Downsizing a heavyweight: factors and methods that revise weight estimates of the giant fossil whale Perucetus colossus

Extremes in organismal size have broad interest in ecology and evolution because organismal size dictates many traits of an organism’s biology. There is particular fascination with identifying upper size extremes in the largest vertebrates, given the challenges and difficulties of measuring extant and extinct candidates for the largest animal of all time, such as whales, terrestrial non-avian dinosaurs, and extinct marine reptiles.

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“We deployed two different ways to estimate body size, one was a volumetric approach, to estimate body size using the shape and outline of the organism…. there are also estimates based on skeletal mass to body mass and where those approaches have similar answers you can have higher confidence in making that estimate”

Dr. Nicholas D Pyenson

Co-Author, Smithsonian Institution

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Unraveling the Weight Mystery of Perucetus colossus

What is Perucetus colossus, and why is it significant in the realm of paleontology?

Perucetus colossus is a colossal basilosaurid whale discovered in the Eocene era of Peru. Its significance lies in challenging assumptions about the upper limits of organismal size, particularly among vertebrates like whales, dinosaurs, and marine reptiles.

How did the weight estimates of Perucetus colossus compare to those of blue whales?

Initially, weight estimates for Perucetus surpassed those of blue whales, the largest extant mammals. However, recent research suggests that these estimates may have been overstated.

What factors and methods were examined to revise the weight estimates of Perucetus colossus?

Researchers analyzed various factors and methodologies, including data from large modern cetaceans, the assumption of isometry between skeletal and body masses, the role of pachyostosis in reconstructions, and comparisons with living whales’ physiological and ecological limits.

What were the revised weight estimates for Perucetus colossus?

Depending on assumptions and methodologies, estimates ranged from 41.3 to 114 tons. However, researchers deemed estimates exceeding 100 tons as unrealistic, suggesting a likelier range of 60 to 70 tons, still considerably larger than its close relative, Basilosaurus.

What implications do these revised estimates have for our understanding of Perucetus colossus and its ecological role?

The revised estimates provide a clearer picture of Perucetus colossus’s size and suggest it more closely resembles the weight of sperm whales than blue whales. Further fossil discoveries, especially cranial and dental material, will help refine these estimates and shed light on its ecological significance in Eocene oceans.