Science - USA (2021-12-24)

and data S5] more than 25 Ma later. These
examples illustrate the short time spans of size
increases in ichthyosaurs compared with those
of cetaceans, but does this pattern hold when
analyzed in the context of time-calibrated
phylogenies?

Computational modeling of
body-size evolution

We first compared the evolutionary patterns
of body-size evolution through traitgrams that
account for the uncertainty tied to the time
calibration of the phylogenies ( 10 ). These
evolutionary traitgrams of ichthyosaur and
cetacean body sizes, normalized to maximum
body size to enable a better comparison of the
pattern, reinforce the notion of fast body-size
evolution in ichthyosaurs (Fig. 4). The maxi-

mum body size of ichthyosaurs increased

dramatically early in their history, whereas

cetacean maximum body size increased up

to the present day. In our clade-wide model-

fitting approaches, the early-burst and trend

models are strongly preferred over all other

evolutionary models for ichthyosaurs [Fig. 4

and fig. S10; see ( 10 )]. By contrast, no strong

preference is indicated for any particular model

in cetaceans (Fig. 4). Resampling of the ceta-

cean dataset to adjust for differences in sample

size, trophic specialization, and the mixture of

fossil and extant data did not reveal strong

support for an early-burst model (see fig. S11).

When restricting the cetacean dataset to the

Pelagiceti to account for the differing degrees

of aquatic adaptation in the early-branching

lineages, an early-burst model is slightly preferred

over the others tested (see fig. S11), but the pattern

is not nearly as strong as for ichthyosaurs.

To explore the evolutionary patterns at

smaller phylogenetic scales and therefore

avoid problems related to the impact of taxon

selection, we assessed the heterogeneity of

the rate of body-size evolution across both

the ichthyosaur and cetacean trees (Figs. 5 and

6 and fig. S12). Models that allow for rate

heterogeneity can identify specific regions of

the tree that signify evolutionary changes and

thus circumvent the problem of a clade-wide

approach that would ignore the various de-

grees of aquatic adaptations in the early ichthyo-

saurs and cetaceans.

Results from a variable-rate model based

on Brownian motion, which uses penalized

likelihood to estimate the evolutionary rates

along all branches in the tree ( 31 ), support

fastevolutionofbodysizeinearlyichthyosaurs

(Fig. 5A). In congruence with observations from

the fossil record and clade-wide model fitting,

the highest evolutionary rates are found in the

Sanderet al.,Science 374 , eabf5787 (2021) 24 December 2021 5 of 14

0 50 100 150

Time since the root in million years

0.0

0.2

0.4

0.6

0.8

1.0

Normalized body size proxy

C.C.

S.S.

L.L.

B.B.

IchthyosauriaIchthyosauria

CetaceaCetacea

0.00 0.25 0.50 0.75 1.00

trend

drift

EB

OU

BM

0.00 0.25 0.50 0.75 1.00

trend

drift

EB

OU

BM

Fig. 4. Body-size evolution in ichthyosaurs and cetaceans compared.Traitgram of body size, normalized such
that 0 corresponds to the smallest body size in each group and 1 to the largest for ichthyosaurs (lilac; based on an
early-burst model) and cetaceans (ochre; based on a Brownian motion model) (see Methods). Lilac dots indicate
Cymbospondylusspecies from the Fossil Hill Fauna. B.,Balaenoptera musculus, blue whale; C.,C. youngorumsp. nov.;
L.,Llanocetus denticrenatus, early giant baleen whale; S.,S. sikanniensis, the largest named ichthyosaur. The inset
shows model-fitting results expressed as Akaike weights for five different evolutionary models from 1000 iterations.
Boxes represent the interquartile ranges (IQRs), with whiskers extending 1.5 times the IQR outside the boxes.
Vertical lines inside the boxes show the median. BM, Brownian motion; EB, early burst; OU, Ornstein-Uhlenbeck.

1.24

0.01

0.02

0.03

0.06

0.09

0.16

0.26

0.44

0.74

Evolutionary rate (σ

)

B

Crown Mysticeti

Balaenidae

Balaenopteridae

Pan-Physeteroidea

Ziphiidae

Crown Odontoceti

Pelagiceti

Neoceti

A 2.61

0.05

0.08

0.12

0.18

0.29

0.44

0.69

1.08

1.68

Evolutionary rate (σ

)

Thunnosauria

Parvipelvia

M

C Cymbospondylidae

Mixosauridae

Shastasauridae

Fig. 5. Exploration of the rate heterogeneity

of body-size evolution.(AandB) Ichthyosaurs

(A) feature the fastest rates of body evolution

early in their history, whereas cetaceans (B) show a

more complex pattern, with fastest rates occurring

in later stages of their history. C, Cymbospondylidae;

M, Merriamosauria.

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