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Marsupial reproduction<\/h3>\n
In a study published in the journal The American Naturalist<\/em><\/a>, a team led by researchers at the University of Washington and its Burke Museum of Natural History and Culture present evidence that another group of mammals\u2014the extinct multituberculates<\/a>\u2014likely reproduced in a placental-like manner. Since multituberculates split off from the rest of the mammalian lineage before placentals and marsupials evolved, these findings question the view that marsupials were “less advanced” than their placental cousins.<\/p>\n
\u00a0Multituberculates and their bones<\/h3>\nMultituberculates arose about 170 million years ago in the Jurassic. Most were small-bodied creatures that resembled rodents. For much of their history, multituberculates were the most abundant and diverse group of mammals. But scientists know very little about their life history, including how they reproduced, because of their generally poor fossil record. The last multituberculates died out about 35 million years ago.<\/p>\n
Weaver reasoned that the microscopic structure of fossilized bone tissues can house useful life-history information about multituberculates, such as their growth rate.<\/p>\n
Working under coauthor Gregory Wilson Mantilla, a University of Washington professor of biology and curator of vertebrate paleontology at the Burke Museum, Weaver and his colleagues obtained cross sections of 18 fossilized femurs\u2014thigh bones\u2014from multituberculates that lived approximately 66 million years ago in Montana.<\/p>\n
All 18 samples showed the same structural organization: a layer of disorganized bone “sandwiched” between an inner and outer layer of organized bone. Disorganized bone, or woven bone, indicates rapid growth and is so named because, under a microscope, the layers of bone tissue are laid out in a crisscrossed fashion. In organized bone, which reflects slower growth, layers are parallel to one another.<\/p>\n
The researchers then examined femoral cross sections taken from 35 small-bodied mammalian species that are living today\u201428 placentals and seven marsupials, all from Burke Museum collections.<\/p>\n
Nearly all of the placental femurs showed the same “sandwich” organization as the multituberculates. But all of the marsupial femurs consisted almost entirely of organized bone, with only a sliver of disorganized bone. The team believes that this stark difference likely reflects their divergent life histories.<\/p>\n
“The amount of organized bone in the outermost layer, or cortex, of the femur strongly correlates with the length of the lactation period,” Weaver says. “Marsupials have long lactation periods and a lot of organized bone in the outermost cortex. The opposite is true for placentals: a short lactation period and much less organized bone in the outermost cortex.”<\/p>\n
The outermost layer of organized bone was laid down after birth as the femur’s diameter increased. For tiny marsupial newborns, bones must grow much more to reach adult size, so they deposit a greater amount of outer organized bone compared to placentals, according to Weaver.<\/p>\n
“This is compelling evidence that multituberculates had a long gestation and a short lactation period similar to placental mammals, but very different from marsupials,” he says.<\/p>\n
Based on this correlation, the researchers estimate that multituberculates had a lactation period of approximately 30 days\u2014similar to today’s rodents.<\/p>\n
Marsupial and mammal evolution<\/h3>\nThese findings cast further doubt on an old view that marsupials have a “more primitive” and placentals “more advanced” reproductive strategy. The common ancestor of multituberculates, placentals, and marsupials may have had a placental-like mode of reproduction that was retained by placentals and multituberculates. Alternatively, multituberculates and placentals could have evolved their long-gestation and short-lactation reproductive methods independently.<\/p>\n
Future studies of multituberculate life history may clarify which explanation is true, as well as other outstanding questions of this, and other, ancient branches of our mammalian family tree.<\/p>\n
“The real revelation here is that we can cut open fossil bones and examine their microscopic structures to reconstruct the intimate life history details of long-extinct mammals,” Wilson Mantilla says. “That’s really incredible to me.”<\/p>\n
Additional coauthors are from Indiana University, the University of Washington, and Harvard University. Funding came from the National 糖心视频 Foundation, the University of Washington, Burke Museum, the Society of Vertebrate Paleontology, the Paleontological Society, and the American Society of Mammalogists.<\/p>\n
Source: University of Washington<\/a><\/em><\/p>\n","protected":false},"excerpt":{"rendered":"
These findings cast further doubt on an old view that marsupials have a “more primitive” and placentals “more advanced” reproductive strategy. The common ancestor of multituberculates, placentals, and marsupials may have had a placental-like mode of reproduction that was retained by placentals and multituberculates. Alternatively, multituberculates and placentals could have evolved their long-gestation and short-lactation reproductive methods independently.<\/p>\n
Future studies of multituberculate life history may clarify which explanation is true, as well as other outstanding questions of this, and other, ancient branches of our mammalian family tree.<\/p>\n
“The real revelation here is that we can cut open fossil bones and examine their microscopic structures to reconstruct the intimate life history details of long-extinct mammals,” Wilson Mantilla says. “That’s really incredible to me.”<\/p>\n
Additional coauthors are from Indiana University, the University of Washington, and Harvard University. Funding came from the National 糖心视频 Foundation, the University of Washington, Burke Museum, the Society of Vertebrate Paleontology, the Paleontological Society, and the American Society of Mammalogists.<\/p>\n