Varanopidae


Varanopidae is an extinct family of amniotes that resembled monitor lizards and may have filled a similar niche, hence the name. Typically, they are considered synapsids that evolved from an Archaeothyris-like synapsid in the Late Carboniferous. However, some recent studies have recovered them being taxonomically closer to diapsid reptiles. A varanopid from the latest Middle Permian Pristerognathus Assemblage Zone is the youngest known varanopid and the last member of the "pelycosaur" group of synapsids.

Description

No known varanopids developed a sail like Dimetrodon. The length of known varanopids, including the tail, varies from. Varanopids already showed some advanced characteristics of true pelycosaurs such as their deep, narrow, elongated skulls. Their jaws were long and their teeth were sharp. However, they were still primitive by mammalian standards. They had long tails, lizard-like bodies, and thin legs. The varanopids were mostly carnivorous, but as they were reduced in size, their diets changed from a carnivorous to an insectivorous lifestyle. Compared to the other animals in Early Permian, varanopids were agile creatures.
The genus Ascendonanus provides the first extensive skin impressions for pelycosaur-grade synapsids, revealing scales akin to those of squamates. Parental care is known in Heleosaurus, suggesting that it is ancestral to synapsids as a whole.

Classification

Class Synapsida
Apsisaurus was formerly assigned as an "eosuchian" diapsid. In 2010, it was redescribed by Robert R. Reisz, Michel Laurin and David Marjanović; their phylogenetic analysis found it to be a basal varanopid synapsid. The cladogram below is modified after Reisz, Laurin and Marjanović, 2010.
Varanopinae has been called "Varanodontinae" in the literature. However, according to Article of the International Code of Zoological Nomenclature, whichever subfamily of Varanopidae contains Varanops is automatically called Varanopinae.
The poorly known Basicranodon and Ruthiromia were tentatively assigned to Varanopidae by Reisz, but have been neglected in more recent studies. They were included for the first time in a phylogenetic analysis by Benson. Ruthiromia was found to be most closely related to Aerosaurus. Basicranodon was found to be a wildcard taxon due to its small amount of known materials, as it is based on a partial braincase from the ?Kungurian stage Richards Spur locality in Oklahoma. It occupies two possible positions, falling either as a mycterosaurine, or as the sister taxon of Pyozia. Although Reisz et al. considered Basicranodon as a subjective junior synonym of Mycterosaurus, Benson found some differences in the distribution of teeth and shape of the dentigerous ventral platform medial to the basipterygoid processes that may indicate taxonomic distinction. Below is a cladogram modified from the analysis of Benson, after the exclusion of Basicranodon: