An examination of forearm bone mobility in Alligator mississippiensis (Daudin, 1802) and Struthio camelus Linnaeus, 1758 reveals that Archaeopteryx and dromaeosaurs shared an adaptation for gliding and/or flapping

摘要

Inresponsetoincreasedlimbboneloadsmanytetrapodcladeshaveconvergeduponsimilaradaptationstoreinforcetheelbowjointbyreducingindependentmovementsoftheforearmbones.Howeverpriorstudieshavenotexaminedhowthesechangesoccurredphylogeneticallyorfunctionally,suchasduringthetransitionfromprehensileforelimbsindinosaurstogliding/flappingflightinbirdwings.Here,afunctionalanalysisofforearmbonemobilityinextantarchosaursshowsthatcrossinganduncrossingoftheradiusandulnacanbeforcedinalligatorsviaapassiveglidingmechanismrecentlydescribedinlacertilians,whilebirdsareadaptedtoinhibitthismotion.Acomparisonofthesefindingswithasampleofextinctquadrupedalarchosaurforearmsstronglysuggeststhat,duetothehighlyconservedmorphologyoftetrapodforearmsingeneral,thelacertilianmechanismbroadlydescribestheplesiomorphicmechanismviawhichtetrapodforearmbonespassivelycrossinresponsetolocomotor-inducedtorsion.Bipedaldinosaursretainedadaptationsforthispassivemechanism,whichindicatesthattheywereunabletoperformactivelong-axisrotationstoaligntheirsemi-pronated,misalignedforearmjoints.Bycontrast,analogoustobirdsandpterosaurs,quadrupedaldinosaursevolvedimmobilizingadaptationstoreduceorprohibitindependentmovementsoftheradiusandulna.Notably,theelbowjointsofArchaeopteryxvonMeyer,1861anddromaeosaursarebird-like.Thisinformation,coupledwithalackofnon-aerialadaptationsforincreasedlimbboneloads,stronglysuggeststhattheforearmsofdeinonychosaurswereadaptedtoresistthebendingandtorsionalstressesincurredbyleadingedgeairstreamsduringglidingand/orflapping.