The Monstersauria & Venom Delivery

Gila monster teeth. JCM

Lizards and snakes are diverse in both feeding and defense strategies and some used modified teeth to deliver venom to prey and  predators. The oldest known squamate with grooves in its teeth, similar to those found in the modern heloderms (gila monster and beaded lizards), is the monstersaura Estesia mongoliensis
 In living lizards, only members in the genus Heloderma have grooved marginal teeth used for venom delivery. Heloderms are low in diversity and geographically restricted to southwestern United States , Mexico and Guatemala. In contrast, the fossil monstersaurs related to Heloderma, are found in North America, Europe and Asia. 

The Monstersauria clade is a group of mostly large, carnivorous anguimorph lizards, and includes the extant Heloderma and stem taxa. Monstersaurs differ from other anguimorph lizards in the possession of venom grooves in their marginal teeth. In living heloderms the venom grooves extend from the root of a tooth to its tip, and are used for venom delivery. While the venom grooves in fossil monstersaurs are shallower. 
The fossil record of monstersaurs in North America dates back to the Late Cretaceous. In the Cenozoic, fossil monstersaurs are commonly known from the Oligocene and Miocene. Fragmentary materials are known from Nevada as well as  the Pleistocene of southern California and the Mio-Pliocene of Tennessee. Heloderma texana is known from the Miocene of Texas and is represented by articulated skulls. The grooved marginal teeth and thickened osteoderms of H. texana closely resemble those in extant heloderms. Lowesaurus matthewi is known from the Oligocene to the Miocene of Colorado and Nebraska , represented by isolated skull elements and trunk vertebrae. There is also an un-named taxon from the Miocene of Florida that has grooved teeth resembling those of extant Heloderma. Mesozoic records of monstersaurs in North America include Palaeosaniwa canadensis,  Paraderma bogerti, and Primaderma nessovi. Parasaniwa wyomingensis may also be as a basal monstersaur becase it seems to have dental “venom grooves” 

Eurheloderma gallicum is the only monstersaurian taxon known from Europe. It is known to have grooved marginal teeth, but detailed description of these grooves is lacking. In Asia, fossil monstersaurs have also been discovered at several localities in the Upper Cretaceous of the Mongolian Gobi Desert.  
Estesia mongoliensis is the largest monstersaur (skull length 150 mm) and it is known from several skulls and postcranial materials. Estesia mongoliensis has grooved teeth but no osteoderms on the skull. Gobiderma pulchrum is known from articulated skulls and skeletons, has mound-shaped osteoderms that are fused to the skull, but lacks grooved teeth. It has been assigned to various anguimorph lineages by different authors but recent work, based on several new specimens, found strong support for this taxon as a basal monstersaur. 

Estesia mongoliensis was first described in 1992 as the sister group to Lanthanotus and varanidae but it was later suggested to be  more closely related to Heloderma, with new braincase characters.  Several subsequent studies generally found Estesia mongoliensis as a monstersaur  although other authors reported Estesia mongoliensis grouped with varanoid. 

A new study  by Yi and Norell (2013) reports on new Estesia mongoliensis specimens from the Upper Cretaceous of Mongolia, including a three-dimensionally preserved skull. The authors phylogenetically analyze 86 anguimorph taxa coded with 435 morphological characters and four genes and they confirm the placement of Estesia mongoliensis in a monophyletic Monstersauria.

 Yi and Norell found that Estesia mongoliensis has two shallow grooves in the rostral and caudal carinae of its dentary teeth, demonstrating a primary venom-delivery apparatus. The phylogeny supports a single origination of venom grooves in the Monstersauria, and suggests  that grooved teeth are currently the only reliable venom-delivery apparatus to be recognized in fossil lizards.

Yi, H-Y and M. Norell. 2013. New materials of Estesia mongoliensis (Squamata, Anguimorpha) and the evolution of venom grooves in lizards. American Museum novitates, no. 3767, 31 pp.

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