Mead, Jim I.
The Mammoth Site
Hot Springs, South Dakota USA
Assessment of lizard history (beginning Triassic) is typically based on cranial fossils, an understandable approach. My concentration is on remains of Miocene through Quaternary age, centering on anguimorphs (Anguidae and Monstersauria) from North America. Osteoderms are common throughout the integument of many lizards but especially within the anguimorphs. These fossils are typically well preserved in the fossil record, but they have to be recovered (which is not always in the methodology of many paleontologists) and recognized as identifiable (which is not always the scenario). Our lab at the Mammoth Site consistently uses sieves 500-600μm to obtain the minute remains, many times fragmented taphonomically or via excavation. Plio-Pleistocene remains from Safford (AZ) are identified as Heloderma suspectum – it occurs in region today. Specimens from Apache (OK; late Pleistocene?) are identified as Heloderma sp. Osteoderms from the latest Miocene/earliest Pliocene Gray Fossil Site (TN, ~5 Ma) are identified as Heloderma sp. Questions/hypotheses emerge as to which extant species of Heloderma may be related to fossils in Texas, Oklahoma, Florida, and Tennessee. What route did the ancestor(s) take, either: 1) from a possible southern Mexico evolution dispersing to the north, or, 2) from a pan-central USA evolution south and west into Mexico. Options are developing. Found with Heloderma at Gray, TN include sub-rectangular osteoderms of an anguine unlike Ophisaurus (North America) but similar to the Central Asian Pseudopus. The ‘Pseudopus’-like anguine may have dispersed from Asia to North America at same time (latest Miocene) as did the red panda (Pristinailurus) and badger (Arctomeles). A middle Miocene locality (Barstovian, ~13 Ma) and an early Miocene locality (Hemingfordian, ~19 Ma), with both helodermatids and anguids, just now being studied may help resolve this. The advantage of osteoderms is the abundance on the body and in the fossil record.