Glossary

Adaptive radiation. The relatively abrupt emergence of taxonomic diversity over a geologically short period of time.

Amniotic egg. The main synapomorphy of the amniotes (i.e., reptiles, birds, and mammals). The amniotic egg provides yolk to the embryo and includes the amnion and allantois, two membranes that reduce dehydration.

Apomorphy. Derived trait.

Bilateral symmetry. Organisms that exhibit an anterior-posterior axis and paired bodily parts (e.g., humans).

Body plan. A basic set of homologous structural traits common to all the species in a particular phylum.

Burgess Shale fauna. Fossil fauna from the mid-Cambrian (about 520 Ma) first discovered at Burgess Shale, Canada, and now known to have worldwide distribution. Specimens attributable to several living phyla have been clearly identified.

Cambrian explosion. The fossil record revealed a large number of species dated to the Cambrian period (544—520 Ma), including arthropods, mollusks, and equinoderms, among many others. Soft parts are preserved in extraordinary detail.

Clade. A monophyletic group of lineages.

Continental drift. The movement of continental masses that rest on rigid tectonic plates. The plates float on a layer of melted rock called the mantle, approximately 100 km beneath the earth’s surface. They move slowly, changing places, colliding with one another, and modifying the distribution of land masses.

Divergence. An evolutionary outcome representing the split of taxa from a common ancestor. Divergent evolution results in the dissimilarity of homologous traits.

DNA-DNA hybridization. A technique used to develop phylogenies at the taxonomic level of order or family.

Ectothermy. The fluctuation of an animal’s internal temperature with that of the environment. Ecto-thermy is characteristic of lower vertebrates (e.g., amphibians and reptiles).

Ediacaran fauna. A fossil assemblage first discovered in the Ediacara Hills of southern Australia and now known to have worldwide distribution. The oldest Ediacaran fossils are estimated to be 555 Ma. Whereas the animal status of some fossils is disputed, others appear to correspond to cnidarian animals similar to extant jellyfish and sea anemones.

Endothermy. Ability to maintain a constant internal temperature. Endothermy is characteristic of higher vertebrates (i.e., birds and mammals).

Eukaryotes. Unicellular (protists) or multicellular (animals, plants, and fungi) organisms with cells possessing a nuclear membrane surrounding several distinct chromosomes, many cytoplasmatic organelles (ribosomes, mitochondria, chloroplasts, etc.), and a relatively large in size (ranging between 10 and 100 |im).

Fossil record. The evidence of past life forms derived from the study of rocks.

Fossil traces. The fossilized remains of animal tracks. Fossil traces allow for inferences regarding morphological and behavioral features of the animals that made them.

Gastrulation. A stage in embryonic development involving the formation of an internal cavity from a hollow ball of cells.

Grade. Sharing of similar traits by taxa that have evolved them independently

Labyrinthine tooth. A tooth structure characterized by the folding of the enamel, typical of lobe-fin fish and early tetrapods.

Mass extinction. The disappearance of lineages in the fossil record during a relatively brief geological period.

Mesoderm. Intermediate layer of cells present in all the animals that exhibit bilateral symmetry.

Mitochondrial DNA (mtDNA). DNA material present in mitochondria—cell organelles involved in the production of energy.

Molecular clock. The hypothesis that the date of divergence between two taxa can be estimated as a function of the degree of similarity in protein or DNA structure. The molecular clock hypothesis is based on the assumption that rates of change are constant, which may not apply to all cases.

Monophyletic clade. A group of lineages stemming from a common ancestor.

Myotomes. Paired muscles involved in swimming in chordates.

Notochord. A dorsal structure characteristic of chordates. A precursor of the vertebrate column.

Pentadactyl hand. A hand with five digits. A basal trait of tetrapods.

Phylogenetic tree. A graphical representation of genealogical relationships among taxa.

Plesiomorphy. Basal traits.

Prokaryotes. Unicellular (monera) organisms characterized by the lack of a nuclear membrane and a relatively small size (less than 10 pm).

Radial symmetry. A body organized around one (or more than one) central axis (e.g., jellyfish). Relative brain size. The size of the brain relative to body size.

Ribosomal RNA (rRNA). Sequential analysis of rRNA demonstrates a very low rate of change that has been used to develop phylogenies of distantly related taxa (e.g., phyla).

Sister groups. Taxa with an immediate common ancestor.

Speciation, allopatric and sympatric. Evolution of a new species. There are two major speciation scenarios. Allopatric speciation involves the geographic isolation of previously interbreeding populations for a number of generations sufficient to result in reproductive isolation. Islands, lakes, a mountain range, and changes in continental land masses are some of the possible geographical barriers that lead to allopatric speciation. Sympatric speciation involves the emergence of reproductive isolation within subpopulations of animals occupying the same habitat. Mutations affecting species recognition, changes in resource exploitation, and similar factors may induce sympatric speciation.

Subspecies. Subpopulations that are partially isolated from parental populations.

Synapomorphy. Derived characters shared by members of a taxon and absent in the taxon’s outgroup.

Taxon (plural: Taxa). A unit of taxonomic analysis.

Tetrapods. A vertebrate taxon characterized by four limbs and including amphibians, reptiles, birds, and mammals.

Tooth replacement. In reptiles, tooth replacement is a continuous process. By contrast, mammalian teeth are replaced only once (milk teeth to adult teeth). These differences provide clues as to the identity of fossils in the therapsid-mammal lineage.

Trait similarity. The sharing of traits across species.

Tribosphenic molar. A molar characterized by the presence of three main cusps, a variable number of accessory cusps, and crests with sharp edges. It first appeared in therian mammals (the ancestors of living marsupials and placentals).