A heavyweight early whale pushes the boundaries of vertebrate morphology

The fossil record of cetaceans documents how terrestrial animals acquired extreme adaptations and transitioned to a fully aquatic lifestyle(1,2). In whales, this is associated with a substantial increase in maximum body size. Although an elongate body was acquired early in cetacean evolution(3), the maximum body mass of baleen whales reflects a recent diversification that culminated in the blue whale(4). More generally, hitherto known gigantism among aquatic tetrapods evolved within pelagic, active swimmers. Here we describe Perucetus colossus-a basilosaurid whale from the middle Eocene epoch of Peru. It displays, to our knowledge, the highest degree of bone mass increase known to date, an adaptation associated with shallow diving(5). The estimated skeletal mass of P. colossus exceeds that of any known mammal or aquatic vertebrate. We show that the bone structure specializations of aquatic mammals are reflected in the scaling of skeletal fraction (skeletal mass versus whole-body mass) across the entire disparity of amniotes. We use the skeletal fraction to estimate the body mass of P. colossus, which proves to be a contender for the title of heaviest animal on record. Cetacean peak body mass had already been reached around 30 million years before previously assumed, in a coastal context in which primary productivity was particularly high.

Automated summary

The fossil record shows how terrestrial animals transitioned to a fully aquatic lifestyle, with whales undergoing significant increases in maximum body size. Perucetus colossus, a basilosaurid whale from the middle Eocene epoch, displays the highest degree of bone mass increase known to date, showing an adaptation associated with shallow diving. Its estimated skeletal mass exceeds that of any known mammal or aquatic vertebrate, making it a contender for the heaviest animal on record. The peak body mass of cetaceans was reached around 30 million years earlier than previously assumed, in a coastal environment with high primary productivity.