NEWS & VIEWS NATURE|Vol 440|6 April 2006

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however, is not to Panderichthysbut to anotheranimal, Elpistostege, from the early LateDevonian of Canada. Elpistostegeis knownonly from two partial skulls and a length ofbackbone, but it has long been recognized as afish– tetrapod intermediate11,12, probably closerto tetrapods than is Panderichthys. Thisimpression is now confirmed: the authors1,2

demonstrate convincingly that ElpistostegeandTiktaalikfall between Panderichthysand theearliest tetrapods on the phylogenetic tree. So, if Tiktaalikis in effect a better-preservedversion of Elpistostege, why is it important?First, it demonstrates the predictive capacity of palaeontology. The Nunavut field pro-ject had the express aim of finding an inter-mediate between Panderichthysand tetrapods,by searching in sediments from the most probable environment (rivers) and time (early Late Devonian). Second, Tiktaalikadds enormously to our understanding of the fish–tetrapod transition because of its position on the tree and the combination of characters it displays.In some respects, Tiktaalikand Panderich-thysare straightforward fishes: they have smallpelvic fins13, retain fin rays in their pairedappendages and have well-developed gillarches, suggesting that both animals remainedmostly aquatic. In other regards, Tiktaalikismore tetrapod-like than Panderichthys. Thebony gill cover has disappeared, and the skullhas a longer snout (Fig. 1). These changes prob-ably relate to breathing and feeding, which arelinked in fishes because the movements usedfor gill ventilation can also be used to suckfood into the mouth. A longer snout suggests ashift from sucking towards snapping up prey,whereas the loss of the gill cover bones (whichturned the gill cover into a soft flap) probably

correlates with reduced water flow through thegill chamber. The ribs also seem to be larger inTiktaalik, which may mean it was better able tosupport its body out of water1. The only realpeculiarity of Tiktaalikis its poorly ossifiedvertebral column that seems to contain anunusually large number of vertebrae. These character distributions paint anintriguing picture. Tiktaalikis clearly a transi-tional form, more tetrapod-like than Pander-ichthysin its breathing and feeding apparatus,but with similar locomotory adaptations. Crucially, because Tiktaalikoccupies a posi-tion closer to tetrapods on the tree than doesPanderichthys, their shared characters can beinferred to be attributes of the segment of thetree between the branches that carry the twoanimals (Fig. 1, red). Panderichthysshowed us a morphology that could be interpreted as directly intermediate between osteolepi-form and tetrapod. But only the similar yet‘upgraded’ morphology in Tiktaalikdemon-strates that this interpretation is correct: thisreally is what our ancestors looked like whenthey began to leave the water.Two aspects of Tiktaalik’s anatomy relate tothe origin of new structures in tetrapods: theears and limbs. The tetrapod middle ear hasarisen as a modification of the fish spiracle (a small gill slit) and hyomandibula (a bonesupporting the gill cover). Panderichthyspos-sesses a widened spiracle, interpreted as theintake for air or water, and a shortened hyo-mandibula14. Tiktaalikshows an almost identi-cal condition, but with an even wider spiracle,indicating that this morphology too is genu-inely transitional. The pectoral fin skeleton of Tiktaalikis notablenot only because of its transitional nature, butalso because its excellent preservation has

allowed the individual bones to be freed of therock and manipulated to estimate ranges ofmovement2. It turns out that the distal part ofthe skeleton is adapted for flexing gentlyupwards — just as it would if the fin were beingused to prop the animal up. Although thesesmall distal bones bear some resemblance totetrapod digits in terms of their function andrange of movement, they are still very muchcomponents of a fin. There remains a largemorphological gap between them and digits asseen in, for example, Acanthostega: if the digitsevolved from these distal bones, the processmust have involved considerable developmen-tal repatterning. The implication is that func-tion changed in advance of morphology.The body form represented by TiktaalikandPanderichthyswas evidently an actual step onthe way from water to land. Just over 380 mil-lion years ago, it seems, our remote ancestorswere large, flattish, predatory fishes, with croco-dile-like heads and strong limb-like pectoralfins that enabled them to haul themselves outof the water. Further information will emergefrom the full description of the fossils, and fromdetailed comparisons with Devonian tetrapodssuch as the very primitive Ventastega15.Of course, there are still major gaps in thefossil record. In particular we have almost noinformation about the step between Tiktaalikand the earliest tetrapods, when the anatomyunderwent the most drastic changes, or aboutwhat happened in the following Early Car-boniferous period, after the end of the Devon-ian, when tetrapods became fully terrestrial.But there are still large areas of unexploredLate Devonian and Early Carboniferousdeposits in the world — the discovery of Tik-taalikgives hope of equally ground-breakingfinds to come. ■

Hidden beneath small mounds in theKalahari Desert in southern Africa,Damaraland mole-rats (Cryptomysdamarensis, pictured) have developeda remarkable caste system. In the life cycle of these animals, which isspent entirely underground, a single‘queen’ female mates with one or two unrelated males. The rest of the colony members generally investtheir efforts in caring for successivelitters of young, hunting for food andmaintaining the colony’s intricatenetwork of tunnels.These worker mole-rats aredivided into two types: ‘frequent’and ‘infrequent’ workers, the latterbeing evidently lazy types that maycomprise as much as 40% of thecommunity but do less than 5% of

the work. Elsewhere in this issue, M. Scantlebury et al.describe howthey have followed up circumstantialevidence for the reasons behind thisdivision of labour, and show that incertain situations the layaboutsspring into action (Nature440,795–797; 2006). Mole-rat workers are thought topostpone their own reproduction(sometimes indefinitely) because of the difficulties of setting up a new colony in the rock-hard soil.Extensive burrowing, and so thechance of meeting a mate fromanother colony, is restricted to briefperiods, maybe once or twice a year,when heavy rains soften the soil. Is this when infrequent workerspay their dues? To find out,Scantlebury and colleaguesexamined individuals they trappedat burrow entrances. By measuringthe body fat, daily energy

expenditure and resting metabolicrate of several individuals during a dry period and after rainfall, theauthors show that the infrequentworkers are fatter and expend farless energy than the other workerswhen it is dry. Following rainfall,however, they display bursts ofeffort not shown by the other colonymembers. Scantlebury et al.proposethat, by conserving their energyduring dry periods and then diggingfuriously after it has rained, the fatworkers have a good chance ofdispersing far enough to find a mate.As the authors point out,funnelling extra resources into a dispersive caste may well be a sensible strategy for the colony asa whole. These apparent layaboutsmay spend most of their timereaping the benefits of colony life,such as food and protection, withoutpulling their weight. But they seem

to give good returns when it comes to exploiting environmentalconditions to ensure long-termsurvival of the colony’s gene pool.

Lucy Odling-Smee

EVOLUTION

It pays to laze

T. JACKSON/CERU/UNIV. PRETORIA

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