Fossil finds help fill in gaps in the land-to-water transition.
As Reported by ScienceNews.org
By Sid Perkins
It took early whales a while to fully break free of the land: For at least one species, females came back to shore to give birth, a new study suggests.
Newly described fossils of ancient whales, which include the unprecedented discovery of a pregnant female, were unearthed in the hinterlands of central Pakistan in 2000 and 2004. The findings are providing scientists with new clues about the life, times and even the possible social structure of these enigmatic creatures.
The discoveries “are rather spectacular, to say the least,” says Erich Fitzgerald, a vertebrate paleontologist at the Smithsonian Institution in Washington, D.C. “They’re quite incredible.”
The family tree of today’s cetaceans — the varied group of aquatic mammals that includes whales, dolphins and porpoises — probably sprouted in southern Asia around 55 million years ago, as land-dwelling creatures began their march back to the sea. In that region, paleontologists have discovered many semiaquatic protowhales deemed to be charter members in cetacean diversification, including wolf-sized creatures that delved into streams about 50 million years ago (SN: 9/22/01, p. 180) and small fox-sized mammals that lived about 48 million years ago (SN: 1/5/08, p. 5). Many such creatures, some of them apparently evolutionary dead ends, appeared in southern Asia during this era. By 30 million years ago, the modern groups of toothed and baleen whales had evolved (SN: 5/14/05, p. 314).
Now, Philip Gingerich, a paleontologist at the University of Michigan in Ann Arbor, and his colleagues add a new twig to the cetacean family tree, a 2.6-meter-long, mostly aquatic mammal that lived along the coast of southern Asia about 47.5 million years ago. Fossils of the species are described online February 3 in PLoS ONE. The researchers dubbed the creature Maiacetus inuus — Maiacetus means “mother whale” in Greek, and Inuus was a Roman fertility god — in part because one of the fossils includes what the researchers say is a near-term fetus, a first for ancient whales.
Those tiny remains, which include the skull, measure about 33 centimeters long and lie within the remains of the larger animal, says Gingerich. A lack of damage to the skull and other bones support the idea that the fossils are those of a fetus and not of a small, unrelated creature that had been eaten by the ancient whale, he notes. Many of the bones were only partially ossified, another clue that the tiny remains are those of a fetus.
Position and orientation of the fetus within the mother provide important clues about the species, the researchers contend. The head is located near the opening to the birth canal, a sign that the whale would have been born headfirst. That, in turn, is a sign that the species came on land to deliver their young: While all large land mammals are typically delivered headfirst, so they can breathe during their birth, all modern cetaceans are born tail first to ensure they don’t drown during delivery.
“This is really exciting stuff,” says Mark D. Uhen, a paleontologist at the Alabama Museum of Natural History at the University of Alabama in Tuscaloosa. Fossilization of a pregnant female “is a very rare event, but a very nice one.”
Because the fetus’s first molars are well-mineralized, Gingerich and his colleagues suggest that Maiacetus young were precocial, or able to supplement their mother’s milk with other food sources soon after birth, as are all of today’s marine mammals.
“From what we understand of cetacean evolution, you could predict that a fossil like this would be found,” says Fitzgerald. “I’ve been trying to convince myself that there’s a problem with it, but the evidence is there.” These fossils are the remains of a pregnant female, he adds.
Another fossil of Maiacetus, recovered about a kilometer away from the specimen that includes fetal remains, is the most complete fossil yet found of an ancient whale, says Gingerich. Only a few vertebrae from the tip of the creature’s tail and a few bones from the ends of the digits are missing, he notes. Analyses suggest the animal’s feet were webbed.
Several aspects of this particular specimen, including its pelvic structure, suggest that the creature was male. For one thing, most of its bones are about 12 percent longer than those of the female Maiacetus. Also, the canine teeth of the fossil are about 20 percent longer than the female’s, a common trait of males in species that display sexual dimorphism, in which the sexes differ in size or appearance.
The features of this near-complete fossil, especially those of the tail, indicate that Maiacetus didn’t have a fluked tail like modern cetaceans, says Uhen. So, the creature probably dog-paddled its way through the water.
Maiacetus is “a fantastic example of an early whale with aquatic specializations,” an “early experiment” in evolution that isn’t survived by any known descendants, Fitzgerald notes. The newly described species is quite different from living whales and dolphins, but also quite different from other ancestral species in the cetacean lineage, he says.
The new fossils are very important, Fitzgerald continues, because those of many previously described species of protowhales include only one creature and are often very fragmentary. “It’s quite rare to have remains of adults as well as young,” he notes.
In modern semiaquatic mammals such as seals and their relatives, species in which males are more than 16 percent larger than females have a harem-style mating system. During breeding season one male controls a territory and mates with several females. Since Maiacetus doesn’t show such a difference, it probably had a one-to-one, or dispersed, mating structure, Gingerich and his colleagues argue. The environment in which Maiacetus lived — along coasts with plenty of breeding room and plenty of food offshore — bolsters the notion that populations could spread out rather than compete for space and resources.
The evidence described by Gingerich and his team “is consistent with sexual dimorphism, but not 100 percent conclusive,” Fitzgerald suggests. For one thing, most pelvic bones of this female Maiacetus aren’t preserved, so it’s difficult to ensure that the two adult specimens are indeed of different sexes. However, he notes, “I think they’ve gone about as far as they can go with the evidence that’s available.”
Although it’s possible that the presumed male and the female are different sizes because they’re different ages or members of two closely related species, it’s probable that they’re the same species, Uhen says. “Sometimes you can only go with the evidence that you’ve got.”
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