The biodiversity of Beaded Lizards.

Helodermatid Lizards are the only extant lizards that are truly venomous (Monitor Lizards such as the Komodo Dragon, Varanus komodoensis, deliver a bite laced with harmful bacteria, but are not actually venomous). There are two extant groups of Helodermatid Lizards; members of the genus Heloderma, currently restricted to the southwestern United States, western Mexico and Central America, where they inhabit deserts and semi-deserts as well as dry woodlands and grasslands, and the Bearded Dragons of the genus Pogona, which occupy similar areas in Australia.

Within the genus Heloderma the Gila Monster (Heloderma suspectum) is a large (up to 60 cm) slow moving Lizard found in Sonora State in Mexico and Arizona, California, Nevada, Utah and New Mexico in the US, and divided into two subspecies, the more southerly Reticulated Gila Monster (Heloderma suspectum suspectum) and the more northerly Banded Gila Monster (Heloderma suspectum cinctum).

The Mexican Beaded Lizard (Heloderma horridum) is found in seasonal Pine-Oak dry forests from southern Sonora State south to Guatemala. It is larger than the Gila Monster, sometimes exceeding 90 cm in length, and more variable, with four subspecies defined from across its range, defined on the basis of scale patterns, colouration and distribution. The large size and potentially dangerous nature of this Lizard has brought it into conflict with humans across much of its range, both due to habitat loss to agriculture and more direct conflict with rising human populations.

In a paper published in the journal Amphibian & Reptile Conservation in July 2013, Randall Reiserer of the Copperhead Institute in South Carolina, Gordon Schuett of the Copperhead Institute and the Department of Biology and Center for Behavioral Neuroscience at Georgia State University, and Daniel Beck of the Department of Biological Sciences, Central Washington University, discuss the results of a genetic study of the populations of Beaded Lizards from which they conclude that the four subspecies are genetically distinct and ought to be elevated to species level.

Beaded Lizards are primarily inhabitants of lowland dry forests, being encountered rarely at middle elevations. Unfortunately humans also favor low altitude environments for agricultural and other purposes, making the Lizards particularly vulnerable to habitat disturbance.

See also Two new species of Woodlizard from northeast Peru
Based upon this study there are now considered to be four species of Beaded Lizards. Reiserer et al. note that this will require the re-examination and classification of a large number of museum specimens.

The Río Fuerte Beaded Lizard (Heloderma exasperatum) is found in the drainage basins of the Río Mayo and Río Fuerte in the foothills of the Sierra Madre Occidental, where it inhabits an area of subtropical dry forest covering parts of Sonora, Chihuahua and Sinaloa States. Heloderma exasperatum is brown with speckled yellow markings, juveniles being grayer, with larger markings, more spots than speckles.

Daniel Beck (right) with Martin Villa at the Centro Ecologia de Sonora, in Hermosillo, Mexico. Beck is holding a near record length Río Fuerte beaded lizard (Heloderma exasperatum). Thomas Wiewandt in Reiserer et al. (2013).

Juvenile Heloderma exasperatum at 

Álamos in Sonora State, Mexico. Stephanie Meyer 

in Reiserer et al. (2013).


Mexican Beaded Lizard, Heloderma horridum, has the widest distribution of the group, being found in dry forests from Sinaloa State south to Oaxaca, via Jalisco, Nayarit, Colima, Michoacán, and Guerrero, and inland to México and Morelos States. These Lizards are grey-brown in colour with yellow markings, the contrast being more marked in juveniles.

Adult Mexican Beaded Lizard Heloderma horridum at Emiliano Zapata, in Jalisco State, Mexico. Javier Alvarado 

in Reiserer et al. (2013).

Neonate Heloderma horridum Chamela, Jalisco State, Mexico. Kerry Holcomb in 

Reiserer et al. (2013)

.


Chiapan Beaded Lizard, Heloderma alvarezi, is found in dry forests in the Río Grijalva Depression of central Chiapas State in Mexico and the Río Lagartero Depression in western Guatemala. These Lizards are grey or brown with yellow markings as juveniles, but largely lose these markings as adults, becoming essentially monochrome in colour, and the only Beaded Lizards to lack tail banding.

Adult Chiapan Beaded Lizard Heloderma alvarezi from Sumidero Canyon in the Río Grijalva Valley, east of Tuxtla Gutiérrez in Chiapas State, Mexico. Thomas Wiewandt  in 

Reiserer et al. (2013)

.

