Encyclopedia of Evolution.pdf - Online Reading Center

Cambrian period
occurred when animals began to evolve different kinds of
defenses as predators or against predators:
• Natural selection favored many different structures that
permitted predators to search for, catch, and chew their
prey.
• Some evolved hard external skeletons. In many cases, this
required the evolution of new body configurations. For
example, the bivalve body form (as in mussels and oysters)
could not function without hard shells. Hard shell defenses
also evolved in a wide variety of forms. Even single-celled
photosynthetic eukaryotes evolved hard and complex
external coverings.
• Some prey, then as now, have soft bodies but hide from
predators by burrowing into the mud. The complexity and
depth of animal burrows increased rapidly during the Cambrian
explosion.
As a result of these processes, the Cambrian world was
filled with diverse and complex animals, as exemplified by
faunas such as the Burgess shale and others. Animals with
internal skeletons did not evolve until the later part of the
Cambrian period (see fishes, evolution of).
Further Reading
Canfield, Don E., Simon W. Poulton, and Guy M. Narbonne. “Lateneoproterozoic
deep-ocean oxygenation and the rise of animal
life.” Science 315 (2007): 92–95.
Conway Morris, Simon. The Crucible of Creation: The Burgess Shale
and the Rise of Animals. New York: Oxford University Press,
1998.
Knoll, Andrew H. Life on a Young Planet: The First Three Billion
Years of Evolution on Earth. Princeton, N.J.: Princeton University
Press, 2003.
Rokas, Antonis, Dirk Krüger, and Sean B. Carroll. “Animal evolution
and the molecular signature of radiation compressed in time.”
Science 310 (2005): 1,933–1,938.
Cambrian period The Cambrian period (540 million to
510 million years ago) was the first period of the Phanerozoic
Eon and the Paleozoic era (see geological time
scale). During the previous eons, major evolutionary innovations
had occurred, such as the origin of prokaryotic cells, of
eukaryotic cells, and of multicellular organisms. At the beginning
of the Cambrian period, complex animal forms rapidly
evolved (see Cambrian explosion). This is why the Cambrian
period is considered the beginning of the Phanerozoic
Eon, or eon of “visible life.”
Climate. All Cambrian life lived underwater. Temperatures
are much more stable underwater than on land.
Continents. Because there were no continents located
near the poles, ocean currents were able to circulate freely. As
a result there was no significant formation of ice sheets.
Marine life. Most and perhaps all of the major animal
groups evolved before or during the Cambrian period. Fossil
deposits such as the Burgess shale preserve fossils of
many animal phyla. The groups of animals within these phyla
were different from the groups within those phyla today. For
example, the arthropods (see invertebrates, evolution of)
were represented by trilobites and animals such as Anomalocaris,
a large predator with a round mouth, both of which
have long been extinct. The first vertebrates, jawless fishes,
evolved during the Cambrian period. The 530-million-yearold
Chengjiang deposit in China contains two jawless vertebrate
fossils, Myllokunmingia and Haikouichthys (see fishes,
evolution of). Conodonts existed as far back as 540 million
years. These fishlike animals had long bodies, large eyes,
and conelike teeth, but no jaws. Single-celled and multicellular
photosynthetic protists (such as seaweeds) were the basis
of the marine food chain.
Life on land. There is no clear evidence that either animals
or plants existed on land during the Cambrian period.
The continents consisted of bare rock, sand, silt, and clay.
Extinctions. Although the Cambrian period is not considered
a time of mass extinctions, the Ediacaran organisms
and other Precambrian forms apparently became extinct during
this period. There were far fewer species in the Cambrian
period than in the following Ordovician period but, on a
percentage basis, Cambrian extinctions were considerable.
Further Reading
White, Toby, Renato Santos, et al. “The Cambrian.” Available online.
URL: http://www.palaeos.com/Paleozoic/Cambrian/Cambrian.htm.
Accessed March 23, 2005.
Carboniferous period The Carboniferous period (360
million to 290 million years ago) was the fifth period of the
Paleozoic era (see geological time scale). In North
America, the earlier Mississippian time, with extensive marine
limestone deposits, is distinguished from the later Pennsylvanian
time, with extensive coal deposits derived from terrestrial
forests, within the Carboniferous period.
Climate. In land that is now in the northern continents,
climatic conditions were very warm and wet, with shallow
lakes and seas. These conditions were perfect for the growth
of extensive swamps, from which coal deposits formed. This
is why most of the coal and oil deposits are found today in
the Northern Hemisphere. Much of the land that is now in
the southern continents had cold polar conditions. Glaciations
on the large southern continent caused intermittent
reductions in sea level during the Carboniferous period.
Continents. The land that is today the northern continents
(such as Europe and North America) formed the continent
of Laurasia, which was near the equator, creating warm
climates. The land that is today the southern continents (such
as Africa and South America) formed the continent of Gondwanaland,
much of which was over the South Pole. During
the later part of the Carboniferous, Laurasia collided with
Gondwanaland (see continental drift). The resulting geological
forces produced mountain ranges even in the middle
of tectonic plates, such as the Appalachian Mountains of
North America (see plate tectonics). The land that is now
Siberia collided with eastern Europe, creating what is now the
Ural Mountains. The worldwide ocean (Panthalassic Ocean)
was not divided by continents as today.