This post was updated on August 12, 2013 prompted by the addition of several taxa.
Dr. Roger Benson (2012) recently proposed a family tree of basal synapsids (pelycosaurs) that was covered earlier. Benson (2012) added Varanosaurus as a late surviving member of the Ophiacodontia (Fig. 1) following Berman et al. (1995). The long rostrum and short cranial region of Varanosaurus (Fig. 2) is indeed shared with Ophiacodon. They are closely related, along with Apsisaurus. the ophiacodontid, Archaeothyris is basal to them.
Romer and Price (1940) published a reconstruction of Varanosaurus. However it differs in several respects from a complete skull image found on the web (Fig. 2). It is a distinct specimen, MCZ 1366.
Three Different Varanosaurus Specimens
Benson (2012) reports that the earlier Romer and Price (1940) specimen, MCZ 1366, is highly reconstructed. Berman et al. (1995) reported on FMNH PR 1760 (Fig. 2) and BSPHM 1901 XV 20 (Fig. 3.
The large reptile tree includes several times more taxa than either the Benson (2012) tree or the Berman et al. (1995) tree. It recovered a tree in which both of the new Varanosaurus specimens were sisters to one another and nested with Apsisaurus at the base of the Ophiacondontidae. This clade includes therapsids and mammals. The Sphenacodontidae, formerly considered therapsid ancestors, is a sister clade to Ophiacodon and Apsisaurus (Fig. 4). Diapsids evolved from basal synapsids, like Heleosaurus.
The MCZ specimen was not tested.
So Varanosaurus was a late-surviving member of a basal synapsid clade. Varanosaurus had the proportions of a modern Varanus, the monitor lizard, and likely had a similar lifestyle.
Both the Berman (1995) and Benson (2012) studies could have benefitted by including more outgroup taxa (Fig. 4) and dropping Caseasauria, which are not related. Apsisaurus needs to be included.
Benson RBJ 2012. Interrelationships of basal synapsids: cranial and postcranial morphological partitions suggest different topologies. Journal of Systematic Palaeontology, iFirst 2012, 1-24.
Berman D S, Reisz RR, Bolt JR, and Scott D. 1995. The cranial anatomy and relationships of the synapsid Varanosaurus (Eupelycosauria: Ophiacodontidae) from the Early Permian of Texas and Oklahoma. Annals of Carnegie Museum 64: 99-133.
Romer AS and Price LW1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.