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Exceptionally preserved delicate baleen microstructures have been found in association\ud with the skeleton of a late Miocene balaenopteroid whale in a dolomite concretion of the\ud Pisco Formation, Peru. Microanalytical data (scanning electron microscopy, electron probe\ud microanalysis, X-ray diffraction) on fossil baleen are provided and the results are discussed\ud in terms of their taphonomic and paleoecological implications. Baleen fossilization modes\ud at this site include molding of plates and tubules, and phosphatization. A rapid formation of\ud the concretion was fundamental for fossilization. We suggest that the whale foundered in a\ud soft sediment chemically favorable to rapid dolomite precipitation, allowing the preservation\ud of delicate structures. Morphometric considerations on the baleen plates and bristles\ud coupled with the reconstructed calcification of the latter permit speculation on the trophic\ud preferences of this balaenopteroid whale: the densely spaced plates and the fine and calcified\ud bristles provide evidence for feeding on small-sized plankton, as does the modern sei whale

One hundred and ninety-two fossil marine vertebrate specimens, preserved as bone elements cropping out at Cerro Los Quesos (Pisco Basin, Peru), are identified and reported in a 1:4,000 scale geological map and in the corresponding stratigraphic section. All the fossils originate from the Pisco Formation, which is dated in this area to the late Miocene (from 7.55 Ma to ≥ 6.71 Ma, based on 40Ar/39Ar analyses of three volcanic ash layers along the section). Specimens are particularly concentrated near the top of the two main hills, where the geologically youngest portion of the examined section crops out. The impressive fossil assemblage includes cetaceans (91.6%), represented by mysticetes (balaenopteroids and cetotheriids) and odontocetes (phocoenids, physeteroids, and ziphiids, including the holotype of Nazcacetus urbinai). Seals, a crocodile, a seabird, bony fish, and sharks are also reported. Isolated large teeth of Carcharocles and Cosmopolitodus are common and, in several instances, associated to mysticete skeletons. Together with a similar work recently published for the other late Miocene locality of Cerro Colorado, this work represents a case study for the detailed inventory of the extraordinary paleontological heritage of the Pisco Basin. As such, it constitutes the basis for future taphonomic, paleoecological, and systematic studies, as well as for a much needed conservation effort

Hundreds of fossil marine vertebrates cropping out at Cerro Colorado (Pisco Basin, Peru) are identified and reported on a 1:6500 scale geological map and in a joined stratigraphic section. All the fossils are from the lower strata of the Pisco Formation, dated in this area to the late middle or early late Miocene. They are particularly concentrated (88%) in the stratigraphic interval from 40 to 75 m above the unconformity with the underlying Chilcatay Formation. The impressive fossil assemblage includes more than 300 specimens preserved as bone elements belonging mostly to cetaceans (81%), represented by mysticetes (cetotheriids and\ud balaenopteroids) and odontocetes (kentriodontid-like delphinidans, pontoporiids, ziphiids, and physeteroids, including the giant raptorial sperm whale Livyatan melvillei). Seals, crocodiles, sea turtles, seabirds, bony fish, and sharks are also reported. Isolated large teeth of Carcharocles and Cosmopolitodus are common throughout the investigated stratigraphical interval, whereas other shark teeth, mostly of carcharinids, are concentrated in one sandy interval. This work represents a first detailed census of the extraordinary paleontological heritage of the Pisco Basin and the basis for future taphonomic, paleoecological, and systematic studies, as well as a much needed conservation effort for this extremely rich paleontological site

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The Mio-Pliocene Pisco Formation, Peru, is a world renowned Konservat-Lagerstätte. Beside its potential in preserving important information on the history of the Pacific Ocean during the Miocene, the Pisco Fm. provides palaeontologists with the unique opportunity to study in detail the evolution of marine vertebrate fauna during the Neogene. Because diatomites are one of the main lithologies of the Pisco Fm., previous authors have hypothesised exceptionally high sedimentation rates related to the deposition of diatom frustules as the main cause leading to the formation of the Lagerstätte. However, sedimentation rates were never calculated. With the perspective to build a solid stratigraphic framework for the whole Pisco Fm., we started to investigate two important fossiliferous sites: Cerro Colorado and Cerro Los Quesos. Within these two areas, measurement of multiple stratigraphic sections combined with extensive field mapping of a number of distinct marker beds was carried out, which allowed a high-resolution correlation and creation of a local lithostratigraphic framework. Integrated tephrostratigraphy, 40 Ar/ 39 Ar dating and diatom biostratigraphy permitted an accurate chronostratigraphy to be established. Both biostratigraphic and radiometric ages converge to indicate a late Miocene age for the two sites, amending previous attribution of Cerro Colorado and Cerro Los Quesos to the middle Miocene. Age models for the site of Cerro Los Quesos reveal that sedimentation rates are high but in line with those of highproductivity environments, thus invalidating the hypothesis of exceptional sedimentation rates as the main cause for the formation of the Lagerstätte. These results are extremely important also in the perspective of building a complete stratigraphic framework for the Pisco Fm.

The enormous concentration of marine vertebrates documented within the Pisco Formation is unique for Peru and South America and places this unit among the prime fossil Lagerstätten for Miocene to Pliocene marine mammals worldwide. In order to provide a robust stratigraphic framework for the fossil-bearing locality of Cerro Los Quesos, this study presents a 1:10,000 scale geological map covering an area of about 21 km 2 , a detailed measured section spanning 290 m of strata, and a refined chronostratigraphy for the studied succession well constrained by diatom biostratigraphy and high-resolution 40 Ar/ 39 Ar isotopic dating of three interbedded ash layers. Within the apparently monotonous, diatomite-dominated sedimentary section, the Pisco Formation has been subdivided into six local members, with stratigraphic control over the different outcrops facilitated by the establishment of a detailed marker bed stratigraphy based on 15 readily distinguishable sediment layers of different nature. ARTICLE HISTORY

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