GETTING A LEG UP

in the almost four billion years since life on earth oozed into existence, evolution has generated some marvelous metamorphoses. One of the most spectacular is surely that which produced terrestrial creatures ON bearing limbs, fingers and toes from water-bound fish with fins. Today this group, the tetrapods, encompasses everything from birds and their dinosaur ancestors to lizards, snakes, turtles, frogs and mammals, in- cluding us. Some of these animals have modified or lost their limbs, but their common ancestor had them—two in front and two in back, where LAND fins once flicked instead. Recent fossil discoveries cast The replacement of fins with limbs was a crucial step in this transfor- mation, but it was by no means the only one. As tetrapods ventured onto light on the evolution of shore, they encountered challenges that no vertebrate had ever faced be- four-limbed animals from fish fore—it was not just a matter of developing legs and walking away. Land is a radically different medium from water, and to conquer it, tetrapods BY JENNIFER A. CLACK had to evolve novel ways to breathe, hear, and contend with gravity—the list goes on. Once this extreme makeover reached completion, however, the land was theirs to exploit. Until about 15 years ago, paleontologists understood very little about the sequence of events that made up the transition from fish to tetrapod. We knew that tetrapods had evolved from fish with fleshy fins akin to today’s lungfish and coelacanth, a relation first proposed by American paleontologist Edward D. Cope in the late 19th century. But the details of this seminal shift remained hidden from view. Further- more, estimates of when this event transpired varied wildly, ranging from 400 million to 350 million years ago, during the Devonian period. The problem was that the pertinent fossil record was sparse, consisting of essentially a single fish of this type, Eusthenopteron, and a single Devonian tetrapod, Ichthyostega, which was too advanced to elucidate tetrapod roots. With such scant clues to work from, scientists could only speculate about the nature of the transition. Perhaps the best known of the sce- narios produced by this guesswork was that championed by famed ver- tebrate paleontologist Alfred Sherwood Romer of Harvard University, who proposed in the 1950s that fish like Eusthenopteron, stranded under UP FOR AIR: Acanthostega, an early arid conditions, used their muscular appendages to drag themselves to a tetrapod, surfaces in a swamp in what new body of water. Over time, so the idea went, those fish able to cover is now eastern Greenland, some 360 million years ago. Although this animal more ground—and thus reach ever more distant water sources—were had four legs, they would not have been selected for, eventually leading to the origin of true limbs. In other words, able to support its body on land. Thus, fish came out of the water before they evolved legs. rather than limbs evolving as an Since then, however, many more fossils documenting this transforma- adaptation to life on land, it seems that tion have come to light. These discoveries have expanded almost expo- they may have initially functioned to help the animal lift its head out of nentially our understanding of this critical chapter in the history of life oxygen-poor water to breathe. Only on earth—and turned old notions about early tetrapod evolution, diver- later did they find use ashore. sity, biogeography and paleoecology on their heads.

