Encyclopedia of Evolution.pdf - Online Reading Center

00 Triassic period
more resulting lineages are indistinguishable on the basis
of fossils. If one saw the two lineages, the lineages would
at this point look alike. By the time their fossils become
noticeably different, evolutionary divergence has already
been going on for some time. Fossils may not appear in the
record until a particular innovation, for example the evolution
of hard skeletons, occurs.
• Molecules provide overestimates of the age of a lineage.
Benton and Ayala point out that the branch point indicated
by molecular cladograms may represent the origin of a
molecular polymorphism, rather than an actual evolutionary
branch point. The two lineages of molecules may have
diverged even while they were still mixed together in the
same population, constituting a polymorphism within a
population rather than a divergence into different populations.
It is well known that some genes mutate faster than
others, meaning that each of these molecular clocks ticks at
a different rate. Molecular estimates of the time at which
bilateral animals diverged from the sponges and cnidarians
range from 1.2 billion to only about 600 million years ago,
depending on which and how many segments of DNA or
other molecules were used.
Therefore the actual evolutionary branch point will probably
be somewhere between the anatomical and the molecular estimates.
The anatomical and the molecular branch points may
represent two different evolutionary events, rather than two
estimates of the same event.
In some cases, the correspondence between fossils and
evolutionary lineages may not be clear. For example, molecular
evidence suggests that modern birds diversified during
the Cretaceous period. Birds such as arcHaeopteryx
were already in existence by the earlier Jurassic period.
The explanation for this is that modern birds are probably
not the descendants of Archaeopteryx, which proved to be an
evolutionary dead end. Birds diversified even before Archaeopteryx,
but most of the branches of bird evolution became
extinct.
Because of horizontal gene transfer among bacteria,
among archaebacteria, and even between the two lineages,
the prokaryotic tree of life might more closely resemble a
web, from which the eukaryotic tree of life emerges. This
might make it difficult or impossible to find “LUCA,” or the
last universal common ancestor of all extant life-forms.
As evolutionary scientists continue to construct the tree
of life, they will learn answers to many questions that have
puzzled biologists for centuries, particularly with regard to
which organisms are more closely related to which others.
Further Reading
Benton, Michael J., and Francisco J. Ayala. “Dating the tree of life.”
Science 300 (2003): 1,698–1,700.
Ciccarelli, Francesca D., et al. “Toward automatic reconstruction of
a highly resolved tree of life.” Science 311 (2006): 1,283–1,287.
Tree of Life Web Project. “Explore the tree of life.” Available online.
URL: http://www.tolweb.org/tree. Accessed May 13, 2005.
Zimmer, Carl. “Did DNA come from viruses?” Science 312 (2006):
870–872.
Triassic period The Triassic period (250 million to 210
million years ago) was the first period of the Mesozoic
era (see geological time scale). It followed the Permian
period, which ended with the Permian extinction. The
Mesozoic era is also known as the Age of Dinosaurs, because
dinosaurs were the largest land animals during that time.
Climate. Because the continent of Pangaea was so large,
the middle of the continent had arid conditions.
Continents. A single worldwide continent, Pangaea, had
formed during the Permian period (see continental drift).
Pangaea was centered at the equator. The Panthalassian
(“world ocean”) Sea surrounded it, and the Tethys Sea penetrated
into it. The widening of the Tethys Sea separated Pangaea
into the northern Laurasia and the southern Gondwana
by the end of the Triassic period.
Marine life. Marine invertebrates and vertebrates began
to evolve into many new forms after the Permian extinction
had eliminated some groups (such as trilobites) and reduced
the diversity of others (such as brachiopods; see invertebrates,
evolution of). All modern groups of marine organisms
existed during the Triassic, except aquatic mammals.
Large aquatic reptiles began to evolve in the oceans during
the Triassic.
Life on land. Because of the Permian extinction, the
Triassic period began with few but very common species.
Scientists speculate that the dry conditions in the middle of
Pangaea provided an advantage to seed plants over plants
that reproduced by spores, and to reptiles over amphibians
and mammals.
• Plant life. The seed fern Glossopteris dominated vast forest
areas at the beginning of the Triassic. Seed plants later
dominated over seedless plants, forming the first extensive
forests of conifers, which had first evolved in the late
Paleozoic era (see gymnosperms, evolution of; seedless
plants, evolution of).
• Animal life. One of the most common vertebrates of the
early Triassic was the reptile Lystrosaurus. All vertebrates
were small at the beginning of the Triassic period. Some
moderately large forms evolved by the end of the period,
including the first dinosaurs, although the largest dinosaurs
did not evolve until the Jurassic and Cretaceous periods.
Further Reading
Kazlev, M. Alan. “The Triassic period.” Available online. URL:
http://www.palaeos.com/Mesozoic/Triassic/Triassic.htm. Accessed
May 16, 2005.
trilobites Trilobites were among the most abundant and
diverse arthropods in the oceans of the Paleozoic era.
Although they have been described as the cockroaches of the
ancient seas, they represent an evolutionary lineage distinct
from that of insects; they were more closely related to modern
arachnids such as the horseshoe crab and spiders (see invertebrates,
evolution of). Besides being diverse and abundant,
they have been marvelously preserved as fossils, since
they were the first arthropods to have external skeletons (even
though just their upper surfaces) reinforced with calcite. They