Perucetus is an extinct genus of an early whale from Peru that lived during the Bartonian age of the middle Eocene. Perucetus is the largest Eocene whale, with length estimates varying from 15–16 meters (49–52 ft) to 17–20 meters (56–66 ft). It was initially claimed to have rivaled or exceeded the modern blue whale in weight, partly due to the incredibly thick and dense bones this animal possessed, coupled with its already great size, but subsequent studies argued that it was significantly lighter. The ecology of Perucetus also remains largely mysterious. Based on the fossils, it was likely a slow-moving inhabitant of shallow waters. Its diet can only be speculated, but one suggestion proposes that it may have fed on benthic animals like crustaceans and molluscs living on the ocean floor. Only a single species is currently known, P. colossus.
History and naming Perucetus is known from a variety of bones; namely, thirteen vertebrae, four ribs, and some parts of the pelvic region. All bones are from the same individual (MUSM 3248) and were collected from the Yumaque Member of the Paracas Formation. The remains of the Perucetus are currently under protection and on display at the Natural History Museum of Lima, which belongs to the National University of San Marcos, the main institution of the team of Peruvian paleontologists involved in the discovery. The name Perucetus derives from the whale's country of origin, Peru, while the species name references the enormous size of the animal.
Description The innominate bone of Perucetus is highly reduced but still features a well-developed acetabulum, a condition considered ancestral among whales. The innominate, however, differs in shape from that of Basilosaurus, and the proximal end of the ilium is notably more robust than in other early Pelagiceti. The centra of the lumbar vertebrae are greatly elongated like in basilosaurines and pachycetines but do not quite reach the proportions of the most extreme members of said groups. The ends of the ribs are large and club shaped, another feature similar to Basilosaurus.
The most characteristic feature of Perucetus is the high degree of pachyosteosclerosis present in the bones of the body, which means that the bones are simultaneously thicker (pachyostotic) and denser (osteosclerotic) than in any other known cetacean. Pachyosteosclerosis and the associated bone mass increase (BMI) is known in a variety of other marine mammals like sirenians and from some other basilosaurids — namely, members of the subfamily Pachycetinae — but no other whales approach the levels of BMI seen in Perucetus. Bianucci and colleagues highlight several lines of evidence to suggest that the bone mass increase was not the result of any pathologies. Besides the presence of BMI in pachycetines, the increase is uniformly present in Perucetus, while the BMI would be inconsistent if caused by some disease or other condition. Due to pachyostosis, the vertebrae are greatly inflated, making them nearly twice as voluminous as those of a 25 m (82 ft) long blue whale. The increase in bone mass is also observed in the microanatomy of the bone. The ribs are entirely composed of dense bone and lack the medullary cavity seen in the bones of other animals. The vascular channels that penetrate the bone are narrow, not only indicating the maturity of the animal but also adding to the already dense nature of the bones.
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