Perucetus Colossus

By Jaime Bran

***Perucetus*** is an extinct genus of early whale from the Eocene of Peru. With an estimated length exceeding 17.0–20.1 meters (55.8–65.9 ft) and weight ranging from 85–340 t (84–335 long tons; 94–375 short tons), *Perucetus* may have rivalled, if not exceeded, the modern blue whale in weight. This is in part due to the incredibly thick and dense bones this animal possessed, coupled with its already great size. The ecology of *Perucetus*, however, remains largely mysterious. Based on the fossils, it was likely a slow-moving inhabitant of shallow waters. Its diet can only be speculated upon, but one suggestion proposes that it may have fed on benthic animals like crustaceans and molluscs that live on the ocean floor. Only a single species is currently known, ***P. colossus***.

*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.

As only a few vertebrae of *Perucetus* are known, estimates of the whale's total length vary by how many of each type of vertebrae it is assumed to have in the spinal column. When scaling to the skeleton of *Cynthiacetus peruvianus*, which had the most complete fossil skeleton of a basilosaurid at the time and has a 20 thoracic and 17 lumbar vertebrae, Bianucci et al. (2023) yielded a total skeletal length estimate of 20.0 meters (65.6 ft). When using *Basilosaurus isis* (18 thoracic and 19 lumbar) and *Dorudon atrox* (17 thoracic and 20 lumbar) as proxies, a slightly larger maximum length of 20.1 meters (66 ft) is yielded. When scaling to *Pachycetus wardii*, which has the fewest vertebrae in the family and reflects the likelihood that *Perucetus* had fewer vertebrae than most basilosaurids, a conservative length of 17.0 meters (55.8 ft) is calculated.

The innominate bone of *Perucetus* is highly reduced but still features a well-developed acetabulum, a condition considered ancestral amongst 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*.
link=https://en.wikipedia.org/wiki/File:Perucetus\_colossus.png|thumb|Restoration with speculative manatee-like fluke.
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.

*Perucetus* may have ranged in weight from 85–340 t (84–335 long tons; 94–375 short tons) with an average of 180 t (180 long tons; 200 short tons). The 17–20-meter (56–66 ft) skeletal structure alone would have accounted for 5.3–7.6 t (5.2–7.5 long tons; 5.8–8.4 short tons), which is already two to three times the weight of the skeleton of a 25 m (82 ft) long blue whale. The weight estimates are based around the relation between skeletal and total body mass of modern mammals. Notably, whales have much lighter skeletons compared to their total mass, whereas sirenians (dugongs and manatees) are similar to land mammals in having much denser skeletons that contribute more to their total weight. Bianucci and colleagues note the difficulties in determining the weight of basilosaurids. They suggest that the increase in skeletal mass could have been compensated for by larger amounts of blubber, which is less dense than other soft tissue. Ultimately, extreme values were used in the calculations, leading to the wide range for the weight estimate present in the type description. Basing the math on sirenians, a weight of 85 t (84 long tons; 94 short tons) was calculated. Combining the lowest skeletal-weight–to–total-weight ratio found in cetaceans with the highest estimated skeletal mass yields a weight of up to 340 t (330 long tons; 370 short tons). Mean values, on the other hand, result in a weight of 180 t (180 long tons; 200 short tons). This may indicate that, although not as long, the species could have been heavier than modern blue whales.