| Miocene shallow coastal waters of the Americas had diverse communities of marine mammal herbivores. This included the Sirenia and Desmostylia and disappeared near the time of the closure of the Central American Seaway that relied on tropical to subtropical waters with warm water and substrate for seagrasses. Evidence for resource partitioning was explored by study of mechanical properties of aquatic plant foods and morphology and microwear patterns indicative of ingestive and mastication specializations.; Seagrasses are mechanically similar to terrestrial grasses and ingested with horny pads or tusks used for tensile fracture and masticated with shearing edges. Rostral deflection is associated with feeding on seagrasses in all but the Hydrodamalinae of the Sirenia, but not for the Desmostylia, perhaps reflecting their lack of hydrostatic constraints. Tusk tip microwear and tip geometry indicates that some tusks had blade edges (i.e. Corystosiren, Rytiodus, and Dioplotherium) for cutting woody rhizomes with high fracture toughness, and some were probes (Metaxytherium, Desmostylus, Paleoparadoxia) more suited for uprooting less tough rhizomes. Tusk tip apical sharpness is greatest for Metaxytherium and Desmostylus, indicating effective probing in dense rhizome beds. Polishing dental microwear on the lingual surfaces of cheek teeth in the Desmostylia indicates abrasion by substrate particles between the tongue and teeth like lingual piston suction feeding of Odobenus .; Most sirenian enamel microstructure is horizontal HSB for resisting damage from shearing. Modified radial enamel in Desmostylus hesperus and vertical HSB-like microtopography of Metaxytherium enamel were adaptations for resisting vertical loading of hypsodonty or increasing coefficient of friction. Cusp tip enamel thickness is normal in sirenians studied except Potamosiren that may have accommodated more abrasive diets differently from sudsequent trichechids. The fovea apicis tuberculi dentis of desmostylian teeth is found in adult teeth of Desmostylus and Paleoparadoxia and provided rapid exposure of the DEJ, enhancing shearing ability. Functional occlusal surface area is allometric to shear length for Desmostylus due to enamel thickness. Dental microwear data is less informative, but more scratches were found in trichechids and more pits in the deciduous teeth of Desmostylus. Scratch orientation supported transverse jaw motions in Sirenia and protraction/retraction jaw movements in Desmostylus . |
