| Summary: | Deep and worldwide climatic and oceanographic changes occurred between the late Oligocene and the early Miocene (c. 28-16 Ma), involving the redistribution and partition of food resources in marine ecosystems and a consequent global increase in diversity and disparity of marine vertebrates. Despite the proposal for some general models linking these abiotic changes to the marine vertebrate evolution, the fossil record documenting this time span is globally scarce and fragmentary. One of the few exceptions is the fossil assemblage of the uppermost Oligocene to lower Miocene Chilcatay Formation (Pisco Basin, southern coast of Peru), where marine vertebrate remains are exceptionally frequent and well preserved, in a way similar to the better known (and more notorious) late Neogene Pisco Fm. This work reports on the vertebrate assemblage of the Ullujaya locality (Chilcatay Fm), where the holotypes of the archaic toothed cetaceans Chilcacetus and Huaridelphis were found. In the study area, the upper lower Miocene (i.e., Burdigalian) strata of the Chilcatay Fm rest with angular unconformity on the uppermost Eocene to lower Oligocene Otuma Fm and, in turn, they are unconformably overlain by those of the Pisco Fm. The succession of sedimentary facies and internal unconformities in the Chilcatay Fm has been resolved into two smaller stratal packages (allomembers), separated by a widespread intraformational unconformity. The lower of these units (Ct1 allomember, dated at about 19-18 Ma by means of silicoflagellate biostratigraphy) comprises i) a sub-horizontal package of medium- to fine-grained sandstones and siltstones and ii) a stack of clinoformed units of coarse-grained biocalcarenites. Several tens of fossil marine vertebrates have been identified from the lower portion of the Ct1 allomember and reported in a geological map and in a joined stratigraphic section. Cetacean remains dominate the assemblage (87.0% of the specimens), with all specimens identifiable to suborder level belonging to Odontoceti (toothed whales), ...
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