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Major evolutionary transitions transpire in an environmental context. Thus, to understand the advent of novelty or exaptation in evolution, aptations and nonaptations must be resolved with ecological insight. Previous work on the origin of terrestrial vertebrates has focused on reconstructions of paleoenvironments or isolated examples of axial and appendicular function, but little work has been done to synthesize the role of evolutionary morphology and evolutionary theory in the origin of terrestrial locomotion, or to integrate paleoecological reconstructions to address the environments within which these changes evolved.

To understand the origin of axial and appendicular systems in their ecological context, I have combined data from a description of a fully articulated stem-tetrapod taxon and comparative evolutionary and paleoenvironmental analyses of total-group tetrapods to answer four questions: (1) what are the traits that underpin the tetrapod condition?; (2) how well do current phylogenies explain the distribution of character-states among Devonian and Carboniferous stem-tetrapods?; (3) how do the environments of stem-tetrapods inform and contextualize these evolutionary changes?; and (4) given the distribution of synapomorphies, and what is known about how extant and extinct sarcopterygians (including tetrapods) negotiate their aquatic and terrestrial environments, how do insights from evolutionary morphology and evolutionary theory inform the origin of walking on land?

The discovery and description of the marine stem-tetrapod from the Devonian of Nevada helps to clarify the phylogenetic, environmental, and anatomical framework that underpins the tetrapod condition. This new taxon, Tinirau clackae, demonstrates that substantial parallelism pervaded the early history of stem-tetrapods, supports an earlier origin of the tetrapod lineage, and further documents that incipient stages of the terrestrial appendicular condition began when sarcopterygians still retained their median fins and occupied aquatic habitats. Moreover, the phylogeny helps structure the traits that diagnose crown-tetrapods, their paleoenvironmental history, and the origin of their locomotory strategies. Without this result, the early history of elpistostegalians would still begin with Panderichthys and Thomson's (1980) marine origins hypothesis (variant II) would remain uncorroborated.

The integration of these paleontological data with data from extant taxa suggests that (i) the trot evolved at least three times in gnathostome evolution; (ii) the tetrapod myaxial condition evolved in water ~35 million years before the origin of amphibious sarcopterygians; (iii) trackway data from modern and fossil records cannot verify whether the lateral sequence diagonal-couplet gait evolved by the end of the Devonian; (iv) the original function of the physical neck--i.e., a space between head and shoulder--was more likely related to the origin of terrestrial locomotion than to any requirement for neck mobility; and (v) distinguishing aptations and nonaptations in a continuum of historical, constructional, and functional influences is critical to elucidating evolutionary transformations.

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