Manatees, large herbivores that grace the world’s coastal marine waters, estuaries, rivers, swamps and marine wetlands, are gentle creatures often referred to as “sea cows.” Their enormous size — an average manatee is ten feet long and weighs 1,200 pounds — might remind you of their closest relative, the elephant.
According to Ocean Today, manatees are important for the maintenance of ecosystem health because of their love of seagrass. Their consistent munching on seagrasses and other aquatic plants — and they eat a lot of them — keeps the grasses short, helping to keep seagrass beds healthy.
Manatee species that exist today include the West African, Amazonian and West Indian manatee, as well as the Florida manatee, which is a subspecies of the West Indian manatee. All sea cow species are classed as vulnerable to extinction. The imperiled manatee is the official marine mammal of Florida.
Even without any true natural predators, manatees have become endangered due to habitat loss and being struck by boats and ships. In Florida, water pollution has been destroying manatees’ natural source of food, leading many to starve.
A recent examination of fossil evidence suggests that there were once many types of sea cow and that various species coexisted at once, a press release from Duke University said.
In a new paper, scientists Steven Heritage of the Duke University Lemur Center’s Museum of Natural History and Erik Seiffert of the Keck School of Medicine at USC, who is also a research affiliate of the Duke Lemur Center, have put together the most comprehensive account ever of sea cow ancestry.
“The earliest known fossil sea cows are about 47 million years old, and those animals lived along the coasts of northern Africa in the proto-Mediterranean Sea,” Heritage said in the press release. “Our analysis found that this first appearance was about 11 million years after the sea cow lineage diverged from their closest living relatives, the elephants.”
The paper, “Total Evidence Time-Scaled Phylogenetic And Biogeographic Models For The Evolution Of Sea Cows (Sirenia, Afrotheria),” was published in the journal PeerJ.
Throughout sea cows’ long history, the coastlines of all the continents on Earth except Antarctica have been enriched by the gentle marine mammals, the press release said. Once, there was even a species of sea cow that weighed 12 tons natant in the Bering Sea.
The earliest known sea cow fossil had four legs and was capable of walking on land like an elephant. Elephants’ oldest fossil ancestors were alive in northern Africa in the early Cenozoic era, right after dinosaurs went extinct.
Today’s manatees and their relative, the dugong, both of the Sirenia taxonomic order, live exclusively in the water and have no hind limbs.
The largest dataset to date of living and fossil sea cow species was put together for the study. It incorporated geography, genetics, anatomy and geologic time periods.
To determine the sea cows’ ancestry, Heritage and Seiffert used statistical models set on a time scale, and, in mapping the directions and ages of the sea cows’ ocean migrations, the scientists used historical biogeography models.
“Our models suggest that the direct ancestors of manatees evolved within continental South America and the migration of manatees into the Caribbean and towards the coasts of North America was a relatively recent event,” Seiffert said in the press release. “In a sense, manatees are newcomers to these West Atlantic ecosystems.”
Freshwater wetlands blanketed a great portion of the northern part of South America for much of the last 20 million years. Over time, the wetlands became the Amazon River system. Sea cows migrated out of South America after the marshland began draining into the South Atlantic Ocean a few million years ago.
The common ancestor of today’s species of sea cow came from an important transatlantic migration toward the southern part of North American and the Caribbean from the Eastern Hemisphere about 34 million years ago.
At the time, during the transition from the Eocene to the Oligocene geologic periods, the climate was cooling quickly and global sea levels were plunging. Many marine and terrestrial animal species became extinct during this time.
Subsequently, the ancestor of today’s sea cows that lived in the Eastern Hemisphere dwindled and eventually became extinct. Sea cow relatives that had become settled in the Western Hemisphere toward the early part of the Oligocene epoch led to a great number of species that persevered, sometimes in multi-species communities.
In their exploration of sea cow evolution, the study partners discovered that between 23 and five million years earlier, during the Miocene period, at least three of the lineages of sea cow from the Caribbean moved into the Pacific Ocean, before Central and South America were attached.
The colossal coldwater species, Steller’s Sea Cow, was a descendant of the Pacific migrations. The enormous sea cow could grow as long as 33 feet, but was hunted to extinction not long after European naturalists came to the isles of the Bering Sea in the mid-18th century.
Heritage and Seiffert said the largest assortment of sea cow lineages existed about 22 and 16 million years ago. However, the quantity of lineages has declined abruptly in the last nine million years, leaving us with the limited number of peaceful, threatened species currently swimming and floating suspended in our oceans.
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