100 SCIENTIFIC AMERICAN DECEMBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. www.sciam.com SCIENTIFIC AMERICAN 101 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. Finding a Foothold tion. In other words, this animal, though constituting the fin of Eusthenopteron or among the first fossil finds to pave clearly a tetrapod, was primarily an a similar creature. Ordinarily, scientists the way for our modern conception of aquatic creature whose immediate fore- might have dismissed this as an aberrant tetrapod origins were those of a creature runners were essentially fish that had specimen. But a mysterious partial skel- called Acanthostega, which lived about never left the water. The discovery forced eton of Tulerpeton, a previously known 360 million years ago in what is now east- scholars to rethink the sequence in which early tetrapod from Russia, had a six- ern Greenland. It was first identified in key changes to the skeleton took place. digit foot. And specimens of Ichthyo- 1952 by Erik Jarvik of the Swedish Mu- Rather than portraying a creature like stega also found on our expedition to seum of Natural History in Stockholm Eusthenopteron crawling onto land and eastern Greenland revealed that it, too, on the basis of two partial skull roofs. then gaining legs and feet, as Romer pos- had a foot with more than five digits. But not until 1987 did my colleagues and tulated, the new fossils indicated that Findings from developmental biology I finally find specimens revealing the tetrapods evolved these features while have helped unravel some of this mystery. postcranial skeleton of Acanthostega. Although in many ways this animal proved to be exactly the kind of anatom- Many of the critical innovations arose while these ical intermediary between fish and full- beasts were still largely aquatic. And the first changes blown tetrapods that experts might have appear to have been related not to locomotion but to imagined, it told a different story from the one predicted. Here was a creature an increased reliance on breathing air. that had legs and feet but that was other- wise ill equipped for a terrestrial exis- they were still aquatic and only later co- We now know that several genes, includ- tence. Acanthostega’s limbs lacked prop- opted them for walking. This, in turn, ing the Hox series and Sonic Hedgehog, er ankles to support the animal’s weight meant that researchers needed to recon- control elements of fin and limb develop- on land, looking more like paddles for sider the ecological circumstances under ment. The same sets of these genes occur swimming. And although it had lungs, which limbs developed, because Acan- in both fish and tetrapods, but they do its ribs were too short to prevent the col- thostega indicated that terrestrial de- different jobs in each. Hoxd 11 and lapse of the chest cavity once out of wa- mands may not have been the driving Hoxd 13, for instance, appear to play a ter. In fact, many of Acanthostega’s fea- force in early tetrapod evolution. more pronounced role in tetrapods, tures were undeniably fishlike. The bones Acanthostega took pride of place as where their domains in the limb bud are of the forearm displayed proportions the missing link between terrestrial ver- enlarged and skewed relative to those in reminiscent of the pectoral fin of Eusthe- tebrates and their aquatic forebears. the fish fin bud. It is in these regions that nopteron. And the rear of the skeleton There was, however, one characteristic of the digits form. How the five-digit foot showed a deep, oar-shaped tail sporting Acanthostega that called to mind neither evolved from the eight-digit one of Acan- long, bony rays that would have provided tetrapod nor fish. Each of its limbs termi- thostega remains to be determined, but the scaffolding for a fin. Moreover, the nated in a foot bearing eight well-formed we do have a plausible explanation for beast still had gills in addition to lungs. digits, rather than the familiar five. This why the five-digit foot became the de- The piscine resemblance suggested was quite curious, because before this fault tetrapod pattern: it may have helped that the limbs of Acanthostega were not discovery anatomists believed that in the make ankle joints that are both stable only adapted for use in water but that transition from fish to tetrapod, the five- enough to bear weight and flexible this was the ancestral tetrapod condi- digit foot derived directly from the bones enough to allow the walking gait that tet- rapods eventually invented. Acanthostega also drew attention to Overview/The Origin of Tetrapods a formerly underappreciated part of early ■ The emergence of land-going vertebrates was a cornerstone event in the tetrapod anatomy: the inside of the lower evolution of life on earth. jaw. Fish generally have two rows of teeth ■ For decades, a paltry fossil record obfuscated efforts to trace the steps that along their lower jaw, with a large num- eventually produced these terrestrial tetrapods from their fish ancestors. ber of small teeth on the outer row com- ) ■ Fossils recovered over the past 15 years have filled many of the gaps in the plementing a pair of large fangs and some story and revolutionized what is known about tetrapod evolution, diversity, small teeth on the inner row. Acantho- biogeography and paleoecology. stega showed that early tetrapods pos- ■ These recent finds indicate that tetrapods evolved many of their sessed a different dental plan: a small preceding pages

characteristic features while they were still aquatic. They also reveal that number of larger teeth on the outer row ( early members of the group were more specialized and more geographically and a reduction in the size of the teeth and ecologically widespread than previously thought. populating the inner row—changes that

probably accompanied a shift from feed- RAÚL MARTÍN

102 SCIENTIFIC AMERICAN DECEMBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. TURNING TETRAPOD The evolution of terrestrial tetrapods from aquatic lobe-fi nned the tail fi n disappeared, as did a series of bones that joined fi sh involved a radical transformation of the skeleton. Among the head to the shoulder girdle (skeletons). Meanwhile other changes, the pectoral and pelvic fi ns became limbs the snout elongated and the bones that covered the gills and with feet and toes, the vertebrae became interlocking, and throat were lost (skulls).

