They climb walls and scuttle upside 

 down across ceilings, dive to the ocean 

 floor to feed on algae, even glide 

 through the air from treetops. Some, with no 

 limbs and extremely long tails, look like snakes; 

 others are snakes. Many are nearly invisible in 

 their home habitats; others announce their pres- 

 ence to their neighbors and warn off potential 

 rivals by flashing colorful dewlaps, or fanlike 

 structures, that lie just underneath their lower 

 jaws [see photograph on page 32]. They dine on 

 everything from microscopic insects to hefty 

 animals. Some wield lethal toxins through long, 

 hollow fangs. They are members of the order 

 Squamata, "the scaly ones," more popularly 

 known as lizards and snakes. They are also one 

 of the most successful orders of tetrapod (four- 

 legged) vertebrates on Earth. In some people 

 they provoke a phobic reaction (particularly the 

 snakes). But we, among many others, find them 

 irresistibly fascinating. Then again, our interest is 

 also a professional one: we have devoted our ca- 

 reers to them. 



What particularly fascinates us is how, long 

 ago, a common ancestor of squamates could 

 have given rise to so many diverse descendants. 

 Their living representatives — numbering more 

 than 7,200 species — inhabit every continent ex- 

 cept Antarctica and even many oceanic islands. 

 When we look at them, we know that some of 

 their similarities and differences reflect recent 

 evolutionary adaptations, made in response to 

 other species in their present-day environments. 

 Other characteristics are a legacy of more an- 

 cient adaptations in response to unknown con- 

 ditions — early choices that set various groups 

 on separate evolutionary trajectories. 



But with so many species and habitats, how 

 can one reconstruct the group's evolutionary his- 

 tory? Fossils tell only a limited story, and they are 

 relatively rare. (Many of the ancestral species 

 were small, and so their bones were less likely to 

 be preserved — and more likely to be overlooked 

 by paleontologists seeking bigger, more spectacu- 

 lar finds.) Investigators have barely begun 

 to probe the genetic data that might bring us 

 closer to understanding the evolutionary rela- 

 tions among living groups. But we think enough 

 clues exist to sketch a coherent story. 



Our story begins when squamates and their 

 nearest relatives, the Rhynchocephalia 

 ("beak-headed" reptiles, so named because their 

 jaws have a beaklike tip) split from a common 



ancestor. According to Susan Evans, a paleontol- 

 ogist at University College Loudon, the branch- 

 ing probably took place sometime during the 

 Lower or Middle Triassic periods, between 251 

 million and 228 million years ago. At that time 

 Earth's landmasses were united in one supercon- 

 tinent, Pangaea. No one can be sure exactly 

 when the split occurred, because the earliest 

 known squamate fossils date only from Lower 

 Jurassic sediments, between 200 million and 175 

 million years ago, but those fossils suggest the 

 group had already been evolving on its own for 

 some time. Another reason paleontologists think 

 squamates are older than their oldest known fos- 

 sils is that earlier fossils have been discovered be- 

 longing to their "sister" group, the rhyncho- 

 cephalians. That group was widespread and 

 diverse before the end of the Middle Triassic, but 

 only two descendants have survived to the pre- 

 sent: they are the two species of tuataras that 

 occur in New Zealand. 



The common ancestor of squamates and rhyn- 

 chocephalians was likely a small lizardlike reptile 

 that ate insects and spiders. Although many skele- 

 tal features distinguish squamates from their sis- 

 ter group, one of the most important is found in 

 the lower jaw. In the common ancestor, the lower 

 jaw rotated from a pivot point on the bottom 

 rear of the skull, which otherwise was relatively 

 rigid. With the evolution of squamates, the skull 

 bone that connected to the lower jaw — the 

 quadrate — became only loosely attached by liga- 

 ments to the rest of the skull. That new hingelike 

 configuration, known as streptostyly, enabled the 

 back of the jaw to move more freely [see illustra- 

 tion on page 31]. In practice, it enabled the animal 

 to deliver a faster and more powerful bite, and 

 perhaps to open its mouth wider as well, making 

 it much easier for ancestral squamates to capture 

 and handle prey. 



Sometime between 30 million and 60 million 

 years after the split between squamates and 

 rhynchocephalians, the squamates themselves 

 split into two major groups, Iguania and Scle- 

 roglossa. Fossil evidence for the timing of that 

 split is sparse, but it is consistent with the 

 amount of divergence evident in the DNA of 

 living species — a question investigated by J. 



An overfed, captive day gecko (Phelsuma madagascarien 

 sis ) from Madagascar licks the transparent scale covering 

 its eye. Lacking eyelids, the gecko cleans its eyes with its 

 tongue. Another gecko parlor trick is walking upside 

 down, thanks to many millions of microscopic filaments 

 that make up the surface of its toe pads. 



July /August 2006 naickm HISTORY 29 



