Early Pleistocene (Blancan) Helodermatid Lizard from Arizona, USA
Two cranial osteoderms and a maxilla assigned to Heloderma suspectum (Squamata, Helodermatidae) were recovered from the 111 Ranch fossil locality, San Simon Valley, Graham County, southeastern Arizona. A minimum age of the deposits and fossils is approximately 2.7–2.4 million years ago, latest Blancan Land Mammal Age, spanning the Pliocene-Pleistocene boundary. The presence of a pronounced deep venom grove and a tooth count of eight indicate that the maxilla belongs to Heloderma and not the closely related and extinct Lowesaurus or Eurheloderma. The highly reduced nasolacrimal fossa and the shortened projection of the maxillary lappet on the fossil agree with the condition in H. suspectum and not in Heloderma horridum. During this transition time, the locality was a mosaic grassland–woodland ecosystem with open water and riparian niches occupied with grazing and browsing mammals.Abstract
Map showing historic distribution of H. horridum and H. suspectum (see text). Numbers refer to localities listed in Table 1. Dots represent Heloderma sp. or H. suspectum. Circle (9) refers to H. texana. Open box refers to Lowesaurus matthewi (10, 11). The 111 Ranch locality presented here is number 6.
Right maxilla (MSM P 8529a) of H. suspectum from 111 Ranch, Graham County, southeastern Arizona: (A) lateral view; (B) lingual view (anterior ends oppose each other); (C) ventral view. (D) External view of cranial osteoderm of Heloderma sp. See text for explanation of identification. Abbreviations: mxl, maxillary lappet; nlf, nasolacrimal fossa. Scale bars = 2 mm.
Recent H. horridum (ETVP 7081). (A) Dorsal view of cranium showing overall pattern of osteoderms (anterior to right); (B) close-up of osteoderms with an acute vermiculate pattern where spicules form on the ridges. This acute, spicule sculpturing can occur on H. suspectum. Figure modified from Mead et al., 2012:fig. 3.
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