Sauropsida

Sauropsida is the amniote lineage that includes reptiles, birds, and their extinct relatives, but excludes mammals. Living sauropsids range from turtles and lizards to crocodilians and birds.

Amniota: Amniota is the vertebrate clade whose embryos develop within an amnion or related extraembryonic membranes. Sauropsida is one of the two major living amniote lineages.
Cladistics: Cladistics classifies organisms by shared derived traits and represents their hypothesized evolutionary relationships as clades. Sauropsida is defined through inferred common ancestry rather than a fixed list of familiar animals.
Carboniferous: The Carboniferous is a geological period spanning roughly 359 to 299 million years ago. The earliest known amniotes appeared during this period, before sauropsid and synapsid lineages were clearly distinct.
Turtles: Turtles are reptiles characterized by a bony or cartilaginous shell formed from expanded ribs and other skeletal elements. Their distinctive body plan belongs to a deeply branching living sauropsid lineage.
Turtle phylogeny: Turtle phylogeny is the study of evolutionary relationships among turtles and their ancestors. Evidence has shifted turtles from an uncertain position toward placement within diapsid reptiles.
Synapsida: Synapsida is the amniote lineage that includes mammals and their extinct relatives. It is Sauropsida’s sister lineage, distinguished by a separate evolutionary history.
Amniotic egg: The amniotic egg is a reproductive structure whose membranes support an embryo in a fluid-filled environment. The amniotic egg marks the larger evolutionary transition shared by sauropsids and mammals.
Hylonomus: Hylonomus is an extinct small reptile from the Late Carboniferous of Canada. Its fossils are among the oldest widely accepted records of a reptile-grade amniote.
Crocodilia: Crocodilia is the order of living crocodiles, alligators, caimans, and gharials. Crocodilians are the surviving non-avian branch most closely related to birds.
Molecular phylogenetics: Molecular phylogenetics infers evolutionary relationships from DNA, RNA, or protein sequences. Genetic analyses have helped resolve relationships that skull anatomy alone left disputed.

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