The fossil record of the Cetacea gives us one of the most remarkable examples of an evolutionary transition in a group of organisms, from small, Mouse Deer-like, and entirely terrestrial, Artiodactyls to the largest Animals ever to have lived in out oceans. The early marine Whales developed specialisms for life in the water during one of the Earth's most extreme greenhouse phases, quickly growing in body size, and by the end of the Eocene becoming extremely elongate as well. However, the extreme gigantism seen in modern Whales is a apparently a relatively recent development which appeared in Baleen Whales in the cooling oceans of the Late Cainozoic. One of these, the extant Blue Whale, Balaenoptera musculus, is currently considered to be the heaviest Animal ever to have lived on Earth. This trend in developing large body-size as a development to active pelagic swimming is not unique in Cetaceans, among Tetrapods it can be observed in several clades of Mesozoic Marine Reptiles, such as the Ichthyosaurs.
As Animals make the transition from living in a terrestrial one to living in a marine one, buoyancy control becomes a key aspect of their biology. In Vertebrates, bone is the densest tissue, and large amounts of this tissue are present in the bodies of most Vertebrates. This has resulted in a variety of specialist bone-adaptations appearing in Tetrapods that have returned to an aquatic lifestyle. In Animals with a slow-moving, shallow-diving lifestyle, such as Sirenians, this typically manifests as an increase in bone mass. This increase has been observed in the earliest Whales, and is seen in many Basilosaurids (an extinct Cetacean family which was the most numerous and widespread Whale group in the Eocene). The reverse situation is seen in more active, pelagic swimming marine Tetrapods, including all modern Whales, with bone mass becoming severely reduced, and other tissues taking on a greater role in providing structural support to the body. The Basilosaurids were a unique group, growing to lengths of up to 20 m and having a much higher relative bone mass than modern Whales, although, this has not until now been shown to be close to that of the extant Sirenians.
In a paper published in the journal Nature on 2 August 2023, Giovanni Bianucci of the Dipartimento di Scienze della Terra at the Università di Pisa, Olivier Lambert of the Direction Opérationnelle Terre et Histoire de la Vie at the Institut Royal des Sciences Naturelles de Belgique, Mario Urbina of the Departamento de Paleontología de Vertebrados at the Museo de Historia Natural of the Universidad Nacional Mayor de San Marcos, Marco Merella and Alberto Collareta, also of the Dipartimento di Scienze della Terra at the Università di Pisa, Rebecca Bennion, also of the Direction Opérationnelle Terre et Histoire de la Vie at the Institut Royal des Sciences Naturelles de Belgique, and of the Evolution & Diversity Dynamics Lab at the Universite de Liege, Rodolfo Salas-Gismondi, also of the Departamento de Paleontología de Vertebrados at the Museo de Historia Natural of the Universidad Nacional Mayor de San Marcos and of the Facultad de Ciencias y Filosofía and Centro de Investigación para el Desarrollo Integral y Sostenible at the Universitad Peruana Cayetano Heredia Lima, Aldo Benites-Palomino, again of the Departamento de Paleontología de Vertebrados at the Museo de Historia Natural-Universidad Nacional Mayor de San Marcos, and of the Department of Paleontology at the University of Zurich, Klaas Post of the Natuurhistorisch Museum Rotterdam, Christian de Muizon of the Département Origines et Évolution at the Muséum National d’Histoire Naturelle, Giulia Bosio of the Dipartimento di Scienze dell’Ambiente e della Terra at the Università degli Studi diMilano-Bicocca, Claudio Di Celma of the School of Science and Technology at the University of Camerino, Elisa Malinverno, also of the Dipartimento di Scienze dell’Ambiente e della Terra at the Università degli Studi di Milano-Bicocca, Pietro Paolo Pierantoni, also of the School of Science and Technology at the University of Camerino, Igor Maria Villa of the Institut für Geologie at the Universität Bern, and Eli Amson of the Staatliches Museum für Naturkunde Stuttgart, describe a new species of Basilosaurid Whale from the Late Eocene Paracas Formation of Peru, which combines a gigantic size with the highest degree of bone mass increase ever seen in a Whale, which was potentially heavier than the living Blue Whale.
The new species is described from a single specimen, MUSM 3248, which was recovered from the upper part of the Yumaque Member of the Paracas Formation exposed in the Ica valley a few kilometres south of the Zamaca locality, and is now housed in the collection of the Museo de Historia Natural of the Universidad Nacional Mayor de San Marcos. This was five metres below a tephra layer radioisotope dated to 36.8 million years ago. Based upon this, and biostratigraphic data from the silts of the Yumaque Member, the specimen is estimated to be between 39.8 and 37.84 million years old, making it Bartonian, or Middle Eocene in age. The species is named Perucetus colossus, where 'Perucetus' means 'Peru Whale' and 'colossus' means gigantic.
