Millerosaurus nests with Milleropsis basal to Diapsida (sans Lepidosauriformes, of course)

Millerosaurus ornatus (Broom 1938, 1948; Watson 1957; Late Permian (Changhsingian), 30 cm est length; RV78 holotype; Figs. 1, 2) is based on a chimaera of over a dozen skeletons with many common elements distinct from one another… but thankfully, not that distinct.

The chimaera issue is why
Millerosaurus earlier entered and left the large reptile tree (LRT, 2008+ taxa; subset Fig. 5) in an attempt at keeping the LRT entirely built on specimens. This avoids the possibility of cobbling together a ‘Frankenstein’ taxon (like the recent “pterosaur precursor” based on lagerpetid parts from various locations around the world). Millerosaurus is here accepted as a rare exception to that general rule. Just keep in mind it is not built on a single specimen, but all specimens are from the same Late Permian Karoo Formation.

The entry of Millerosaurus
permits another look at this misunderstood corner of the reptile family tree (Figs. 5, 6). For instance, Eudibamus (Fig. 3) nests here, far from the bolosaurids. Here’s the backstory:

Millerina is now Milleretta (Fig. 4) an unrelated basal lepidosauromorph taxon in the LRT basal to diadectomorphs, turtles, caseasaurs and lepidosauriformes.

The label ‘cotylosaur’ is rarely used now. Perhaps someday the same thing will happen to other invalid clades like ‘Ornithodira‘, ‘Avemetatarsalia‘, ‘Pseudosuchia‘ and ‘Cetacea‘.

Watson 1957 continues,
“To it [the Cotylosauria] he [Broom 1938] added several other genera, including one, Milerosaurus, with a pelycosaur-like temporal opening.”

This is what Cisneros et al. 2008 considered the clade Millerettidae, a traditional and invalid clade composed of taxa that share the first six letters of their generic name (Fig. 7). Prior workers (e.g. Gow 1972) made the same mistake due to taxon exclusion, academic training and lack of a valid alternative.

By contrast, in the LRT Millerosaurus and Milleropis nest in the protodiaspsid clade (Fig. 6), previously misidentified (due to taxon exclusion) as members of the pelycosaur-like Varanopidae on the basis of a few traits (= “Pulling a Larry Martin”), not on the basis of a last common ancestor and a wide gamut taxon list.

“Very well-preserved specimens of this last genus make possible a nearly complete description of the whole skeleton of these animals.”

See figures 1 and 2.

“They are shown by the occurrence of a typical lizard-like columella auris and tympanic cavity to be sauropsids, and are evidently far more primitive in general structure than any other members of that group.”

The columella auris (yellow in figure 2) is the stapes in tetrapods, the hyomandibular in fish. It is so rarely noted in tetrapods that it is not scored in the LRT.

“The group founded for them is shown to include, with great probability, Mesenosaurus from near the beginning of the Russian Permian reptilecontaining deposits.”

You’ll find Mesenosaurus (Fig. 3) near the base of the Protodiapsida in the LRT (subset Fig. 5).

Watson 1957 continues,
“The real resemblance of the millerosaurs to primitive captorhinids and pelycosaurs is evidence of a common ultimate derivation from anthracosaurs.”

Anthracosauria has had many definitions. Generally Anthracosauria refers to reptile-like amphibians and amphibian-like reptiles. Ironically, Anthracosaurus is not related to these. Like modern workers, Watson was not aware of the basal dichotomy that split basal Lepidosauromorpha (including Milleretta) from Archosauromorpha (including Milleropsis and Millerosaurus) first documented in the LRT in 2011.

“The Millerosauria provide a starting point for the development of all sauropsids except perhaps the Chelonia.”

In the LRT only Milleretta is basal to what was then called, Chelonia (turtles).
By contrast, the milleropsids (Figs. 3, 5) are basal to archosauromorph diapsids.
Sauropsida is an antiquated term, invalidated by the archosauromorph/lepidosauromorph basal dictomy in amphibian-like reptiles recovered by the LRT in 2011.

“Thus the first appearance of ‘diapsid’ reptiles in the Upper Permian Cistecephalus Zone, and the immensely rapid development they show in the Lower Trias, is related to the effective disappearance of Dicynodon, and of the carnivorous gorgonopsids and Therocephalia which preyed on it, at the end of Permian time. The break is as great as that which separates the beginning of Tertiary from the end of Cretaceous times amongst land-living vertebrates.”

Since 1957 so many taxa have been described that blend this former break. The earliest diapsid in the LRT, Petrolacosaurus, (Fig. 3) is from the Late Carboniferous. Several Early Permian protodiapsid taxa are known, so these phylogenetically more primitive taxa must have radiated before the genesis of Petrolacosaurus.

References
Broom R 1938. On a new type of primitive fossil reptile from the Upper Permian of South Africa. Proceedings of the Zoological Society of London Series B 108: 535–542.
Broom R 1948. A contribution to our knowledge of the vertebrates of the Karroo
beds of South Africa. Transactions of the Royal Society of Edinburgh B 61: 577–629.
Cisneros JC, Rubidge BS, Mason R and Dube C 2008. Analysis of millerettid parareptile relationships in the light of new material of Broomia perplexa Watson, 1914, from the Permian of South Africa. Journal of Systematic Palaeontology 6(4):453–462.
Gow CE 1972. The osteology and relationships of the Millerettidae (Reptilia: Cotylosauria). Journal of Zoology, London 167:219-264.
Watson DMS 1957. On Millerosaurus and the Early History of the Sauropsida. Philosophical Transactions of the Royal Society of London Series B 240(673):325-400.

wiki/Milleropsis
wiki/Millerosaurus
wiki/Varanopidae
wiki/Millerettidae