Acrophyseter undetermined

ACROPHYSETER SP.

Specimen: MUSM 2182, a skull with both mandibles and teeth. The whole dorsal part of the supracranial basin, as well as the right supraorbital region, is worn away. The medial surface of the right mandible is closely appressed to the basioccipital basin and right orbital region, whereas the medial surface of the left mandible crosses the ventral surface of the rostrum at about mid-length, in an oblique direction. Five or six teeth or tooth fragments are in situ in the right upper tooth row, three in the left upper tooth row, and four or five in the left mandible. Twelve teeth of the same specimen were found detached around the skull and mandibles.

Locality: Cerro los Quesos, 50 km south of Ica, at the top of the main hill ( Fig. 1A View Figure 1 ). Geographical coordinates: 14 ° 30 0 52.8 ″ S, 75 ° 43 0 05.8 ″ W, altitude 696 m. The specimen was discovered by Mario Urbina in 2008 and collected in November 2010. The locality of Cerro los Quesos yielded other marine vertebrate remains: the ziphiid Nazcacetus urbinai Lambert et al., 2009 , a phocoenid tentatively referred to Lomacetus ginsburgi Muizon, 1988 , a kogiid displaying affinities with Scaphokogia cochlearis Muizon, 1988 , several mysticetes including a small cetotheriid sharing similarities with Piscobalaena nana , and at least two large balaenopteroids, phocids, and the sharks Carcharocles megalodon and Cosmopolitodus hastalis ( Lambert et al., 2009; Bianucci et al., in press; our pers. observ.).

Horizon: The specimen was discovered in the upper layers of the Pisco Formation outcropping in Cerro los Quesos, namely in Member F of the local section (Di Celma et al., in press; Bianucci et al., in press; Fig. 22 View Figure 22 ). 40 Ar / 39 Ar dating of the biotite from ash layers under (lower portion of Member F) and above the level of the specimen gave absolute ages of 6.9 and 6.7 Ma, respectively; this Messinian (latest Miocene) interval is confirmed biostratigraphically with diatoms providing an interval of 7.3 – 7.1 Ma for the base of Member F (Di Celma et al., in press; Fig. 22 View Figure 22 ). The beds having yielded MUSM 2182 were tentatively correlated to the AGL level as defined by Muizon & DeVries (1985) and Muizon (1988) in Aguada de Lomas, Sacaco area on the basis of the possible occurrence of the phocoenid Lomacetus ginsburgi and a kogiid sharing similarities with Scaphokogia cochlearis (Bianucci et al., in press). This correlation is compatible with the age estimates (c. 7.5 – 7.0 Ma) provided by Muizon (1988) for the AGL level.