Evolution—The Theory and Its Supporting Evidence - The Changing Earth: Exploring Geology and Evolution - page 485

TABLE 18.3 Types of Fossil Preservation
Body fossils—unaltered remains Original composition and structure retained

Freezing Large Ice Age mammals in frozen ground (rare)

Mummifi cation Air drying and shriveling of soft tissues (rare)

Preservation in amber Insects in hardened tree resin

Preservation in tar Bones in asphalt-like substance at oil seeps (rare)

Body fossils—altered remains Change in composition and/or structure of original

Permineralization Addition of minerals to pores and cavities

Recrystallization Change in crystal structure—for example, aragonite recrystallized as calcite

Replacement One chemical compound replaces another—for example, pyrite replaces calcium carbonate;

silicon dioxide replaces wood

Carbonization Carbon fi lm of leaf, insect, and so on, when volatile elements are lost

Trace fossils Burrows, tracks, trails, nests, droppings (coprolites), or any other indication of organic activity

Molds and casts Mold, a cavity having the shape of a bone or shell; cast, a mold fi lled by minerals or sediment

The fossil record does show a sequence of different or-
ganisms, but not one based on density, size, shape, mobility, or
habitat. Rather, the sequence consists of fi rst appearances of
various groups of organisms through time (

If fossils actually provide evidence to support the theory
of evolution, we might ask, Where are the fossils, the so-called
missing links, that connect descendants and ancestors? Paleon-
tologists call these “missing links” transitional fossils to empha-
size the fact that they show characteristics of different groups,
and there are, in fact, many of them. For instance, Jurassic-age
fossils from Germany have anatomic features much like those
of small carnivorous dinosaurs, yet they have feathers and

Figure 18.17).
One-cell organisms appeared before multicelled ones, plants
before animals, and invertebrates before vertebrates. Among
vertebrates, fi sh appear fi rst, followed in succession by am-
phibians, reptiles, mammals, and birds.

◗

Birds Mammals

Cenozoic Era

Reptiles

Cretaceous

Bony
fishes

Jurassic

Triassic

Permian Amphibians

Pennsylvanian

Cartilagenous
fishes

Mississippian

Jawed
fishes

Devonian

Silurian

Jawless
fishes

Ordovician

Cambrian

◗

Figure 18.17 The Fossil Record and Evolution The spindles indicate the times of appearance and
the relative abundance of the members of each group. So, the jawless fi shes appeared fi rst and there
are far more species of fi shes than there are of amphibians, reptiles, birds, and mammals. A mass
extinction at the end of the Mesozoic Era is shown by the marked restriction of the reptile spindle.