Science - USA (2021-12-24)

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ILLUSTRATION: STEPHANIE ABRAMOWICZ/DINOSAUR INSTITUTE,NATURAL HISTORY MUSEUM OF LOS ANGELES COUNTY

similar to that of an adult fin whale. Even

more interesting is the age of the species—it

evolved about 2.5 million years after the ap-

pearance of Cartorhynchus, the oldest pro-

posed ichthyosaur relative from China, which

is no bigger than a laptop computer ( 7 ). Thus,

the early evolutionary history of ichthyosaurs

seems to show a rapid rise of gigantism, with

C. youngorum representing the largest verte-

brate of its time, either at sea or on land.

Previous discoveries from Early Triassic

rocks of China have established that the first

ichthyosaurs were relatively small ( 7 ), but

Early Triassic Cymbospondylus fossils from

Svalbard (an archipelago in the Arctic) ( 8 )

have shown that large bodies evolved in the

earliest phases of ichthyosaur macroevolu-

tion. Fossils like these and others prompted

an “early burst” evolutionary model in the

wake of the Permian-Triassic mass extinction

( 9 ). In this context, Sander et al.’s discovery

of C. youngorum provides a new size maxi-

mum that is notably close to the time origin

for the oldest ichthyosaurs, compressing the

timeline of earlier models into just the first

few million years of ichthyosaur history (see

the figure).

The speed of ichthyosaur gigantism might

be unparalleled among marine vertebrates.

One of the major contributions of Sander et

al. is the comparison of this rapid increase

in size with comparative data from the evo-

lutionary history of whales, which are the

closest ecomorphological comparatives of

ichthyosaurs. Whereas it took whales about

90% of their 55-million-year history to evolve

into giants, ichthyosaurs—as exemplified by

C. youngorum—did it in the first 1% of their

150-million-year history.

There is an ongoing debate about how

fast ecosystems can recover from mass ex-

tinctions. In the Middle Triassic, marine

ecosystems were still recovering after the

Permian-Triassic mass extinction, an event

during which more than 80% of marine

genera died out. Ichthyosaurs were the first

vertebrates to achieve dominance in the top

spots of ocean food webs, but the earliest

phases of their transition from land to sea

are still obscure ( 4 ), unlike that of whales

( 10 ). Today’s whales and other ocean giants

take advantage of short food chains to ac-

cess high densities of zooplankton ( 1 ) and

are themselves vital contributors to nutri-

ent cycling ( 11 ). However, this configuration

PERSPECTIVES

Ichthyosaurs evolved in Triassic oceans where

common prey items were shelled cephalopods,

such as ammonoids.

INSIGHTS

EVOLUTION

Early and fast rise of

Mesozoic ocean giants

A whale-sized ichthyosaur shows

how fast these reptiles evolved

By Lene Liebe Delsett1,2 and

Nicholas D. Pyenson1,3

W

e live in a time of ocean giants:

Today’s blue whales, right whales,

sperm whales, and killer whales

are not just extremely large ma-

rine predators, they each represent

the largest-sized examples ever for

their respective lineages ( 1 , 2 ). Notably, whale

gigantism appears to be a relatively recent

phenomenon, only having evolved in the

past few million years, which is but a frac-

tion of their total evolutionary history ( 3 ).

Does this delayed rise of gigantism hold true

for ocean giants at other times in the past?

After all, dozens of reptile and mammal lin-

eages have invaded ocean ecosystems for the

past 300 million years ( 4 ), and whale-sized

vertebrates have evolved many times ( 5 ).

On page 1578 of this issue, Sander et al. ( 6 )

describe a very large species of ichthyosaur,

Cymbospondylus youngorum sp. nov., from

early Middle Triassic rocks of Nevada that

appears to be the first ocean giant.

Ichthyosaurs were among the first verte-

brates to invade the oceans after the Permian-

Triassic mass extinction (252 million years

ago), and they persisted through most of the

Mesozoic era. The specimen described by

Sander et al. is well preserved and very large.

The skull alone is about the size of a grand

piano, which places total length estimates

for C. youngorum at more than 18 m long,

(^1) Department of Paleobiology, National Museum of Natural
History, Smithsonian Institution, Washington, DC, USA.
(^2) Natural History Museum, University of Oslo, Oslo, Norway.
(^3) Burke Museum of Natural History and Culture, Seattle, WA,
USA. Email [email protected]; [email protected]
1554 24 DECEMBER 2021 • VOL 374 ISSUE 6575