A heavyweight early whale pushes the boundaries of vertebrate morphology
Giovanni Bianucci cs 2023 Nature
https://doi.org/10.1038/s41586-023-06381-1

Cetacean fossil record documents how terrestrial animals acquired extreme adaptations, and transitioned to a fully aquatic lifestyle.
In whales, this is ass.x a substantial increase in max.body-size.
An elongate body was acquired early in cetacean evolution,
but the max.body-mass of baleen whales reflects a recent diversification that culminated in the blue whale.
Hitherto known gigantism among aquatic tetrapods evolved within pelagic, active swimmers.

Here we describe Perucetus colossus (mid-Eocene basilosaurid, Perù).
AFAWK, it displays the highest degree of bone-mass increase known to date, an adaptation ass.x shallow diving.
The estimated skeletal mass of P.colossus exceeds that of any known mammal or aquatic vertebrate.
We show:
the bone structure specializations of aquatic mammals are reflected in the scaling of skeletal fraction (skeletal mass vs whole-body mass) across the entire disparity of amniotes.
We use the skeletal fraction to estimate the body-mass of P.colossus,
this proves to be a contender for the title of heaviest animal on record.
Cetacean peak body-mass had already been reached c 30 My before previously assumed, in a coastal context in which primary productivity was particularly high.