Introduction: If you have collected fossils to any great extent in marine Tertiary depostis of tropical or once-tropical latitudes, you have probably come across cylindrical pieces of thick, dense vertebrate bone several centimeters in diameter that someone told you were the ribs of sirenians or seacows. You may even have learned to apply the term "pachyostoic" to such bones. Inquiring further, you probably learned that seacows (manatees and dugongs) are the legendary mermaids, that they are the nearest living relatives of elephants, and that they all replace their teeth horizontally, back to front, just like elephants do. Such is education: a random mixture of truth and error, hopefully favouring the former.

Whatever debatable connection they may have had with legends of sirens, sirenians are quite real (Fig. 1), and for over 50 million years they have played an important, if under-appreciated, role in some of Earth's most biologically productive ecological communities: seagrass beds. They are the only herbivorous marine mammals living today, and through most of the Cenozoic they have been the main (and almost the only) consumers of the larger sorts of marine vegetation. With only four living species, and a fossil record overshadowed by many more diverse groups, in the last 30 years they have still produced a number of palaeontological surprises, including evidence of an adaptive radiation that was unsuspected only a decade ago.