Heloderma texana is known from early Miocene aged sediments of Big Bend National Park, West Texas. It is a member of Scleroglossa (see also Shinisaurus, Lanthanotus), one of the two major squamate clades (the other being Iguania, e.g., Ctenosaura, Phrynosoma).
Within Scleroglossa, H. texana belongs to Helodermatidae, a monophyletic group of lizards that includes only two extant taxa, both of which are venomous: the beaded lizard (H. horridum) and the Gila monster (Heloderma suspectum). While these two taxa have a modern range from the southwestern United States south to Guatemala, Helodermatidae as a whole is known from the Late Eocene of France, and from the latest Paleocene to Recent in North America.
Helodermatids are closely related to varanids, which include the monitor lizards (e.g. Varanus gouldii) that are native to Australoasia and Africa. Within Helodermatidae, H. texana appears to be the sister taxon to extant Heloderma.
About the Species
This specimen was collected by Ms. Margaret Stevens, Geology Department, Lamar University, Beaumont, Texas, from exposures of the Delaho Formation (Arikareean age) of Rancho de Roman de la Ho in Big Bend National Park, West Texas. Funding for scanning was provided by Dr. Timothy Rowe of The University of Texas at Austin.
About this Specimen
The specimen was scanned by Richard Ketcham on 25 June 1999 along the coronal axis for a total of 409 slices, each slice 0.095 mm thick, with an interslice spacing of 0.095 mm. The dataset displayed was reduced for optimal Web delivery from the original, much higher-resolution CT data.
Pregill, G. K., J. A. Gauthier, and H. W. Greene. 1986. The evolution of helodermatid squamates with description of a new taxon and an overview of Varanoidea. Transactions of the San Diego Society of Natural History 21:167-202.
Stevens, M. 1977. Further study of Castolon local fauna (Early Miocene), Big Bend National Park, Texas. Pearce-Sellards Series 28:1-69.
Heloderma horridum (Mexican beaded lizard) on The Animal Diversity Web (The University of Michigan Museum of Zoology)
Heloderma suspectum (Gila Monster) on The Animal Diversity Web (The University of Michigan Museum of Zoology)