An abrupt episode of global warming and major changes in plant and animal life marked the transition between the Paleocene and Eocene epochs about 55 million years ago.
Several groups of mammals, including early primates and modern hooved mammals, made their first appearances in Asia, Europe, and North America around this time.
A new study published in today's issue of the journal Science supports the idea that Asia was the center of origin for at least one important group of mammals, and probably for several others as well.
The study allows paleontologists for the first time to compare the fossil sequences of Asia, Europe, and North America for this dramatic period in Earth's history, said Paul Koch, an associate professor of Earth sciences at the University of California, Santa Cruz, and a coauthor of the paper.
"Now we can finally see what was happening in Asia at the same time that there were dramatic changes in the faunas of North America and Europe," Koch said.
The results are consistent with phylogenetic studies that have pointed to Asia as the center of origin for several important groups of mammals, including primates and the two orders of hooved mammals, artiodactyls (even-toed ungulates, such as deer, cows, and sheep) and perissodactyls (odd-toed ungulates, such as horses, rhinos, and tapirs).
The study also shows that an extinct family of carnivorous mammals, the hyaenodontids, definitely appeared first in Asia and later migrated to North America.
"These groups probably spread to North America across the Bering land bridge in response to the warming of the climate that occurred at the Paleocene/Eocene boundary," said Gabriel Bowen, a Ph.D. candidate in Earth sciences at UCSC and first author of the paper.
The boundary is marked in the geologic record by an anomalous blip in carbon isotope ratios. Geochemists have linked this anomaly to a massive release of methane gas from the ocean, which altered the isotopic composition of the carbon pool over the entire surface of the Earth.
Since methane is a powerful greenhouse gas, this oceanic belch of methane may well have driven the transient global warming that occurred at this time, dramatically altering the global climate for about 100,000 years.
"The very same global change that may have let the animals wander more widely is also the event that lets us nail down the timing," Koch noted.
Koch and Bowen worked with William Clyde of the University of New Hampshire in a cooperative project with the Chinese Academy of Science's Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) led by Suyin Ting of Louisiana State University. They joined IVPP paleontologists Yuanqing Wang and Yuan Wang to collect and analyze samples from the Hengyang Basin in southern China. Ting has been working in this area for many years, studying the rich fossil fauna found in layers of brick-red to tan sandstones and mudstones known as the Lingcha Formation.
The researchers used a combination of techniques, including isotope analysis and magnetic markers, to develop a chronological sequence of fossil assemblages from China that they could compare with similar sequences for North America and Europe.
Carbonate nodules in the rocks of the Lingcha Formation yielded carbon isotope ratios that pinpointed the time of methane release and global warming in the fossil-bearing layers. Changes in Earth's magnetic field were also recorded in the layers of rock, providing additional markers for correlating the positions of fossils from different sites.
John Alroy of the National Center for Ecological Analysis and Synthesis at UC Santa Barbara and Takehisa Tsubamoto of Kyoto University performed a "quantitative biochronology" analysis of fossil data that helped extend the correlations from the Hengyang Basin to other fossil sites in China.
"This is a rare situation in the fossil record where we can pin down a point in time and look at the distribution of groups of animals on several continents," Bowen said.
The most conclusive results were for the hyaenodontid creodonts, a group of dog-like predators that were clearly present in Asia before they appeared in Europe or North America. The hyaenodontids were the dominant predators of the Eocene, ranging from small mongoose-like creatures to huge wolf-like animals with powerful jaws. Now extinct, they were eventually supplanted by modern carnivores.
With respect to the primates and the hooved mammals (perissodactyls and artiodactyls), the researchers found that representatives of these modern orders first appeared in Asia no later than they did in North America, and possibly earlier.
"Based on the best data we have now for correlating different Asian faunas, it looks quite possible that these groups were present in Asia first, but we can't say definitively yet," Bowen said.
Additional geochemical and paleomagnetic studies of fossil sites in Asia may clear up the remaining ambiguities. But Koch noted that many groups of mammals may have originated during the Paleocene in tropical parts of Asia or even Africa, where preservation of fossils is poor.
"If in the Paleocene they were present in Asia in the tropics, we would only get to see their fossils when they got to the desert regions at higher latitudes. So it might be that the warming at the Paleocene/Eocene boundary allowed them to get into the higher latitudes for the first time, and then they continued to extend their range into Europe and North America," Koch said. - By Tim Stephens
[Contact: Paul Koch, Gabriel Bowen, Tim Stephens]