The specimen comprises a partial skeleton including 13 vertebrae (two tentatively referred to the last thoracics and the others to the anterior lumbars), four ribs and right innominate lacking the distal portion of the ilium. It is believed to have been nearing skeletal maturity when it died.
Perucetus colossus differs from all other known Cetaceans in the extremely pachyosteosclerotic (thick and dense) nature of the bones of its post-cranial skeleton. Pachyosteosclerosis has been reported in Cetaceans before, and is a defining feature of the Basilosaurid Subfamily Pachycetinae, but never to the extent seen in Perucetus colossus.
Although heavily modified by the presence of additional dense bone material, the transverse processes of the vertebrae are sufficiently similar to those of other Basinosaurids for Biannucci et al. to be confident about their placement as the two thoracic and the first eleven lumbar vertebrae. About half the vertebrae are missing their centrum epiphyses, a sign that these bones were not fused at the time of death. In a terrestrial Mammal, this would be taken as a sign of immaturity, but in Whales this condition frequently persists into adult life, and is therefore considered to be of no significance for determining the age of the specimen.
The best preserved rib displays a simple proximal end without distinct tuberculum and capitulum and a weak overall curvature in anterior or posterior view, which is consistent with a placement in the posterior region of the rib cage. Since pachyosteosclerosis of the ribs tends to develop in the foremost ribs first, this is consistent with the entire rib cage being pachyosteosclerotic.
Specimen MUSM 3248, shows no signs of having been pathological in any way (pathological pachyosteosclerosis tends to produce highly variable bone density distribution), but rather appears to have been a healthy specimen of a species with a highly pachyosteosclerotic skeleton. The apophyses of the vertebrae are distinctive in having a 'bloated' appearance, unlike that of any other Mammal, whereas in all other Cetaceans, including the largest Balaenopterids and Balaenids, the vertebral apophyses tend to be relatively thin.
The ribs which were found were also highly pachyosteosclerotic, leading Bianucci et al. to conclude that this state would have applied to the whole skeleton. If this was the case, then the volume of the entire skeleton would have been 2.9-4.1 m², and would have weighed 5.3 to 7.6 tonnes, which is 2.0–2.9 times as much as that of a 25 m long Blue Whale.
The total range of skeletal masses found in Tetrapods varys between about 26 mg in the Dwarf Chameleon Brookesia nana, and an estimated 9.9 tonnes in the giant Titanosaur, Argentinosaurus huinculensis. Modern Cetaceans have relatively light skeletons, compared to the rest of their tissues, perhaps making up 2.2-5.1% of their entire body mass, while that of terrestrial Mammals varies between 4 and 10%. The skeletons of Sirenians, aquatic Mammals with a lifestyle similar to that presumed for pachyosteosclerotic Basilosaurs such as Perucetus colossus. tend to have skeleton-mass-to-body-mass ratios similar to that of terrestrial Mammals.
Estimating the body mass of extinct Animals such as Basilosaurs is complicated, and in the case of Perucetus colossus is made more challenging by the extreme skeletal morphology. It is likely that the heavy skeleton would have been counterbalanced by a large volume of buoyant blubber, which would have had a strong impact on the overall density and mass of the soft tissue of the living Whale. If it is assumed that Perucetus colossus has a skeleton-mass-to-body-mass ratio similar to that of modern Sirenians, then the living Animal would have had a total mass of about 85 tonnes, but if the ratio was comparable to that of living Whales, then it would have been between about 180 and 340 tonnes. Adult Blue Whales typically have a total body mass of about 130-150 tonnes, so it is possible that Perucetus colossus was more massive than a Blue Whale.
Among Tetrapods, high levels of pachyosteosclerosis are only known in fully aquatic species capable of bearing young in the water. The morphology of Perucetus colossus makes it highly unlikely that it was ever able to pull itself onto land for any reason, and therefore it is presumed to have given birth at sea. Its presumed morphology and high level of pachyosteosclerosis make it likely that it lived in shallow waters, such as those inhabited by living Manatees, although the large mass of the Animal may have made it more resilient to wave actions, something which has also been proposed for the more recently extinct t Steller’s Sea Cow, Hydrodamalis gigas. Coastal habitats have previously been proposed for Basilosaurids, based upon their skeletal anatomies and stable isotope data.