EUSTHENOPTERON Noninterlocking A lobe-fi nned fi sh Very short ribs vertebrae Three Short snout with many bones midline Opercular bones fi ns covering gills and throat

Pelvic fi n Skull joined to Pectoral fi n with with bony rays shoulder bony rays Small pelvis unattached to spine

ACANTHOSTEGA Interlocking vertebrae One midline fi n An early tetrapod Longer ribs Longer snout with fewer bones

Absence of opercular bones

Front limb with Hind limb with eight-digit foot eight-digit foot Separation of Larger pelvis skull from shoulder attached to spine

IGUANIA Skull decoupled from shoulder to form neck A modern iguana No midline fi ns Long snout with Interlocking few bones vertebrae Absence of opercular bones

Long curved Large pelvis ribs attached to spine Weight-bearing front Weight-bearing hind limb with fi ve-digit foot limb with fi ve-digit foot

ing exclusively in the water to feeding on these genera paleontologists have known and name this species formally, but it is land or with the head above the water. about for several decades but have only shaping up to be a fascinating animal. This insight enabled experts to recog- recently scrutinized: 380-million- to nize additional tetrapods among remains 375-million-year-old Panderichthys A Breath of Fresh Air that had long sat unidentifi ed in museum from Europe’s Baltic region, a large fi sh thanks to these recent fi nds and drawers. One of the most spectacular of with a pointy snout and eyes that sat atop analyses, we now have the remains of these fi nds was that of a Late Devonian its head, and 375-million- to 370-mil- nine genera documenting around 20 mil- genus from Latvia called Ventastega. In lion-year-old Elpistostege from Canada, lion years of early tetrapod evolution and the 1990s, following the discovery of which was very similar in size and shape an even clearer idea of how the rest of the Acan thostega, researchers realized that to Panderichthys. Both are much closer vertebrate body became adapted for life a lower jaw collected in 1933 was that of to tetrapods than is Eusthenopteron. on land. One of the most interesting rev- a tetrapod. Further excavation at the And just last year an expedition to Elles- elations to emerge from this work is that, original Ventastega site soon yielded mere Island in the Canadian Arctic led as in the case of limb development, many more material of exceptional quality, in- by paleontologist Neil Shubin of the Uni- of the critical innovations arose while cluding an almost complete skull. versity of Chicago produced some out- these beasts were still largely aquatic. Meanwhile a number of near-tetra- standingly well preserved remains of a And the fi rst changes appear to have been pod fi sh have also been unveiled, bridg- fi sh that is even more tetrapodlike than related not to locomotion but to an in- ing the morphological gap between Eus- either Panderichthys or Elpistostege. creased reliance on breathing air.

ANDREW RECHER thenopteron and Acanthostega. Two of Shubin and his team have yet to describe Oddly enough, this ventilation shift

www.sciam.com SCIENTIFIC AMERICAN 103 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. PRIMEVAL PROMENADE: Ichthyostega is the earliest known tetrapod to hind limbs, as well as an ear that appears to have been specialized for show adaptations for nonswimming locomotion, although it seems likely underwater use. How Ichthyostega divided its time between the to have moved more like a seal than a typical land vertebrate. This animal terrestrial and aquatic realms is uncertain. But it may have dug nests also had some aquatic features, including a large tail and flipperlike for its eggs on land and hunted and fed in the water. may have kicked off the gradual morph- Breathing above water also required the ones that initially ventured farthest ing of the shoulder girdle and pectoral a number of changes to the skull and jaw. out of the water to get them. fins. Indeed, evolutionary biologists have In the skull, the snout elongated and the Meanwhile, farther back in the skel- struggled to explain what transitional bones that form it grew fewer in number eton, a series of bones that joins the head forms like Acanthostega did with their and more intimately sutured together, to the shoulder girdle in fish disappeared. proto-limbs, if not locomote. The hy- strengthening the snout in a way that en- As a result, tetrapods, unlike fish, have a pothesis favored on current evidence is abled the animal to lift it clear of water muscular neck that links the head to the that as the backwardly directed fins and into an unsupportive medium. The rest of the skeleton and allows for move- gradually turned into sideways-facing bones at the back of the head, for their ment of the head separate from the body. limbs with large areas for muscle attach- part, became the most firmly integrated The gill system also underwent substan- ments, they gained in strength. And al- of any in the skull, providing sturdy an- tial renovation, losing some bones but though it would be millions of years be- chors for muscles from the vertebral col- increasing the size of the spiracle—an fore the forelimbs developed to the point umn that raise the head relative to the opening on the top of the head that led of being able to support the body on body. And the fusing of bones making up to an air-filled sac in the throat region, land, they may well have functioned in the lower jaw fortified this region, facili- making the entire respiratory apparatus the interim to allow the animal to raise tating the presumed “buccal pump” better suited to breathing air. its head out of the water to breathe. The mode of tetrapod ventilation. In this type But why, after millions of years of toes could have facilitated this activity by of breathing, employed by modern am- successfully breathing underwater, did 280 million helping to spread the load on the limbs. phibians and air-breathing fish, the some fish begin turning to the air for years ago

