User:Tony Sidaway/Reptilia
This page is for personal notes on the phylogenetic systematics of amniota, particularly reptilia. The key source for this is Modesto and Anderson (2004), "The phylogenetic definition of reptilia."
General notes on phylogenetic nomenclature
Types of group
Crown
A crown group is a group consisting of living representatives, and their ancestors, back to the most recent common ancestor of that group. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms relative to extinct ones. Though formulated in the 1970s, it was not commonly used until its reintroduction in the 2000s.
Stem
A paraphyletic group whose members are more closely related to the crown group than to any other living group but are not members of the crown group. All members of a stem group are extinct. Example: archeopteryx is a member of the stem group of modern birds. It lacks some characteristics of birds but is more closely related to modern birds than any other crown group.
More group types
Group names (nomens)
Amniota
Synapsida
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Synapsids ('fused arch') are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name. Primitive synapsids are usually called pelycosaurs; more advanced mammal-like ones, therapsids. The non-mammalian members are described as mammal-like reptiles in classical systematics, but are referred to as "stem-mammals" or "proto-mammals" under cladistic terminology.