Neonate Heloderma alvarezi from the Río Lagartero 

Depression in western Guatemala. 

Quetzal Dwyer 

in 

Reiserer et al. (2013)

.


Guatemalan Beaded Lizard, Heloderma charlesbogerti, is found in Río Motagua Valley on the Atlantic coast of Guatemala, with reports of additional populations on the Pacific coast. These Lizards are dark grey, with mottled yellow markings, the juveniles tending to be paler than the adults.

Adult Guatemalan Beaded Lizard, Heloderma charlesbogerti, from the Motagua Valley, Guatemala. Daniel Ariano-Sánchez 

in 

Reiserer et al. (2013)

.

Neonate Heloderma charlesbogerti hatched at 

Zoo Atlanta in late 2012. David Brothers 

in 

Reiserer et al. (2013)

.


This reclassification of Beaded Lizards has implications for their conservation. Beaded Lizards were formerly classified as 'Vulnerable' by the International Union for the Conservation of Nature, but have recently been raised to 'Least Concern' after a review of the criteria used, though the Guatemalan Beaded Lizard remained classified as 'Critically Endangered', and all Beaded Lizards in Mexico are regarded as 'Threatened' under Mexican law, giving them similar legal protection to species classified as internationally Vulnerable. This review, which suggests that not only are the Lizards separated by habitat fragmentation but by reproductive boundaries, is likely to lead to a re-examination of this status.

See also Two new species of Woodlizard from northeast Peru, A Venomous Lizard from the Late Cretaceous of Mongolia, Three new species of Dragon Lizard from the western arid zone of Australia, Genetic Diversity in Atlas Dwarf Lizards and A giant Monitor Lizard from the Miocene of Samos, Greece.

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A Venomous Lizard from the Late Cretaceous of Mongolia.

The modern Lizards of the genus Heloderma, from North and Central America, are the only surviving members of a group called the Monstersauria, which dates back to the Cretaceous and is known from Asia and North America. They are related to Slow-worms and Alligator Lizards.

In a paper published in the American Museum Novitates on 25 January 2013, Hongyu Yi of the Division of Paleontology at the American Museum of Natural History and Mark Norell of the Division of Paleontology at the American Museum of Natural History and the Department of Earth and Environmental Sciences at Columbia University describe a Monstersaurid Lizard from the Late Cretaceous of Mongolia.

The new specimen is placed in the species Estesia mongoliensis, which was described by Mark Norell in 1992, though this specimen is more complete than previously described specimens, allowing comparison of the venom delivery system and skull with that of modern Monstersaurs.

The skull of the new Estesia mongoliensis specimen in lateral view. Scale bar is 10 mm. Yi & Norell (2013).

Modern Monstersaurs have deep groves in their side teeth, running from the root to the tip. This is not present in all fossil species from North America, and deep groves similar to those seen in Heloderma do not appear before the Miocene. All North American species posses skulls with fused osteoderms (boney plated that develop in the skin). Previously known Asian Monstersaurs do not possess venom grooves, but also have fused osteoderms, suggesting that the group developed osteoderms, then shallow venom grooves, then deeper grooves. The new Estesia mongoliensis specimen lacks osteoderms, but has shallow, but well developed venom grooves, suggesting that the evolution of the group is more complicated, with venom grooves and/or osteoderms having more than one origin.

See also A giant Monitor Lizard from the Miocene of Samos, Greece and Helodermatid Lizard from the Late Miocene-Early Pliocene of Tennessee.

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Helodermatid Lizard from the Late Miocene-Early Pliocene of Tennessee.

Helodermatid Lizards are the only extant lizards that are truly venomous (Monitor Lizards such as the Komodo Dragon, Varanus komodoensis, deliver a bite laced with harmful bacteria, but are not actually venomous). There are two extant groups of Helodermatid Lizards; the Gila Monster, Heloderma suspectum, and Mexican Bearded Lizard, Heloderma horridum, currently restricted to the southwestern United States, western Mexico and Central America, where they inhabit deserts and semi-deserts as well as dry woodlands and grasslands, and the Bearded Lizards of the genus Pogona, which occupy similar areas in Australia.

The Gila Monster, Heloderma suspectum. A. Holycross/Reptiles of Arizona.

