in the living species males have the larger 

 tusks, we have no evidence for a differ- 

 ence in tusk size between the sexes in an- 

 cient dugongs. I believe that these big, 

 bladelike tusks were used by both males 

 and females to dig up and consume the 

 large, woody rhizomes, or underground 

 stems, of the largest sea grasses, for ex- 

 ample, those of turtle grass (Thalassia), 

 which are inaccessible to tuskless sireni- 

 ans such as manatees. (Dugongs eat the 

 whole plant, half of which is the nutritious 

 rhizome. Manatees can chew gritty grass 

 but can't get at the rhizomes.) 



Another dugong that inhabited the an- 

 cient Caribbean at the same time as the 

 great tuskers was Metaxytherium. Some 

 ten feet long, this creature also sported a 

 pair of tusks at the front of its upper jaw, 

 but these appendages were so tiny, with 

 conical crowns only about half an inch 

 long, that they appear useless compared 

 with the daggers and hoes of other du- 

 gongs. Metaxytherium was probably a rel- 

 atively unspecialized feeder. It most likely 

 grazed on the leaves of various sea grasses 

 and on the nutritious rhizomes of the 

 smaller sea grasses, which would not have 

 been hard to uproot. This is the strategy 

 that the completely tuskless Horida mana- 

 tee uses in salt water today. 



Was the ancient Caribbean full of big 

 sea grasses with tough rhizomes that filled 



the bill for an array of sea cows? Evidence 

 in the form of fossil sea grasses is rare. At 

 one Florida site, however, fossil sea grass 

 some forty-five million years old was 

 found, giving us a window on the past 

 plant life of the Caribbean. These fossils 

 reveal that, while sea-grass beds must 

 have looked much the same for as long as 

 sea cows have been on earth, at one time, 

 sea-grass communities in the Caribbean 

 were somewhat more diverse than those of 

 today, which comprise a mere four genera. 

 Did the abundance of robust sea grasses 

 permit the evolution of several kinds of 

 large-tusked dugongs? Did the plants sur- 

 vive throughout the dugongs' twenty-mil- 

 lion-year heyday? We have only clues, but 

 after studying them, I find the following 

 scenario to be a plausible one. I suspect 

 that sea-grass beds supported diverse spe- 

 cies of plants until about two to three mil- 

 lion years ago, and that these sea grasses in 

 turn supported a contingent of large- 

 tusked rhizome eaters. Turtle grass, for ex- 

 ample, is considered a climax species and 

 characterizes the stable composition to- 

 ward which sea-grass communities tend if 

 left to themselves. Suppose, however, 

 these grasses were not left alone, but were 

 periodically ripped up by mammaUan dig- 

 ging machines in the form of dugongs? 

 Rather than maintain a static climax com- 

 munity, this would enhance plant diversity 



and productivity and maintain ecological 

 niches that could have supported other, 

 less capable diggers such as tiny-tusked 

 Metaxytherium. The large-tusked dugongs 

 would have acted as keystone species in 

 the ecosystem, keeping both sea-cow and 

 sea-grass diversity at higher levels than 

 they would otherwise have attained. 



Two to three million years ago, in the 

 grip of a major ecological upheaval, the 

 Caribbean saw the extinction of many 

 shallow-water mollusks and other inverte- 

 brates and most likely some of the marine 

 plant life. This upheaval, like most in the 

 earth's history, stemmed from the move- 

 ments of crustal plates and the building of 

 mountains. The isthmus of Central Amer- 

 ica was completed, joining North and 

 South America but also separating the 

 Caribbean and Pacific and disrupting cur- 

 rents that had flowed between them. The 

 changes in water circulation and salinity 

 that produced the mass extinction of Car- 

 ibbean invertebrates could explain the dis- 

 appearance of dugongs from the area at 

 roughly the same time. 



At this time too, manatees made their 

 first appearance in the Caribbean and in 

 southern North America. They had 

 evolved in the rivers of South America 

 (see "Marching Teeth of the Manatee," 

 Natural History, May 1983) and only now 

 spread northward into marine waters. Per- 

 haps then- 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 panorarrux 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 calf feed on 

 Caribbean sea grasses. Before 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 waters. 



Mural by Ely Kish; courtesy of the Smittisonian Institution 



73 



