Taphonomy of fossil whales in the Miocene/Pliocene Pisco Fm., Peru

Leonard Brand, Ph.D., Prof of Biology and Paleontology, Loma Linda University

Esperante, R., L. Brand, A. Chadwick, and O. Poma.  2015. Taphonomy and paleoenvironmental conditions of deposition of fossil whales in the diatomaceous sediments of the Miocene/Pliocene Pisco Formation, southern Peru – a new lagerstätte.  Palaeogeography, Palaeoclimatology, Palaeoecology, 417: 337-370.  (publication date 2015; available online in 2014) (Download pdf here)


Brand, L., M. Urbina, A. Chadwick, R. T DeVries, R. Esperante.  2011.  A high resolution stratigraphic framework for the remarkable fossil cetacean assemblage of the Miocene/Pliocene Pisco Formation, Peru.  South American Journal of Earth Science, 31:414-425. (Download pdf here)

Esperante, R., L. Brand, K. Nick, O. Poma, and M. Urbina. 2008. Exceptional occurrence of fossil baleen in shallow marine sediments of the Neogene Pisco Formation, Southern Peru. Palaeogeography, Palaeoclimatology, Palaeoecology, 257:344-360. (Download pdf here)

Brand, L. R., R. Esperante, A. Chadwick, O. Poma, and M. Alomia. 2004. Fossil whale preservation implies high diatom accumulation rate in the Miocene-Pliocene Pisco Formation of Peru. Geology, 32:165-168. (Download pdf here)

Esperante-Caamano, R., L. Brand, A. Chadwick, and O. Poma. 2002. Taphonomy of fossil whales in the diatomaceous sediments of the Miocene/Pliocene Pisco Formation, Peru. pp. 337-343 In: De Renzi, M., M. Alonso, M. Belinchon, E. Penalver, P. Montoya, and A. Marquez-Aliaga (eds.). Current Topics on Taphonomy and Fossilization. International Conference Taphos 2002. 3rd Meeting on Taphonomy and Fossilization, Valencia, Spain. (Download pdf here)

Pisco Formation exposed in Cerro Blanco (foreground) and Cerro Baleena (background). Notice lack of vegetation, and well-exposed stratigraphy, because there is no rain. This made it easy to find fossils and study the sediments, and facilitated accurate determination of the stratigraphic position of fossils. There has been only limited tectonic activity here after deposition of the Pisco Fm., which facilitates determination of the original Miocene/Pliocene paleogeography. The lower part of these hills (steeper slopes) is largely sandstone with some diatomaceous sediment, and the upper part (containing the highest concentration of whales) is mostly diatomaceous siltstone.

 

Completely articulated fossil whale, except for one missing flipper, on Cerro Blanco. Front of skull damaged by modern erosion. Otherwise the bones are very well preserved.

 

Most whales in the Pisco Formation are cf. Balaenoptera, relatives of modern fin, sei, and blue whales.

 

Arthur Chadwick and the high-precision GPS unit used to determine the location of all whales and sedimentary marker beds.

 

Distribution of fossil whales on Cerro Blanco, superimposed on an aerial photo. Red dots = individual whales (N = 180). Black dots = specimens that cannot be determined to be complete whales, usually because of post-exposure erosional damage (N = 166). It appears likely that most whales were essentially complete before the effects of modern erosion processes.

 

Stratigraphic distribution of whales on Cerro Blanco study area. The horizontal centerline of each whale symbol is located at the stratigraphic position of an individual whale. Whales are mostly in the diatomaceous siltstone in upper part of section. The lower part of section (120 meters, not shown here) is mostly sandstone, and contains only a few whales. Whales are distributed throughout the diatomaceous sediment, and not in discrete, mass death units.

 

Another excavated whale. This whale is complete, and nearly articulated. The skull is sitting on top of part of the vertebral column and rib cage. Limited pre-burial movement of skull relative to the associated post cranial skeleton is fairly common, and our hypothesis is that floating dead whales sank when there was enough decay to allow some movement and separation of body parts as they sank.

 

Some of the whales are completely articulated. Others, like this one, are partly disarticulated, but associated. This whale's skull is several meters from the lower left of the photo. All whale bones that we excavated (11 whales were excavated) are very well preserved, with no evidence of boring by invertebrates, or other erosion or dissolution processes. The diatomaceous sediment encasing the whales did not contain any invertebrates or evidence of bioturbation. Invertebrates and bioturbation were abundant in some units of the Pisco, but not associated with the assemblage of abundant whales.

 

Excavation of the best preserved whale, Fernanda. Researchers are (from left) Merling Alomia, Orlando Poma, Arthur Chadwick, and Raul Esperante.

 

The whale Fernanda (the specimen on the right) completely uncovered. This whale is complete except for the end of the tail. A slab of baleen was lying on top of the flipper in the left side of this photo (see next photo also).

 

Baleen from Fernanda. Left - surface view of the baleen (~ 8 cm from top to bottom of photo). Upper right - cross section of baleen. The dark colored v-shaped structures are sheets of baleen. Lower right - microscopic enlargement of surface of a sheet of baleen. Notice the fine ridges preserved in the baleen. We found fossilized baleen in several other whales also, but the baleen was best preserved in this whale. This type of preservation requires rapid burial, before the baleen tissue decayed. Fernanda was also buried before the spinal cord decayed, as black mineral replacement of the spinal cord was present in its normal position within the vertebral column (but nowhere else).

An unexcavated, complete whale.

 

The whales are mostly in diatom-rich sediment, as shown in this photo and the next photo. The diatoms are often broken, but do not show evidence of dissolution. The most abundant diatoms in all samples are indicators of diatom blooms.

 

Conclusions: The whales and other vertebrates were apparantly killed by toxic plankton blooms and/or suffocation from volcanic ash. The well-preserved whale skeletons, absence of invertebrate fauna associated with the whales (modern dead whales are fairly quickly colonized by large numbers of invertebrate scavengers), and fine preservation of the original texture and position of non-bony structures (baleen and spinal cord) indicate rapid burial of the whales. Paleogeography and sedimentary structures indicate that these whales were buried in a shallow bay, above wave base, in an environment not likely to be anoxic. It appears that a combination of factors led to rapid accumulation of diatoms and burial of whales. These probably included: high levels of nutrients from upwelling and from volcanic input, leading to rapid diatom reproduction; self-sedimentation of diatom flocks and mats (from secretion of sticky gels that form diatom aggregates), as occurs in modern blooms; lack of dissolution of diatoms because of the shallow water; possible concentration of diatoms in the bay from storm-related currents (as indicated by sedimentological evidence).


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