Araeoscelidia

Araeoscelidia or Araeoscelida is a clade of extinct tetrapods (traditionally classified as diapsid reptiles) superficially resembling lizards, extending from the Late Carboniferous to the Early Permian. The group contains the genera Araeoscelis, Petrolacosaurus, the possibly aquatic Spinoaequalis, and less well-known genera such as Kadaliosaurus and Zarcasaurus. This clade is usually considered to be the sister group to all (currently known) later diapsids.

Araeoscelidans Temporal range: Carboniferous–Permian 302–275.6 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Life restoration (top) and skull reconstruction (bottom) of Petrolacosaurus kansensis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Diapsida
Order:	†Araeoscelidia Williston, 1913
Genera
†Aphelosaurus †Araeoscelis †Halgaitosaurus †Kadaliosaurus †Petrolacosaurus †Spinoaequalis †Zarcasaurus

Description

Araeoscelidians were small animals (less than one meter in length) looking somewhat like lizards, though they are only distantly related to true lizards. They differ from other, earlier sauropsids by their slender limbs, their elongated tail, and of course by the presence of two temporal openings, the feature defining the diapsid condition. In Araeoscelis, only the upper temporal opening remains, thus resulting in a derived euryapsid condition.

Genera

Araeoscelidia includes well-known genera such as Araeoscelis Williston 1910,^[1][2] Petrolacosaurus Lane 1945^[3][4] and Spinoaequalis,^[5][6] known from virtually complete skeletons. Zarcasaurus,^[7] Aphelosaurus^[8][9][10] and Kadaliosaurus^[11] belong to this clade but are known only from post-cranial remains and a mandible fragment for Zarcasaurus.

The genus Dictybolos has been included in Araeoscelidia by Olson (1970)^[12] but this inclusion has been criticized e.g., by Evans (1988),^[13] especially since Olson also included distantly related groups such as protorosaurs and mesosaurs.

New specimens have been discovered in the United States state of Oklahoma,^[14][15] but lack a scientific description as of 2023.

Phylogeny

The majority of historical phylogenetic studies recovered araeoscelidians as the most basal group of diapsids:

Cladogram after Bickelmann et al., 2009^[16] and Reisz et al., 2011:^[17]

Sauropsida †Parareptilia †Millerettidae †Eunotosaurus †Hallucicrania †Lanthanosuchidae †Procolophonia †Procolophonoidea †Pareiasauromorpha Eureptilia †Captorhinidae Romeriida †Paleothyris Diapsida †Araeoscelidia Neodiapsida †Orovenator †Lanthanolania †Tangasauridae †Younginidae †Claudiosaurus †Palaeagama †Saurosternon †Coelurosauravus †Thalattosauria †Hupehsuchia †Ichthyopterygia Sauria Lepidosauromorpha Archosauromorpha

However, Simões et al. (2022) recover them as stem-amniotes instead, as the sister group to the clade including Captorhinidae and Protorothyris archeri.^[18]

Reptiliomorpha	Seymouriamorpha Diadectomorpha Araeoscelidia Protorothyris Captorhinidae Amniota Synapsida Sauropsida Acleistorhinidae Millerettidae Mesosauria Neoreptilia Procolophonia Neodiapsida (including modern reptiles and birds)

Stratigraphic and geographic distribution

Araeoscelidia are known from the Late Carboniferous in the United States (Petrolacosaurus, Spinoaequalis) to the Early Permian in France (Aphelosaurus), Germany (Kadaliosaurus) and the United States (Dictybolos, Zarcasaurus, Araeoscelis, Halgaitosaurus^[19]). Apart from araeoscelidans, only one other diapsid is known before the Late Permian: Orovenator from the Early Permian of Oklahoma.^[17]