Science - USA (2021-12-24)

deep regions of the ichthyosaur tree, with
some indication that fastest size evolution
may have occurred in cymbospondylids and
the early phase of merriamosaurs (Fig. 5A).
After the initial burst, the estimated evolu-
tionary rates slowed over time.
The distribution of evolutionary rates in the
cetacean tree forms a stark contrast to the
ichthyosaur pattern (Fig. 5B). The deep parts

of the cetacean tree feature average rates.

Surprisingly, the body size of both early crown

mysticetes and odontocetes evolved slowly,

before accelerating in two clades of mysticetes

with living representatives, the Balaenidae

(right and bowhead whales) and Balaenopter-

idae (rorquals), as found in a previous study

( 6 ). Among odontocetes, the fossil lineages of

the Pan-Physeteroidea, in particular, a clade

that contains living sperm whales, are charac-

terized by fast rates, whereas Ziphiidae (beaked

whales) have comparatively slow rates of size

evolution (Fig. 5B).

Whereas rate heterogeneity models can

identify phases of fast and slow morphological

evolution, the determination of the direction

of evolution, that is, evolution toward larger

and smaller body sizes in specific lineages,

Sanderet al.,Science 374 , eabf5787 (2021) 24 December 2021 6 of 14

Fig. 6. The adaptive landscape
of body-size evolution.(AandB)
Ichthyosaurs (A) feature two early
adaptive shifts toward larger skull
length, whereas cetaceans (B) entered
selective regimes that promoted larger
skull width much later in their evolution.

Platypterygius australis

Hupehsuchus nanchangensis

Cartorhynchus lenticarpus

Sclerocormus parviceps

Chaohusaurus chaoxianensis

Chaohusaurus geishanensis

Utatsusaurus hataii

Gulosaurus helmi

Parvinatator wapitiensis

Xinminosaurus catactesCymbospondylus buchseri

Cymbospondylus nichollsi

Cymbospondylus petrinus

Cymbospondylus duelferiCymbospondylus youngorum sp. nov.

Barracudasauroides panxianensis

Contectopalatus atavus

Phalarodon callawayi

Phalarodon fraasi

Mixosaurus xindianensis

Mixosaurus cornalianus

Mixosaurus kuhnschneyderi

Phantomosaurus neubigi

Qianichthyosaurus zhoui

Qianichthyosaurus xingyiensis

Wimanius odontopalatus

Besanosaurus leptorhynchus

Thalattoarchon saurophagis

Shastasaurus liangaeShastasaurus sikkaniensis

Guizhouichthyosaurus tangae

Shonisaurus popularis

Callawayia neoscapularis

Hudsonelpidia brevirostris

Macgowania janiceps

Temnodontosaurus platyodon

Temnodontosaurus trigonodon

Suevoleviathan integer

Leptonectes tenuirostris

Excalibosaurus costiniEurhinosaurus longirostris

Ichthyosaurus communis

Ophthalmosaurus icenicus

Ophthalmosaurus natans

Aegirosaurus leptospondylus

Caypullisaurus bonapartei

Platypterygius americanus

Brachypterygius extremus

skull length 0 2000 mm

Merriamosauria

skull length

decrease

skull length

decrease

skull

length

increase

skull

length

increase

A

B

edentulous mysticetes

Pan-Physeteroidea

Kogiidae

skull width

increase

skull width

increase

skull width

skull width 0 2000 mm

decrease

Cymbospondylidae

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