Encyclopedia of Evolution.pdf - Online Reading Center

dinosaurs
what DeVries thought were mutations were actually recombinations
of existing genes.
DeVries was one of three biologists (along with Erich
von Tschermak-Seysenegg and Carl Correns) who rediscovered
the work of Gregor Mendel (see Mendel, Gregor;
Mendelian genetics). Mendel had demonstrated that traits
were passed on from one generation to another in a particulate
fashion rather than by blending, and DeVries believed
it was pangenes that did this. Largely because of DeVries,
who influenced later geneticists such as Henry Bateson and
Thomas Hunt Morgan, Mendel’s mutations were considered
inconsistent with Darwinian evolution. It was not until the
modern synthesis of the late 1930s that Darwinian evolution
and Mendelian genetics were brought together in a way
that showed DeVries to have been wrong. Interestingly, this
might not have happened had DeVries not helped to rediscover
and publicize the work of Gregor Mendel. The rediscovery
of Mendel’s work remains DeVries’s major impact on
evolutionary science. He died May 21, 1935.
dinosaurs The term dinosaur usually refers to reptiles,
many of them large, that dominated the Earth during the
Mesozoic era. The earliest dinosaurs lived during the Triassic
period. Dinosaurs diversified into numerous forms
during the Jurassic period and Cretaceous period, perhaps
because one of the mass extinctions, which occurred
at the end of the Triassic period, removed competition from
other vertebrate groups. The dinosaurs themselves became
extinct at the end of the Cretaceous period (see Cretaceous
extinction).
Modern phylogeny (see cladistics) classifies organisms
based on evolutionary divergence. Since birds and mammals
diverged from the group of vertebrates usually called
reptiles, there is no such thing as reptiles unless one includes
birds and mammals in the group (see reptiles, evolution
of). Dinosaurs had many characteristics of skin, teeth, and
bones that would remind observers of the reptiles with which
they are familiar. In fact, since birds evolved from dinosaurs,
the cladistic approach would indicate that the dinosaurs still
exist: They are birds. Therefore the dinosaurs that most people
think of are more properly called non-avian dinosaurs.
Although dinosaurs are truly ancient, the therapsid (mammallike)
reptile lineage had diverged before the dinosaur lineage,
and true mammals already existed by the time the first dinosaurs
roamed the Earth.
The first dinosaurs to be studied were large, which is
why Sir Richard Owen (see Owen, Richard) gave them the
name dinosaurs (Greek for “terrible lizards”) in 1842. Many
famous dinosaur discoveries came from the northern plains
of the United States. In the late 19th century, almost 150 new
dinosaur species were revealed by the excavations and reconstructions
of American paleontologists Edward Drinker Cope
and Othniel Charles Marsh. They began as collaborators but
ended up in a three-decade intense rivalry. Marsh did not perform
very much fieldwork. One account claims that he visited
a spot where dinosaur bones were lying about like logs but
did not recognize them. Marsh was, however, rich enough to
buy fossils and to hire field workers. His uncle, George Peabody,
endowed a museum at Yale specifically so that Marsh
could study dinosaurs. Cope, who was also wealthy, spent
much productive time in the field. He worked in Montana at
the same time that General Custer’s army was being annihilated
nearby. When suspicious Natives checked out his camp,
he entertained them by taking out his false teeth until they
went away. A schoolteacher who found some dinosaur bones
genially informed both Marsh and Cope about them. Cope
sent the schoolteacher money and told him to ask Marsh to
send all the specimens back to him. The rivalry grew intense
enough that the excavators of the Marsh and Cope camps
threw rocks at one another. Cope lost his money, but not his
ego, in financial speculation, dying poor. He willed his body
to a museum, hoping to have his skeleton used as the “type
specimen” to represent the human species. When the skeleton
was prepared, it was found to have syphilitic lesions and was
unsuitable as a type specimen.
Most of what scientists know about dinosaurs comes
from the study of fossilized bones, in the tradition of Cope
and Marsh. Recently this has included the microscopic study
of the spaces left by blood vessels in the bones. Other kinds
of fossil evidence have also allowed many insights into the
lives of dinosaurs:
• Gastroliths. These are “stomach stones” that helped to
grind coarse plant material in the stomachs of some dinosaurs
that ate branches and leaves. Many modern birds
swallow stones that help them to grind their food.
• Eggs. Fossilized eggs with embryos inside reveal some of the
process of dinosaur development. In addition, the discovery
of caches of eggs reveals that dinosaurs cared for nests of
eggs in a manner similar to that of most modern birds.
• Soft material. In 2005 paleontologist Mary Higby Schweitzer
discovered preserved soft tissue, including what
may be blood vessels and cells, in a dinosaur bone.
Since the time of Owen, Marsh, and Cope, dinosaurs
have entered the popular imagination as emblematic both of
huge size and of evolutionary failure. In both ways, the image
is unfair. First, many dinosaurs were small or medium sized.
Many of the smaller dinosaurs were sleek and rapid, and
some had feathers (see birds, evolution of). Second, there
were many kinds of dinosaurs, all over the world; they were,
for more than a hundred million years, an astounding evolutionary
success.
Some dinosaurs were extremely large. Sauroposeidon
proteles, which lived 110 million years ago in what is now
Oklahoma, had a body 100 feet (30 m) in length, with a
neck more than 30 feet (10 m) long. The size estimate of the
neck of this dinosaur is based upon the fact that most vertebrates
have the same number of neck vertebrae, and the
longest vertebrae found for this dinosaur were about five feet
(140 cm) long. The sauropod Supersaurus weighed nearly
a hundred tons, the largest animal ever to live on land (see
figure on page 123). They were also the largest four-legged
animals that would possibly have lived; if they had grown to
140 tons, the size of the legs necessary to support the weight