A novel modification of the tetrapod scapula during development of turtles with shell closing systems
The general evolutionary trend of the shoulder girdle in four-legged backboned animals (i.e. tetrapods) is characterized by the loss or reduction of bones. The turtle’s shell is perhaps the most recognizable exception to this trend. Little is known about other skeletal modifications in turtles that may have accompanied the evolution of the shell during its evolutionary history. We investigated changes in the development of two bones (episcapula and suprascapula) in the shoulder girdle among turtles of the Emydidae family that feature shell-closing systems (i.e. shell kinesis) and those that do not. Development and morphology of extant turtle species were compared to phylogenetic representative of extant tetrapod lineages. Scapula structures were analyzed for changes in cartilage and ossification composition. Our observations suggest that turtles with shell-closing systems regain the capacity to develop additional skeletal elements derived from the scapula. The suprascapula becomes a separate component of the shoulder girdle and is characterized by de novo formation of a synovial joint. We hypothesize that plasticity in bone development promotes the evolution of novel skeletal arrangements in tetrapods.