posits in Scotland that are around 376 million years 

 old. Taken together, the evidence seems to peg the 

 origin of tetrapods neatly to a period of less than 

 ten million years in the Late Devonian, and to a rec- 

 ognizable geographical area. 



Because of its nearly complete preservation, Tik- 

 taalik has also answered major questions about the 

 order in which tetrapodlike physical features arose, 

 and perhaps even what they evolved to do. Changes 

 to the head came first, in response to a greater re- 

 liance on air breathing. Tiktaalik had lost a series of 

 bones that join the head to the shoulder in fishes and 

 that protect and help operate the gill-breathing sys- 

 tem. Thus, Tiktaalik haci a neck — an outstanding 

 tetrapod feature. The neck gave the head a greater 

 range of movement, enabling the animal to raise its 

 head out of the water to gulp air. Compared to 

 Eusthenopteron and similar fish, the pocket in which 

 air was exchanged or stored had become enlarged, 

 and some of the gill apparatus was already evolving 

 toward the configuration that tetrapods eventually 

 used for hearing. 



Crucially, though, the forelimbs of Tiktaalik com- 

 bine traits of fishes and tetrapods. Although its fins 

 still had fin rays and webbing, they were much short- 

 er than those of its nearest fish relatives. The skele- 

 tons of the fins were robust and flexible in much the 

 same ways as tetrapod limbs. The fins could have 

 served as props for the body — Tiktaalik 

 had enlarged ribs that served as a kind £gQ§ 

 of scaffold for trunk musculature that 

 stiffened the body — as well as a rudi- 

 mentary means of walking. The fin 

 bones, however, were still more 

 similar to the fin bones of closely 

 related fishes than to the digits that 

 eventually evolved in the ear- 

 liest tetrapods. Tiktaalik helps 

 confirm that key anatomi- 

 cal changes took place while 

 the animals were still techni- 

 cally fish, and that develop- 

 ments to the limbs began 

 with modifications in func- 

 tion, before the main changes 

 in form evolved. 



The next step for us is to 

 go beyond the bones. If 

 we could look at sediments 

 in which tetrapods were pre- 

 served along with other con- 

 temporary plants and ani- 

 mals, we could put tetrapods 

 in their ecological context. 



Right hind limb of Ichthyostega, measuring six 

 inches long, could not have borne its share of 

 the animal's weight. It is paddlelike and shows 

 no real knee or ankle joint. The triangular bone 

 at the top of the fossil is the pelvis. 



A chance to do just that emerged in a Pennsylvania 

 site known as Red Hill. It has yielded several frag- 

 ments belonging to at least three distinct tetrapod 

 species. The surrounding sediments showed that the 

 climate was warm and subtropical, yet seasonally 

 variable. The Red Hill tetrapods lived in a river basin 

 surrounded by diverse flora and fauna. 



About 385 million years ago, or a little less, dense 

 forests were beginning to grow beside rivers and lakes. 

 The onset of a seasonal climate in the Late Devon- 

 ian had promoted the evolution of deciduous trees. 

 As they dropped their leaves into the water, bacteria 

 that feasted on their remains used up most of the oxy- 

 gen in the water. At the same time, oxygen levels in 

 the atmosphere were beginning to rise, and so any 

 creature that was more effective than other animals 

 at breathing air would have had an advantage. 



Invertebrates such as millipedes, mites, scorpions, 

 and primitive wingless insects had crawled onto land 

 and filled an array of damp, green habitats. They had 

 also gotten bigger and become potential sources of 

 food for four-footed predators. Once true limbs 

 evolved, tetrapods were better able to exploit areas 

 of densely vegetated shallow water and swamp that 

 fish with fins could not. 



In the past twenty years, discoveries have radical- 

 ly changed what we paleontologists think about the 

 origin of four-legged animals. Important finds have 

 altered what we thought we 

 knew about the gradual 

 transition from water to 

 land, as some pieces of the 

 puzzle have been moved 

 about and others added. 

 With Tiktaalik, an inter- 

 mediate stage between 

 finned Eusthenopteron and 

 footed Ichthyostega has come 

 into focus. Acanthostega and 

 Ichthyostega have revealed 

 their bone structures, modes 

 of moving, and lifestyles. The 

 Red Hill fossils have provided 

 an ecological context. More ex- 

 ploration will only make the story 

 richer. The same holds for the knowl- 

 edge of how tetrapods diversified and 

 spread out. The early ones traveled quick- 

 ly on their four feet. From one point of ori- 

 gin, they soon fanned out into 

 far-flung parts of the Late De- 

 vonian world and adaptivelv 

 made themselves at home — on 

 on feet. The rest is evolu- 



i.inii 



tionarv history. 



□ 



July/August 2006 NATURAL H1STOK.Y 



41 



