pacemaker Another name for the sinoatrial (SA)
node.
See alsoSA NODE.

paedogenesis The ability to reproduce while still in
immature or larval stage; acceleration of reproductive
ability.

paedomorphosis Retainment of ancestral juven-
ile characteristics in the adult form. Can occur
through neoteny or progenesis (acceleration of gonad
development).

paleontology The scientific study of past life forms,
both plant and animal, in their geological and paleoen-
vironmental context. It can be in the form of study of
fossils of organisms or their by-products. The field has
several subdisciplines, including micropaleontology,
paleobotany, palynology, invertebrate and vertebrate
paleontology, human paleontology or paleoanthropolo-
gy, taphonomy, and ichnology. James Hall (1811–98),
New York State’s first paleontologist, is considered the
father of American paleontology.
See alsoGEOLOGICAL TIME.

Pangaea The name given to the one huge landmass,
or supercontinent, that existed during the Permian (280

to 248 million years ago) through the JURASSIC PERIOD

(206 to 144 million years ago). Pangaea began to break

apartduring the Jurassic, forming two more large conti-

nents, GONDWANALANDand LAURASIA. Gondwanaland,

or Gondwana, formed the southern supercontinent, and

its remnants are now the continents South America,

Africa, India, Australia, and Antarctica. Laurasia, the

northern supercontinent, has as its remnants current

North America, Europe, Asia, Greenland, and Iceland.

paraphyletic Refers to a group of individuals

(taxon) that includes the most recent common ancestor

of all of its members, but does not include all of the

descendants of that most recent common ancestor.

parasite Any plant or animal that lives on or in

another organism, the host, to obtain its nutrients and

eventually harming or killing the host; only the parasite

benefits. Examples of parasites are ticks, fleas, trema-

todes, lice, Giardia lamblia, Sacculina,and plants such

as rafflesia and dodder (Cuscutasp.) Parasites can be

ectoparasites (living on the host, e.g., tick) or endopar-

asites (living within the host, e.g., tapeworm).

parasitism The symbiotic relationship between two

organisms where one species, the parasite, benefits, but

the other, the host, is harmed. If the parasite kills the

host it endangers its own survival.

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