Last year Shubin’s team announced mouth cavity expands and contracts like their oxygen? Clues have come from the ga a LIVING a g te eg TETRAPODS the discovery of a 365-million-year-old bellows to gulp air and force it into the overall shape of the skull, which in all te os eton as th yost rp nt th le tetrapod upper arm bone, or humerus, lungs. Buccal pumping may have de- early tetrapods and near-tetrapods dis- an h Ve Ic Tu 362 that has bolstered this idea. The bone, manded more jaw power under the influ- covered so far is quite flat when viewed n Ac on to r e te rp dug from a fossil-rich site in north central ence of gravity than in the water, where head-on. This observation, combined p s ege s hy UPPE R Pennsylvania known as Red Hill, ap- organisms are more or less weightless. with paleoenvironmental data gleaned ost gine heno cht st El st ri a DE VON IAN pears to have joined the rest of the body Might the strengthening of the jaws from the deposits in which the fossils Eu de Elpi ian olepidid n eolepidids on via a hingelike joint, as opposed to the have instead come about as an adapta- have been found, suggests that these tet Pa Os Liv 382 ball-and-socket variety that we and oth- tion for feeding on land? Possibly. The creatures were shallow-water specialists, er terrestrial vertebrates have. This ar- earliest tetrapods were all carnivorous, going to low-water places to hunt for s MIDD LE rangement would not have permitted a so it is unlikely that, as adults, they fed smaller fish and possibly to mate and lay hy DE VON IAN walking gait, but it would have enabled much on land during the first phases of their eggs. Perhaps not coincidentally, 394 just the kind of push-up that a tetrapod their evolution, because the only prey vascular plants were flourishing during Kenicht LLOOWEWE R needing a gulp of air might employ. It they would have found there were in- the Devonian, transforming both the ter- OTHER LOBE-FINNED FISH DE VON IAN also might have helped the animal hold sects and other small arthropods. The restrial and aquatic realms. For the first its position in the water while waiting to babies, on the other hand, needed just time, deciduous plants shed their leaves

ambush prey. this type of prey, and they may have been into the water with the changing seasons, RAÚL MARTÍN

104 SCIENTIFIC AMERICAN DECEMBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. creating environments that were attrac- fish this air sac “catches” sound waves, JENNIFER A. CLACK, a Reader in verte- tive to small prey but difficult for big fish preventing them from simply passing brate paleontology and doctor of sci- to swim in. Moreover, because warm wa- straight through the animal’s body. From ence at the University of Cambridge, ter holds less oxygen than colder water there they are transmitted by the sur- has been studying tetrapod origins for does, these areas would have been oxy- rounding bones to the inner ear. The en- 25 years. A fellow of the Linnean Soci-

