TWO NEW SPECIES OF ADELOMELON DALL, 1906 (GASTROPODA: VOLUTIDAE) FROM THE TERTIARY OF PATAGONIA (ARGENTINA)

Journal of Paleontology, Sep 2004 by Scarabino, Fabrizio, Martinez, Sergio, R�o, Claudia J Del, Oleinik, Anton E, Et al

ABSTRACT-

Two new species of gastropods belonging to the volutid genus Adelomelon Dall, 1906 are described from Tertiary strata of Patagonia, Argentina. Adelomelon posei n. sp. was found in the lower part of the Gran Bajo del Gualicho Formation (R�o Negro Province), which is currently dated as Late Eocene through Middle Miocene in age. Adelomelon valdesiense n. sp. comes from the Puerto Madryn Formation (Chubut Province) of early Late Miocene age. Both species are characterized by angulated biconical teleoconch whorls and strongly developed rows of axial nodes, similar in shape to those of some Recent species of Adelomelon, but positioned differently. Discovery of two new species of the genus Adelomelon in the Tertiary of Patagonia significantly extends the known stratigraphic range of this genus from Late Eocene(?)-Miocene to Recent.

INTRODUCTION

THE VOLUTID subfamily Zidoninae H. and A. Adams, 1853 in the waters off South America includes the genera Zidona H. and A. Adams, 1853, Provocator Watson, 1882, Nanomelon Leal and Bouchet, 1989, Adelomelon DaIl, 1906, and Pachycymbiola Ihering, 1907 (Clench and Turner, 1964; Weaver and "duPont, 1970; Leal and Bouchet, 1989). The genus Adelomelon, represented by four species, is presently known to occur only in the southwestern Atlantic region, from southeastern Brazil south to the Falkland (Malvinas) Islands and Straits of Magellan (Clench and Turner, 1964; Kaiser, 1977; Leal and Bouchet, 1989). These living species inhabit subtidal-to-bathyal depths.

Until recently, the genus Adelomelon sensu stricto was known only from the Patagonian Tertiary and represented by only one valid species, Adelomelon pilsbryi (Ihering, 1899) from the Early Miocene Monte Leon Formation. Two new Tertiary species of Adelomelon from Patagonia, from the Late Eocene(?) and Miocene, are described in this paper.

GEOLOGICAL SETTING AND STRATIGRAPHY

The two new Tertiary fossil volutid species described herein have been found in the Gran Bajo del Gualicho (Lizua�n and Sep�lveda, 1978) (R�o Negro Province) and in the Puerto Madryn (Haller, 1978) (Pen�nsula Vald�s, Chubut Province) formations (Rg. 1).

Adelomelon posei n. sp. was found in the Gran Bajo del Gualicho Formation in the northern cliffs of the Salina del Gualicho, 45 km northwest of San Antonio Oeste (Fig. 1). The Salina del Gualicho area is characterized by exposures of fossiliferous Cenozoic marine rocks overlying Jurassic porphyrites of the Marifil Formation. The Gran Bajo del Gualicho Formation at this locality is represented by an intercalation of up to 20 m of gray oxidized sandstone and thin fossiliferous sandy mudstone beds. Camacho (1987) recognized two thick layers of coquina, containing silicified fossils, at the base and at the top of the section. The upper layer, recognized as the "Estratos con Cypraea" by Figueiras (1985), contains the following molluscs: Venericardla (Venericor) sp., "Turritella" hauthali Ihering, 1907, Conus sp., Conus (Leptoconus) patagonicus Frenguelli, 1932, Cypraea posei Figueiras, 1985, and Iheringiella sp. The assemblage of the lower coquina bed includes Area sp., Ostrea sp., Venericardia (Venericor) sp., Ficus sp., "Turritella" hauthali, and Adelomelon posei. Figueiras (1985) originally named these strata "Estratos con Cypraea" and placed them in the Roca Formation of Danian age. Pose (1986) reassigned these beds to the Gran Bajo del Gualicho Formation.

In this area the Gran Bajo del Gualicho Formation was first referred to by Wichmann (1919) as the "Molasa Patagonica" of "Tertiary age." Later Camacho (1987), based on the presence of the carditid bivalve Venericor Stewart, 1930, correlated these horizons with the "Monophoraster and Venericor Beds," an informal biostratigraphic unit defined by Camacho (1974). The "Monophoraster and Venericor Beds" extend along the Atlantic coastline from the Colorado River (R�o Negro Province) southward to the Comodoro Rivadavia region (Chubut Province), where its stratotype is located. Horizons containing Venericor are also recorded in the southwestern Patagonia (R�o Turbio and Man Aike Formations, Santa Cruz Province), and to the west in the Vaca Mahu�da Formation (R�o Negro Province). The Gran Bajo del Gualicho Formation at the Salina del Gualicho overlies the marine Paleocene Arroyo Barbudo Formation, and is overlain by the early Late Miocene Puerto Madryn Formation. The Puerto Madryn Formation is overlain by the Puerta del Diablo Formation, which, based on the diatom content, was assigned to the Late Miocene-Pliocene (Sep�lveda and Mart�nez-Macchiavello, 1985). However, the exact age of the Gran Bajo del Gualicho Formation remains uncertain. In the Salina del Gualicho area, where A. posei was found, the presence of the widely distributed bivalve genus Venericor, a worldwide Paleogene index fossil, suggests a Late Eocene age (Camacho, 1967, 1987; see Camacho et al., 2000 for a detailed discussion of the age of Venericor), but the age of Venericor in Patagonia deserve more detailed study. Venericor also has been recovered from the Rio Turbio and Man Aike formations, units that are considered Eocene based on palynological and foraminiferal content (Romero, 1977; Carrizo et al., 1990; Malumi�n et al., 2000). However, beds containing Venericor in Comodoro Rivadavia area have been included by BeIlosi (1990) in the upper part of the Chenque Formation, which according to its palynological content was assigned to the earlymiddle Miocene (Barreda and Palamarczuk, 2000). Due to this conflicting evidence, the age of the Gran Bajo del Gualicho is considered herein as ranging from Late Eocene to Middle Miocene.