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h2s: secrets of a smelly gas SCIENCE colon screening—beyond polyps dating the grand canyon supernova’s first sparks spied

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NEWSTHE WEEKLY NEWSMAGAZINE OF SCIENCE diverging views NEW FOSSIL FINDS REWRITE MAMMALIAN HISTORY AM.BOB.3-83/5/083:05PMPage154 T a duck-billedplatypus outofplaceintheagedinosaurs. saw heldwhatremainedof thegrooveandrealizedthey the imagesofjaw’s Richandcollaborators structure, yearsearlier. inAustraliaafew unearthed Reviewing 154 times. Since the fossil record is incomplete, therehasbeenevery times. Sincethefossilrecordis incomplete, ancestoracquiredthe pouchonce,notthreeseparate pouch, andsodotheirfossilrelatives. Sobiologistsinferthata Kangaroos, wallabies,andwombats—all —beara during animalhistory, indicatesotherwise. unlessfossilevidence numberofchangesoccurred assume thefewest mony whenthey tiple timeswithinthemammals. poppedupmul- rabble-rousing paleontologistsallegethatthey apparatuses,some theseintricatehearing andchewing evolved hasever noreptileorothervertebrate to lessthannovel.Though downgrade therefined mammalianmiddleearandridgedmolars mostcontentiousrecent findings The even otherwise. suggest willy-nilly.”do notevolve paleontologist Jacques “Complicatedsystems Gauthierasserts, the long-heldnotionthat,asYaleeasy toachieve—challenging haveistics mightactually beencommonplaceandthusrelatively thatthesedefiningBut someresearchersnowbelieve character- todiversify.vations” thatpresumablyallowedlineagesthereafter might have independentlyindifferent evolved groups. thought,butalsothatthey millions ofyearsearlierthanpreviously featuresevolved only thatcomplex not suggests of mammalhistory emergingview are.This the landorinwaterastoday’s andspecializedforlifeon as motley seemtohave beenjust mals. They the simplicity ofprimitivemam- of have long-heldviews contradicted aren’tplain. webbed feet—certainly bills, water-repellentfur, beaver-like tails,and ows of aroundtheshad- were allsimple,shrewlike creaturesthatscurried belonged toaplatypus fromatimewhenmammalssupposedly infreshwater. tosensetheirprey nerves Butthisgroovedjaw ing billsbearnotoriouslywidegroovesequippedwith It’s biologistsinvoke acontroversialidea:Evolutionary parsi- ofadvancedearlymammals, suchasRich’sReports platypus, For centuries,relatedanimalshave beendefined inno- by“key In fossil finds thepastdecade,new jawbone’sThe groovegave away itsowner’s identity becauseliv- T. rex T. AC ,20 VL 173 2008 VOL. MARCH 8, researchers scrutinized the fossil they had researchers scrutinizedthefossilthey jaw. Using amodified CTscanner, the inch–long findingreported aslightgrooveinhalf unlikely places.In January, heandhisteam forfindingom Richhasaneye bitsofskullsin . , however—mammals with rubbery duck- withrubbery . Platypuses, however—mammals Jaw fossilspointtomultiple originsofthemost TWICE UPONATIME mammalian offeatures BY AMY MAXMEN BY AMY mals withthismolarcouldmunch onplantsoranimals. tribosphenic molars,couldcrush, pulverize,andgrind.Mam- called Onceformed,theelaborateteeth, like pestlesintomortars. interlocked withthe lowerteeth, inupperteeth projections The ofpointsusedtobe. wherethe apex withraisedprojections teeth wouldfuseinatriangular pattern,back teeth formingthicker later.it wasthoughtthatmolarsdeveloped Overtime,thepointy cally—the moderncrushingmolar, Until Richreported. thisfind, pair ofscissors,slicingcrunchybugs. swipedpastoneanotherlikeCrocodilelike, bladeson a theteeth on mostfossiljaws teeth fromthistimewerepointed. molars. The mammals. Yet have 120millionyears ago,mostmammalsdidn’t tinguishes modernmarsupialsandplacentalmammalsfromother individuals fromeitherlinemadetheirway south. North America,andGreenland.And,millionsofyearslater, some Europe, toformAsia, whichbroke apart supercontinent Laurasia, groups descendedfromacommonancestoronthenorthern liveyoung,paleontologistsinferredthatthe of bothgroupsbirth andthepouch-bearingmarsupials.Sincemembers examples), in auterusthroughplacenta(dogs,whales,andhumansare lineages includetheplacentalgroup,whonourishtheiryoung period)thesetwo to 119millionyearsago(earlyintheCretaceous Hemispherebeen foundinNorthern fossilbeds.Datingto144 mammalianlineageshad ancestors oftwo ofthethreesurviving then,fossilsoftheearliest andAustralia.Before in Antarctica digs intheSouthernHemisphere, in hard-to-mine,remotelands like themolarandmiddleear, onetime.Not evolved many. thatcomplicatedstructures, reason tothinkfornearlyacentury the lowerjaw of thinking,however,lenged existing wasahefty molarembeddedin ALL IN A JAW — during mammalianhistory. complex innovationsevolved how manytimesthesame trusleri platypus, in thisjawofanextinct Yet, herewas But Tremors shakingtheoldconsensusbegantenyearsagowith Ausktribosphenos stirdebateover , Teinolophos Ausktribosphenos Ausktribosphenos. That was the accepted story until1997, wastheacceptedstory That when ern ,duguptheirfirst contentious Rich andhiscolleagues,workinginsoutheast- Furrows jaw—a 120million-year-oldfossil ’ toothwasclearly—andproblemati- belonging toamammalnamed tribosphenos nyktos measured barelyoverhalfaninch,its features ledtheteamtodescribe Ausktribosphenos of aplacentalmammalinthe cental .Findingafossil ’ advancedmolar, adefining trait I BITE BIG reported ina1997 reported Australia, andhiscolleagues Rich, oftheMuseum in Victoria hypothesis, northern-origin cast doubtontheLaurasian back totheearlyCretaceous, Southern Hemisphere dating It wasthetype oftooththatdis- What reallychal- . Though thejaw. Though as anearlypla- SCIENCE NEWS Science paper. Ausk-

