Amniote - WikiHQ

Traditional classification

Older classifications of the amniotes traditionally recognised three classes based on major traits and physiology:

Class Reptilia (reptiles)
Subclass Anapsida ("proto-reptiles", possibly including turtles)
Subclass Diapsida (majority of reptiles, progenitors of birds)
Subclass Euryapsida (plesiosaurs, placodonts, and ichthyosaurs)
Subclass Synapsida (stem or proto-mammals, progenitors of mammals)
Class Aves (birds)
Subclass Archaeornithes (reptile-like birds, progenitors of all other birds)
Subclass Enantiornithes (early birds with an alternative shoulder joint)
Subclass Hesperornithes (toothed aquatic flightless birds)
Subclass Ichthyornithes (toothed, but otherwise modern birds)
Subclass Neornithes (all living birds)
Class Mammalia (mammals)
Subclass Prototheria (Monotremata, egg-laying mammals)
Subclass Theria (metatheria (such as marsupials) and eutheria (such as placental mammals))

This rather orderly scheme is the one most commonly found in popular and basic scientific works. It has come under critique from cladistics, as the class Reptilia is paraphyletic—it has given rise to two other classes not included in Reptilia.

Most species described as microsaurs, formerly grouped in the extinct and prehistoric amphibian group lepospondyls, have been placed in the newer clade Recumbirostra, and share many anatomical features with amniotes, which indicates they were amniotes themselves.

Classification into monophyletic taxa

A different approach is adopted by writers who reject paraphyletic groupings. One such classification, by Michael Benton, is presented in simplified form below.

Series Amniota
(Class) Clade Synapsida
A series of unassigned families, corresponding to Pelycosauria †
(Order) Clade Therapsida
Class Mammalia – mammals
(Class) Clade Sauropsida
Subclass Parareptilia †
Family Mesosauridae †
Family Millerettidae †
Family Bolosauridae †
Family Procolophonidae †
Order Pareiasauromorpha
Family Nycteroleteridae †
Family Pareiasauridae †
(Subclass) Clade Eureptilia
Family Captorhinidae †
(Infraclass) Clade Diapsida
Family Araeoscelididae †
Family Weigeltisauridae †
Order Younginiformes †
(Infraclass) Clade Neodiapsida
Order Testudinata
Suborder Testudines – turtles
Infraclass Lepidosauromorpha
Unnamed infrasubclass
Infraclass Ichthyosauria †
Order Thalattosauria †
Superorder Lepidosauriformes
Order Sphenodontida – tuatara
Order Squamata – lizards and snakes
Infrasubclass Sauropterygia †
Order Placodontia †
Order Eosauropterygia †
Suborder Pachypleurosauria †
Suborder Nothosauria †
Order Plesiosauria †
(Infraclass) Clade Archosauromorpha
Family Trilophosauridae †
Order Rhynchosauria †
Order Protorosauria †
Division Archosauriformes
Subdivision Archosauria
Infradivision Crurotarsi
Order Phytosauria†
Family Ornithosuchidae †
Family Stagonolepididae †
Family Rauisuchidae †
Superfamily Poposauroidea †
Superorder Crocodylomorpha
Order Crocodylia – crocodilians
Infradivision Avemetatarsalia
Infrasubdivision Ornithodira
Order Pterosauria †
Family Lagerpetidae †
Family Silesauridae †
(Superorder) Clade Dinosauria – dinosaurs
Order Ornithischia †
(Order) Clade Saurischia
(Suborder) Clade Theropoda – theropods
Class Aves – birds

Phylogenetic classification

With the advent of cladistics, other researchers have attempted to establish new classes, based on phylogeny, but disregarding the physiological and anatomical unity of the groups. Unlike Benton, for example, Jacques Gauthier and colleagues forwarded a definition of Amniota in 1988 as "the most recent common ancestor of extant mammals and reptiles, and all its descendants". As Gauthier makes use of a crown group definition, Amniota has a slightly different content than the biological amniotes as defined by an apomorphy. Though traditionally considered reptiliomorphs, some recent research has recovered diadectomorphs as the sister group to Synapsida within Amniota, based on inner ear anatomy.

Cladogram

The cladogram presented here illustrates the phylogeny (family tree) of amniotes, and follows a simplified version of the relationships found by Laurin & Reisz (1995), with the exception of turtles, which more recent morphological and molecular phylogenetic studies placed firmly within diapsids. The cladogram covers the group as defined under Gauthier's definition.

Following studies in 2022 and 2023, with Drepanosauromorpha placed sister to Weigeltisauridae (Coelurosauravus) in Avicephala based on Senter (2004):

References