- Acanthostega
- one of the earliest known tetrapods
- Actinopterygii
- ray-finned fishes
- allantois
- membrane of the egg that stores nitrogenous wastes produced by the embryo; also facilitates respiration
- amnion
- membrane of the egg that protects the embryo from mechanical shock and prevents dehydration
- amniote
- animal that produces a terrestrially adapted egg protected by amniotic membranes
- Amphibia
- frogs, salamanders, and caecilians
- ampulla of Lorenzini
- sensory organ that allows sharks to detect electromagnetic fields produced by living things
- anapsid
- animal having no temporal fenestrae in the cranium
- anthropoid
- monkeys, apes, and humans
- Anura
- frogs
- apocrine gland
- scent gland that secretes substances that are used for chemical communication
- Apoda
- caecilians
- Archaeopteryx
- transition species from dinosaur to bird from the Jurassic period
- archosaur
- modern crocodilian or bird, or an extinct pterosaur or dinosaur
- Australopithecus
- genus of hominins that evolved in eastern Africa approximately four million years ago
- brachiation
- movement through trees branches via suspension from the arms
- brumation
- period of much reduced metabolism and torpor that occurs in any ectotherm in cold weather
- caecilian
- legless amphibian that belongs to the clade Apoda
- Casineria
- one of the oldest known amniotes; had both amphibian and reptilian characteristics
- Catarrhini
- clade of Old World monkeys
- Cephalochordata
- chordate clade whose members possess a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail in the adult stage
- Chondrichthyes
- jawed fish with paired fins and a skeleton made of cartilage
- Chordata
- phylum of animals distinguished by their possession of a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some point during their development
- chorion
- membrane of the egg that surrounds the embryo and yolk sac
- contour feather
- feather that creates an aerodynamic surface for efficient flight
- Craniata
- clade composed of chordates that possess a cranium; includes Vertebrata together with hagfishes
- cranium
- bony, cartilaginous, or fibrous structure surrounding the brain, jaw, and facial bones
- Crocodilia
- crocodiles and alligators
- cutaneous respiration
- gas exchange through the skin
- dentary
- single bone that comprises the lower jaw of mammals
- diapsid
- animal having two temporal fenestrae in the cranium
- diphyodont
- refers to the possession of two sets of teeth in a lifetime
- dorsal hollow nerve cord
- hollow, tubular structure derived from ectoderm, which is located dorsal to the notochord in chordates
- down feather
- feather specialized for insulation
- eccrine gland
- sweat gland
- Enantiornithes
- dominant bird group during the Cretaceous period
- eutherian mammal
- mammal that possesses a complex placenta, which connects a fetus to the mother; sometimes called placental mammals
- flight feather
- feather specialize