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in modern placental and marsupial lineages. Could it have devel- MORE THAN ONCE Despite such objections, recent reports by oped an advanced feature, and then millions of years later, given Rich, Luo, and a handful of paleontologists have gone further, say- rise to mammals in the north with less-pronounced molars? ing that the middle ear, the very hallmark of all true mammals, was Suspicious of the ramifications of Rich’s claim, Zhe-Xi Luo, a obtained by different groups, rather than by one common ancestor. paleontologist at the Carnegie Museum of Natural History in Only mammals have evolved the finely tuned architecture that Pittsburgh, questioned whether the fossil truly belonged in the pla- characterizes the middle ear, composed of three tiny bones embed- cental group. After all, Rich’s identification was based on jaw ded in the skull by the eardrum. Scientists think the structure details alone. To reinvestigate where Ausktribosphenos fits in the evolved from the jaw. An exquisite series of fossils along the path mammalian family tree, Luo and others analyzed the relationships from reptiles toward mammals shows step-by-step changes in rep- among living and extinct mammals having the mortar-and-pes- tilian jaw bones. Less refined bones at the rear of the reptilian jaw tle-type molars. The scientists constructed a new tree by meas- can transmit sound. And in mammal- uring similarities in 55 features preserved in the teeth and jaws “A minor like reptiles, those bones became smaller of 21 modern and fossil specimens. Two distinct clans of mam- alteration and progressively moved back toward mals bearing the crushing molars emerged from the analysis, the skull. Next, they became loosely published in Science in 2001. One clan included Ausktribosphenos during attached to the remaining, and enlarged, and two other molar-bearing fossils from the Southern Hemi- embryonic major jaw bone. In true mammals, mid- sphere, along with the —furry Australian egg-laying dle ear bones are free from the jaw, mammals that include the platypus and spiny anteater. development allowing for acutely sensitive hearing. The second clan comprised fossils from the Northern Hemi- Paleontologists debate not whether the sphere and the placental mammals and marsupials. Thus, could have bones originated from the reptilian jaw, according to Luo’s analysis, Ausktribosphenos wasn’t in the same led to a major but rather how many times the bones group as placental mammals after all. separated from it. The finding cleared up one problem, but introduced another. If innovation.” Enter again: Rich’s team’s jaw, the tree was correct, the molar thought to belong exclusively to live- — ZHE-XI LUO Teinolophos trusleri, the Australian bearing mammals had evolved separately in the egg-laying CARNEGIE MUSEUM platypus. Years before the researchers monotremes. Perhaps the crushing molar appeared to have only OF NATURAL HISTORY had scanned for the jaw’s finer charac- evolved once because of evidence. teristics, they reported finding a trough Luo’s team then suggested the unthinkable: Egg-layers had inde- by the jaw joint in a 2005 Science. Cyn- pendently evolved a molar that was similar to that of other mam- odonts, extinct mammal-like reptiles that gave rise to true mam- mals. By the time modern platypuses and spiny anteaters arose mals, bore just such a trough. In it, their incompletely formed without adult teeth, the biting evidence was buried in the past. middle ear bones sat attached to the jaw bone. Seeing the same Not everyone is convinced. Regarding Luo’s finding of repeated thing in a sophisticated mammal meant that the ear bones of the origins of the molar, Tim Rowe of the University of Texas at Austin true mammalian ancestor must not have been fully disconnected says, “I find that there’s room for a different interpretation.” Disagree- from the jaw. The group concluded that the middle ear must have ments often ensue when whole are defined by a handful of broken off from the jaw twice—after egg-laying and live-bearing eroded bones or cracks in jaws. Paleontologists determine age by mammals split off from a common ancestor. signs such as cracks in the skull. If bones are missing, incompletely “We suddenly had a clear indication that the middle ear didn’t formed features like molars could come from either an evolution- evolve once in this perfect stage where everything separated from arily primitive adult mammal or from a juvenile, Rowe says. the jaw in one clean sweep. In fact, there were earlier, cruder

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