Neogene Distribution of Gila Monsters and Beaded Lizards (Heloderma), North America

Mead, Jim I.

Swift, Sandra L

The Mammoth Site

Hot Springs, SD USA.

Czaplewski, Nicholas J.

Oklahoma Museum of Natural History

University of Oklahoma

Norman, OK USA.

Smith, Kent S.

Office for the Advancement of American Indians in Medicine and Science

Oklahoma State University

Tulsa, OK USA.

The phylogenetic clade Monstersauria contains robust, carnivorous, anguimorph lizards that includes the extant and extinct Helodermatidae (Paraderma + Eurheloderma + Lowesaurus + Heloderma). Two species are extant (H. suspectum, H. horridum) with a possibility of three additional forms being recognized within the latter taxon (H. charlesbogerti, H. exasperatum, H. alvarezi). Today these large lizards live from southern-most Nevada and Utah, south through Arizona, south along the Pacific coast into southern Mexico and southern Guatemala. The fossil record of Heloderma is sparse. The oldest known species, H. texana, is a small form from the earliest Miocene (~ 23 Ma [millions of years]; Arikareean Land Mammal Age, LMA) of Texas, which is disjunct from the extant forms. Based on osteoderms, Heloderma sp. was recognized from the early Miocene (~18 Ma; Hemingfordian LMA) at Thomas Farm, Florida. Some researchers interpret a dentary with teeth from Thomas Farm as an intermediate form between the European Eocene-age Euroheloderma and the extant Heloderma. Alternatively it fits between the extinct Lowesaurus and Paraderma, yet not Heloderma. Osteoderms also place Heloderma sp. well outside its extant range, from the latest Miocene/earliest Pliocene (~5 Ma; Hemphillian/Blancan LMAs) of eastern-most Tennessee. Heloderma suspectum was recovered from 111 Ranch, Arizona, dating to the early Pliocene (~2.7-2.4 Ma; Blancan LMA) and within its extant range. Heloderma sp. was recovered at Anza Borrego, SE California dating to the early Pleistocene (~0.75 Ma; Irvingtonian LMA). Latest Pleistocene (~250k-10k; Rancholabrean LMA) remains are known from its current distribution in Arizona and New Mexico. A new and poorly understood find of Heloderma sp. is currently being studied from the Apache local fauna in southwestern-most Oklahoma, 800 km NE of its current distribution – with a preliminary age of Pleistocene.