Estesia mongoliensis

Yi, Hong-Yu & Norell, Mark A., 2013, New materials of Estesia mongoliensis (Squamata: Anguimorpha) and the evolution of venom grooves in lizards, American Museum Novitates 2013 (3767), pp. 1-32 : 19

ESTESIA MONGOLIENSIS AS A BASAL MONSTERSAUR

As mentioned before, previous studies report varying phylogenetic positions for Estesia mongoliensis ( Norell et al., 1992; Norell and Gao, 1997; Conrad et al., 2011a; Yi and Norell, 2010), but these studies differ in taxon and character sampling. In a preliminary analysis ( Yi and Norell, 2010), we suggested that Estesia mongoliensis is the most basal member of the Varaniformes (sensu Conrad, 2008), but our subsequent work shows that this was probably due to inadequate taxon sampling. In comparison to the matrix used in Yi and Norell (2010), the current matrix is updated in several aspects: first, we expanded our taxon sampling to genus level in the Anguimorpha, including broad sampling in the Mosasauria. Although Estesia mongoliensis never grouped with mosasaurs, adding mosasaurs in the matrix changed the character optimization of Varanus -like characters (e.g., the fusion of nasals). In the updated data matrix, Estesia mongoliensis “shifted back” to the Monstersauria, possibly due to the extra steps of reversals required if Estesia mongoliensis remained within the Varaniformes.

In addition to expanded taxon sampling, we added eight new morphological characters based on the observations in this study; we also updated the characters codings for Gobiderma pulchrum based on the recent work by Conrad et al. (2011b). Gobiderma pulchrum has been assigned to different positions in the anguimorph phylogeny by different authors. When first reported by Borsuk-Bialynicka (1984), Gobiderma pulchrum was considered as a “necrosaurian grade” anguimorph lizard, convergent with Helodermatidae . Pregill et al. (1986) argued that it should be more closely related to Xenosaurus . Conrad et al. (2011b) provided detailed descriptions on the morphology of Gobiderma pulchrum based on new specimens, supporting the monstersaurian affinity of this taxon. Updating character codings for Gobiderma pulchrum not only augmented information for this particular taxon, but also changed optimizations of “ Heloderma -like” characters in the anguimorph tree. Finally, the current data matrix included molecular characters to further explore the possible phylogenetic positions for Estesia mongoliensis within a total evidence framework.

The morphology-only and combined analysis supported Estesia mongoliensis as a basal monstersaur. The strict consensus tree of the combined analysis recovered 34 unambiguous morphological synapomorphies for the Monstersauria (appendix 5).