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Mononykus and Birds: Methods and Evidence

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Mononykus and Birds: Methods and Evidence 300 ShortCommunications and Commentaries [Auk, Vol. 114 The Auk 114(2):300-302, 1997 Mononykus and Birds: Methods and Evidence LuISCHIAPPE? 4 MARK NORELL, 2 AND JAMES CLARK 3 lDepartmentof Ornithology,American Museum of NaturalHistory, Central Park West at 79thStreet, New York, New York 10024, USA; 2Departmentof Vertebrate Paleontology, American Museum of Natural History, Central Park West at 79thStreet, New York, New York 10024, USA; and 3DepartmentofBiological Sciences, George Washington University, Washington, D.C. 20052, USA In a recent article Zhou (1995) criticized our hy- Gauthier 1986).Yet, all of theseorganisms are bipe- pothesis(Perle et al. 1993, 1994;Chiappe 1995a,b) dal, and none has ever been regarded(or observed) that the Late CretaceousMononykus is the sister to be fossorial. taxonto all otherbirds exceptArchaeopteryx. He con- Zhou's secondpoint is that the similaritiesamong cluded that "The most-parsimoniousexplanation is Mononykusand otherbirds are due to homoplasy.For that Mononykusis not a bird and that its ancestors example,Zhou acceptsthat Mononykusshares a lon- neverpossessed the capacityfor flight," althoughhe gitudinallyoriented and carinatesternum with birds doesnot provide a new hypothesisfor the relation- moreadvanced than Archaeopteryx(p. 959).Neverthe- shipsof this taxon.Here, we reply.Our responseis less,despite the absenceof sucha sternumin Archae- lessconcerned with the specificsof Zhou's character opteryx(Wellnhofer 1993) or in any non-aviandino- analysis,and insteadfocuses on fundamentaldiffer- saur (Barsbold 1983)--a fact acknowledgedby encesin approachbetween Zhou and ourselves.These Zhou•he regardsthese similarities as "mostreason- differences are rooted in our conviction that estima- ably explained"by convergentevolution. Zhou's tion of genealogyis contingentonly on empiricalevi- argument hinges on functionalconsiderations. He dence(i.e. character distribution among taxa), and that seems to believe that if similar structures have differ- phylogenetichypotheses need to be testedand refined ent functionsthey cannot be homologous.He em- by the additionof charactersand taxa. phaticallypoints out (p. 960) that "... among the This method is in sharp contrastto Zhou's ap- five purportedavian characters,the first two almost proach,which focuseson attemptsto correlatethe pe- certainly are digging adaptations.The other three culiarmorphology of Mononykuswith digginghabits. probablyare relatedto diggingdirectly or indirectly. Fromthis proposalhe offersthe phylogeneticconclu- Hence, the five charactersare not phylogenetically sionthat this creaturecannot be a bird. Here, we point informative"(italics added). In Zhou's argument,the out severalmethodological problems and inconsisten- explanationof a particularstructure as an adaptation ciesin Zhou's approach.Several mischaracterizations for burrowingtakes precedence over the explanation of the evidencein Zhou'spaper also require clarifica- of this structureas evidencefor a closerelationship. tion. For simplicity,the following discussionis di- The fallacyof suchan argument,whereby untestable vided between these issues. adaptationistscenarios overturn the powerfultest of Methodology.--Zhoupresents two major conclu- phylogenyprovided by shared derived characters, sions:(1) apomorphicsimilarities shared by Mono- hasbeen frequently pointed out (e.g.Gould and Vrba nykusand birds are Mononykus'adaptations for dig- 1982, Lauder 1994, 1995). Furthermore, the fact that ging, and (2) thesesimilarities evolved convergently Mononykusshares structures with extant burrowing in Mononykusand birds.Tying morphology of extinct mammals(e.g. moles)is irrelevantin establishingits organismsto a particular functionis a difficult task phylogeneticrelationship to other vertebratesunless (Lauder 1995). Several of the features correlated with a closerelationship between this archosaurand this diggingactivities by Zhou apparentlyare basedonly groupof placentalmammals is seriouslybeing enter- on his own intuition. This is best exemplifiedby his tained. statement(p. 960) that "Sincedigging and bipedalism In phylogeneticinference, hypotheses are testedby are both characteristicof Mononykus... the devel- the distributionof charactersamong taxa. Phyloge- oped trochantericcrest also may be relatedto the ani- netic hypothesesare rejectedonly by their replace- mal's fossorial habit . ." All birds more advanced mentwith othermore-parsimonious hypotheses. A life thanArchaeopteryx and basalAlvarezsauridae (a taxon style (e.g. fossorial)can be regardedas a behavioral includingMononykus and its South American allies; character,but by itselfis incapableof replacinga well- seeNovas 1996),along with