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By Mike Gannis

• In addition to horns, these lizards have another defense—they can actually squirt blood from their eyes; coyotes and foxes apparently dislike the taste and drop them.

• Horned lizards are specialized for eating ants. They are very difficult to feed in captivity, and often die of starvation.

• Being cold-blooded, horned lizards bask in the sun to warm up and cover their bodies with sand to avoid temperature extremes.

• Females of some species lay eggs, while females of others bear live young, which is thought to be an adaptation for cold environments.

• Several species exhibit “rock mimicry,” in which they change the shape of their bodies to mimic the stones in their habitat.

• They drink during rainstorms by “rain harvesting,” standing with hind legs up and head down, allowing water to drip into their mouths.

• Horned lizards tend to stay within a moderate home range, but sometimes go on long walkabouts, moving hundreds of meters in a day. They have been known to move total distances of up to a mile.

It looks like something from the Mesozoic Era–a squat body covered with sharp spines, head encased in bony armor, dark penetrating eyes, and horns worthy of a Triceratops dinosaur. Phrynosoma cornutum would indeed be a terrifying sight if you were the size of an ant, its usual prey. But many humans find the docile and harmless Texas Horned Lizard (sometimes called a "horny toad") rather endearing. UC Riverside biologist Wendy Hodges certainly likes the creatures, and has studied horned lizards for a decade. Her latest studies of the reptiles involve an attempt to reconstruct the physical features of the common ancestor of the 13 living species of North American horned lizards.

Assisted by visualization expert Reuben Reyes at the Texas Advanced Computing Center (TACC) at the University of Texas at Austin, a member of the National Partnership for Advanced Computational Infrastructure (NPACI), Hodges is applying advanced computer analysis and graphics techniques to three-dimensional data sets acquired through computed tomography (CT). The results are both scientifically informative and visually striking.

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 species of North American horned lizards, each developing a unique set of cranial horns and spikes. But what did this common ancestor look like, and how did the horns develop?

"CT reconstructions enable us to analyze and compare the morphologies of different species, in this case the Texas Horned Lizard, Phrynosoma cornutum, and the Mexican Horned Lizard, Phrynosoma taurus," Hodges said. "The goal of this project is to visualize the evolution of horns in this group of lizards and to determine how horn number increased through evolutionary time."

In their initial efforts, Hodges and Reyes are applying "morphing" programs and algorithms for computing ancestral states in combination with three-dimensional morphologies from CT scans to visualize an intermediate form between two species.

The fossils of horned lizards and other small desert-dwelling

animals are small, fragile, and easily overlooked. The rarity of fossils is one reason for Hodges’ and Reyes’ efforts to reconstruct ancestral forms through computer-based morphing.

PROJECT LEADERS
Wendy L. Hodges, University of California, Riverside
Reuben Reyes,
Texas Advanced Computing Center, University of Texas at Austin

Analysis of DNA sequences plays a key role in determining evolutionary relationships. Over 2,500 base pairs representing four genes from horned lizards were sequenced, aligned, combined with morphological data used in previous studies, and used in phylogenetic analyses to reconstruct the relationships between horned lizard species. But genetic information alone isn’t everything–small differences in gene expression during embryonic development can translate into significant changes in adult morphology. Reconstructing and visualizing the ancestral forms of organisms requires the full combination of comparative genetics, phylogenetics, and 3-D morphology, with a major assist from advanced computer modeling.

Structural information was acquired 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. In its five years of operation, this instrument has scanned a variety of specimens, from rocks, meteorites, and fossils to plant and animal structures.

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.

The reconstruction and morphing software source code is written in C++ using "Performer" graphics libraries. Computational resources used in the effort included a Silicon Graphics Onyx2 computer with 24 processors, 25 gigabytes of main memory, and six "Infinite Reality2" graphics pipelines, and other NPACI computer resources.

"We’re still working on the morphing reconstruction," said Reyes. "After we create visualizations from the combination of data from the Mexican and Texas Horned Lizard species as a test case, we intend to integrate the data for all species. In the future we plan to include non-linear morphing effects, and we hope to be able to visualize how the morphologies varied over time as the various species branched off from one another."

Hodges’ and Reyes’ efforts are conducted in coordination with the University of Texas Digital Morphology Group, an informal association of students, researchers, and educators from many different universities that collaborate to develop useful computer-based tools and exploit digital technologies for research and education. The research is funded in part by the NSF through a Postdoctoral Fellowship in Biological Informatics to Hodges; Ted Garland of UC Riverside is the primary sponsor and Tim Rowe of UT Austin is a co-sponsor. Funding for CT sections is provided through NSF support to Rowe. Additional support for Reyes is provided through TACC.

Mike Gannis is a senior science writer at the San Diego Supercomputer Center.