Heaviest animal ever

The record for heaviest animal ever is jointly held by two whale species – one living, one long-extinct. The living species is the blue whale (Balaenoptera musculus) of worldwide oceanic distribution. The heaviest confirmed specimen is a female weighing 190 tonnes (418,878 lb) and measuring 27.6 m (90 ft 6 in) long, caught in the Southern Ocean on 20 March 1947. The longest confirmed specimen is a female landed in 1909 at the whaling station Grytviken in South Georgia in the South Atlantic, which measured 107 Norwegian fot (=33.57 m; 110 ft 1.6 in). This specimen probably weighed more, therefore, than the 27.6-m one, but unfortunately its weight was not recorded.

The long-extinct whale species is the newly named and described Perucetus colossus, an ancient archaeocete that lived 39 million years ago, during the mid-Eocene epoch as described in Nature on 2 August 2023. It is presently known only from an 18-bone fossil partial skeleton uncovered in the Ica desert on Peru's southern coast. Due to the paucity of Perucetus remains, estimates for its total weight when living vary greatly, from 85 to 340 tonnes (187,400–749,570 lb) – the top range thereby far exceeding the blue whale's maximum recorded weight. But even if the median value of 212.5 tonnes (468,480 lb) from that range is utilized as a more conservative estimate until further fossil remains are found, Perucetus is likely to have been at least equal in weight to the 33.57-m-long South Georgia specimen of blue whale.

The reason why Perucetus colossus ("colossal Peru whale") may have been so incredibly heavy is due not only to its estimated length of 17–20 m (55–66 ft) but also to the exceptional thickness and density of its bones so far discovered, a combined condition known technically as pachyosteosclerosis, and recorded to a much lesser degree in some additional archaeocete species as well as in certain other animals. If all its bones were of the same density, then it's likely that Perucetus had the heaviest skeleton of any mammal (and aquatic vertebrate) in history, estimated at between 5.3 and 7.6 tonnes (11,685–16,755 lb), which is almost twice the weight of a blue whale's skeleton.

The first Perucetus vertebra was discovered in 2013 by the Peruvian palaeontologist of the Department of Vertebrate Palaeontology at the Natural History Museum of the Universidad Nacional Mayor de San Marcos de Lima. He was a co-author of the 2023 published paper, along with fellow scientists from the Cayetano Heredia University (Peru), the University of Pisa, University of Camerino, University of Milano-Bicocca (all Italy), the Royal Institute of Natural Sciences of Belgium, the University of Liège (both Belgium), University of Zurich, Bern University (both Switzerland), the Natural History Museum Rotterdam (Netherlands), the National Museum of Natural History (France) and the Stuttgart State Museum of Natural History (Germany)

Archaeocetes constitute an early group of whales that still possessed lengthy, non-telescoped skulls (unlike those of modern whales), and the members of one taxonomic family, Basilosauridae, which includes Perucetus as a member and existed exclusively during the Eocene, were notably elongate in shape, though their bodies were not as flexible as once believed.