Summary
The amniotes are distinguished from amphibians by the presence of a terrestrially adapted egg protected by amniotic membranes. The amniotes include reptiles, birds, and mammals. The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids (mammals) and sauropsids. Sauropsids can be further divided into anapsids (turtles) and diapsids (birds and reptiles). Reptiles are tetrapods either having four limbs or descending from such. Limbless reptiles (snakes) are classified as tetrapods, as they are descended from four-limbed organisms. One of the key adaptations that permitted reptiles to live on land was the development of scaly skin containing the protein keratin, which prevented water loss from the skin. Reptilia includes four living clades: Crocodilia (crocodiles and alligators), Sphenodontia (tuataras), Squamata (lizards and snakes), and Testudines (turtles).
Glossary
amniote
animal that produces a terrestrially adapted egg protected by amniotic membranes
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
anapsid
animal having no temporal fenestrae in the cranium
archosaur
modern crocodilian or bird, or an extinct pterosaur or dinosaur
brumation
period of much reduced metabolism and torpor that occurs in any ectotherm in cold weather
Casineria
one of the oldest known amniotes; had both amphibian and reptilian characteristics
chorion
membrane of the egg that surrounds the embryo and yolk sac
Crocodilia
crocodiles and alligators
diapsid
animal having two temporal fenestrae in the cranium
Hylonomus
one of the earliest reptiles
lepidosaur
modern lizards, snakes, and tuataras
sauropsid
reptile or bird
Sphenodontia
clade of tuataras
Squamata
clade of lizards and snakes
synapsid
mammal having one temporal fenestra
temporal fenestra
non-orbital opening in the skull that may allow muscles to expand and lengthen
Testudines
order of turtles