On the other hand, Archelosauria had not been historically supported by most morphological analyses, which have instead found turtles to either be descendants of parareptiles, early-diverging diapsids outside of Sauria, or close relatives of lepidosaurs within the clade Ankylopoda.

Crawford et al. defined Archelosauria as the clade formed by the descendants of the most recent common ancestor of Crocodylus niloticus (the Nile crocodile) and Testudo graeca (the Greek tortoise).

[1] Sphenodon Anolis Python Chelidae Pelomedusidae Podocnemididae Carettochelys Trionychidae Durocryptodira Crocodylus Gallus Analyses based on morphological data have generally recovered turtles either as non-diapsid reptiles nested within Parareptilia (a group of basal reptiles that lived from the Carboniferous to the Triassic), as early-diverging diapsids outside of Sauria, or as close relatives of Lepidosauria.

Archelosauria was diagnosed by two unambiguous synapomorphies (shared derived traits): a sagittal crest on the supraoccipital bone, and the lack of an entepicondylar foramen on the humerus.

A cladogram adapted from their analysis is shown below:[9] Younginiformes Eunotosaurus Coelurosauravus Sphenodontia Squamata Pappochelys Odontochelys Kayentachelys Proganochelys Ichthyosauromorpha Sauropterygia Thalattosauria Protorosauria Allokotosauria Rhynchosauria Prolacerta Archosauriformes Wolniewicz et al (2023) also found evidence for an expanded Archelosauria containing the three Mesozoic marine reptile clades of uncertain placement:[10] Lepidosauromorpha Coelurosauravus Choristodera Testudines Thalattosauria Sauropterygia Ichthyosauromorpha Archosauromorpha