ly backward, but then, in the lumbar region, each suc- 

 ceeding one tilted a bit farther forward and was taller 

 and broader than the one in front. Even farther back 

 they began to change again, tilting backward but with 

 a forward curve at the top. The vertebral column could 

 thus be divided into regions, which suggests that those 

 regions, and the muscles attached to them, were spe- 

 cialized for distinct jobs. Just how it all worked is not 

 yet clear, but any spinal and muscular division of la- 

 bor is extraordinary in such an early tetrapod. It oc- 

 curs today only in mammals. 



Skull of Acanthostega is shown about three-quarters actual size. The 

 creature had the gills of a fish and the fingers and toes of a tetrapod. 



How did Ichthyostega move? The most recent ev- 

 idence told us that it moved like nothing we had 

 imagined. Ichthyostcga\ body plan would have re- 

 stricted sideways bending — the lateral moves one as- 

 sociates with, say, Acanthostega, or a modern sala- 

 mander. But Ichthyostega could have readily flexed its 

 body up and down. It turned out that our first guess- 

 es about the hind limbs were wrong — they were not 

 very small after all. One possibility is that Ichthyostega 

 moved its forelimbs forward in tandem for a power 

 stroke and thus inchwormed along, a gait more com- 

 mon in modern mammals than in modern reptiles. 

 Think of a seal using its two forelimbs to haul itself 

 out of the water and clamber onto a beach. 



Although Iclithyostega and Acanthostega were con- 

 temporaries in the same geographic area, they 

 lived in contrasting environments. Acanthostega was 

 mostly aquatic; Ichthyostega was partly terrestrial. In 

 locomotion and lifestyle, too, Ichthyostega was as dif- 

 ferent from Acanthostega as chalk is from cheese; pret- 

 ty much all they have in common is four legs and 

 the endings of their names. But beyond those two 

 examples, the understanding of other tetrapods, 

 their diversity, and the environments that hosted 

 them is exploding. The 1987 Acanthostega fossils had 

 unexpectedly revealed distinctive features of the 

 teeth and jaws, features that became the key for rec- 



ognizing the difference between the jaws of early 

 tetrapods and those of the contemporary fish. We 

 reexamined caches of previously indeterminate or 

 largely ignored fossil fragments in museum collec- 

 tions. Sure enough, a series of bone fragments from 

 Scotland, even older than the ones from Greenland, 

 proved to be from Devonian tetrapods. 



The pace of discovery picked up with further ex- 

 amples from Belgium, China, Latvia, Russia, and the 

 United States. Trackways have now confirmed the 

 presence of early tetrapods in Ireland and even in Aus- 

 tralia. Fresh field collecting is 

 proceeding in several of those 

 regions, with an eye toward 

 finding tetrapods. 



In short, Devonian tetra- 

 pods have gone global. Nine 

 species have been named, and 

 more are in the pipeline. With 

 the possible exception of the 

 specimen from Belgium 

 (which may be a close relative 

 of Ichthyostega), each tetrapod 

 is distinct and unique to its 

 own geographic area. Some of 

 the bones are mere frag- 

 ments — strangely, mostly 

 jaws — but the more complete specimens are showing 

 how creatures adapted anatomically to their habitats. 



But what about that first step? Which creature 

 was the first to venture out of the water, and how, 

 when, where, and in what circumstances was that 

 move accomplished? Pinning down the order in 

 which fish gained the necessary anatomical equip- 

 ment would go a long way toward explaining the 

 transition. One of the most important recent dis- 

 coveries helping to answer some of the questions is 

 not a tetrapod. It is, however, as close a relative to 

 a tetrapod as it could be while still being fishlike. In 

 2005 Neil Shubin of the University of Chicago and 

 his team discovered spectacular fossils of a "near 

 tetrapod" on Ellesmere Island in Nunavut Territory, 

 Canada. They named it Tiktaalik. 



T 



iktaalik lived between 385 and 383 million 

 years ago and is one of three kinds of near 

 tetrapods known from the early part of the Late De- 

 vonian. Each one lived in what were then estuaries 

 and river channels around the coasts of the old con- 

 tinent of Laurussia, which straddled the equator and 

 encompassed parts of what are now North Ameri- 

 ca and northern and eastern Europe. Their presence 

 suggests that tetrapods originated in that region, 

 perhaps just a short time later, geologically speak- 

 ing. The earliest evidence of limbs comes from de- 



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NATURAL HISTORY July/August 2006 



