EXTRACCIÓN CON SOLVENTES

The Stenopterygii include the bristlemouths, small, luminescent filter feeders that live in the deep oceans and may be the world’s most abun-dant vertebrate. Lanternfishes (Scopelomorpha) use luminous lures to catch prey. Lizardfishes (Cyclosquamata) are well-camouflaged sit-and-wait predators who live on coral reefs. SEE ALSO Phylogenetic Relationships of Major Groups.

Gil G. Rosenthal

Bibliography

Moyle, Peter B. Fish: An Enthusiast’s Guide. Berkeley: University of California Press, 1993.

Moyle, Peter B., and Joseph J. Cech, Jr. Fishes: An Introduction to Ichthyology, 3rd ed.

Upper Saddle River, NJ: Prentice-Hall, 1996.

Nelson, Joseph S. Fishes of the World, 3rd ed. New York: John Wiley & Sons, 1994.

Paleontologist

Paleontologists study the history of life on Earth as shown in the fossil record. Fossils are the traces of organisms that lived in the past and are pre-served in Earth’s crust. Paleontology involves the identification and nam-ing of fossil species and organisms and the determination of the environment in which they lived. Paleontology is considered a subcategory of geology. It is a very broad science that uses biology, geology, chemistry, and physics.

There are many subdivisions in the field of paleontology, including:

• vertebrate paleontology, the study of fossils of animals with back-bones;

• invertebrate paleontology, the study of fossils of animals without backbones;

• micropaleontology, the study of fossils of single-celled organisms;

• paleobotany, the study of plant fossils;

• paleoecology, the study of ancient environments;

• biostratigraphy, the study of the fossils in rock layers from different areas to determine their relative ages.

As can be seen by this list, paleontology is more than just the study of dinosaurs. Modern paleontology attempts to understand life-forms as they are related to extended family trees, some of very ancient origins. Thus, pa-leontologists are frequently involved in studies of evolutionary biology and can be considered systematists, which means that they study the evolution-ary relationships among organisms.

Paleontologist

P

filter feeders animals that strain small food particles out of water

vertebrate an animal with a backbone invertebrate an animal without a backbone fossil record a collec-tion of all known fossils

Most paleontologists work in geology programs of colleges or universi-ties. They do research and teach classes. Smaller numbers of paleontologists work in museums. There, they carry out their own research and sometimes teach and assist with exhibits. A much smaller number of paleontologists work for government geological surveys. Until recently, paleontologists found work with oil companies, helping to search for oil. However, this field has declined as a source of employment for paleontologists.

Research in paleontology generally involves doing fieldwork, analyzing the fossils, and writing up one’s findings for publication and presentation.

Analysis of fossils begins with carefully measuring and describing them.

Next, the fossils are dated by various methods. Then the fossils and the rocks in which they were found are used to learn information about the history of Earth. Finally, the fossils are used to fill in missing information about the fossil record and are related to present-day organisms.

A paleontologist must have a doctoral (Ph.D.) degree. A bachelor’s de-gree can be obtained in either geology or biology. Graduate schools gener-ally require a full year of chemistry, physics, and mathematics (through calculus) at the undergraduate level. It is also important to have strong writ-ing and computer skills. After gettwrit-ing a bachelor’s, one can get a master’s and then a doctoral degree or, alternatively, enter a doctoral program di-rectly. If an individual has not had much experience with research in col-lege (such as writing a senior thesis), then it might be best to get a master’s degree first. It generally takes from two to three years to complete the mas-ter’s program. A Ph.D. program usually takes from four to six years if the candidate already has a master’s, and from six to eight years if he or she does not. The courses most important to paleontology include mineralogy, stratigraphy and sedimentation, sedimentary petrology, invertebrate pale-ontology, ecology, invertebrate and vertebrate zoology, evolutionary biol-ogy, and genetics. S E E A L S O Paleontology.

Denise Prendergast

A paleontologist cleans the remains of the jawbone of a Tyrannosaurus rex.

ecology study of how organisms interact with their environment

Bibliography

Cosgrove, Holli R., ed. Encyclopedia of Careers and Vocational Guidance, 11th ed.

Chicago: Ferguson Publishing Company, 2000.

The Year 2000 Grolier Multimedia Encyclopedia. Danbury, CT: Grolier Interactive Inc., 1999.

