The Once Prolific Dugong
The dugong is a large marine mammal which, together with the manatees, is one of four living species of the order Sirenia. It is the only living representative of the once-diverse family Dugongidae; its closest modern relative, Steller's sea cow (Hydrodamalis gigas), was hunted to extinction in the 18th century. Smithsonian scientists have discovered that this was not always the case. According to the fossil record of these marine mammals, which dates back 50 million years ago, it was more common to find three, or possibly more, different species of seacows living together in one locality at one time. This suggests that the environment and food sources for ancient seacows were also different than today. The team's findings are published in the journal PLoS ONE.
Dugongs, along with other sirenians, are referred to as "sea cows" because their present day diet consists mainly of sea-grass. When eating they ingest the whole plant, including the roots, although when this is impossible they will feed on just the leaves. A wide variety of seagrass has been found in dugong stomach contents, and evidence exists they will eat algae when seagrass is scarce. Although almost completely herbivorous, they will occasionally eat invertebrates such as jellyfish, sea squirts, and shellfish.
Today there are only four species of seacows; three species of manatees, which are found in different coastal waters of the Atlantic Ocean, and one species of dugong, found along the coasts of the Indo-Pacific Ocean.
There were once many other species. How did they co-exist or compete? What did they eat? To answer these questions, the team, lead by Smithsonian predoctoral fellow Jorge Velez-Juarbe, examined three localities from separate time periods, from the late Oligocene (about 23-28 million year ago) in Florida, the early Miocene (about 16-23 million years ago) in India and the early Pliocene (about 3-5 million years ago) in Mexico. All three areas showed conclusive fossil evidence that two or more species of seacow had once coexisted there.
The scientists examined the size and dimensions of the ancient skulls as well as estimating the body sizes, the team deduced that the different species of seacows had characteristics that allowed each to feed on different types of seagrass. Such separation among physical features, the team suggests, reduced any competition for food and allowed multiple species of seacows to coexist. This also suggests that, unlike today's seacow habitats that are dominated by one or two species of seagrass, many species of seagrass must have once coexisted.
What caused the many species to change to the few of today? Causes for the demise of multispecies sirenian communities remain unclear, but in the Western Atlantic and Caribbean, regional changes in the seagrass communities may have played a role. The sparse fossil record of seagrasses minimally sets a middle Eocene antiquity to their presence in the region. Notably, the diversity of this middle Eocene assemblage is more similar, in richness, to extant Indo-West Pacific communities than to those currently found in the region. Though no other fossil seagrasses are known from the region, the persistence of multispecies sirenian communities until relatively recently suggests that Neogene seagrass communities were still richer and more biodiverse than they are today.
For further information: http://www.plosone.org/article/info:doi/10.1371/journal.pone.0031294