Pleistocene

Pleistocene

This most recent sequence of geologic time is somewhat complicated to describe in terms of animal science since there is much more information available from the fossil record. Scientifically, it is described as the period of time from 1.9 million year ago to 10,000 years ago. It is identified with a noticeable change in the animal fossils, which usually indicates some kind of extinction or massive change in the environment. It is difficult to summarize what happened from region to region since there was as much climatic variety then as there is today. However, the general consensus among scientists is that the beginning of the Pleistocene epoch began with an overall global cooling. This cooling was significant in that many cold-intolerant species disappeared and some new more resistant species appear in the fossil record.

Every geologic time period is defined by what scientists call a type section. A type section is a place that is considered to be the first discovered well-defined area in which evidence of a time-period shift, or difference between plant and animal communities, can be observed. In short, it is the first discovery of some important geological event characterized by a change in the kinds of species and populations of plant and animal fossils.

The type section for the Pleistocene was first proposed in 1839 by British geologist Charles Lyell after he examined a sequence (one of many layers) of rocks in southern Italy. He noticed that within and between the layers of rock, there was a distinct change between fossils of marine mollusks of warm-water species to fossils of species which were similar to modern cold-water species. After further investigation it was determined that this new set of geologic strata contained almost 70 percent living or historical species. Later studies in Europe by other geologists revealed that glaciation had occurred at about the same time as the strata in Italy were deposited. Eventually researchers pieced together evidence that indicated the Pleistocene was a time of great global cooling. During the epoch, immense glaciers and ice sheets occurred at the North and South Poles and at all high altitudes.

The Pleisocene was a relatively short span of geologic time that fluctuated between episodes of warming and cooling, but the general climate was very cold for much of the seas and regions of the continents.

The Pleistocene cooling had a tremendous effect on animal life on Earth; faunas , or ecological populations of animals, were severely disrupted or eliminated altogether. Some species became extinct, while others flourished. Many new species have been identified as occurring around this time change and after. New species appeared both on land and at sea. As the ice sheets bound up more and more water, sea levels dropped. Land bridges appeared from beneath the sea, the most famous of which were the Bering land bridge and the land bridge between North and South America. Waves of animal migrations occurred on the continents.

Animal Migrations to the Americas

Before the Pleistocene, North and South America contained their own distinctive sets of animals. Marsupials abounded in South America and the horse flourished in North America. With the emergence of the Panamanian land bridge between the two Americas, a great migration, or swapping of animal species, began. Marsupials (mammals with no placenta but with a pouch in which their young develop), sloths, and other animals such as glyptodonts, which looked like an armadillo, headed north. The proboscideans—including a group called the gomphotheres, with elongated lower jaws that looked like shovels—as well as mammoths, and mastodons, moved south. Relatives of modern horses, lions, camels, and wolves migrated after the gomphotheres.

The Bering land bridge that connects Russia and Alaska supported the invasion from Asia of animals such as the mammoth, deer and their relatives, and bison to the Americas. Perhaps the most influential animal to come across the land bridge was another mammal, Homo sapiens.

Animal Adaptations to Climate

Animals were also on the move in other parts of the world. The mammoths continued to migrate over Europe and Asia. The woolly mammoth developed a thick fur and began to graze in the spruce forests that bordered the ice. The rhinoceros also moved into Europe and central Asia and developed a coat of thick fur for surviving in the cold conditions. Its front horn grew to extreme lengths, reaching nearly a meter, and some researchers have suggested that legends describing the survivors of this species may have led to the myth of the unicorn. The massive and dangerous archaeocyonids, or bear dogs, were enormous predators whose bones are still found in caves today. In Europe, Panthera leo spelaea, a large species of cave lion, roamed the mountains in search of bison and other prey. In North America, Smilodon, the saber-toothed cats, traveled over the more warm and savanna-like regions of what is now the southwest United States.

Pleistocene Extinctions

Terrestrial invertebrates flourished and died with the fluctuating climate. Records of species of snails show scientists how the climate cooled and warmed throughout the period. Scientists identify these changes by the silt deposits left by the advancing and retreating glaciers in the North. They also use tree ring thicknesses (a part of dendrochronology) to determine periods of dry or wet years. The best data for calculating oxygen and carbon dioxide isotopes (different numbers of electrons) come from ice cores in Greenland. Small, single-celled marine animals called foraminifera were able to secrete specialized shells. These small eukaryotes are extremely sensitive to temperature change and their tiny fossils leave an excellent record of shifting climate for paleontologists to observe. The records of these animals, found in mud recovered from oil wells off coastal waters read like a book of temperature fluctuations. When the water is warm a certain species will abound. They die and sink to the mud where they are fossilized. When the water is colder other species survive. These also leave their fossils in the mud. An expert can read the sequence of fossils in the mud.

Curiously, amphibians and reptiles of the Pleistocene did not suffer the extinctions that befell the mammals. Since these animals appear to be very sensitive to climate today, it was assumed they would be affected by changes in climate during the ice ages. Apparently they were not, and the fossils of these animals did not change over time either in species or abundance. Their geographic distribution may have changed as climates fluctuated, but there are very few known extinctions of species. Birds also managed to survive. Most of the birds that disappeared did so as a result of human interference in recent times. The great moa of New Zealand was hunted to extinction in the Holocene and is not considered a Pleistocene casualty.

The Pleistocene is famous for its extinctions rather than for its migrations. Some researchers believe the extinction event is not over and point to the increasing list of endangered species throughout the world. Scientists are still not sure what caused the Pleistocene extinctions. Some hypothesize that many species could simply not tolerate the continuous climate fluctuations, others that temperatures were too cold. This is called the "Overchill Hypothesis." Other scientists note that wherever evidence of human migration is discovered, the large animals, or megafauna, disappear. These scientists believe that humans overhunted, and although not all the bison, deer, mammoths, and other large herbivores were killed for food, their disappearance led to the starvation of predators that relied on these animals. This is the "Overkill Hypothesis." Both hypotheses have merit, but still raise many questions. We may never know what caused the Pleistocene extinctions, but today, loss of habitat and increasing pollution are the most lethal killers of animal life on Earth.

see also Phylogenetic Relationships of Major Groups.

Brook Ellen Hall

Bibliography

Carroll, Robert. Vertebrate Paleontology and Evolution. New York: W. H. Freeman and Company, 1988.

Macdonald, D. The Encyclopedia of Mammals. New York: Facts on File Publications, 1987.

Martin, Paul, and Richard Klein. Quaternary Extinctions: A Prehistoric Revolution. Tucson, University of Arizona Press, 1989.

Woodburne, M. Cenozoic Mammals of North America. Berkeley: University of California Press, 1987.

Sir Charles Lyell (1797-1875) was a British geologist who opposed the catastrophic theory advanced at the time to account for great geologic changes. A proponent of uniformitarianism, Lyell is considered the father of modern geology.