Ward's World Activity Guides

Paleontology (continued) + ward ' s science The term prehistoric is also a key part of the definition of fos- sils. Remains of recent animals and plants are not considered as fossils simply because they are not old enough. Paleontologists commonly use, as an arbitrary definition of "prehistoric," the approximate end of the last glacial advance, about 10,000 years before the present (BP). In some cases, ancient organism re- mains that are somewhat less than 10,000 years old are termed "subfossils." Notably, artifacts made by people are not termed fossils; instead, these artifacts constitute the data of the related science of archeology, the study of human civilizations. The fossil record thus includes a very diverse class of objects ranging from molds of microscopic bacteria in rocks more than 3 × 109 years old to unaltered bones of fossil humans in ice-age gravel beds formed only a few thousand years ago. The quality of preservation ranges from the occasional occurrence of soft parts (skin and feathers, for example) to barely decipherable impressions made by shells in soft mud that later hardened to rock. The most common fossils are hard parts of various animal groups. Therefore, the fossil record is not an accurate account of the complete spectrum of ancient life, but is biased in overrepresenting those forms with shells or skeletons. Fos- silized worms are extremely rare, but it is not valid to make the supposition that worms were any less common in the geologic past than they are now. Applications of paleontology Paleontology lies on the boundary between two disci- plines—biology and geology. The field of paleontology has been subdivided in various ways. For example, a distinction can be made between paleozoology (invertebrate and ver- tebrate paleontology) and paleobotany (the study of fossil plants and algae). Micropaleontology deals with those fossils that must be studied with microscopes, including compound light and scanning electron microscopes. This field unites a highly varied series of fossil groups, including shelled protists such as radiolaria and foraminifera, tiny skeletons of planktonic diatoms, and coccoliths (the latter two sometimes referred to as nannofossils in recognition of their extremely small size), as well as conodonts (minute toothlike fossils that formed parts of the feeding apparatuses in early eel-like chordates). Because of their tiny size and great abundance in small samples, microfos- sils have traditionally been the most economically valuable fos- sils; in fact, many micropaleontologists have been employed by petroleum companies to aid in the identification of particular stratigraphic levels, including those bearing petroleum. Most invertebrate paleontologists are employed in geology depart- ments, where they are involved in a wide array of stratigraphic, paleobiological, and paleoenvironmental studies. Conversely, many vertebrate paleontologists are allied with biology or medical departments because of their studies of vertebrate skeletal anatomy and evolution. Paleobotanists are traditionally plant anatomists, employed in botany departments. Palynology involves the study of organic-walled microfossils, such as pollen and spores, but also including enigmatic marine microfossils such as acritarchs (algal resting cysts of uncertain affinities) and chitinozoans (possible egg capsules of Paleozoic organisms). However, much modern paleontology defies simple categori- zation and is more commonly classified along lines reflecting application of fossil data in various disciplines; these include taxonomy/systematics, evolutionary studies, biostratigraphy, taphonomy, ichnology (the study of trace fossils), and paleo- ecology/paleoenvironmental studies (Fig. 2) Systematics and taxonomy The classification of fossil species into a hierarchical group- ing of taxa is usually based on similarities in morphological characters. Many thousands of fossil species have been de- scribed and hundreds more are discovered and described each year; it has been estimated that the known fossil record comprises only 1–5% of the total number of easily preservable organisms that once lived on Earth. Beyond the very necessary, if mundane, description, illustration, and naming of new spe- cies lies the more challenging exercise of arranging these spe- cies into a classification that reflects evolutionary relationships Fig. 2: Some of the divisions of the field of paleontology. (Credit: Adapted from an original drawing by E. N. K. Clarkson)

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