gen-poor. If so, the changes to the skele- larged air chamber evident in Panderich- T H E A U T H O R ety, Clack’s outside interests include ton described here may have given early thys would have been able to intercept choral singing (particularly of early sa- tetrapods access to waters that sharks more sound waves, thereby enhancing cred music) and gardening. She is also and other large fish could not reach by the animal’s hearing ability. a motorcyclist and rides a Yamaha Di- putting them literally head and shoulders Modifications to the ear region were version 900. above the competition. It was just hap- also closely tied to those in the gill sys- penstance that these same features would tem. To wit: a bone known as the hyo- es have necessitated a sea change in the later come in handy ashore. mandibula—which in fish orchestrates way we regard early tetrapods. Gone are These breathing-related innovations feeding and breathing movements— the clumsy chimeras of popular imagina- sent tetrapods well on their way to be- shrank in size and got lodged in a hole in tion, fit for neither water nor land. What coming land-worthy. Getting a grip on the braincase, where it became the sta- were once considered evolutionary works terra firma required further modifica- pes. In modern tetrapods the stapes in progress—an incompletely developed tions to the skeleton, however. An over- magnifies sound waves and transmits limb or ear, for example—we now know haul of the ear region was one such devel- them from the eardrum across the air were adaptations in their own right. opment. Many of the details of this trans- space in the throat to the inner ear. (In They were not always successful, but formation are still largely unknown. But mammals, which have a unique hearing they were adaptations nonetheless. At it is clear that even in the tetrapodlike fish system, the stapes is one of the three os- each stage of this transition were innova- that still had fins, Panderichthys among sicles making up the middle ear.) The tors pushing into new niches. Some, in them, the part of the skull behind the eyes first stage of conversion must have oc- fact, were highly specialized to do this. had already become shorter, following a curred rapidly, given that it was in place shrinking of the capsules that house the by the time of Acanthostega. Quite pos- Breaking the Mold inner ears. If, as paleoenvironmental evi- sibly it proceeded in tandem with the by and large, the limbed tetrapods dence suggests, Panderichthys dwelled in shift from fins to limbs with digits. But and near-tetrapods unearthed thus far shallow tidal flats or estuaries, the reduc- the stapes would not take on its familiar have been sizeable beasts, around a me- tion in the inner ear may reflect the grow- role as a component of the terrestrially ter long. They preyed on a wide variety ing influence of gravity on the vestibular adapted tympanic ear for millions of of invertebrates and fish and were prob- system, which coordinates balance and years. In the meantime, it apparently ably not fussy about which ones. We are orientation. At the same time an increase functioned in these still aquatic tetrapods beginning to find exceptions to this gen- in the size of the air chamber in its throat as a structural component of the skull. eralist rule, however. One is Livoniana,

LUCY READING-IKKANDA may have aided hearing. In some modern Taken together, these skeletal chang- discovered in a museum in Latvia by Per Erik Ahlberg of Sweden’s Uppsala Uni- versity in 2000. This animal is repre- 280 million years ago sented by some lower jaw fragments

ga a LIVING that exhibit a bizarre morphology: in- a g te eg TETRAPODS te os eton stead of the usual two rows of teeth lin- as th yost rp nt th le an h ing each side of the jaw, it had seven Ve Ic Tu 362 n Ac rows. Exactly what Livoniana might on to r e te rp p s ege have been consuming with this corn-on- s hy UPPE R ost gine the-cob dentition we do not know. But heno cht st El st ri a DE VON IAN Eu de Elpi ian it most likely had a diet apart from that olepidid n eolepidids on tet Pa of its brethren. Os Liv 382 Renewed work on the first known Devonian tetrapod, Ichthyostega, is s MIDD LE hy DE VON IAN showing that it, too, diverged from the — 394 norm contrary to earlier preconcep- Kenicht tions. The ear region and associated parts LLOOWEWE R of the braincase of Ichthyostega have OTHER LOBE-FINNED FISH DE VON IAN long baffled researchers because they dis- TETRAPOD RELATIONS: Tetrapods arose from lobe-finned fish like Eusthenopteron some 380 million to 375 million years ago, in the late Middle Devonian period. play a construction unlike that of any other tetrapod or fish from any period.

www.sciam.com SCIENTIFIC AMERICAN 105 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. DEVONIAN DISCOVERIES Fossils of early tetrapods and near-tetrapod fi sh tropics and subtropics of the ancient landmasses Mountain- have turned up at sites as far-fl ung as northwestern Laurasia and Gondwana. And the earliest tetrapods, building areas China and the eastern U.S. As a result, it is now it seems, inhabited freshwater and brackish water apparent that these animals lived throughout the environments rather than strictly marine ones. Land areas

Shallow epicontinental Tulerpeton, Tula region, Russia seas Livoniana, Latvia and Estonia

Acanthostega, eastern Greenland Panderichthys, Latvia and Estonia

Ichthyostega, eastern Greenland Sinostega, Ningxia Hui autonomous L A U R A S I A region, China

Humerus from unknown genus, Red Hill, Pa.