Helodermatid Lizards have distinctive osteoderms (bony plates in their skin) that are circular or hexagonal, dome shaped, and cover their entire body, fusing to the skull on the head. This is not seen in any other form of lizard, making fossil Helodermatid Lizards easy to identify (though these are not numerous, the group having apparently never been numerous).

The skull of The Gila Monster, Heloderma suspectum, showing the fused osteoderms. Will's Skull Page.

In a paper in the March edition of the journal Acta Palaeontologica Polonica, a team of scientists from the Department of Geosciences and the Don Sundquist Center of Excellence in Paleontology at East Tennessee State University led by Jim Mead describe the discovery of Helodermatid Lizard osteoderms from the Miocene-Early Pliocene Gray Fossil Site in Washington County, Tennessee. These are not enough to identify the lizards to species level but are distinctive enough that they can only come from a Helodermatid Lizard.

Osteoderms from the Gray Fossil Site (A-C) and the extant Mexican Bearded Lizard, Heloderma horridum (D-F). A-C₁ and D-F₁ apical view, C₂ & F₂ basal views of C₁ & F₁. From Mead et al. (2012).

The Gray Fossil Site was laid down in a sub-tropical forest at the boundary between the Miocene and the Pliocene. The forest was made up largely of Oaks (Quercus) and Hickory Trees (Carya) with some conifers, Elms (Ulmas), Birches (Betula), Ash (Fraxinus), Hackberry (Celtis), Alder (Alnus) and Willow (Salix). It had an understory of Buttercup Shrub (Corylopsis) and numerous vines including three types of wild grapes (Vitis) and Sinomenium, a woody vine today restricted to lowland tropical and subtropical forests in East Asia.

It is not clear whether the Gray Fossil Site Forest was a wet or dry ecosystem; some of the plant species there, such as Alders, are known to prefer wetlands others, such as grapes, prefer a drier climate. The animal fossils from the site include Alligator, and lungless Plethodontid Salamanders, which would tend to suggest a moister climate, but the presence of Helodermatid Lizard osteoderms, would suggest a drier climate over at least part of the forrest, it the animals had similar environmental preferences to today.

The location of the Gray Fossil Site. From Shunk, Driese & Clark (2006).

The earliest Helodermatid Lizard known is Primaderma nessovi from the Early Cretaceous of Utah. Other Cretaceous forms known are Paraderma bogerti from Wyoming, and Gobiderma pulchrum and Estesia mongoliensis, both from Mongolia. Known Tertiary forms are Eurheloderma from the Palaeocene of Wyoming, Eurheloderma gallicum from the Eocene of France, Lowesaurus matthewi from the Oligocene to Early Miocene of Colorado and Nebraska, Heloderma texana from the Miocene of Texas and some unnamed Miocene remains from Florida.

Fossil Helodermatid Lizard ellements (A) Left maxilla (upper jaw bone) from Eurheloderma gallicum, from the Eocene Phosphorites du Quercy of France. (B) Right maxilla of same. (C) Maxilla of Paraderma bogerti, from the Early Cretaceous of Wyoming. (D) Right frontal (forehead bone) of Lowesaurus matthewi, from the Oligocene White River Formation of Logan County, Colorado. (E) Cranial bone of Heloderma texana, from the Miocene Delaho Formation of Texas. (F) Skull of same. (G) Right maxilla of (D). From Mead et al. (2012).

This suggests that Helodermatid Lizards originated in what is now Eurasia or North America before the breakup of Pangaea in the Jurassic, and that the common ancestor of Australian and North American Helodermatid Lizards lived in the Cretaceous (rather than having a later, Gondwanan origin and reaching North America from the south with the join-up of the Americas at the beginning of the Pliocene). This would suggest that the preference for hot, dry climates is fairly ancient within the group, making it likely that at least part of the Gray Fossil Site Forest had a dry climate.

Ventral view of the skull of the Gila Monster, Heloderma suspectum, with osteoderms in place. From Mead et al. (2012).

The end of the Miocene and beginning of the Pliocene was a time of global cooling, as North and South America were joined by Central America, preventing the flow of ocean currents between the tropical Pacific and Atlantic Oceans (from this time onwards, all flow between Oceans has been in Antarctic waters). It also saw the spreading of grasslands replacing forests in many parts of the world, and the mixing of North and South American faunas.

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