Vertebral anatomy in the Florida manatee, Trichechus manatus latirostris: a developmental and evolutionary analysis.

The vertebral column of the Florida manatee presents an unusual suite of morphological traits. Key among these are a small precaudal count, elongate thoracic vertebrae, extremely short neural spines, lack of a sacral series, high lumbar variability, and the presence of six instead of seven cervical vertebrae. This study documents vertebral morphology, size, and lumbar variation in 71 skeletons of Trichechus manatus latirostris (Florida manatee) and uses the skeletons of Trichechus senegalensis (west African manatee) and Dugong dugon (dugong) in comparative analysis. Vertebral traits are used to define morphological, and by inference developmental, column modules and to propose their hierarchical relationships. A sequence of evolutionary innovations in column morphology is proposed. Results suggest that the origin of the fluke and low rates of cervical growth originated before separation of trichechids (manatees) and dugongids (dugongs). Meristic reduction in count is a later, trichechid innovation and is expressed across the entire precaudal column. Elongation of thoracic vertebrae may be an innovative strategy to generate an elongate column in an animal with a small precaudal count. Elimination of the lumbus through both meristic and homeotic reduction is currently in progress.