The sea cows originated in the Tethys Sea, between southern Europe and northern Africa, during the latest part of the Paleocene epoch, according to a new study by scientists from Duke University, the University of Southern California and the Natural History Museum of Los Angeles County.
The sea cows are members of Sirenia, a mammalian order that includes four living species: the dugong (genus Dugong) and three manatees (genus Trichechus).
Also known as sirenians, these animals are unique in being the only fully aquatic mammals that are also obligate herbivores, and their habitats are predominantly warm, near-shore, and relatively shallow waters.
The fossil record of sea cows begins near the onset of the middle Eocene, about 47 million years ago, and since that time their evolving diversity and biogeography has ensued to leave fossil occurrences on every continent except Antarctica.
While the living species of sea cows lack hindlimbs and are equipped with several anatomical adaptations that suit them to wholly aquatic lifeways, some of the earliest fossil sirenians were, by contrast, quadrupedal and amphibious.
“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,” said Dr. Steven Heritage, a researcher in the Duke Lemur Center Museum of Natural History at Duke University.
“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 earliest fossil ancestors of elephants are also from northern Africa and lived during the early Cenozoic, the era that followed the extinction of the dinosaurs.”
“While modern manatees and dugongs have no hind limbs and are strictly aquatic, the oldest known fossil sea cows had four limbs and could walk on land.”
Dr. Heritage and his colleague, Dr. Erik Seiffert from the Duke Lemur Center Museum of Natural History at Duke University, the University of Southern California and the Natural History Museum of Los Angeles County, assembled the largest-yet dataset of living and fossil species of sea cows, combining genetics, anatomy, geography, and geologic ages.
The analyses included time-scaled statistical models for the ancestry of sea cows and models of historical biogeography that identify the ages and directions of their migrations across Earth’s oceans.
“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,” Dr. Seiffert said.
“In a sense, manatees are newcomers to these West Atlantic ecosystems.”
For much of the past 20 million years, a large portion of northern South America was covered by fresh water wetlands. These eventually gave rise to the Amazon River system with drainage into the South Atlantic Ocean beginning only a few million years ago.
Manatees from that vast marshland seem to have made their way out of South America after that time.
Around 34 million years ago, a key trans-Atlantic migration from the eastern hemisphere towards southern North America and the Caribbean gave rise to the common ancestor of the living sea cow species.
The age of that migration corresponds to the Eocene-Oligocene boundary, which was a time of plummeting global sea levels, a rapidly cooling climate, and widespread and severe extinctions of numerous animal species both on land and in the sea.
After that time, the ancestral sea cows of the eastern hemisphere faced a steep decline and eventually disappeared.
But the western hemisphere lineage that was established near the beginning of the Oligocene gave rise to many species of sea cows that flourished and persisted for tens of millions of years, sometimes living in communities of multiple species.
The authors also found that at least three Caribbean sea cow lineages migrated into the Pacific Ocean during the Miocene epoch, between 23 and 5 million years ago, before Central and South America were connected.
One descendant of those Pacific migrations was the Steller’s sea cow (Hydrodamalis gigas).
This gigantic species was described in 1741 by the German biologist Georg Wilhelm Steller, who participated in the Vitus Bering’s Great Northern Expedition. It became extinct 27 years later, presumably as a result of human-driven habitat change and overexploitation.
Adult Steller’s sea cows reached a length of 10 m (33 feet), weighed over 10 tons, and stored up to 10 cm (3.9 inches) of blubber in some areas of the body.
“The highest diversity of sea cow lineages occurred around 22 and 16 million years ago,” the researchers said.
“But over the past 9 million years, the number of lineages has dropped precipitously resulting in the few species that remain today.”
Their findings are published in the journal PeerJ.
S. Heritage & E.R. Seiffert. 2022. Total evidence time-scaled phylogenetic and biogeographic models for the evolution of sea cows (Sirenia, Afrotheria). PeerJ 10: e13886; doi: 10.7717/peerj.13886
Source link: https://www.sci.news/biology/sirenia-evolution-11135.html