litoptern, (order Litopterna), any of various extinct hoofed mammals that first appeared in the Paleocene Epoch (which began about 65.5 million years ago) and died out during the Pleistocene Epoch (which ended about 11,700 years ago). The order was restricted to South America, but in many ways, the evolution of the litopterns paralleled that of hoofed mammals in the Northern Hemisphere. Two distinct lineages of litoptern evolution are discernible in the fossil record.
One line of litopterns, the proterotheres, strongly resembled horses. Their limbs were modified for running and also had special features for locking their knees, allowing them to stand for long periods of time. The proterothere skull was long and low and contained cheek teeth resembling those of deer. Proterotheres became extinct in the Pliocene Epoch (5.3–2.6 million years ago), about the time that true horses appeared in South America.
The other litoptern group, the macrauchenids, resembled camels. The nasal opening was set high on the skull, which probably supported a short proboscis, or trunk. Some of the macrauchenids survived the intrusion of more advanced mammals from North America and persisted well into the Pleistocene Epoch.
fossil record, history of life as documented by fossils, the remains or imprints of organisms from earlier geological periods preserved in sedimentary rock. In a few cases the original substance of the hard parts of the organism is preserved, but more often the original components have been replaced by minerals deposited from water seeping through the rock. Occasionally the original material is simply removed while nothing is deposited in its place; in this case, all that remains is a mould of the shape of the plant or animal. A brief treatment of the fossil record follows. For full treatment, see geochronology.
In some places, such as the Grand Canyon in Arizona, it is possible to recognize a great thickness of nearly horizontal strata representing the deposition of sediment on the seafloor over many hundreds of millions of years. It is often observed that each layer in such a sequence contains fossils that are distinct from those of the layers that are above and below it. In such sequences of layers in different places, the same, or similar, fossil floras or faunas occur in the identical order. By comparison of overlapping sequences, it is possible to build up a continuous record of faunas or floras that have progressively more in common with present-day life-forms as the top of the sequence is approached.
Study of the fossil record has provided important information for at least three different purposes. The progressive changes observed within an animal group are used to describe the evolution of that group. In general, but not always, successive generations tend to change morphologically in a particular direction (e.g., the progressive acquisition or loss of specific features), and these changes are often interpreted as better adaptation (through preferential selection of beneficialmutations) to a particular environment.
Fossils also provide the geologist a quick and easy way of assigning an age to the strata in which they occur. The precision with which this may be done in any particular case depends on the nature and abundance of the fauna: some fossil groups were deposited during much longer time intervals than others.
fossil-containing strataFossils help geologists establish the ages of layers of rock. In this diagram, sections A and B represent rock layers 200 miles (320 km) apart. Their ages can be established by comparing the fossils in each layer.
Fossil organisms, furthermore, may provide useful information about the climate and environment of the site where they were deposited and preserved. Certain species of coral, for example, require warm shallow water; certain plants require warm swampy conditions such as are found today in the FloridaEverglades, South America’sPantanal, and Botswana’s Okavango delta. Thus, when rocks containing fossils of this kind are found in rocks of the present-day polar regions, there is a strong presumption that the crust on which they were deposited has shifted its position on the surface of Earth since that time.
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