Perucetus colossus

### Synonyms

SulaimanitheriumPerucethusBasiloterusBasilosaurusAncalecetusChrysocetusCynthiacetusDorudon

### Weight

ranging from 85–340 t (84–335 long tons; 94–375 short tons)

### Diet

Carnivorous

***Perucetus*** is an extinct genus of early whale from the Eocene of Peru. With an estimated 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 of this animal 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***.

## History and naming

*Perucetus* is known from a variety of bones, namely 13 vertebrae, four ribs and some parts of the pelvic region. All bones, which represent a singular individual (MUSM 3248), have been collected from the Yumaque Member of the Paracas Formation.

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 amongst whales. The innominate however differs in its shape from that of *Basilosaurus* and the proximal end of the illium 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 (pachyostosis) and denser (osteosclerosis) 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 point at 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 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 twice 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 combined with the highest estimated skeletal mass on the otherhand 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, it could have been heavier than modern blue whales.

## Classification

*Perucetus* was identified as a member of the Pelagiceti based on the high amount of lumbar vertebrae with cicular centra and a highly reduced innominate bone. Within pelagiceti, the well-defined acetabulum suggests closer affinities with basilosaurids (like *Basilosaurus*, *Pachycetus* *Cynthiacetus* and *Chrysocetus*) and llanocetids (*Mystacodon*). Bianucci and colleagues subsequently assing *Perucetus* to the family Basilosauridae.

## Paleobiology

The immense size and bone density both make it impossible for *Perucetus* to have gone on land, which is in line with its classification as a basilosaurid. The pachyosteosclerosis is taken as a sign that *Perucetus* lived in shallow waters, using it as buoyancy control like modern manatees do. Given its size and weight, *Perucetus* could have resisted crashing waves in more turbulent waters, something also inferred for the similarly buoyant Steller's sea cow. The animals affinity for shallow waters is congruent with the interpretation that basilosaurids preferred coastal waters, rather than living in the open ocean.

While the fragmentary nature of this animal renders precise statements on its locomotion uncertain, some suggestions have been made. The elongated centra of the vertebrae for instance may suggest that it, like manatees but not dugongs, swam with the use of axial undulation. This further indicates shallow water rather than pelagic habits for the animal. The great size of the vertebrae does impose limits on the swimming style of *Perucetus*, as does the shape of the transverse processes of the vertebrae. Using the methods of a previous study would suggest that *Perucetus* was limited in its ability to flex upwards and from side to side, but with an increased ability to flex downward (ventrally). This could suggest that *Perucetus* swam with slow up and down movements of its tail while not making use of any side to side movements as has been suggested for *Basilosaurus*. The strong ventral flexion in particular may have been of great importance for the animal when pushing itself off the ocean floor in order to breathe at the surface. The precise reasoning for this exact combination of features, pachyosteosclerosis and gigantism, is not fully understood, but may be linked to the energetic cost of undulating movements or the ability to dive for longer periods of time.

The diet and feeding style remain even more mysterious given that no skull material of this animal is currently known. Still, some possibilities could be inferred based on the lifestyle as inferred by the potscrania. While the many noted similarities to sirenians could be taken as a sign for a grazing lifestyle, this notion is deemed unlikely as no other cetacean is known to have been herbivorous. It is deemed more likely that *Perucetus* could have fed on molluscs, crustaceans and other animals that live on the sea floor, either through suction-feeding or filter-feeding. Such a lifestyle would be comparable to that of the modern grey whale. Another hypothesis mentioned by Bianucci *et al.* is that it is theoretically possible that *Perucetus* could have been a scavenger like large demersal sharks. Ultimately, until better material is found the precise ecology of *Perucetus* remains unknown.

## Sources

* https://www.nature.com/articles/s41586-023-06381-1
* https://www.earth.com/news/colossal-whale-peru-perucetus-colossus-largest-animal-ever-lived/
* https://www.bbc.com/news/science-environment-66384632
* https://edition.cnn.com/2023/08/02/world/ancient-colossal-whale-perucetus-colossus-scn/index.html
* https://www.reuters.com/science/ancient-whale-peru-may-be-most-massive-animal-ever-earth-2023-08-02/
* https://www.businessinsider.com/ancient-whale-among-heaviest-animals-could-weighed-up-to-340-tons-2023-8
* https://www.washingtonpost.com/climate-environment/2023/08/02/largest-animal-whale-p-colossus/
* https://www.nbcnews.com/news/world/heaviest-animal-ever-whale-peru-desert-rcna97912
* https://theprint.in/scientifix/move-over-blue-whale-peru-fossils-indicate-perucetus-colossus-from-38-mn-yrs-ago-was-largest-mammal/1701026/
* https://www.nationalgeographic.com/premium/article/ancient-whale-perucetus-colossus-largest-animal-ever