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morphologists to re-examine what mighthave misled them for more than a century.The authors report phylogenetic interpreta-tions of nine retropositional events that ledto the insertion of so-called ‘short inter-spersed elements’ at particular loci in thenuclear genome of various artiodactyl andcetacean ancestors. Three of these eventsunambiguously support the grouping ofcetaceans, hippos and ruminants in a clade,and the other six provide a partial resolutionof the relationships within that clade.Because the likelihood of these elementsbeing independently inserted at the samelocus in different lineages (or preciselyexcised) seems virtually nil, these markerscan reasonably be considered to be essen-tially noise-free.

So, these molecular results may prompt aserious revision of how we view morphol-ogical transformations in whales and artio-dactyls. Three salient features of artiodactylsare: first, the axis of symmetry of hand andfoot runs between the third and fourth digit(paraxony); second, the heel has developedan extremely mobile joint at a place wheremost mammals have a barely mobile joint9;and third, the last lower milk molar consistsof three rows of cusps (three-lobed deci-duous lower premolar 4, DP/4). Althoughparaxony is a striking feature, it also occurs inprimitive whales10, so it is uninformative forthe issue at hand. But the remodelled heel is adifferent matter — because it is present in allartiodactyls and in no other mammal, thischaracter is classically interpreted as derivedand, therefore, as supporting artiodactylmonophyly.

The presence of the mobile joint in artio-dactyls can be inferred from bones: there is apulley (or trochlea) on the distal part of theastragalus (one of the heel bones) — a so-called trochleated astragalar head. This led toefficient and fast locomotion in the earliestartiodactyls. Although some features of theartiodactyl heel are present in other mam-mals such as rabbits, the astragalar head isnever trochleated. In modern cetaceans, thehindlimb is so greatly reduced that the heelcannot be recognized, and no completefunctional astragalus is known for a fossilwhale. However, the mesonychians — agroup of land mammals that is considered tobe the closest extinct relative of cetaceans —also lack a trochleated astragalus. So if Shi-mamura and colleagues’ hypothesis is cor-rect, then either mesonychians are not close-ly related to cetaceans (and many dentalcharacters are convergent), or the specializedheel morphology is not the exclusive charac-ter that many morphologists take it to be. Itmay have evolved several times indepen-dently in artiodactyls, or have been lost in themesonychian/cetacean clade. The completeastragalus of an early cetacean wouldprobably shed light on this issue.

The hypothesis put forward by Shima-

mura et al. also clashes with the DP/4 charac-ter — the tooth has three lobes in allartiodactyls, but not in early cetaceans ormesonychians. If the new molecular data arecorrect, the morphology of this tooth has,like the trochleated astragalus, a complicatedphylogenetic history that includes reversals,convergences or both. Dental differencescould reflect dietary differences, becauseboth early cetaceans and mesonychians wereprobably carrion feeders or carnivores,whereas all early artiodactyls were omni-vores or herbivores. But many morphologi-cal systematists are reluctant to let functionalarguments influence their evaluation ofcharacters.

What characters, besides the molecularones described by Shimamura et al. and byGatesy7, do hippos, ruminants and cetaceanshave in common that makes them differentfrom pigs, peccaries and tylopods (camelsand llamas)? Morphological studies haveusually upheld close genealogical ties amongpigs, peccaries and hippos (and the largergroup that includes hippos: anthraco-theroids), but the similarities are usuallyuninformative primitive characters that arealso present in the ancestral artiodactyl, orfeatures that are subject to rampant conver-gences (such as certain dental characters, seeFig. 1). Shimamura et al. also suggest thatartiodactyls and cetaceans diverged in theCretaceous — about 15 million years beforethey are found in the fossil record. However,if hippos are closely related to cetaceans, andif mesonychians are not, then the discrepan-cy in the times of origin is considerably less: itamounts to the difference between the originof cetaceans (approximately 50 million yearsago) and that of anthracotheroids (around49 million years ago). But because of therapid and unique specialization of cetaceanmorphology, few characters can be recruitedto bolster the grouping of ruminants, hipposand cetaceans into a clade. Recovery andstudy of the earliest cetaceans wouldprobably help to resolve this problem.

Few molecular studies rule out previouslyaccepted morphological trees as convincinglyas that of Shimamura et al. However, the faceof phylogenetic science itself is changingrapidly — it is becoming more objective andless inductive. For example, phylogeneticistsno longer need to ask whether molecular dataare superior or inferior to morphologicaldata, because the signal-to-noise ratio in mor-phological, molecular and combined data setscan now be measured directly, even beforeexamining phylogenetic trees11. In any case,the new analyses indicate that the use ofretropositional events as molecular markersmay define a new power of resolution in esti-mating phylogenies. This method could evenbring to a close some of the most intense con-troversies2,12,13 in the field, such as whether thetoothed whales2,12 are monophyletic or para-phyletic, and likewise for the rodents13.

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NATURE | VOL 388 | 14 AUGUST 1997 623

100 YEARS AGO'Cyclone sail' — I have sent to you, forpublication, if you think desirable, aphotograph of a type of an ideal sail —ideal, in that the wind acting on it has notendency whatever to incline the boat. The wind pressure acts practically atright angles to the mean surface of thesail. When the wind is making a largeangle with the sail, the centre ofpressure is almost at the centre of thesurface, but when the wind strikes thesail at an acute angle, as in all sails orkites, the centre of pressure movestowards the weather edge; but bysuitably adjusting the sail, the desirableresult of obliterating all heelingmovement has been achieved. Percy S. PilcherFrom Nature 12 August 1897.

50 YEARS AGOOn June 6, 1942, there was a secondmeeting in Berlin, when the results of theuranium project were reported to Speer,as Minister for War Production. The factsreported were as follows: definite proofhad been obtained that the technicalutilization of atomic energy in a uraniumpile was possible. Moreover, it was to beexpected on theoretical grounds that anexplosive for atomic bombs could beproduced in such a pile.... Following thismeeting, which was decisive for theproject, Speer ruled that the work was togo forward as before on a comparativelysmall scale. Thus the only goal attainablewas the development of a uranium pileproducing energy as a prime mover — infact, future work was directed entirelytowards this one aim. Prof. W. HeisenbergFrom Nature 16 August 1947.

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