A Pompeii-like preservation for Dosinia ponderosa (Gray, 1838): two volcanic ash casts from the East Pisco Basin (Miocene, Peru)

In the East Pisco Basin, in Peru, a highly unusual example of Pompeii-like preservation has been discovered. At the locality of Cerros la Mama y la Hija, along the western side of the lower Ica River Valley, south of Ocucaje, two bivalve specimens were found preserved as volcanic ash casts. As is the case for most fossil invertebrates from the Pisco Formation, the original carbonate shell is not preserved, but these specimens are nonetheless exquisitely preserved, even featuring taxonomically informative characters of the shell exterior that allow for their assignment to the locally common species Dosinia ponderosa (Gray, 1838). Both casts are mainly constituted by fine-grained volcanic glass shards and biotite crystals. The high abundance of glass shards and the absence of non-volcaniclastic material such as biogenic or terrigenous particles indicates that the casts are the product of primary deposition of a volcanic ash layer on the seafloor. These specimens testify to an intriguing taphonomic history that features burial underneath a distal volcanic ashfall. During the Late Miocene, pyroclastic ashfalls coming from the Central Andes were indeed frequent in the East Pisco Basin, allowing this exceptional type of fossil preservation.