Encyclopedia of Evolution.pdf - Online Reading Center

Permian period
upon the exploitation of natural resources, even the slightest
interruption of these resources would send humankind, very
few of whom know how to survive in the wild, into a desperate
tailspin. Humans would probably not have survived
the Permian extinction.
Further Reading
Benton, Michael J. When Life Nearly Died: The Greatest Mass
Extinction of All Time. London: Thames and Hudson, 2003.
Department of Geological Sciences, University of California, Santa
Barbara. “Evidence of meteor impact found off Australian coast.”
Available online. URL: http://beckeraustralia.crustal.ucsb.edu/.
Accessed March 22, 2005.
Erwin, Douglas H. Extinction: How Life on Earth Nearly Ended 250
Million Years Ago. Princeton, N.J.: Princeton University Press,
2006.
Grice, Kliti, et al. “Photic zone euxinia during the Permian-Triassic
superanoxic event.” Science 307 (2005): 706–709.
Huey, Raymond B., and Peter D. Ward. “Hypoxia, global warming,
and terrestrial late Permian extinctions.” Science 308 (2005):
398–401.
Ward, Peter D., et al. “Abrupt and gradual extinction among Late
Permian land vertebrates in the Karoo Basin, South Africa.” Science
307 (2005): 709–714.
Permian period The Permian period (290 million to
250 million years ago) was the sixth and final period of the
Paleozoic era (see geological time scale). It followed
the Carboniferous period. At the beginning of the Paleozoic
era, almost all life was aquatic; by the end of the Paleozoic
era, forests filled with vertebrates and insects covered
large areas of dry land. The Permian period had the greatest
diversity of species of the entire Paleozoic era. It ended with
the Permian extinction, the most important of the mass
extinctions in the history of the Earth.
Climate. The climate of the Permian was cooler and drier
than that of earlier periods of the Paleozoic, especially in the
center of the massive continent of Pangaea. Today, deserts
with extreme temperatures are found in the middle of the
Eurasian continent and were probably found in the middle of
Pangaea as well. Glaciation occurred in the southern portions
of Pangaea. Then global warming, caused initially by volcanic
eruptions, occurred near the end of the Permian period,
which contributed to massive extinctions.
Continents. All of the continents collided to form one
massive continent called Pangaea.
Marine life. Marine invertebrate life had reached its
greatest diversity to that time, with many species in all of the
phyla that are known from the fossil record. Some invertebrates
that are now rare, such as stalked echinoderms and
brachiopods, were common in the Permian. Reefs, with an
abundance of species, were common. The exception was the
trilobites, which had gradually declined from being the dominant
marine invertebrates from the Cambrian period onward.
Diversity was also high among the vertebrates. Many species
of fishes had evolved in both marine and freshwater environments,
including bony fishes, sharks, lungfishes, and coel-
acanths (see fishes, evolution of). There were no aquatic
reptiles or aquatic mammals.
Life on land. Forests covered much of the land surface:
• Plants. The tree-sized horsetails and club mosses that had
characterized the “coal swamps” of the Carboniferous
were close to extinction (see seedless plants, evolution
of), replaced by tree-sized ferns, seed-ferns (a type of plant
now extinct), and conifers (see gymnosperms, evolution
of). There were no flowering plants.
• Animals. The drier conditions encouraged the diversification
and spread of reptiles. There were two major groups
of reptiles: the synapsids, and the diapsids. There were no
dinosaurs, but some of the reptiles of the Permian were
rather large. None of these reptiles were of recognizably
modern groups; however, the diapsids were the ancestors
of most of the modern animals commonly called reptiles,
as well as extinct reptiles such as archosaurs and dinosaurs
(see reptiles, evolution of). The diapsids were also the
ancestors of birds. The synapsids are today extinct except
for one line of descendants: mammals. Some Permian synapsids
grew as large as hippos. The most famous Permian
synapsid was Dimetrodon, whose ribs formed a huge
“sail” along its back. Some synapsids had many mammal-like
characteristics; for example, unlike diapsids and
modern reptiles, they had several kinds of specialized teeth.
They may also have had whiskers: their skulls have small
nerve openings where these whiskers would have been. If
they had whiskers, they had hair, and therefore were probably
warm-blooded. The dissipation of body heat may have
been the main function of the sail along the back of Dimetrodon.
In other words, although these synapsids were not
mammals, they closely resembled them. A nearly complete
set of synapsid transitional forms has been found between
reptiles and mammals (see mammals, evolution of).
Extinctions. The Permian extinction brought many evolutionary
lineages to an end and severely reduced the abundance
of many others. Scientists will never know what might
have happened if evolution could have continued uninterrupted.
Would the synapsids have developed into the dominant
vertebrates, ushering in the age of mammals 250 million
rather than 65 million years ago? Would this have prevented
the evolution of the dinosaurs? These questions will never be
answered.
Further Reading
Kazlev, M. Alan. “Permian Period.” Available online. URL: http://
www.palaeos.com/Paleozoic/Permian/Permian.htm. Accessed May
2, 2005.
Museum of Paleontology, University of California, Berkeley. “Permian
Period.” Available online. URL: http://www.ucmp.berkeley.
edu/permian/permian.html. Accessed May 2, 2005.
phenotypic plasticity See adaptation.
photosynthesis, evolution of Photosynthesis is the process
by which some bacteria, some protists, and most plants