RESEARCH ARTICLE SUMMARY
◥PALEONTOLOGY
Early giant reveals faster evolution of large body
size in ichthyosaurs than in cetaceans
P. Martin Sander, Eva Maria Griebeler, Nicole Klein, Jorge Velez Juarbe, Tanja Wintrich,
Liam J. Revell, Lars Schmitz*
INTRODUCTION:The iterative evolution of sec-
ondarily marine tetrapods since the Paleozoic
offers the promise of better understanding how
the anatomy and ecology of animals change
when returning to the sea. Recurring patterns
of convergence in the geological past may sug-
gest predictability of evolution when transi-
tioning from full-time life on land to full-time
life in the ocean. Ichthyosaurs (fish-shaped
marine reptiles of the Mesozoic) and today’s
cetaceans (whales, dolphins, and porpoises)
are two of the most informative lineages to
exemplify secondary returns to the sea. The
notable resemblance in body shape and life-
style of ichthyosaurs and cetaceans contrasts
with their separation in time by nearly 200 mil-
lion years, providing an often-cited example
of convergent evolution. Ichthyosaurs arose
249 million years ago and populated the oceans
for the next 150 million years. Cetaceans did
not evolve until about 56 million years ago.
As tail-propelled swimmers, ichthyosaurs and
cetaceans evolved not only convergent body
shapes but also large body sizes.RATIONALE:The integration of fossil and extant
data can improve understanding of aquatic
adaptation and gigantism as patterns of con-vergent evolution, particularly when interpreted
in an ecological context. Our paleontological
fieldwork in the Fossil Hill Member (Middle
Triassic, Nevada, USA) provided the basis for
the marine reptile data and resulted in finds
of giant ichthyosaurs as part of the pelagic
Fossil Hill Fauna. We compiled data for both
fossil and living whales from the extensive
literature. Together, these data provide the
basis for computational analyses of maximum
body size and its evolution over time. Modeling
of energy flux in the Fossil Hill Fauna helps in
understanding how the Fossil Hill ecosystem
could have supported several large to giant
tetrapod ocean consumers so early in ichthyo-
saur evolutionary history.RESULTS:We describe an ichthyosaur with a
2-m-long skull from the Fossil Hill Fauna as a
new species ofCymbospondylus. At present,
this is the largest known tetrapod of its time,
on land or in the sea, and is the first in a series
of ocean giants. The Fossil Hill Fauna includes
several other large-bodied ichthyosaurs in
theCymbospondylusradiation. The body-size
range in this Triassic fauna rivals the range
seen in modern whale faunas, from a total
length of about 2 m inPhalarodonto more
than 17 m in the new species. As preserved
in the fossil record, the Fossil Hill Fauna rep-
resents a stable trophic network and could
even have supported another large ichthyo-
saur if it bulk fed on small, but abundant, prey
such as ammonoids. In absolute time, the
new ocean giant lived 246 million years ago,
only about 3 million years after the appearance
of the first ichthyosaurs. Our research suggests
that ichthyosaurs evolved large body size very
early on in the clade’s history, comparatively
earlier than whales.CONCLUSION:Ichthyosaurs and cetaceans both
evolved very large body sizes, yet their respec-
tive evolutionary pathways toward gigantism
were different. Ichthyosaurs seem to have
benefited from the abundance of pelagic
conodonts and ammonoids after the recov-
ery from the end-Permian mass extinction,
even in the absence of modern primary
producers. Cetaceans took different routes,
but all appear to be related to trophic
specialization, including the loss of teeth in
baleen whales (Mysticeti) and the evolution of
raptorial feeding and deep diving in toothed
whales (Odontoceti).
▪RESEARCH
1578 24 DECEMBER 2021•VOL 374 ISSUE 6575 science.orgSCIENCE
Fossil Hill
Fauna0 50 100 150
Time in million years after the origin of the group0.00.20.40.60.81.0Normalized body sizeIchthyosauriaCetaceaTriassicNowIchthyosaurs evolved large body sizes earlier in their history than cetaceans.The Fossil Hill Fauna of the
Middle Triassic of Nevada, USA, is critical for recognizing this pattern. It features the first ocean giant among tetrapods, only
3 million years after ichthyosaurs first appeared. Whales took comparatively longer to attain similarly large body sizes.
The list of author affiliations is available in the full article online.
*Corresponding author. Email: [email protected]
(P.M.S.); [email protected] (E.M.G.);
[email protected] (L.S.)
Cite this article as P. M. Sanderet al.,Science 374 ,
eabf5787 (2021). DOI: 10.1126/science.abf5787READ THE FULL ARTICLE AT
https://doi.org/10.1126/science.abf5787 CREDIT: STEPHANIE ABRAMOWICZ