Araeoscelidia

Araeoscelidans Temporal range: Carboniferous–Permian
Life restoration (top) and skull reconstruction (bottom) of Petrolacosaurus kansensis
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Reptiliomorpha
Order:	†Araeoscelidia Williston, 1913
Genera
†Aphelosaurus †Araeoscelis †Halgaitosaurus †Kadaliosaurus †Petrolacosaurus †Spinoaequalis †Zarcasaurus

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 has historically been considered to be the sister group to all (currently known) later diapsids, though studies from the early 2020s onwards have found it much closer to the base of Sauropsida unrelated to other diapsid reptiles, or even outside Reptilia entirely, suggesting their physically diapsid skull is unrelated to those of modern diapsids.

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.

Araeoscelidia includes well-known genera such as Araeoscelis Williston 1910,¹² Petrolacosaurus Lane 1945³⁴ and Spinoaequalis,⁵⁶ known from virtually complete skeletons. Zarcasaurus,⁷ Aphelosaurus⁸⁹¹⁰ and Kadaliosaurus¹¹ 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,¹⁴¹⁵ but lack a scientific description as of 2023.

The majority of historical phylogenetic studies recovered araeoscelidians as the most basal group of diapsids, however this has recently been challenged, and it has been suggested that the araeoscelidians convergently evolved their diapsid condition separately from those of other diapsids (Neodiapsida).¹⁶

Cladogram after Bickelmann et al., 2009¹⁷ and Reisz et al., 2011¹⁸, showing classical position of Araeoscelidia at the base of Diapsida sister to Neodiapsida:

However, Simões et al. (2022) recover them as stem-amniotes instead, as the sister group to the clade including Captorhinidae and Protorothyris archeri.¹⁹

 Cladogram of Reptilia after Jenkins et al 2025, which found Araeoscelidia as the most basal group of sauropsids:¹⁶

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, Halgaitosaurus²⁰). Apart from araeoscelidans, only one other diapsid is known before the Late Permian: Orovenator from the Early Permian of Oklahoma.¹⁸