New Fossils Show Sea Cows Had Their Roots In The Old World
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| Top to bottom: A life restoration of Pezosiren portelli, as it may have appeared, and our modern day sirenian: the West Indian manatee. (Life restoration courtesy Calvert Marine Museum; Manatee photo ® David Schrichte.) |
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By Dr. Daryl P. Domning,
Professor of Anatomy,
Howard University, Washington, D.C.
Where in the world did sirenians come from? For all of their nearly 50-million-year recorded history, they have lived nearly everywhere the seas were warm – from the East Indies to the West Indies, all through what is now the Mediterranean, and a few other places as well. But where did they start – where did they, like the whales, first evolve from land mammals and go back into the water?
Science first confronted this question back in the 1800s, when fossil relatives of the living manatees and dugongs had become known in Europe, but then turned up in the Western Hemisphere as well. A surprise was the 1855 report by British paleontologist Sir Richard Owen of a skull from middle Eocene rocks in Jamaica that he named Prorastomus sirenoides. It was by far the most primitive sea cow ever seen, and the oldest – now known to be some 47 million years old.
By the early 1900s, other fossils almost as old and primitive had surfaced in Egypt. So, had sirenians evolved in the New World and then spread to the Old, or vice versa? Some wondered if Prorastomus was the ancestor of the manatees of the Atlantic basin, while the Egyptian sea cows had given rise to the dugongs in the Old World.
There the question sat for most of a century, while new techniques for understanding the fossil record were developed. Scientists came to agree that the closest relatives of sirenians were the elephants, hyraxes, and other living and extinct mammals of Africa and Asia, which pointed to a common Old World ancestry for all of these animals. But that meant the very earliest sirenians should be found on that side of the Atlantic, and so far they hadn’t been (except for one tail bone from Israel that might or might not even be a sirenian).
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| Scientists came to agree that the closest relatives of sirenians were the elephants, hyraxes, and other living and extinct mammals of Africa and Asia, which pointed to a common Old World ancestry for all of these animals. |
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The issue was highlighted further in the 1990s, when I led excavations in Jamaica that discovered another member of the same zoological family as Prorastomus. Named Pezosiren portelli, it was not quite as old or primitive as Owen’s animal. We were, however, able to show that it could still walk on four legs – proving that these prorastomid sea cows as a group were indeed evolutionary intermediates between land mammals and the fully aquatic sirenians of later times. But if they evolved in the Old World, why were we still finding them only in the New? Was this just a quirk of a spotty fossil record, or was something more complicated going on?
Only in the last couple of years has this problem begun to be resolved. First, a group of French and Senegalese paleontologists reported a single, distinctive vertebra of a prorastomid from Senegal in West Africa. This showed that the prorastomid family had at least been represented on that side of the Atlantic (although that backbone seems to be less primitive and no more ancient than the Jamaican ones). And then, another group of scientists, this time from France and Tunisia, discovered in that North African country an ear bone that they believe represents a sirenian (so far known only as “the Chambi sea cow” after the place it was found) that is much more primitive than even Prorastomus, and maybe earlier – possibly up to 50 million years old.
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| A skull drawing of Prorastomus sirenoides. Scientists have recently discovered a single, distinctive vertebra of a prorastomid in West Africa and an ear bone in North Africa that represents a sirenian much more primitive than even Prorastomus and possibly earlier. (Drawing courtesy Calvert Marine Museum)
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So at last we are starting to have convincing fossil evidence that the sea cows really did start out in the Old World, as we expected from where their closest relatives are. And, knowing that the earliest ones to reach the Americas were four-footed amphibious animals rather than sophisticated swimmers equipped to cross deep oceans, we can surmise that they probably made the trip by following the coastlines around the northern end of the Atlantic. Back in the early Eocene epoch, a land bridge there connected Europe with North America, and tropical climates extended as far north as Greenland; so these early sea cows needed to brave neither high seas nor low temperatures to make the journey. Some paleontologists, however, still suspect that they came straight across the (then-narrower) Atlantic, much closer to the Equator; so there are still unanswered questions for future fossil finds to address. Stay tuned!
Dr. Domning's research on the evolution of sirenians has most recently involved paleontological fieldwork in Jamaica, Austria, and France. He has long been active as an advisor on manatees to the federal government, the state of Florida, and in other conservation efforts, and is presently secretary of the Board of Directors of Save the Manatee Club.
Read more about his work>>
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| Dr. Daryl Domning examines a life-size skeletal reconstruction of Pezosiren portelli, discovered in Jamaica. Another member of the same zoological family as Prorastomus, the discovery of this four-legged sea cow proved that prorastomid sea cows as a group were indeed evolutionary intermediates between land mammals and the fully aquatic sirenians of later times. (Photo © Calvert Marine Museum) |
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For Further (Technical) Reading:
Benoit, J., S. Adnet, E. El Mabrouk, H. Khayati, M. Ben Haj Ali, L. Marivaux, G. Merzeraud, S. Merigeaud, M. Vianey-Liaud, and R. Tabuce. 2013. “Cranial remain from Tunisia provides new clues for the origin and evolution of Sirenia (Mammalia, Afrotheria) in Africa.” PLoS ONE 8: e54307. doi:10.1371/journal.pone.0054307
Domning, D.P. 2001. "The earliest known fully quadrupedal sirenian." Nature 413(6856): 625-627.
Gheerbrant, E., D.P. Domning, and P. Tassy. 2005. "Paenungulata (Sirenia, Proboscidea, Hyracoidea, and relatives)." Chap. 7 in: K.D. Rose and J.D. Archibald (eds.), The Rise of Placental Mammals: Origin and Relationships of the Major Extant Clades. Baltimore, Johns Hopkins University Press: 84-105.
Goodwin, M.B., D.P. Domning, J.H. Lipps, and C. Benjamini. 1998. "The first record of an Eocene (Lutetian) marine mammal from Israel." Journal of Vertebrate Paleontology 18(4): 813-815.
Hautier, L., R. Sarr, R. Tabuce, F. Lihoreau, S. Adnet, D.P. Domning, M. Samb, and P.M. Hameh. 2012. “First prorastomid sirenian from Senegal (western Africa) and the Old World origin of sea cows.” Journal of Vertebrate Paleontology 32(5): 1218-1222.
Savage, R.J.G., D.P. Domning, and J.G.M. Thewissen. 1994. "Fossil Sirenia of the West Atlantic and Caribbean region. V. The most primitive known sirenian, Prorastomus sirenoides Owen, 1855." Journal of Vertebrate Paleontology 14(3): 427-449.
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