EXCEPTIONAL OCCURRENCE OF FOSSIL BALEEN IN THE MIOCENE/PLIOCENE PISCO FORMATION, PERU

Fossil baleen is very rare and only a few occurrences have been documented in the scientific literature. We report the occurrence of thirty specimens of fossil baleen in the Miocene/Pliocene Pisco Formation of the East Pisco Basin, in southern Peru. They occur in shallow water siliciclastic, dolomitic and diatomaceous layers that were deposited during the last of three Tertiary marine transgressions. Whales are generally articulated or disarticulated to varying degrees but with the bones well preserved and closely associated. Sediment associated with the skeletons is often dolomitized and tends to be more indurated than the surrounding sediment. There is no evidence of bioturbation of the associated sediment. Some bones show microborings and microfractures, the latter possibly due to compaction. There is no invertebrate fauna associated with and feeding on the whales, as occurs with modern whale carcasses on the seafloor. Most of the specimens show exceptional occurrence of the baleen plates in life position.

Fossil baleen shows three classes of preservation :1) Well preserved, thick isotropic material of organic nature, with very little shrinkage. This type is the less abundant 2) Replacement by anhydrite and/or gypsum, with moderate to extreme shrinkage, resulting in thin strips that may be bifurcated. 3) Replacement by carbonate (calcite or dolomite).

Observations of modern whale carcasses on the seafloor indicate that baleen does not remain long within the mouth of a dead whale. This is due to the non-bony nature of baleen and the fact that the plates are not rooted in the upper mandible but just glued to it. Sedimentological and taphonomic features suggest that the exceptional occurrence of the baleen apparatus of these fossil is the result of very rapid burial of the carcasses, which likely occurred after most of the soft tissue decayed but before baleen was removed.