Fossil History and Morphology of Heloderma and other Venom-producers in Anguimorpha
Mead, Jim I.
The Mammoth Site
Hot Springs, South Dakota USA
With 130 million years of global history, Anguimorpha is a clade of lizards (squamates) with both limbed and limbless members within approximately 200 extinct and extant species. Phylogenetic analyses of Anguimorpha exist, however, there are disagreements largely due to the general discrepancy between morphology-based and molecular-based data. The fossil history of the group, although diverse, it is not robust. There are morphological characters that do allow some insight to their evolutionary history. A few select skeletal traits permit a better understanding about the evolution of venom use within the group, especially within the clade Monstersauria. The distinct base-to-tip groove in marginal teeth do imply direct use of venom and therefore can be tracked through geological time. Serration on the tooth edge (xiphidonty) once thought to imply potential ability for envenomation is now not a character for venom use because it occurs in non-venomous vertebrates such as crocodilians, dinosaurs, and sharks to name a few. Discussion and comparisons will focus on morphology and history of Monstersaurians: Paleosaniwa + Primadema + Gobiderma + Estesia + Helodermatidae (Paraderma + Eurheloderma + Lowesaurus + Heloderma).