Biology: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 has historically been considered to be the sister group to all (currently known) later 'diapsids', though studies from the early 2020s onwards have found as among the earliest (if not the earliest) members of Sauropsida unrelated to neodiapsid reptiles, or even outside Sauropsida entirely, suggesting their physically diapsid skull is unrelated to those of modern diapsids.

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, although most recent studies considered this to be only convergent with the condition in neodiapsids. 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, Petrolacosaurus Lane 1945 and Spinoaequalis, known from virtually complete skeletons. Zarcasaurus, Aphelosaurus and Kadaliosaurus 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) but this inclusion has been criticized e.g., by Evans (1988), 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, but lack a scientific description as of 2023.

Phylogeny

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).

Cladogram after Bickelmann et al., 2009 and Reisz et al., 2011, showing classical position of Araeoscelidia at the base of Diapsida sister to Neodiapsida:

Sauropsida †Parareptilia     †Millerettidae 50px           †Eunotosaurus     †Hallucicrania   †Lanthanosuchidae 50px   †Procolophonia   †Procolophonoidea 50px     †Pareiasauromorpha 60px           Eureptilia     †Captorhinidae 50px     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.

Reptiliomorpha	Seymouriamorpha 100px       Diadectomorpha 100px         Araeoscelidia 100px       Protorothyris 100px     Captorhinidae 100px       Amniota   Synapsida 100px   Sauropsida   Acleistorhinidae 100px       Millerettidae 100px       Mesosauria 100px   Neoreptilia   Procolophonia 100px     Neodiapsida (including modern reptiles and birds)

 Cladogram of Reptilia after Jenkins et al 2025, which found Araeoscelidia as the most basal group of sauropsids, a placement found by later versions of this dataset as well as studies by Buffa et al :

Reptilia/Sauropsida

†Araeoscelidia 60px       †Bolosauridae 60px   Neoreptilia †Ankyramorpha   †Acleistorhinidae 60px       †Pareiasauromorpha 60px     †Procolophonoidea 60px           †Mesosauridae 60px         †Cabarzia       †Ascendonanus     †Orovenator 60px       Parapleurota   †Millerettidae 60px   Neodiapsida   †Younginidae 60px       †Tangasauridae 60px       †Weigeltisauridae 60px       †Claudiosaurus 60px   Sauria   Lepidosauromorpha     Archosauromorpha 60px

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

Bibliography

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