Paleo-Tethys Ocean -- n. A large ocean that originated between eastern Gondwana, Siberia, Kazakhstan, and Baltica in the Ordovician and finally closed in the Jurassic; replaced by the Tethys Ocean as eastern Pangea was assembled.
Pangea -- n. A supercontinent that existed from the the end of the Permian to the Jurassic, assembled from large continents like Euramerica, Gondwana, and Siberia, as well as smaller landmasses like the Cathaysian and Cimmerian terranes; Greek for ďall lands.Ē
Panthalassic Ocean -- n. A vast ocean that existed from the Late Precambrian to the Jurassic, circling the globe and connecting to smaller oceans that developed throughout the Phanerozoic; also known as the Panthalassa.
paraphyletic -- Term applied to a group of organisms which includes the most recent common ancestor of all of its members, but not all of the descendants of that most recent common ancestor. More?
parasite -- n. An organism that lives on or within a host (another organism); it obtains nutrients from the host without benefiting or killing (although it may damage) the host; parasitic- adj.; parasitism- n. a type of symbiotic relationship in which one organism benefits and the other does not.
parataxonomy -- n. The grouping of organisms based on common morphology; does not imply evolutionary relationships. For example, fossil eggs can be assigned to parataxons without any knowledge of which specific organism produced them.
pedipalps -- The second pair of appendages of cheliceromorphs. In many arachnids, such as spiders, the pedipalps are enlarged in the male and used for copulation.
peptidoglycan -- carbohydrate polymer cross-linked by proteins. It is found in the cell wall of Gram positive bacteria, where it stains with the dye crystal-violet.
perianth -- The sepals and petals of a flower are together called the perianth; literally "around the anthers". More info?
periphyton -- Dense strands of algal growth that cover the water surface between the emergant aquatic plants. Spirogyra is commonly responsible for this growth.
petal -- One of the outer appendages of a flower, located between the outer sepals and the stamens. Petals often display bright colors that serve to attract pollinators. More info?
pharyngeal slits -- Characteristic of chordates, pharyngeal slits are openings through which water is taken into the pharynx, or throat. In primitive chordates the pharyngeal slits are used to strain water and filter out food particles; in fishes they are modified for respiration. Most terrestrial vertebrates have pharyngeal slits only in the embryonic stage.
phylogenetics -- Field of biology that deals with the relationships between organisms. It includes the discovery of these relationships, and the study of the causes behind this pattern.More?
phytomelanin -- a papery "sooty" black layer over the seed of plants in the Asparagales, which includes agaves, aloes, onions and hyacinths. It is an important character for defining the group.
pigment -- any colorful compound, used by living things to absorb or block sunlight, and in sexual displays. More info?
pistil -- The central set of organs in a flower; it is composed of one or more carpels. More info?
placenta -- n. In mammals, a tissue formed within the uterus through which nutrients are passed from the mother to the embryo (and later the fetus) and its wastes are removed. It is analogous to the protective membranes in the egg of other amniotes. placental n. A mammal that gives live birth to well-developed young that have prolonged embryonic development within the motherís uterus. Marsupial mammals also have a placenta, but the embryo spends less time developing in the uterus before birth. Placentals include animals as diverse as humans, elephants, dogs, and mice.
plankton -- n. Very small, free-floating organisms of the ocean or other aquatic systems, including phytoplankton, which produce their own nutrients through photosynthesis, or zooplankton, which get their nutrients from organisms.
plate -- n. Rigid parts of the Earth's crust and part of the Earth's upper mantle that move and adjoin each other along zones of seismic activity. The theory that the crust and part of the mantle are divided into plates that interact with each other causing seismic and tecotnic activity is called plate tectonics.
polarity of characters -- The states of characters used in a cladistic analysis, either original or derived.Original characters are those acquired by an ancestor deeper in the phylogeny than the most recent common ancestor of the taxa under consideration. Derived characters are those acquired by the most recent common ancestor of the taxa under consideration.
pollen -- The microspore of seed plants.
pollen tube -- In seed plants, the extension of the male gametophyte as it emerges from the pollen grain in search of the female gametophyte.
pollination -- Process of transferring the pollen from its place of production to the place where the egg cell is produced. This may be accomplished by the use of wind, water, insects, birds, bats, or other means. Pollination is usually followed by fertilization, in which sperm are released from the pollen grain to unite with the egg cell.
pollinator -- Animal which carries pollen from one seed plant to another, unwittingly aiding the plant in its reproduction. Common pollinators include insects, especially bees, butterflies, and moths, birds, and bats.
pollinia -- A mass of fused pollen produced by many orchids.
polyphyletic -- Term applied to a group of organisms which does not include the most recent common ancestor of those organisms; the ancestor does not possess the character shared by members of the group. More?
precocial -- adj. Describes young that are mobile and fairly self-sufficient at birth. Precocial young are generally well-developed (and large) at birth, born with their eyes open, and able to walk. Chickens and grebes are examples of precocial birds; sheep and guinea pigs are examples of precocial mammals.
primitive -- adj. Refers to a character or feature in a group of organisms that is inherited from a common ancestor. Teeth are a primitive characteristic of birds since birds inherited them from their dinosaur ancestors.
proboscis -- Elongated organ, usually associated with the mouth. The proboscis is an important feeding appendage in echiurans.
producer -- Any organism which brings energy into an ecosystem from inorganic sources. Most plants and many protists are producers.
pseudoelaters -- Moisture-sensitive cells produced in the sporangium of hornworts.
pseudoextinction -- The apparent disappearance of a taxon. In cases of pseudoextinction, this disappearance is not due to the death of all members, but the evolution of novel features in one or more lineages, so that the new clades are not recognized as belonging to the paraphyletic ancestral group, whose members have ceased to exist. The Dinosauria, if defined so as to exclude the birds, is an example of a group that has undergone pseudoextinction.
pteridophyte -- Plant in which the sporophyte generation is the larger phase and in which the gametophyte lives an existence independent of its parent sporophyte. Pteridophytes are almost all vascular plants, and include the lycophytes, trimerophytes, sphenophytes, and ferns.
pteridosperm -- An extinct group of seed plants which bore fern-like leaves.