from Part I - Ancient invaders
Published online by Cambridge University Press: 05 February 2014
Introduction
To understand the biological invasions of dugongs in relation to ecological theory, we first need to look into the early evolution of tetrapods and their return to an aquatic habitat, and then study the evolution and spread of seagrass habitats and their interactions with modern sirenians.
Whereas amphibians never quite parted from the water, all other tetrapod groups (mammals, birds and reptiles) have a number of representatives that have returned to marine or aquatic environments from a fully terrestrial or an amphibious lifestyle. We know that mammals, in particular, have returned to the marine and aquatic environment in at least seven separate taxonomic lineages: Cetacea (whales), Pinnipedia (seals, sea lions and walruses), Sirenia, Ursus maritimus Phipps, 1774 (polar bear), Enhydra lutris nereis Merriam, 1904 (sea otter), and the extinct Desmostylia and Thalassocnus spp. (aquatic sloths). Five of these lineages are still extant, and two are extinct. The ancestry of Sirenia is distinct from that of Cetacea and Pinnipedia, although they are thought to have evolved an aquatic lifestyle around the same time as the cetaceans.
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