The body shape of Perucetus colossus, the extremely heavy basilosaurid from the middle Eocene of Peru

The basilosaurid cetacean Perucetus colossus, recently described from the middle Eocene (ca. 40-38 Ma) of the Pisco Basin, Peru, has garnered great attention for its remarkable body mass (Bianucci et al., 2023). The analysis of its holotype - a partial skeleton comprising 13 vertebrae, 4 ribs, and the right innominate - hints at a staggering total skeleton mass of ca. 5–8 t, at least twice that of the largest living animal, the blue whale. This suggests a body mass for Perucetus between 85 and 340 t, making it a contender for the title of the heaviest animal ever. Assuming neutral buoyancy, the estimated medial body mass of Perucetus allows its body volume to be estimated at 180 m3 . Comparison with the 3D body shapes of extant marine mammals, scaled to Perucetus median length estimate, points to a striking resemblance to the West Indian manatee Trichechus manatus. This basic analysis reinforces our previous interpretation of Perucetus as a benthic feeder inhabiting hydrodynamic coastal environments. Newly discovered ribs from the holotype allowed to reconstruct the maximum cross-section of the ribcage of Perucetus. The cross-sectional surface area estimated from a 3D model of a double-headed rib from the central portion of the thorax is approximately 1.4 m2 , notably smaller than our volumetric reconstruction above. However, it is crucial to note that this area represents only a fraction of the total cross-sectional surface area of the body. Indeed, in the extant Tursiops truncatus, the ribcage's cross-sectional surface area at a similar anteroposterior level accounts for only 40% of the entire body's cross-section. Moreover, it is likely that Perucetus had a greater volume of blubber and muscle tissue around the ribcage compared to the bottlenose dolphin. Therefore, the use of ribs to reconstruct the body shape of Perucetus requires caution.