Internet Resources

Paleontological Research Institution. Cornell University.
http://www.priweb.org.

Paleontology

Paleontology is the study of the history of life as revealed in the fossil record. Fossils are remnants or traces of living organisms from past geo-logic ages that have become preserved in Earth’s crust. They include not only the skeletons or shells of deceased creatures, but also burrows, foot-prints, eggs, and fossilized feces (excrement), known as coprolites.

Paleontology draws extensively from both biology and geology. Some subdisciplines of paleontology are defined by the types of organisms that are studied. Examples are vertebrate paleontology, invertebrate paleontology, paleobotany, and micropaleontology (study of single-celled fossils). Paleoe-cologists study extinct ecosystems. Related areas include biostratigraphy, the study of fossil distributions in different strata (rock layers), and taphon-omy, which examines the process of fossil formation. Biological disciplines in which contributions of paleontology are particularly critical include sys-tematics and taxonomy. They focus on determining phylogenetic rela-tionships (the sequence of branching events in evolutionary history which have resulted in the production of divergent species) between extinct as well as extant organisms. Another such discipline is comparative anatomy, which examines the morphology (form) and structure of organisms. Still another is evolutionary biology, which examines how biological organisms change over time.

The study of the fossil record also permits the identification of periods of major change in biological diversity. Sudden shifts in flora and fauna re-sult from major events involving the extinction of organisms, such as the one that eliminated the dinosaurs at the end of the Cretaceous period. In fact, geological eras are bounded by these sudden changes.

Taphonomy examines the processes by which fossils are formed. Any event that occurs between the death of an organism and its fossilization is of interest to taphonomists. The first step to fossilization is burial. Burial can occur in a number of ways; corpses may be buried by sediments in rivers, by sand, or in the bottoms of lakes or oceans. After burial, corpses may be compressed and distorted by the surrounding sediment. There is also a lengthy period of remineralization following burial. During this time, bone is replaced by minerals carried through the rock by water. Remineralization does not necessarily obscure fine detail because the replacement occurs on a minute scale.

During the process of fossilization, much information about the biol-ogy of organisms is lost. Damage to the corpse, either by scavengers or from weather or erosion, may occur prior to burial, and distortion from a Paleontology

fossil record a collec-tion of all known fossils

vertebrate an animal of species or group of related species

number of sources can occur afterwards. Soft parts of organisms are fos-silized much less frequently than hard parts, and information on color, phys-iology, or behavior is particularly likely to be lost. It is because of the incompleteness of most fossils that paleontologists have developed a well-deserved reputation for inferring (deducing) huge amounts of information on the biology of organisms from fragmentary, or partial, remains.

The proper dating of fossil material is often critical to paleontological studies. Relative dating considers the relative placement of different rock strata; younger rock layers are formed on top of older layers. Also, similar sequences of strata that are found in different locations are likely to date from the same period. Absolute dates for fossil material are usually estimated using radioisotopes. This method makes use of the fact that radioactive atoms decay into more stable atoms at a known rate. S E E A L S O Fossil Record; Geological Time Scale; Paleontologist.

Jennifer Yeh

Bibliography

Futuyma, Douglas J. Evolutionary Biology, 3rd ed. Sunderland, MA: Sinauer Associ-ates, 1998.

Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W.

W. Norton and Co., 1996.

Parasitism

Parasitism describes a relationship between two species, a parasite and its host, in which the parasite benefits, while the host is harmed. Parasitism is one form of symbiosis, which more generally describes any situation in-volving a close relationship between organisms of different species.

Parasites are different from predators and parasitoids (which also derive benefits from certain interspecific interactions while harming the other ticipant) in that the host of a parasite is not necessarily killed. Instead, par-asites derive benefits from their hosts, most often nutritional resources and shelter, over a longer period of time. It is in fact advantageous to parasites if they do not harm their hosts too badly, because that prolongs the period during which parasites can obtain benefits from hosts. However, in some cases, the impact of parasites on a host is great enough to cause disease, and in extreme cases, the death of the host may also occur.

Parasitism is a common survival strategy among biological organisms, and many species are characterized by parasitic lifestyles for all or part of their lives. All the major kingdoms of life include some parasitic species. In addition, there are very few biological species that are free of parasites al-together.