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Heaviest animal ever to live discovered in Peru

"Nature" features research that was coordinated by the palaeontologists of the University of Pisa

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The fossilized remains of an extraordinary animal dating back to almost 40 million years ago have resurfaced in the Ica Desert, along the southern coast of Peru. The fossil in question belongs to an ancestor of the present-day whales and dolphins – one that was characterised by very large and heavy bones, which in turn evoke a sea monster of titanic proportions. An article that has just published in the prestigious scientific journal Nature presents a first analysis of this exceptional cetacean, which has been given the name of Perucetus colossus in honour of the South American country in which it was found and in reference to its literally colossal size. The international team of scientists that authored this paper features the strong participation of the palaeontologists of the Department of Earth Sciences of the University of Pisa, namely, professor Giovanni Bianucci (first author and coordinator of the research group), PhD student Marco Merella and assistant professor Alberto Collareta. Other Italian geologists and paleontologists from the universities of Milano-Bicocca (researcher Giulia Bosio and professor Elisa Malinverno) and Camerino (professors Claudio Di Celma and Pietro Paolo Pierantoni) also took part in the study, joined by researchers from Peru and various European countries.

Artistic reconstruction of Perucetus colossus (artwork by Alberto Gennari).

“Although the Perucetus’ skeleton is not complete, rigorous estimates based on the measurement of preserved bones as well as on comparisons with a broad database of living and fossil organisms - explains Giovanni Bianucci - indicate that the mass of the skeleton of Perucetus was about 5–8 tonnes - at least twice the skeletal mass of the largest living animal, the blue whale. The very heavy skeleton of Perucetus - which in life would have reached 20 meters in total body length - suggests that the body mass of this ancient cetacean may have been as much as 340 tonnes, almost twice that of the largest blue whales and more than three times that estimated for Argentinosaurus, one of the largest dinosaurs ever found”. Thus, Perucetus represents an excellent candidate for the role of heaviest animal of all times, a record from which the blue whale would be undermined. The palaeobiological implications of such a discovery are of extreme importance. "The enormous body mass of Perucetus - continues Bianucci - indicates that cetaceans developed phenomena of gigantism at least twice: in relatively recent times, with the evolution of the large baleen whales that inhabit the modern oceans, and some 40 million years ago, with the radiation of the basilosaurids of which Perucetus is the most extraordinary representative". Studying such a 'heavyweight' was certainly exciting but not without its challenges: "Each of the vertebrae of Perucetus is so heavy - the lightest weighs over 100 kg! - that several strong persons was required to handle it” says Marco Merella. “In addition to making the excavation and preparation phases more difficult, heaviness greatly complicated the osteoanatomical analysis of the skeleton. We therefore turned to the innovative methodologies of virtual palaeontology, and in particular to structured light scanning, to acquire and elaborate detailed three-dimensional models of all the collected bones. These models allowed us to continue the study once back to Pisa. In fact, thanks to structured light scanning, that it was possible to rigorously estimate the volume of the skeleton, thus providing quantitative support for the reconstruction of the body shape and lifestyle of this exceptional extinct cetacean".

Skeletal mass vs. body mass across amniotes (mammals and reptiles, including birds) (credits: Giovanni Bianucci).

“The titanic size of the bones of Perucetus certainly represents the most striking feature of this new species - says Alberto Collareta - but the enormous mass reconstructed for the entire skeleton also reflects the high density of the type of bone tissue it is made of. Indeed, all the bones of Perucetus are made up of extremely dense and compact bone, similar to that found, though in a much less marked way, in the present-day sirenians. The latter live in shallow coastal waters, where a particularly heavy skeleton acts as a 'ballast', thus facilitating feeding at the seabed and increasing the inertia to the action of waves. The kind of thickening and heaviness of the skeleton - in technical terms, pachyosteosclerosis - which Perucetus shares with the sirenians is not found in any living cetacean. Although it is difficult to provide a palaeoecological interpretation of this extraordinary adaptation, it is thus likely that it provided Perucetus with the stability needed to inhabit the high-energy waters close to the coastline. Perucetus probably fed near the seabed, perhaps searching for the carcasses of other marine vertebrates as some large-bodied sharks do today."

The research team at the fossil find site (Giovanni Bianucci).

Research at the University of Milano-Bicocca focused on reconstructing the stratigraphy and geological age of this ancient ancestor of the present-day whales. “Based on the micropaleontological analysis of planktonic species and radiometric dating of a volcanic ash found in the vicinity of the fossil - add Elisa Malinverno and Giulia Bosio - we were able to determine that the fossil is between 39.8 and 37.84 million year old. Thus, Perucetus lived during the Eocene epoch, when the forebears of the modern cetaceans were abandoning the terrestrial lifestyle in favour of a marine one".

Claudio Di Celma and Pietro Paolo Pierantoni from the Geology section of the University of Camerino carried out the geological-stratigraphic study of the area where Perucetus was discovered. “Through the study of the sedimentary rocks that embed the fossil - explains Claudio Di Celma - we have contributed to the reconstruction of the environment in which this ancient marine mammal lived. Where is now a desert that extends for hundreds of kilometers along the coast of Peru - explains the geologist Claudio Di Celma - in the past there was a broad marine embayment, the Pisco Basin, characterized by a great abundance of nutrients and a rich biodiversity”.

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Although Perucetus was by all means an unexpected find, the place and mode of this discovery were not. “The find of the first bones of Perucetus dates back to thirteen years ago and is due to Mario Urbina, a field researcher and true ‘living legend’ of Peruvian palaeontology”, explains Bianucci. “It is only thanks to Mario's perseverance that the multi-year excavation of this extraordinary (and very heavy) fossil was possible. It was Mario himself who realized that the Desert of Ica - one of the driest areas on Earth - is home to one of the largest deposits of fossil vertebrates worldwide".

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The palaeontological heritage of the Ica Desert

For about fifteen years, and thanks to a number of national and international research projects (many of which were led by the University of Pisa), this exceptional paleontological heritage has been adequately valued through scientific research carried out by a close-knit and multidisciplinary equipe of which Peruvian palaeontologists are an integral part. The Desert of Ica has thus become the scenario of many record-breaking discoveries: from the most ancient quadrupedal cetacean to have reached the Pacific Ocean, to the earliest ancestor of the modern baleen whales and the enormous macropredatory sperm whale Livyatan melvillei. All these discoveries confirmed the leading role of the University of Pisa in the study of marine mammal palaeontology.


Thanks also to a new ministerial grant (PRIN) obtained by the University of Pisa, the research efforts on the Ica Desert cetaceans as well as on the exceptional conditions that led to the formation of this extraordinary fossil deposit will continue in the years to come. “I would bet the surprises aren't over yet”, concludes Bianucci.

  • 2 August 2023

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