Updated July 7, 2020
the LRT moves Meyasaurus, Indrasaurus and Hoyalacerta to the base of the Yabeinosaurus + Sakurasaurus clade within the Scleroglossa and Squamata.
Carusia intermedia (Borsuk-Bialynicka 1985, Kegin and Norell 1998, Late Cretaceous, Mongolia, ZPAL MgR-111/34, Fig. 1) was originally described as a questionable Scincomorphan, with the name Carolina. In 1987 Borsuk-Bialynicka renamed it after learning Carolina was preoccupied. Kegin and Norell recovered it in the Anguimorpha. Unfortunately the name Carusia is already taken by the Solomon Islands skink, Carusia zebrata. And the two are not related.
Figure 1. Carusia intermedia, a basal lepidosaur close to Meyasaurus now, but looks a lot like Scandensia. Note the primitive choanae and broad palatal elements. None of the data I have shows the caudoventral process of the jugal, so I added it here from the description. Same with the epipterygoid. The medially touching palatines is currently an autapomorphy, but we don’t know what the palatines of Meyasaurus look like.
Be that as it may, the large reptile tree (not updated yet) recovered this little round-headed lizard was ancestral to the Tritosauria, a sister to Meyasaurus and both are sisters to the clade that includes Homoeosaurus and Dalinghosaurus, all derived from Scandensia (Fig. 3) and “Renestosaurus” rossi.
Figure 2. Shinisaurus, a taxon Kequin and Norell recovered as a sister to Carusia. The resemblances are striking indeed, but phylogenetic analysis sends Carusia over to Meyasaurus.
Kegin and Norell (1998) did not add Scandensia to their inclusion list as it was published in the same year. The Kegina nd Norell analysis included over 37,000 trees, which is over 36,999 more trees than the large reptile tree. They nested Carusia with Shinisaurus (Fig. 2) a close relative of Bahndwivici, a taxon published in 2006. The resemblance is strong. The teeth and palate are quite distinct, though with Shinsaurus having fewer, larger sharper teeth, a more gracile palate and an expanded occiput, best seen here at Digimorph.org. In the large reptile tree shifting Carusia closer to Bahndwivici adds 11 to 16 steps.
Figure 3. Scandensia a basal lepidosaur not far off from Carusia and Meyasaurus.
Overall smaller than Scandensia, the skull of Carusia was slightly shorter and rounder, with a longer postorbital region and a smaller naris. As bit as the orbit is in Carusia, it was actually smaller than in Scandensia. The palate is quite telling in that the nares and vomernasal orgains were just beginning to separate in this taxon. The teeth were set side-by-side in a comb-like arrangement, better seen from the medial side with only the chisel-shaped tips appearing on the lateral side.
The skull is heavily incrusted with osteoderms (large scales). For all the similarities to Scandensia, Carusia nests with Meyasaurus (Fig. 4), where, beyond obvious appearances, more traits are shared. Moving Carusia to Scandensia adds 12 steps. Note how the postorbital blends into the shape of the dorsal jugal. In Carusia either these two bones are fused or the suture was overlooked beneath the osteoderms.
Figure 4. Meyasaurus, a sister to Carusia.
At first I followed Kequin and Norell
by noting that the lacrimal was fused to the maxilla and the postorbital was fused to the postfrontal. Even so, Carusia nested with Meyasaurus.
Kequin and Norell 1998 description of Carusia
Distinguished from all other closely related taxa including Xenosaurus and Shinisaurus by the following autapomorphies: lacrimal absent*, with anteroventral process of jugal contacting prefrontal*; maxilla contributes to formation of ventral border of orbit*; caudoventral process of jugal ventrally directed; fenestra exochoanalis (naris sans vomernasal organ) distinctively short and elliptic; presence of midline contact of palatines anteriorly; palatine rectangular and strongly widened, suppressing suborbital fenestra into narrow and elongate opening; presence of vertical crest anteriorly on quadrate; condyle-fossa articulation for both squamosaland supratemporal with cephalic condyle of quadrate; descending process of parietal subrectangular, elongated, and laterally compressed; epipterygoid slanted strongly posterodorsally; marginal process of supra occipital prominent and well-ossified; presence of strong adductor crest dorsolaterally on surangular; marginal teeth extremely fine, numerous, and having comblike arrangement; strong posterior extension of neural spine of axis over entire third cervical.
* maybe not, but this doesn’t matter phylogenetically.
The midline contact of the palatines is unique. I don’t see it in other basal tritosaurs other than Meyasaurus.
Kequin and Norell 1998 diagnosed Carusioidea on the basis of seven synapomorphies: fused frontals**; lateral borders of frontals constricted between orbits**; postorbital branch of jugal sculptured (or is that not the branch but the actual postorbital?)**; surangular extends anteriorly slightly beyond the level of the coronoid eminence**; presence of a jugal−squamosal contact on the upper temporal bar (or not, see above); anterior tapering of surangular reduced**, presence of cranial osteoderms with vermiculate rugosities**.
** note similarities to Meyasaurus.
Lepidosaurs are difficult to categorize even when you have the entire collection of the AMNH down the hall. That’s why Dr. Norrell at the AMNH could not resolve their tree.
And now you know at least a few of the reasons why.
Sticking with the old categories doesn’t cut it. Sometimes you need to produce large gamut studies to see if maybe there’s another clade, in this case the Tritosauria, that all the misfits are attracted to.
Borsuk-Bialynicka M 1985. Carolinidae, a new family of xenosaurid-like lizards from the Upper Cretaceous of Mongolia. Acta Palaeontologica Poonica. 30: 151-176.
Borsuk-Bialynicka M 1987. Carusia, a new name for the Later Cretaceous lizard CarolinaBorsuk-Bialynicka, 1985. Ibid. 32:153.
Kequin G and Norell MA 1998. Taxonomic Revision of Carusia (Reptilia: Squamata) from the Late Cretaceous of the Gobi Desert and Phylogenetic Relationships of Anguimorphan Lizards. American Museum Novitates 3230: 51pp.