Helodermatidae Gray, 1837

Conrad, J. L., 2008, Phylogeny And Systematics Of Squamata (Reptilia) Based On Morphology, Bulletin of the American Museum of Natural History 2008 (310), pp. 1-182 : 121

Helodermatidae Gray, 1837

(figs. 54E, 55E, 56D)

DEFINITION: Heloderma horridum , Lowesaurus matthewi , Eurheloderma gallicum , and all descendants of their last common ancestor.

DIAGNOSIS: The node here referred to as Helodermatidae is diagnosed by seven unambiguous synapomorphies in the current analysis. These are 37(1) prefrontal-postfrontal/postorbitofrontal contact present, 57(1) frontals rhomboid in dorsal view, 77(3) absence of a pineal foramen, 115(1) absence of palatine teeth, 118(2) absence of pterygoid teeth (reversed in some Heloderma suspectum ), 182(1) absence of a dentary shelf (fig. 38E), and 220(1) presence of a distinct venom groove in the dentary teeth (fig. 38E).

COMMENTS: Helodermatidae as defined here generally follows common usage ( McDowell and Bogert, 1954; Bogert and Del Campo, 1956; Yatkola, 1976; Estes, 1983; Norell et al., 1992; Gao and Fox, 1996) and is similar to the usage of Pregill et al. (1986) who first applied this term to a node on a cladogram. This differs from the usage of some subsequent authors who prefer a crown based application of Helodermatidae ( Norell and Gao, 1997; Gao and Norell, 1998, 2000).

Estes (1983) tentatively included Paraderma bogerti in his treatment of Helodermatidae , citing plesiomorphic characteristics and the then-forthcoming study by Pregill et al. (1986). Estes (1983) further cautioned that a parietal he had referred to P. bogerti ( Estes, 1964) might be that of Palaeosaniwa canadensis . However, the parietal of P. canadensis is inconsistent with that morphology and the parietal is here tentatively considered to belong to P. bogerti . Pregill et al. (1986) included Heloderma , Lowesaurus (5 Heloderma ) matthewi , Eurheloderma gallicum , and Paraderma bogerti in their Helodermatidae . The latter usage is consistent with that of Estes et al., who defined it as a stem including ‘‘ Heloderma , and all organisms sharing a more recent common ancestor with this taxon than with any other extant organisms,’’ (Estes et al., 1988: 228). Given the phylogenetic topology presented here (figs. 54E, 55E) and a crown-based definition for Helodermatidae , helodermatids would be a subclade of Heloderma . Consequently, maintaining Helodermatidae in a traditional sense is advocated here.

The current application of Helodermatidae to the node specified above is beneficial given that all of the anchor taxa for the name were originally described as helodermatids (as opposed to ‘‘necrosaurids’’/‘‘parasaniwids’’; e.g., Paraderma bogerti and Gobiderma pulchrum ) and are retained as a monophyletic group in all of the principle trees recovered by this study (see also Auge´, 2003). Estesia mongoliensis is variable in its placement in the current analysis. Different principle trees place it as the sister-taxon to L. matthewi , E. gallicum , the L. matthewi + Heloderma clade, or as the sister-taxon to Helodermatidae as defined here. This phylo- genetic hypothesis owes, in part, to the relative quality of known E. gallicum and L. matthewi . Thus, E. mongoliensis may or may not be a helodermatid as the name is defined here, but it is certainly a monstersaur.