Where: Peru (14.8° S, 75.5° W: paleocoordinates 15.3° S, 73.0° W)

• coordinate stated in text

When: Pisco Formation, Serravallian to Serravallian (13.8 - 7.2 Ma)

• poorly explored lowest part of the Pisco Formation (allomember P0 [23]

•

•The strata that yielded the holotype of Tiucetus rosae rest unconformably on the latest Oligocene to Early Miocene Chilcatay Formation. Sediment samples from the skull, associated specimens and the locality of Santa Rosa itself failed to yield microfossils, and there is currently no other direct dating evidence available for P0. Nevertheless, dates from the underlying Chilcatay Formation and the overlying P1 allomember of the Pisco Formation constrain the deposition of P0 to between 17.8 and 9.0 Ma [23]. Furthermore, dated ash beds from within the P1 and P2 allomembers suggest that both were deposited over no more than 1–1.5 Myr each [23]. Assuming similar rates of sedimentation for P0 would yield an approximate age of 11–9 Ma (Tortonian) for Tiucetus, which is consistent with the occurrence of a ‘typical’ Late Miocene cetotheriid morphotype (i.e. one broadly resembling ‘Cetotherium’ megalophysum and Herentalia) in the same unit.

•However, the situation is complicated by an apparent mismatch between faunal and radiometric data from the Pisco-Ica and Sacaco basins. Of particular relevance here is the age of the El Jahuay vertebrate level (ELJ), which is among the oldest vertebrate horizons exposed in the Sacaco Basin, and has been K/Ar-dated to older than 8.8 Ma [18]. Based on its vertebrate fauna, ELJ appears to be stratigraphically above the Cerro La Bruja vertebrate level (CLB) of the Pisco-Ica Basin [18,25,30], yet diatom assemblages and more recent radiometric dates from the localities of Cerro La Bruja and Cerro Los Quesos place CLB between 7.5 and 8.5 Ma [23,31,32]. If CLB has been correctly dated, then a revision of the dating evidence from the Sacaco Basin appears to be in order. Alternatively, if the ELJ estimate is correct, CLB, and indeed all of the Pisco Formation exposed in the Pisco-Ica Basin, may be older than recently suggested [23,31,32]. In this scenario, a conservative estimate for the age of P0 might be late Middle Miocene (Serravallian; 13.8–11.6 Ma), which may be further supported by the markedly archaic aspect of the cetacean assemblage from this unit relative to P1 and P2 (see above).

•In the light of these conflicting hypotheses, the age of Tiucetus rosae must fall somewhere between 17.8 and 9.0 Ma, and plausibly within either the Serravallian or the early Tortonian period, depending on the resolution of the mismatch between the Pisco-Ica and Sacaco basins. A Serravallian age is supported by the mainly archaic cetacean assemblage of P0 and radiometric dates from the Sacaco Basin, although the more recent dating evidence from the Pisco-Ica Basin does not exclude the possibility of an early Tortonian age. Direct evidence for the age of P0 and a re-examination of the radiometric data from the Sacaco area are required to resolve this question.

Environment/lithology: marine; lithology not reported

Size class: macrofossils

Primary reference: F. G. Marx, O. Lambert, and C. Muizon. 2017. A new Miocene baleen whale from Peru deciphers the dawn of cetotheriids. Royal Society Open Science 4:170560 [M. Uhen/M. Uhen]more details

Purpose of describing collection: general faunal/floral analysis

PaleoDB collection 188740: authorized by Mark Uhen, entered by Mark Uhen on 14.09.2017

Creative Commons license: CC BY (attribution)