Meeting Abstract

P2.80  Saturday, Jan. 5  Deep Phylogenetic Character Reversal Enhances Scapula Functionality in Shell-Closing Systems of Recent Turtle Lineages CORDERO, GA; Iowa State University, Ames gcordero@iastate.edu

Skeletal evolution in terrestrial vertebrates (tetrapods) generally proceeds via the modification of preexisting structures. However, in complex skeletal modules, the reappearance of characters present in an ancestor (phylogenetic character reversal) frequently occurs. To distinguish between these processes, I studied the development and evolution of shell-closing systems in recent (8-15 Ma) turtle lineages. Character states of the dorsal scapula (segmented or unsegmented) were mapped onto a phylogeny and development of the scapula was observed. The scapula was shown to arise as a single continuous unit that secondarily becomes segmented during late embryogenesis. Segmentation of the dorsal scapula is required for the formation of a specialized joint that mediates anterior-posterior movement of the pectoral girdle during activation of the shell-closing system. Parsimony ancestral state reconstructions, aided with descriptions from the fossil record, supported the hypothesis of a deep phylogenetic character reversal associated with the segmented dorsal scapula of turtles. The oldest known turtle fossil (220 Ma) did not feature this character state, though it was present in some ancient reptilian lineages (~270 Ma). My observations support theory on the capacity for animal genomes to encode for the patterning of convergent anatomical structures. Observed segmentation of the dorsal scapula during embryogenesis invokes a role for the highly conserved bone morphogenetic protein pathway coupled with homeobox gene expression. To test this hypothesis, I am currently investigating differential gene expression during development of the scapula across turtle phylogeny.