Robert Macey, a molecular biologist then at Wash- 

 ington University in St. Louis, and his colleagues. 

 Intrigumgly, the split appears to coincide with the 

 breakup of Pangaea into two supercontinents, 

 Laurasia and Gondwana. 



For some time, paleontologists reasonably theo- 

 rized that that event, by separating two populations 

 of squamates, initiated the evolutionary divergence 

 of the two major groups. Investigators now know 

 that early fossils of both groups were deposited in 

 both zones of Pangaea before the land mass broke 

 up. Nevertheless, that geological event apparently 

 bore some relation to the evolutionary one. Most of 

 the early diversification of iguanians took place on 

 Gondwana, the southern continent, whereas most 

 of the early diversification of scleroglossans took 

 place on Laurasia, the northern continent. Why that 

 is, however, remains unclear. 



w 



hen Iguania and Scleroglossa diverged, igua- 

 nians retained most of the characteristics of 



their squamate ancestors. Iguanian skulls remained 

 relatively rigid. They continued the early squamate 

 lifestyle of lying in wait for prey and remaining 

 cryptic — hidden or camouflaged — except when 

 pursuing prey. And they continued to rely on vision 

 to detect and discriminate prey and on what Kurt 

 Schwenk, a functional anatomist at the University 

 of Connecticut in Storrs, calls lingual prehension — 

 literally, holding with their tongues — to capture and 

 manipulate prey. Schwenk argues that lingual pre- 

 hension was the ancestral mode of feeding in tetra- 

 pod vertebrates, and so in squamates it may repre- 

 sent the inheritance of a very ancient trait. 



The diversification of iguanians has been spec- 

 tacular. Today they include most insect-eating 

 lizards that tend to pursue their prey from fixed 

 perches, such as chameleons; many other insectiv- 

 orous species; and nearly all large plant-eating 

 lizards around the globe — including iguanas, the 

 group's namesake. Among iguanians is Moloch, the 

 thorny devil of Australia [see photograph below], a 



Australian thorny devil (Moloch horridus), a lizard in the family Agamidae, specializes in eating 

 ants, which it captures with its sticky tongue. Otherwise harmless, the lizard, shown here about 

 life size, protects itself by sporting spiny scales, changing its coloration to match its background, 

 walking slowly and jerkily (or freezing in place), and holding its tail erect (which may make the ani- 

 mal look like a plant). When threatened, it puffs up and bends its head down between its legs, ex- 

 posing the peculiar hump, or "false head, " on the back of its neck, instead of its real head. 



NATURAL HISTORY July/August 2006 



