Encyclopedia of Evolution.pdf - Online Reading Center

diseases, evolution of
• Thyreophores or “shield-bearing” dinosaurs evolved into
large animals with armored plates and, in the stegosaurs, a
paired row of large plates along the back.
• Ornithopods also evolved into large forms but remained
primarily bipedal.
Saurischian dinosaurs had long necks, and hands with
long index fingers, a basic pattern now found in many birds.
They diverged into the sauropods and the theropods.
• Sauropods became very large. An example is the Supersaurus
mentioned above.
• Theropods had long, flexible necks, sharp teeth and claws,
and their hands (with three curved claws) were good at
grasping. Their bones were thin and light and hollow like
those of modern birds. Their collarbones were fused into
a wishbone or furcula, a feature found only in birds today.
These features made the later development of flight possible.
From large theropod ancestors, coelourosaurians
evolved. Some of them, such as Tyrannosaurus, were quite
large, with big heads but very small arms. Others were much
smaller, with larger brains and eyes like modern birds, resembling
an ostrich in body form even to the point of having a
toothless beak. All but one of these birdlike lineages became
extinct. The lineage that survived to eventually become the
birds was the maniraptorans, which were dinosaurs with
feathers.
The maniraptoran dinosaurs apparently had hollow
bones, a wishbone, and feathers before they had the ability
to fly. Evolutionary biologists conclude from this that the
original function of feathers was not for flight but to hold in
body heat. This may have been the first step in warm-bloodedness,
which is important in providing the energy needed for
flight. Specimens of one of them, Oviraptor, have been found
brooding eggs in the fashion of modern birds, another indication
of warm-bloodedness.
One evolutionary branch of maniraptorans not only
had feathers but could fly. The earliest known of these was
arcHaeopteryx from 150 million years ago, during the
Jurassic period. Although skeletally similar to other maniraptorans,
Archaeopteryx resembled modern birds in having
long arms that formed a wing with flight feathers, reduced
teeth, and large brain. Because maniraptorans were runners,
evolutionary biologists believe that flight originated from
rapid running “from the ground up” rather than from gliding
“from the trees down.” The evolutionary lineage of Archaeopteryx
became extinct, but related lineages continued to
evolve more birdlike characteristics, such as a larger breastbone
that allowed massive flight muscles to anchor. By the
early Cretaceous period, before the extinction of non-avian
dinosaurs, there were finch-sized dinosaur birds capable of a
modern type of flight. Other lineages adapted to an aquatic
existence, resembling modern waterfowl, some of them even
losing the ability to fly. All but one lineage, the birds, became
extinct by the end of the Cretaceous period.
Dinosaurs diversified into many forms, from huge consumers
of branches and leaves to small, sleek predators,
mainly during the Jurassic and Cretaceous periods. When
flowering plants evolved at the beginning of the Cretaceous
(see angiosperms, evolution of), some dinosaurs
evolved the ability to consume their leaves, but there was
apparently no coevolution between non-avian dinosaurs
and flowering plants for pollination of flowers or dispersal
of fruits. Insects (and some birds) evolved many pollinating
forms, and mammals were particularly good dispersers
of fruits and seeds. The increasing dominance of flowering
plants in the landscape may have been one reason that
dinosaurs were in decline during the late Cretaceous. Massive
volcanic eruptions changed the climate and further
hastened the decline of dinosaurs. It was apparently the
asteroid that hit the Earth 65 million years ago that sent
all remaining dinosaurs, except the birds, into extinction.
Until almost the last moment, there were at least a hundred
species of dinosaurs.
Further Reading
Achenbach, Joel. “Dinosaurs: Cracking the mystery of how they
lived.” National Geographic, March 2003, 2–33.
Bakker, Robert T. The Dinosaur Heresies. New York: William Morrow,
1986.
Currie, Philip E., and Kevin Padian. Encyclopedia of Dinosaurs. New
York: Academic Press, 1997.
Fiorillo, Anthony R. “The dinosaurs of arctic Alaska.” Scientific
American, December 2004, 84–91.
Fipper, Steve. Tyrannosaurus Sue: The Extraordinary Saga of the
Largest, Most Fought Over T. Rex Ever Found. New York: Freeman,
2000.
Horner, John R., Kevin Padian, and Armand de Ricqlès. “How dinosaurs
grew so large—and so small.” Scientific American, July
2005, 56–63.
Levin, Eric. “Dinosaur family values.” Discover, June 2003, 34–41.
Norell, Mark. Unearthing the Dragon: The Great Feathered Dinosaur
Discovery. New York: Pi Press, 2005.
Parker, Steve. Dinosaurus: The Complete Guide to Dinosaurs. Richmond
Hill, Ontario: Firefly Books, 2003.
Sander, P. Martin, and Nicole Klein. “Developmental plasticity in
the life history of a prosauropod dinosaur.” Science 310 (2005):
1,800–1,802.
Schweitzer, Mary H., Jennifer L. Wittmeyer, and John R. Horner.
“Gender-specific reproductive tissue in ratites and Tyrannosaurus
rex.” Science 308 (2005): 1,456–1,460.
——— et al. “Analyses of soft tissue from Tyrannosaurus rex suggest
the presence of protein.” Science 316 (2007): 277–280.
Yeoman, Barry. “Schweitzer’s dangerous discovery.” Discover, April
2006, 36–41, 77.
Zimmer, Carl. “Jurassic genome.” Science 315 (2007): 1,358–1,359.
diseases, evolution of See evolutionary medicine.
disjunct species Disjunct species contain or consist of
populations (disjunct populations) that are distant from one
another. The distances that separate the populations of disjunct
species are greater than those that would allow frequent,
or even occasional, gene flow between them. Gene
flow occurs when pollen or seeds (of plants), or animals,
travel from one population, or part of a population, to