oviraptoridtheropods, supportedphylogenetic hypothesis. That is not to say have a trochanteric crest (the result of the fusion of the that sucha characteris invalid in phylogeneticstudy primitive theropodlesser and greatertrochanters; see (Wenzel 1992, Lauder 1994). However, it must be evaluated en masse with the ensemble of other char- acters.Together, it is the congruenceof all the evidence 4 E-mail:[email protected] that determineswhich charactershave a singleorigin April 1997] ShortCommunications andCommentaries 301 and whichoriginated via convergence(Patterson 1982, (e.g. dromaeosauridtheropods) and presentin birds de Pinna 1991,Wenzel 1992,Rieppel 1994). morederived than Metornithesprovides further sup- Zhou is also carelessregarding the phylogenetic port for the avianrelationship of Mononykus(Chiappe utility of primitive versusderived characters.Right- et al. 1996).Among these characters are a wide verte- fully, he emphasizesthe deficiencyof primitive char- bral foramenin the dorsalvertebrae, a laterallypro- acteristicsfor discoveringphylogenetic relationships jectingfibular tuberclefor m. iliofibularis,a quadra- (p. 961).However, he hasnot givenup on the phylo- tojugalnot contactingthe squamosal,absence of a me- geneticimportance of primitivecharacters, stating that dial fossaon the proximalend of the fibula, and the (p. 962) "the lack of an avian appearancein these absenceof a postorbital-jugalcontact. [primitive]structures casts doubt on the 'avian'status A phylogenetichypothesis is opento testby the ad- of this specializedanimal" (i.e. Mononykus). dition of new characters,the addition of new taxa and Zhou's approach epitomizes a widespread but the reevaluationof othercharacters. In this way phy- flawed view of the early evolution of birds, in which logeniescan be corroboratedor rejectedand replaced taxa a prioriare expectedto fit an intuitive notion of with others.Rejection of our phylogenetichypothesis '"oird"and accordingly,to have a particular"avian" requiresonly that our criticspropose an alternative life style (Chiappe 1995b).Thus, the assumptionof hypothesisthat bettersummarizes the evidence.Then digging habits in Mononykussupposedly invalidates we would be able to arguespecific points of this de- its phylogeneticplacement within Aves (=Avialae bate. Paradoxically,our criticsdo not want to restrict sensu Gauthier 1986). Under this view, however, themselvesto this arena of data and evidential sup- whalesand bats would be placedoutside Mammalia port (seeChiappe et al. 1995,1996). Instead, Zhou and simply because they are completely aquatic and others(e.g. Martin and Rinaldi 1994;Feduccia 1994, capableof flight, respectively,characters that are far 1996)prefer to keepthis argumentin the realmof un- unlike those of the "ideal"mammal. testablescenarios where special knowledge of the Anatomy.--Zhou'sanatomical comparisons also fail mechanismsof the evolutionary processare tanta- to supporthis claim. For example,he treatsthe anti- mount to evidential criteria (see Chiappe et al. 1995, trochanterand the supracetabularcrest as synonyms. 1996;Norell and Chiappe1996). Thus, Zhou's conclu- Yet,these structures clearly are non-homologousas is sion that "The most-parsimoniousexplanation is that demonstratedby thepresence of bothfeatures together Mononykusis not a bird . ." does not rely on the in severaltaxa (e.g.Mononykus and Patagopteryx;see modernuse of parsimonyas an optimalitycriterion Perle et al. 1994,Chiappe 1996). Incidentally,and in for choosingamong alternative phylogenetic hypoth- contrastto Zhou'sclaim (p. 959), the antitrochanterof eses(Farris 1983), but on the subjectivecriterion of Mononykusis illustratedby a stereo-pairphotograph what appearsto be more reasonablefor him. in Perle et al. (1994),a paper cited by Zhou. In sum, Zhou seems to have no doubts about the The phylogeneticplacement of Mononykuswithin fossorialspecializations of Mononykus;we are skepti- birdsis supportedby an extensivelist of synapomor- cal (seeNorell et al. 1993, Chiappe 1995b).More to phies(Perle et al. 1993,Chiappe 1995b, Chiappe et al. the point, even if Mononykuswere fossorial,this has 1996). These include synapomorphiesdiagnosing no bearingon phylogeneticinference. Furthermore, Aves (= Avialae sensu Gauthier 1986) and those Zhou dismissesthe phylogeneticsignificance of char- sharedbetween Mononykus and more advancedbirds actersthat are uniqueto Mononykusand birds among (Metornithesof Perle et al. 1993).Avian synapomor- archosaurs.We have pointed out several of these phiespresent in Mononykusinclude a caudalvertebral important characters,and various others, although count smaller than 25-26 elements, teeth with unser- not exclusiveto birds,provide further supportof our rated crowns,a caudaltympanic recess that opensin- hypothesiswithin the framework of cladisticanaly- side the columellarrecess and
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