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A coast horned lizard
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A coast horned lizard

June 1, 2010
A coastal horned lizard (Phrynosoma coronatum). The coastal horned lizard may look formidable with its sharp body scales and horns on its head, but it is a harmless species that can easily be picked up. The coastal horned lizard is one of 13 living species of North American horned lizards.

Horned lizards are cold-blooded and bask in the sun to warm up or cover their bodies with sand to avoid temperature extremes. The females of some species will lay eggs, while females of others bear live young, which is thought to be an adaptation for cold environments. Several species have been known to exhibit rock mimicry, where they'll change the shape of their bodies to mimic the stones in their habitat. Although horned lizards usually stay in close range of home, they have been known to travel a long distance, moving hundreds of meters in a day and in some cases as far as a mile!

Between 23 and 30 million years ago, the first North American horned lizards branched off from sand lizards, becoming stockier and spinier and evolving their distinctive crowns of horns. A single common ancestor gave rise to the 13 North American species, each developing a unique set of cranial horns and spikes. Researchers are trying to determine what this prehistoric, common ancestor may have looked like and how the horns of these species developed.

Coastal horned lizards were once common in chaparral and coastal sage scrub habitats of California and Baja, California, but heavy exploitation by the curio, biological supply and pet trades have greatly diminished their populations. Their most serious threat today is habitat loss. [Image 1 of 9 related images. See Image 2.]

More about this Image Wendy Hodges, a biologist at the University of California, Riverside, has studied horned lizards for a decade. Her latest studies of the reptiles involve an attempt to reconstruct the physical features of their ancient common ancestor. With the assistance of Reuben Reyes, a visualization expert from the Texas Advanced Computing Center (UT-Austin), Hodges is applying advanced computer analysis and graphics techniques to 3-D data sets acquired through computed tomography (CT). CT reconstructions allows the analyses and comparison of the morphologies of different species of horned lizards. The final goal of the project is the visualization of the evolution of horns in this group of lizards and determining how horn number increased through evolutionary time.

Hodges and Reyes have been applying "morphing" programs and algorithms for computing ancestral states in combination with 3-D morphologies from CT scans to visualize an intermediate form between two horned lizard species. Hodges acquired structural information from preserved horned lizard museum specimens using a high-resolution CT scanner at the National Science Foundation (NSF)-supported University of Texas High-Resolution X-ray Computed Tomography Facility. The scanner is comparable to a conventional medical diagnostic CT device, but was custom-built to have greater resolution and penetrating power and was specifically designed to explore the internal structures of natural objects and materials at macro- and microscopic levels.

Eventually, images of each horned lizard species' head will be created from the CT data using visualization software, to reveal both the external configurations and the internal structures of the specimens. One visualization application depicts the external skin and internal skeleton using a color-mapping scheme that represents the distance from the skin to the bone, giving some of the lizard head reconstructions a polychromatic, "tie-dyed" appearance.