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Mammal growth spurt

About 50 million years ago, mammals started to breathe easier — and also to grow more easily, paleontologists say. A new study suggests that a large increase in oxygen levels provided the physiological opportunity for mammals to increase dramatically in size.

Small mammals coexisted with dinosaurs until the dinosaurs’ extinction 65 million years ago. Some scientists have suggested that the extinction opened up ecological niches for other animals; however, that idea has not proved adequate to explain the sudden growth of individual mammals, seen in the fossil record 15 million years later, in the Eocene.

Paul Falkowski, a research scientist in the geology and marine sciences departments at Rutgers University in New Brunswick, N.J., and colleagues have turned to deep-sea core samples for answers. The team says that results from their core analysis, published in the Sept. 30 Science, depict significant increases in oxygen levels at distinct times, which they say correlate with the rise of large placental mammals (those that carry their young in the placenta).

Falkowski and colleagues measured carbon isotopes in 500 deep-sea core samples that spanned more than 200 million years, which they used to determine oxygen levels in the atmosphere for timescales of millions of years, Falkowski says. Scientists have previously constructed similar records, but, he says, the new measurements provide finer resolution, about every 500,000 years.

The team’s measurements show a rise in oxygen from 10 percent of atmospheric gases at the time of the dinosaurs to 17 percent 50 million years ago during the early Eocene, and then to 23 percent 40 million years ago during the middle Eocene. “People who are familiar with paleontology would associate that with a lot of things happening, but the big thing is the rise of placental mammals,” Falkowski says. “That’s when the real big diversification of placental mammals occurred.”

Co-author Mike Novacek, a paleontologist at the American Museum of Natural History in New York City, noticed that the timing corresponded not only to records of mammal diversification, but also to increased size. The team says that the relationship can be explained by mammal physiology: In similar amounts of tissue, large animals have fewer oxygen-carrying capillaries than small animals do, so they need more oxygen in order to function.

“If you start to think about buffalos stampeding and tigers running after deer, these animals require a very high concentration of oxygen in order to have an active metabolic rate,” Falkowski says. “If you provide larger concentrations of oxygen, then these animals can get larger — and that’s exactly what we see in the Eocene.”

Not all large animals need abundant oxygen; dinosaurs, for example, lived with far less. “But I would bet that those dinosaurs were not running around very quickly,” Falkowski says. And they also did not carry their young inside a placenta like most mammals do, which requires high levels of oxygen.

In addition to increased oxygen, other factors, such as the expansion of jungle vegetation to the poles, could have contributed to ecological diversity, says Don Prothero, geology professor at Occidental College in Los Angeles, Calif. Prothero also points out that mammals were large in times of reduced oxygen: Gigantic 20-ton rhinoceroses lived 30 to 20 million years ago, as well as mammoths and ground sloths from 2 million years ago — two time periods that were colder, drier and lower in oxygen, he says. “The hypothesis may work for the early Eocene, but does not seem to apply to any other period of mammalian evolution,” Prothero says.

Falkowski’s team’s next step is to determine the cause of the oxygen trends, which the researchers say occurred during the opening phase of the so-called Wilson cycle — the 300-million- to 500-million-year periodic opening and closing of Earth’s ocean basins. Falkowski says that opening oceans store more organic matter, which oxygenates the atmosphere, while closing oceans have the opposite effect.

Kathryn Hansen

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