Metaxygnathus, New South Wales, Hynerpeton, Australia Red Hill, Pa. G O N D W A N A

Ventastega, Elpistostege, Quebec Pavari / Ketleri Formation, Latvia

But with the aid of new fossils, fresh prep- on land. It had incredibly powerful shoul- seem to have contributed much forward aration of previously collected material ders and forearms. And the ribs of the thrust during locomotion—the robust and, crucially, CT scanning of key speci- chest region were very broad and over- forelimbs and large shoulders provided mens, my colleagues and I have begun to lapping, forming a corset that would have that. Thus, on land Ichthyostega may make sense of this mysterious construc- prevented the chest cavity and lungs from have moved rather like a seal, fi rst raising tion. The best interpretation seems to be collapsing when on the ground. Even so, its back, then advancing both forelimbs that Ichthyostega possessed a highly spe- Ichthyostega probably did not locomote simultaneously, and fi nally hauling the cialized ear, but one that was geared for like a standard-issue land vertebrate. For rest of its body forward. use underwater. Instead of having an ear- one thing, its ribcage would have restrict- In September, Ahlberg, Henning drum, as many modern terrestrial ani- ed the lateral undulation of the trunk that Blom of Uppsala University and I pub- mals do, at each side of the back of the typically occurs in tetrapod movement. lished a paper detailing these fi ndings in head lay a chamber with strengthened And in contrast to fi sh, Acanthostega or the journal Nature. If we are correct, top and side walls that was probably fi lled other early tetrapods, Ichthyostega had Ichthyostega is the earliest vertebrate on ) with air. Into the membranous fl oor of spines on its vertebrae that changed di- record that shows some adaptations for map this chamber stretched a spoon-shaped rection along the spinal column, hinting nonswimming locomotion. It is impos- and very delicate stapes, which presum- that the muscles they supported were sible to say with certainty what Ichthyo- ably vibrated in response to sound im- specialized for different jobs and that it stega was doing ashore. It may have been pinging directly on the air in the cham- moved in a unique fashion. This multidi- eating stranded fi sh there but reproduc- ber, transmitting these vibrations to the rectional arrangement of the vertebral ing in water, in which case it could have

inner ear through a hole in the wall of the spines parallels that in mammals today, used its specialized ear to listen for po- ; LUCY READING-IKKANDA ( ) braincase. This arrangement would im- but it was unheard of in Devonian tetra- tential mates. (This scenario implies that ply that Ichthyostega spent a good deal of pods until we studied Ichthyostega. All Ichthyostega was making noises as well time in water. Likewise, the animal’s tail told, this latest evidence suggests that, as listening to them.) Alternatively, Ich- tetrapods fi n and fl ipperlike hind limbs suggest an rather than bending in the horizontal thyostega may have been eating in the aquatic lifestyle. plane, as the body of a fi sh does, the body water and listening for prey there, where- Yet other parts of the Ichthyostega of Ichthyostega bent mainly in a vertical as it was using its forelimbs to dig nests

skeleton bespeak an ability to get around plane. The paddlelike hind limbs do not for its eggs on land. Ultimately, however, ANDREW RECHER (

