseudocardinal tooth joining proximal ends of lunule and cardinal tooth. Right cardinal tooth thick, wedge-shaped, inclined anteriorly 10−15°. Right posterior pseudocardinal tooth lamellar, diverging 45° posterioventrally from midpoint of cardinal tooth. Ventral margin not preserved.

Remarks.—Field notes of Woodring (1958: 8–9) put locality WP27 and the type locality of Hybolophus disenum, WP 28, close to the town of Mancora in soft sandstone beds of the basal Heath Formation, which were observed to overlie massive sandstone and conglomerate of the Mancora Formation. The two formations are lower and upper Oligocene, respectively ( Higley 2004). Neither formation, however, is now mapped near Mancora ( Palacios 1994; also, César Chalcatana [INGEMMET], personal communication 2015). Woodring’s (1958) localities do plot in an area mapped by Palacios (1994) as the upper Eocene Mirador Formation. Adding to the confusion, distinctive black carbonaceous conglomerate near Mancora is attributed to the Mancora Formation by both Olsson (1931) and Palacios (1994).

Turritella woodsi Lisson, 1925 (= T. conquistadorana Hannah and Israelsky, 1925 ; see DeVries 2007) is found at locality WP 27. The gastropod also occurs about 30 km northeast at localities WP 31 and WP 32, near the mouth of Quebrada Plateritos (= Quebrada Culebra), an area mapped as the Mancora Formation by Palacios (1994). In southern Peru, T. woodsi is found in Priabonian to Chattian strata of the East Pisco Basin ( DeVries 2007). Thus, specimens of Hybolophus disenum could have an age between late Eocene and latest Oligocene. The stratigraphic and paleontological evidence for locality WP 28 indicates a late Eocene or early Oligocene age.

In a related aside, Deniaud et al. (1999) assigned the Ecuadorian Subibaja Formation, in which are found specimens of Turritella woodsi and Marks’s (1951) type specimens of Hybolophus carrizalensis , to the Langhian. In the Talara and East Pisco basins of Peru and Progreso Basin of Ecuador,. woodsi -bearing beds underlie beds with specimens of the Langhian T. infracarinata Grzybowski, 1899 ( Marks 1951; DeVries 2007). Therefore, H. carrizalensis might have an Oligocene to early Miocene age, not Langhian.

Hybolophus disenum may be contemporaneous or nearly so with Crassatella neorhynchus , being from the same outcrop area west of Mancora ( Olsson 1931; Woodring 1958). Specimens of H. disenum have a flattened umbo, typical of Hybolophus but absent in Crassatella ; are closer to being equilateral with dorsal margins descending ventrally less steeply than on specimens of C. neorhynchus ; and have a less angular and more widely divergent posterior ridge. The damaged left anterior cardinal tooth ( Fig. 9C ) may be separated from the beak by the lunule, a feature typical of Crassatella but present in more than one species of Hybolophus . The inner ventral margin of H. disenum is not preserved and so cannot be compared with the crenulate margin in large specimens of East Pisco C. neorhynchus .

Stratigraphic and geographic range.—Upper Eocene or lower Oligocene, Mirador or Mancora Formation, northern Peru.

Figures

Fig. 9. Veneriform Hybolophus crassatellines from Peru. A−D. Hybolophus disenum sp. nov., late Eocene or Oligocene, Mancora, northern Peru. A. USNM 618239, holotype, exterior (A1) and dorsal margin (A2) of right valve. B. USNM 618238, paratype, exterior (B1) and interior (B2) of left valve. C. USNM 618241, paratype, hinge plate of left valve. D. USNM 618240, paratype, dorsal margin of right valve (D1), anterior at right, hinge plate (D2). E−J. Hybolophus terrestris sp. nov., late Miocene, mid-section in Pisco depositional sequence, south-central Peru. E. UWBM 101874, paratype, exterior of left valve. F. UWBM 101873, paratype, interior of right valve. G. UWBM 101879, dorsal margin of paired valves, anterior at right. H. UWBM 101877, paratype, exterior of posteriorly elongated left valve. I. UWBM 101875, paratype, exterior of left valve with posterior shortening. J. UWBM 101871, holotype, interior of left valve. Scale bars 10 mm.