Monstersauria is aclade ofanguimorphlizards, defined as alltaxa more closely related toHeloderma thanVaranus. It includesHeloderma, as well as several extinct genera, such asEstesia,Primaderma andGobiderma, but this group was found to be polyphyletic in the most recent and complete squamate phylogenetic analysis by Reederet al. (2015).[3]
Traditionally, Monstersauria was thought to include the modernHelodermatidae along with fossil genera such asGobiderma andEstesia on the finding that it was a sister toVaranidae. But in more recent years, such as 2004 and 2008, more precise molecular studies have shown that the extantHeloderma is closer toAnguidae & kin than toVaranoidea. A large-scale integrated analysis on squamate phylogeny incorporating 737 characters of morphological and molecular data in 2015 analyzed the traditionally-monstersaurian fossil taxa along with the rest of the dataset, and what it found was a well-supported separation of the extinct monstersaurians from the extantHeloderma. In total, three different possibilities exist: eitherHeloderma is sister to the rest ofNeoanguimorpha and fossil monstersaurians nest withinVaranoidea (based on molecular and combined data; optimal arrangement); they both nest with each other inNeoanguimorpha (unlikely possibility based on parsimony analysis of combined data), or they both nest with each other inVaranoidea (based on morphology only). The most likely tree chosen by the authors, based on the combined dataset of 691 morphological characters and 46 molecular characters across 210 operational taxonomic units, is as shown, focusing onAnguimorpha:[4]
^Tadahiro Ikeda, Hidetoshi Ota, Tomonori Tanaka, Kenji Ikuno, Katsuhiro Kubota, Kohei Tanaka and Haruo Saegusa. 2022. A Fossil Monstersauria (Squamata: Anguimorpha) from the Lower Cretaceous Ohyamashimo Formation of the Sasayama Group in Tamba City, Hyogo Prefecture, Japan. Cretaceous Research. 130, 105063. DOI: 10.1016/j.cretres.2021.105063
^M. A. Norell and K. Gao. (1997). Braincase and phylogenetic relationships ofEstesia mongoliensis from the Late Cretaceous of the Gobi Desert and the recognition of a new clade of lizards.American Museum Novitates 3211:1-25
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