background extinction
The average rate of low-level extinction that has occurred
continuously throughout much of evolutionary history.
cast
A type of fossil, formed when the decay of a body part leaves a void
that is then filled with minerals that later harden.
Cenozoic era
The most recent interval of geologic time, beginning 65.5 million
years ago, during which mammals became the dominant vertebrates and
angiosperms became the dominant plants.
cladistic approach
A method for constructing a phylogenetic tree that is based on
identifying the unique traits (shared, derived characters, called
synapomorphies) of each monophyletic group.
convergent evolution
The independent evolution of similar traits in distantly related
organisms due to adaptation to similar environments and a similar way
of life.
fauna
All the animal species characteristic of a particular region, period,
or environment.
homoplasy
(adjective: homoplastic) Similarity among organisms of different
species due to reasons other than common ancestry, such as convergent
evolution. Features that exhibit such similarity (e.g., the wings of
birds and bats) are said to be homoplastic, or convergent. Compare
with homology.
impact hypothesis
The hypothesis that a collision between the Earth and an asteroid
caused the mass extinction at the K�P boundary, 65 million years ago.
mass extinction
The extinction of a large number of diverse evolutionary groups
during a relatively short period of geologic time (about 1 million
years). May occur due to sudden and extraordinary environmental
changes. Compare with background extinction.
Mesozoic era
The interval of geologic time, from 251 million to 65.5 million years
ago, during which gymnosperms were the dominant plants and dinosaurs
the dominant vertebrates. Ended with extinction of the dinosaurs
(except birds).
monophyletic group
An evolutionary unit that includes an ancestral population and all of
its descendants but no others. Also called a clade or lineage. Compare
with paraphyletic group and polyphyletic group.
Paleozoic era
The interval of geologic time, from 542 million to 251 million years
ago, during which fungi, land plants, and animals first appeared and
diversified. Began with the Cambrian explosion and ended with the
extinction of many invertebrates and vertebrates at the end of the
Permian period.
paraphyletic group
A group that includes an ancestral population and some but not all of
its descendants. Compare with monophyletic group.
polyphyletic group
An unnatural group based on convergent (homoplastic) characteristics
that are not present in a common ancestor. Compare with monophyletic group.
Precambrian
The interval between the formation of the Earth, about 4.6 billion
years ago, and the appearance of most animal groups about 542 million
years ago. Unicellular organisms were dominant for most of this era,
and oxygen was virtually absent for the first 2 billion years