Soon after the origin of the Order Sirenia in the Eocene, ancestral members of the group began the transition from terrestrial to obligate aquatic habits. Among functional systems transformed during the transition was locomotion. Ancestral members were terrestrial quadrupeds with paraxial oscillatory movements of the limbs; living descendants are aquatic axial oscillators. The goals of this project are: 1) to document the locomotor style of individual transitional fossil taxa, 2) to reconstruct the sequence of locomotor styles in Sirenia based on current understanding of the phylogenetic history of the group, and 3) to compare the trajectory of the sirenian transition with that of an independent transition to axial undulation in the Order Cetacea, the subject of previous analysis.
Prediction of locomotor style will be based on analysis of the vertebral column. Regional variation of the column is a powerful but underutilized tool for analysis of segmental neural supply, body regionalization, and postcranial mobility. Paleoneorological methods developed during a study of marine Carnivora, and functional/morphological methods developed in the study of cetacean locomotor style will be used, adapted, and augmented for this project. These techniques have been useful in the determination of size and innervation territory of limbs, the presence/absence of a fluke and peduncle, the degree of cervical shortening, the regions of prefluke stabilization, and the extent of anatomical (and functional) deviation from ancestral (terrestrial) patterns of vertebral regionalization.
Specimens of fossil Sirenia with significant postcranial material have been identified. The necessity of examination of original specimens and the distribution of their repositories require that the principal investigator travel for data collection, but analysis of data can occur at her home institution.
Data analysis will include determination of fossil swimming styles and placement of those styles in phylogenetic context. It will also include a comparison of the inferred Sirenian transition with that already predicted for Cetacea. In cetacean history, axial undulation of the vertebral column preceded limb loss, and first served to propel enlarged hind feet that served as the propulsive surface. Hind limbs were progressively reduced in size after they were replaced by the wide torso, and finally by the fluke as the dominant propulsive surface. Origin of a peduncle postdated the fluke, and was accompanied first by elongation and finally by the secondary stabilization of the lumbos.
The comparison of two independent and broadly contemporary terrestrial/aquatic transitions from terrestrial paraxial to aquatic axial locomotion in taxa of different ancestry and lifestyle offers a test case for an examination of sequence and constraint in evolutionary transitions.
