Skeletal repatterning enhances the protective capacity of the shell in African hinge-back tortoises (Kinixys)
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Changes inthestructuralassociationof skeletal traitsarecrucial totheevolutionofnovel formsand functions. Invertebrates, suchrearrangementsoften occurgraduallyandmayprecedeorcoincidewiththefunctionalactivationof skeletal traits.Toillustratethisprocess,weexaminedtheontogenyofAfrican hinge-backtortoises (Kinixys spp.).Kinixys species featureamoveable"hinge" onthedorsalshell(carapace)thatenablesshellclosure(kinesis)whenthehind limbsarewithdrawn.Thishinge,however, isabsent injuveniles.Herein,we describehowthisunusualphenotypearisesviaalterationsinthetissueconfigurationandshapeof thecarapace.Theontogeneticrepatterningofosseousand keratinous tissue coincidedwithshifts inmorphological integrationand the establishment of anterior (static) andposterior (kinetic) carapacialmodules. Basedonexvivoskeletalmovementandrawanatomy,weproposethatKinixys employsa"slidinghinge"shell-closingsystemthatovercomesthoracicrigidity andenhancestheprotectivecapacityofthecarapace.Universalpropertiesofthe vertebrate skeleton, suchasplasticity,modularity, andsecondarymaturation processes,contributedtoadaptiveevolutionarychangeinKinixys.Wediscussa hypotheticalmodel toexplainthedelayedemergenceof skeletal traitsandits relevancetotheoriginsofnovelform-to-functionrelationships.
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