Reconstruction of Perucetus colossus in its coastal habitat. Because portions of the skeleton are unknown, several aspects of the reconstruction are tentative: the overall proportions of the axial postcranium are based on a close relative Cynthiacetus peruvianus, which was scaled-up and dilated according to the elements recovered for Perucetus colossus; the skull and limbs were only scaled-up; the tail fluke and forelimb use (bottom-walking) are based on the Manatee, Trichechus, sp, the extant Marine Mammal with the closest degree of pachyosteosclerosis in the postcranial skeleton; the hind limb of Perucetus colossus was not recovered, but the anatomy of its innominate indicates the presence of a reduced, articulated leg. The associated Sawfish, Pristis sp., was recovered from the same unit in the East Pisco Basin, the Yumaque Member of the Paracas Formation. Alberto Gennari in Bianucci et al. (2023).
Although Perucetus colossus is known only from a single fragmentary skeleton, lacking a head or any teeth (which would be needed to make any assumption about its diet or feeding habits), some conclusions can be made about the way in which it lived. The vertebrae of Perucetus colossus have elongated centra, something also found in other Basilosaurids, as well as in living Manatees, Trichechus spp., where it is associated with swimming by means of axial undulation. Interestingly, another living Sirenian, the Dugong, Dugong dugon, which lives in more open waters and swims in a similar way to living Whales, by oscillation of a lunate tail, lacks such elongate centra.
A previous study examined motion in another Basilosaurid, Cynthiacetus peruvianus, providing a methodology to analyse the movement of Perucetus colossus. Doing this showed that dorsal and lateral flexation was extremely limited in Perucetus colossus, but that its capacity for ventral flexation was actually higher than in Cynthiacetus peruvianus, suggesting that swimming was driven entirely by downward strokes of the posterior part of the body. Such a form of motion has also been proposed for early Whales Antaecetus and Pachycetus, and would have been particularly useful for pushing the Animal away from the seafloor suddenly; in the smaller Whales it has been suggested that this might have indicated they were Ambush predators, but this seems unlikely for an Animal as large as Perucetus colossus; instead a strong upward swimming capacity might have been needed to help it breath.
Estimates of the osteological range of motion.
Extension and flexion of the preserved portion of vertebral column of
Perucetus colossus holotype (MUSM 3248) is compared with an equivalent
vertebral column portion of Cynthiacetus peruvianus holotype (MNHN.F.PRU10)
using the respective 3D models. Intervertebral spaces were reconstructed
based on the Common Dolphin, Delphinus delphis. Scale
bar is 50 cm. Bianucci et al. (2023).The large size and undulatory motion of early Whales such as Perucetus colossus and Basilosaurus spp., is similar to that seen in the large Ichthyosaurs of the Early Triassic. In these Marine Reptiles it has been suggested that their large size might have helped to compensate for the high energetic cost of undulatory locomotion. It has also been observed that the earliest members of several Mesozoic Marine Reptile groups shared a small size (less than 1 m), pachyosteosclerotic skeleton, and serpentiform body adapted for undulatory motion, suggesting that all these Animals were the subjects of similar selective pressures, associated with shallow diving in a coastal environment. Having adopted such a lifestyle, all would then have been under further selective pressure to grow in size, allowing longer and deeper dives.
While the absence of a skull makes it impossible to accurately reconstruct the feeding habits of Perucetus colossus, its large size and a body apparently adapted to slow swimming and diving in near-shore environments, do make some speculation possible. Such a large Animal must have required very large quantities of food, and therefore that food must have been fairly abundant. Perucetus colossus has been observed to share a number of features with extant Sirenians, a group which feeds almost exclusively on Seagrass and Macroalgae, but no other Cetacean has ever been recorded to follow a herbivorous lifestyle, so this seems a little unlikely. An alternative might be sessile or sedentary benthic Animals such as Crustaceans, demersal Fish, and Molluscs. Such organisms could be harvested with a sucking and/or filter feeding technique, similar to that used by the living Grey Whale, Eschrichtius robustus, or possibly to have fed on sunken carrion, in a way similar to many large, bottom-dwelling Sharks.
The discovery of Perucetus colossus extends the range of known skeletal phenotypes for Vertebrates, and our understanding of the maximum achievable body mass for a healthy Animal. It further shows that masses close to or exceeding 100 tonnes are not a recent phenomenon in Cetaceans, as has previously been assumed, but had also been achieved by the second half of the Eocene; a time during which the maximum size was also achieved by several other Mammal groups. While the remains of the only known specimen of Perucetus colossus are fragmentary, preventing a full reconstruction of the living Animal and its lifestyle, it appears to have been adapted for a slow-swimming, most likely coastal, benthic habit, showing that a fully pelagic lifestyle is not necesarily a prerequisite for achieving large size in Cetaceans. It further expands the known ecological niche of the Basilosaurids, but supports the hypothesis that this group was essentially restricted to coastal environments, which are known to have been highly productive during the Eocene.
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