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824 | Nature | Vol 620 | 24 August 2023
Article
A heavyweight early whale pushes the
boundaries of vertebrate morphology
Giovanni Bianucci1, Olivier Lambert2, Mario Urbina3, Marco Merella1, Alberto Collareta1,
Rebecca Bennion2,4, Rodolfo Salas-Gismondi3,5, Aldo Benites-Palomino3,6, Klaas Post7,
Christian de Muizon8, Giulia Bosio9, Claudio Di Celma10, Elisa Malinverno9,
Pietro Paolo Pierantoni10, Igor Maria Villa11 & Eli Amson12 ✉
The fossil record of cetaceans documents how terrestrial animals acquired extreme
adaptations and transitioned to a fully aquatic lifestyle1,2. In whales, this is associated
with a substantial increase in maximum body size. Although an elongate body was
acquired early in cetacean evolution3, the maximum body mass of baleen whales
reects a recent diversication that culminated in the blue whale4. 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 diving5. The estimated
skeletal mass of P. colos sus exceeds that of any known mammal or aquatic vertebrate.
We show that the bone structure specializations of aquatic mammals are reected 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.
The fossil record of cetaceans provides one of the most notable docu-
mentations of an evolutionary transition of lifestyle1,2. This transition
brought a fully terrestrial group of mammals back to water, over 300 mil-
lion years after tetrapods first gained ground
6
. Early to middle Eocene
artiodactyls with a chevrotain-like morphology (such as Indohyus) are
understood to be the closest known relatives of cetaceans7. Becoming
increasingly specialized for aquatic life during this global greenhouse
stage, cetaceans quickly evolved larger body sizes, with a first notable
trend of body elongation seen in late Eocene basilosaurines8. The true
gigantism and associated body mass seen in baleen whales is neverthe-
less a recent acquisition, probably associated with the cooling trend
and seasonality installed in the late Cenozoic era
4
. Furthermore, the
largest cetacean is the extant blue whale (Balaenoptera musculus),
which is also the heaviest animal currently known. More generally, the
largest taxa among aquatic tetrapods (which include marine reptiles
such as ichthyosaurs9) known to date are all pelagic, active swimmers.
As animals acquire more aquatic habits, buoyancy becomes a critical
aspect of their biology. Bone is dense relative to most other tissues and
present in large quantities in the tetrapod body, probably explaining
why bone mass specializations are documented in a myriad of tetrapod
lineages that independently evolved aquatic habits
10
. The adaptations
of shallow-diving, slow-swimming species often comprise bone mass
increase (BMI). This is produced by the infilling of the inner cavities of
skeletal elements with compact bone (that is, osteosclerosis) and, in the
more extreme cases, by additional deposition of bone on their exter-
nal surface
5
(that is, pachyostosis sensu stricto). BMI is documented
in cetaceans’ amphibious close relatives11, as well as early members
of the clade, the basilosaurids in particular. Extant cetaceans have
conversely acquired an entirely different bone microanatomy, with
an osteoporotic-like structure typical of pelagic, secondarily aquatic
tetrapods with more active swimming. Basilosaurids are therefore
unique in the sense that they acquired large sizes (up to around 20 m
in body length
3
) and BMI. The degree of their BMI nevertheless did
not match, up until now, that of some sirenians, for example, of which
the whole rib cage is both strongly osteosclerotic and pachyostotic5.
Here we describe a basilosaurid whale that substantially pushes the
upper limit of skeletal mass in mammals, as well as in aquatic verte-
brates in general. This early whale combines a gigantic size and, to our
knowledge, the strongest degree of BMI known to date. It also poten-
tially represents the heaviest animal ever described.
https://doi.org/10.1038/s41586-023-06381-1
Received: 11 April 2023
Accepted: 28 June 2023
Published online: 2 August 2023
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1Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy. 2D.O. Terre et Histoire dela Vie, Institut Royal des Sciences Naturelles de Belgique, Brussels, Belgium. 3Departamento de
Paleontología de Vertebrados, Museo de Historia Natural-Universidad Nacional Mayor de San Marcos, Lima, Perú. 4Evolution & Diversity Dynamics Lab, UR Geology, Universite de Liège, Liège,
Belgium. 5Facultad de Ciencias y Filosofía/Centro de Investigación para el Desarrollo Integral y Sostenible, Laboratorios de Investigación y Desarrollo, Universitad Peruana Cayetano Heredia
Lima, Lima, Perú. 6Department of Paleontology, University of Zurich, Zurich, Switzerland. 7Natuurhistorisch Museum Rotterdam, Rotterdam, The Netherlands. 8Département Origines et
Évolution, CR2P (CNRS, MNHN, Sorbonne Université), Muséum National d’Histoire Naturelle, Paris, France. 9Dipartimento di Scienze dell’Ambiente e della Terra, Università degli Studi di
Milano-Bicocca, Milano, Italy. 10School of Science and Technology, University of Camerino, Camerino, Italy. 11Institut für Geologie, Universität Bern, Bern, Switzerland. 12Staatliches Museum für
Naturkunde Stuttgart, Stuttgart, Germany. ✉e-mail: eli.amson@smns-bw.de
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