106 SCIENTIFIC AMERICAN DECEMBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. its particular body plan was doomed, vironments in which tetrapods evolved. mysteries, as will insights from evolu- because no fossil dating later than 360 But the discoveries of such creatures as tionary developmental biology. To that million years ago can be reliably attrib- Ventastega and Tulerpeton in deposits end, studies of the genetic-control mech- uted to the Ichthyostega lineage. No representing settings of varying salinity anisms governing the formation of the doubt there were many such superseded have called that notion into question. gill region in fish and the neck area in designs over the course of early tetrapod The Red Hill site in Pennsylvania has mammals and birds are just beginning evolution. Further work will be needed proved particularly rich in providing a to provide hints about which processes to confirm these ideas, but the latest data context for the tetrapods, yielding many characterize both tetrapods and fish and demonstrate that Devonian tetrapods fish species as well as invertebrates and which are unique to tetrapods. For ex- were more diverse than previously sus- plants. Like the eastern Greenland de- ample, we know that tetrapods have lost pected. We are learning to expect more posits, it represents a river basin. Yet pa- all the bones that protect the gills in fish such surprises as these animals and their leoenvironmental studies suggest that but that the genes that govern their for- relatives become better known. the region had a temperate climate, rath- mation are still present in mice, where er than the monsoonal conditions associ- they function differently. We have also Have Legs, Will Travel ated with the Greenland finds. That is to ascertained that in the neck region, the the fossils uncovered over the say, early tetrapods may have been even biochemical pathways that preside over past two decades have done more than more widespread than we thought. the development of limbs have broken allowed scientists to trace many of the changes to the tetrapod skeleton. They have also provided fresh insights into Although we now have a good explanation for why the when and where these creatures evolved. front limbs evolved the way they did, we lack one for We are now reasonably certain that tet- the origin of the hind limbs because none of the fossils rapods had emerged by 380 million to 375 million years ago, in the late Middle recovered so far contains any clues about them. Devonian, a far tighter date range than the one researchers had previously pos- Unfinished Business down. Although biologists can easily in- tulated. We have also determined that we still have much to learn about duce extra limbs to grow on the flank of the early representatives of this group changes in anatomy that accompanied a tetrapod, this cannot be done in the were nothing if not cosmopolitan. the rise of tetrapods. Although we now neck. Something special happened when Devonian tetrapods were scattered have a reasonable hypothesis for why the tetrapods first evolved a neck that pre- across the globe, ranging from locations shoulder girdle and front limbs evolved vented limbs from sprouting there. that are now China and Australia, where the way they did, we lack an adequate ex- Other questions may be more diffi- creatures known as Sinostega and Meta- planation for the origin of the robust cult to answer. It would be wonderful to xygnathus, respectively, have turned up, hind-limb complex—the hallmark of a know which one of the many environ- to the eastern U.S., where the Red Hill tetrapod—because none of the fossils re- mental contexts in which tetrapod fos- humerus and a beast called Hynerpeton covered so far contains any clues about it. sils have turned up nurtured the very were found. Placing the fossil localities Only specimens of Ichthyostega and first members of this group (the available onto a paleogeographic map of the time, Acanthostega preserve this part of the evidence indicates only that these ani- we see that these animals dwelled anatomy, and in both these animals the mals did not debut in strictly marine set- throughout the tropics and subtropics of hind limbs are too well formed to reveal tings). We would also like to compre- a supercontinent comprising Laurasia to how they took shape. Almost certainly no hend fully the evolutionary pressures at the north and Gondwana to the south. single scenario can account for all the work during each phase of the transi- Their near-ubiquitous distribution in the stages of the transition. We also want to tion. Lacking a perfect fossil record or warmer climes is a testament to how suc- acquire a higher-resolution picture of the recourse to a time machine, we may nev- cessful these creatures were. order in which the changes to the skeleton er piece together the entire puzzle of tet- Within these locales, Devonian tetra- occurred, say, when the hind limb evolved rapod evolution. But with continued pods inhabited a startlingly wide range relative to the forelimb and the ear. work, we can expect to close many of the of environments. Deposits in eastern The discovery and description of ad- remaining gaps in the story of how fish Greenland that were the first to yield ditional fossils will resolve some of these gained ground. such creatures indicate that the area was once a broad river basin dominated by MORE TO EXPLORE periodic floods alternating with drier Gaining Ground: The Origin and Evolution of Tetrapods. Jennifer A. Clack. Indiana University conditions. The river was unequivocally Press, 2002. freshwater in origin and thus formed the The Emergence of Early Tetrapods. Jennifer A. Clack in Paleogeography, Paleoclimatology, basis for received wisdom about the en- Paleoecology (in press).

www.sciam.com SCIENTIFIC AMERICAN 107 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.