Taphonomy and paleoenvironmental conditions of deposition of fossil whales in the diatomaceous sediments of the Miocene/Pliocene Pisco Formation, southern Peru—A new fossil-lagerstätte

The Pisco Formation in Peru contains abundant fossil cetaceans in middle Miocene to lower Pliocene sandstone, siltstone, and tuffaceous and diatomaceous mudstone. Fossil whales are especially abundant in the uppermost diatomaceous units. Most specimens are well articulated or partially disarticulated but associated. Degree of preservation is exceptionally high and uniform among articulated and disarticulated specimens and on the lower and upper surfaces of the bones. Some specimens have baleen preserved in anatomical position. Bones show no evidence of bioerosion by macro- or micro-invertebrates, except for a very limited amount of microbial borings. Diatomaceous layers seem to lack bioturbation. However, bioturbation did occur in a few tuffaceous silty and sandy layers of the lower part of the formation. Shark teeth are found associated with many of the specimens; however, despite abundance of whale skeletons, shark tooth marks are extremely rare. Several lines of evidence indicate that sediments and whales were deposited in a shallow-to-deep shelf environment (an embayment), and not in a beach environment. The thick diatomaceous successions record conditions of strong ocean upwelling indicated by the abundant occurrence of the diatom species Thalassionema nitzschioides. The exceptional preservation of diatom frustules suggests that they reached the seafloor very rapidly and were not successively reworked. The hundreds of whale specimens found, their excellent preservation, and their high degree of articulation make the Pisco Formation fossil whales the best representative assemblage of fossil Mysticeti known so far. The abundance of whale skeletons and other vertebrates in the diatomaceous beds, their excellent preservation, including evidence that soft tissue (baleen) was still present at burial, and the degree of articulation, point to frequent mortalities followed by rapid sedimentation that prevented decay of the skeletons and colonization by invertebrates. Sedimentation rates appear much higher than rates inferred from some other modern and ancient settings where whale skeletons have been found.