these ancient dugongs abundant, they were diverse. From the Oligocene to the Pliocene..:...._tbat is, from more than thirty to West ndion less than five million years ago-at least three, probably more, kinds of dugongs lived together in the Caribbean. This newly discovered diversity raises Tuskers the question of how these different spe cies, which had such similar diets, could by Oaryl P Oomning have coexisted in the same environment. Today, no place in the world supports more than a single species of sirenian. Amid the contemporary traffic of "the ing softer, less abrasive sea grasses that What, if anything, was different about the Florida and Caribbean coasts, the rotund they uproot with a pair of tusks in their Caribbean during much of the Age of marine mammals known as West lndian upper jaws. Mammals that promoted a degree of sea manatees attempt to live the slow, deliber Sirenians have a long history, first ap~ cow diversity unknpwn today? And what are life of aquatic grazers. Found in both pearing on earth sorne fifty million years caused these animals to later die out? tropical salt water and the fresh waters of ago, and their family tree has included Much of m y work with fossil sirenians has inland springs, these sirenians, or sea denizens of cold as well as warm waters. focused on how various combinations of cows, placidly paddle through warm wa The huge Steller's sea cow, for example, anatomy and behavior might ha ve allowed ters, grazing on a wide assortrnent of fi inhabited the waters of the North Pacific these separare species to share the avail brous-leaved water plants, including the and Bering Sea, until it was hunted to ex able marine plant foods. rntroduced water hyacinth. Intermittently, tinction in 1768, just twenty-seven years Most of the extinct Caribbean dugongs, a manatee snout breaks the surface; after a after its discovery (see "A Sea Cow Fam like their .living Indopacific relati ves, breath of air, the animal clases its nóstrils .ily Reunion," Natural History, April wielded impressive tusks. Sorne were and silently submerges. Half a world 1987). Nor have dugongs and manatees more than a foot long and were shaped like away, the manatees' look-alike but strictly always so neatly divided their tropical .knives or chisels, with self-sharpening saltwater cousins, the dugongs, quietly ply realms between the Atlaotic and Indopa enamel edges. These were not carried for warm shallows of the Indian and south cific oceans. West Indian manatees are, ge show; lodged solidly in deep sockets in the westem Pacific oceans. While manatees ologically speaking, relative newcomers to upper jaw, with only a few inches of tip have an ever-growing series of teeth the Caribbean; for millions of years, their exposed, they were powerful tools that adapted to the abrasive grasses that grow cousins the dugongs dominated the tropi could have been used in combat, as are the in fresh water, dugongs specialize in eat- cal Westem Hemisphere. Not only were tusks of modem male dugongs. But while
72 NATURAL HlSTORY 4/94 in the living species males have the larger the bill for an arra y of sea cows? Evidence and productivity and maintain ecological tusks, we have no evidence for a differ in the form of fossil sea grasses is rare. At niches that could have supported other, ence in tusk size between the sexes in an one Florida site, however, fossil sea grass less capable diggers such as tiny-tusked cient dugongs. 1 believe that these big, sorne forty-five million years old was Metaxytherium. The large-tusked dugongs bladelike tusks were used by both males found, giving us a window on the past would have acted as keystone species in and females to dig up and consume the plant life of the Caribbean. These fossils the ecosystem, keeping both sea-cow and large, woody rhizomes, or underground reveal that, while sea-grass beds must sea-grass diversity at higher levels than stems, of the largest sea grasses, for ex have looked much the same for as long as they would otherwise have attained. ample, those of turtle grass (Thalassia), sea cows have been on earth, at one time, Two to three rnillion years ago, in the which are inaccessible to tuskless sireni sea-grass communities in the Caribbean grip of a major ecological upheaval, the ans such as manatees. (Dugongs eat the were somewhat more di verse than those of Caribbean saw thé extinction of many whole plant, half of which is the nutritious toda y, which comprise a mere four genera. shallow-water mollusks and other inverte rhizome. Manatees can chew gritty grass Did the abundance of robust sea grasses brates and most likely sorne of the marine but can't get at the rhizomes.) permit the evolution of several kinds of plant life. This upheaval, like most in the Another dugong that inhabited the an large-tusked dugongs? Did the plants sur earth's history, stemmed from the move cient Caribbean at the same time as the vive throughout the dugongs' twenty-rnil ments of crustal plates and the building of great tuskers was Metaxytherium. Sorne lion-year heyday? We have only clues, but mountains. The isthmus of Central Amer ten feet long, this creature also sported a after studying them, 1 find the following ica was completed, joining North and pair of tusks at the front of its upper jaw, scenario to be a· plausible one. 1 suspect South America but also separating the but these appendages were so tiny, with that sea-grass beds supported diverse spe Caribbean and Pacific and disrupting cur- · conical crowns only about half an inch cies of plants until about two to three rnil rents that had fiowed between them. The long, that they appear useless compared liort years ago, and that these sea grasses in changes in water circulation and salinity with the daggers and hoes of other du turn supported a contingent of large that produced the mass extinction of Car gongs. Metaxytherium was probably arel tusked rhizome eaters. Turtle grass, for ex ibbean invertebrates could explain the dis atively unspecialized feeder. lt most likely ample, is considered a climax species and . appearance of dugongs from the area at grazed on the lea ves of various sea grasses characterizes the stable composition to roughly the same tiwe. and on the nutritious rhizomes of the ward which sea-grass communities tend if At this time too, manatees made their smaller sea grasses, which would not have left to themselves. Suppose, however, first appearance in the Caribbean and in been hard to uproot. This is the strategy these grasses were. not left alone, but were southern North America. They had that the completely tuskless Florida mana periodically ripped up by mammalian dig evolved in the rivers of South America tee uses in salt water today. ging machines in the form of dugongs? (see "Marching Teeth of the Manatee," Was the ancient Caribbean full of big Rather than maintain a static climax com Natural History, May 1983) and only now sea grasses with tough rhizomes that filled munity, this would enhance plant diversity spread northward into marine waters. Per haps thelr constantly replenished, wear-re sistant batteries of grinding teeth, which were superior to those of dugongs, gave them a competitive edge; or maybe the de cline of the dugongs simply created an ecological vacuum into which the mana tees expanded. Surviving sea grasses with the biggest rhizomes-such as turtle grass-could now live happily ever after, their manatee-proof root systems undis turbed by hungry plowers of the sea.
An underwater panorama depicts, from left, the ancient whale Basilosaurus, two dugongs, and a variety of other marine mammals and fishes. Metaxytherium, a tiny tusked dugong, and her calffeed on Caribbean sea grasses. Befo re three million years ago, the Caribbean was a garden of sea grasses with large, nutritious roots that were plowed up and savored by resident dugongs, many with long tusks. Today only tuskless manatees inhabit these w.aters. Mural by El y Kish ; courtesy of the Smithsonian lnstitution