Mesozoic Earth and Life History - The Changing Earth: Exploring Geology and Evolution - page 619

and foot stomping in defense, territorial disputes, or attempts
to attract mates.

compared to their volume. With their comparatively smaller
surface area for heat loss, sauropods probably retained heat
more effectively than their smaller relatives.

Warm-Blooded Dinosaurs? Were dinosaurs endotherms
(warm-blooded) like today's mammals and birds, or were they
ectotherms (cold-blooded) like all of today's reptiles? Almost
everyone now agrees some compelling evidence exists for dino-
saur endothermy, but opinion is still divided among (1) those
holding that all dinosaurs were endotherms, (2) those who
think only some were endotherms, and (3) those proposing
that dinosaur metabolism, and thus the ability to regulate body
temperature, changed as they matured.
Bones of endotherms typically have numerous passage-
ways that, when the animals are alive, contain blood vessels,
but the bones of ectotherms have considerably fewer pas-
sageways. Proponents of dinosaur endothermy note that
dinosaur bones are more similar to the bones of living endo-
therms. Crocodiles and turtles have this so-called endother-
mic bone, but they are ectotherms, and some small mammals
have bone more typical of ectotherms. Perhaps bone struc-
ture is related more to body size and growth patterns than to
endothermy, so this evidence is not conclusive.
Endotherms must eat more than comparably sized ecto-
therms because their metabolic rates are so much higher.
Consequently, endothermic predators require large prey pop-
ulations and thus constitute a much smaller proportion of
the total animal population than their prey, usually only a few
percent. In contrast, the proportion of ectothermic predators
to prey may be as high as 50%. Where data are suffi cient to al-
low an estimate, dinosaur predators made up 3% to 5% of the
total population. Nevertheless, uncertainties in the data make
this argument less than convincing for many paleontologists.
A large brain in comparison to body size requires a
rather constant body temperature and thus implies endo-
thermy. And some dinosaurs were indeed brainy, especially
the small- and medium-sized theropods, so brain size might
be a convincing argument for these dinosaurs. Even more
compelling evidence for theropod endothermy comes from
their probable relationship to birds and from the rather re-
cent discoveries in China of dinosaurs with feathers or a
featherlike covering. Today, only endotherms have hair, fur,
or feathers for insulation.
Some scientists point out that certain duck-billed dino-
saurs grew and reached maturity much more quickly than
would be expected for ectotherms and conclude that they
must have been warm blooded. Furthermore, a fossil orni-
thopod discovered in 1993 has a preserved four-chambered
heart much like that of living mammals and birds. Three-
dimensional imaging of this structure, now on display at the
North Carolina Museum of Natural Sciences, has convinced
many scientists that this animal was an endotherm.
Good arguments for endothermy exist for several types
of dinosaurs, particularly theropods, although the large sau-
ropods were probably not endothermic, but nevertheless
were capable of maintaining a rather constant body tem-
perature. Large animals heat up and cool down more slowly
than smaller ones because they have a small surface area

Flying Reptiles Paleozoic insects were the fi rst animals to
fl y; however, the fi rst among vertebrates were pterosaurs, or
fl ying reptiles, which were common in the skies from the Late
Triassic until their extinction at the end of the Cretaceous
( Figure 22.26). Adaptations for fl ight include a wing mem-

brane supported by an elongated fourth fi nger (Figure 22.26c),
light hollow bones, and development of those parts of the
brain associated with muscular coordination and sight.
Pterosaurs are generally depicted in movies as large
creatures, but some were no bigger than today's sparrows,
robins, and crows. A few species, however, had wingspans of
several meters, and one Cretaceous pterosaur found in Texas
had a wingspan of at least 12 m! Nevertheless, even the larg-
est species probably weighed no more than a few tens of
kilograms.
Experiments and studies of fossils indicate that the bones
of large pterosaurs, such as Pteranodon (Figure 22.26b), were
too weak for sustained wing fl apping. These comparatively
large animals probably took advantage of thermal updrafts
to stay airborne, mostly by soaring, but occasionally fl apping
their wings for maneuvering. In contrast, smaller pterosaurs
probably stayed aloft by vigorously fl apping their wings, just
as present-day small birds do.
Because at least one pterosaur species had a coat of hair
or hairlike feathers, possibly it, and perhaps all pterosaurs,
were endotherms. In addition, wing fl apping requires a high
metabolic rate and effi cient respiratory and circulatory sys-
tems as in birds today, so it seems likely that at least some
pterosaurs were warm blooded.

◗

Marine Reptiles Several types of Mesozoic reptiles adapted
to a marine environment, including turtles and some croco-
diles, as well as the Triassic mollusk-crushing placodonts. Here,
though, we concentrate on the ichthyosaurs and plesiosaurs
and the less familiar mosasaurs. All were thoroughly aquatic
marine predators, but other than all being reptiles, they were
not closely related to one another. Furthermore, none were di-
nosaurs, although some popular media depict them as such.
The streamlined, rather porpoise-like ichthyosaurs var-
ied from species measuring only 0.7 m to giants more than
15 m long (

Figure 22.27a). Details of their ancestry are
still not clear, but fossil ichthyosaurs from Japan prompted
researcher Ryosuke Motani to say, “I knew Utatsusaurus was
exactly what paleontologists had been expecting to fi nd for
years: an ichthyosaur that looked like a lizard with legs.”*
Ichthyosaurs used their powerful tail for propulsion
and maneuvered with their flipperlike forelimbs. They
had numerous sharp teeth, and preserved stomach con-
tents reveal a diet of fish, cephalopods, and other marine

◗

*Ryosuke Motani, 2004. “Rulers of the Jurassic Seas.” Scientifi c American ,
vol. 14, no. 2. p. 7.