It is difficult to imagine life on Earth without mammals. They swim in the depths of the ocean, jump through deserts in Australia and travel to the moon.
This diversity can be deceptive, at least when it comes to how mammals create the next generation. Based on how they reproduce, almost all mammals living today fall into one of two categories: placental mammals and marsupials. Placentas, including humans, whales, and rodents, have a long gestation period. They give birth to well-developed young – with all major organs and structures in place – and have relatively short weaning periods, or lactation periods, during which the young are fed on their mother’s milk. Marsupials, such as kangaroos and opossums, are the opposite: they have short gestation periods — they give birth to young that are little more than fetuses — and long lactation periods in which the offspring breastfeed for weeks or months and grow in the mother’s pouch, or marsupium.
For decades, biologists viewed the marsupial’s mode of reproduction as the more “primitive” state, and assumed that placentas developed their more “advanced” method after these two groups diverged from each other. But new research is testing that opinion. In a paper published July 18 in The American naturalist, a team led by researchers from the University of Washington and the Burke Museum of Natural History and Culture presents evidence that another group of mammals — the extinct multituberculates — likely reproduced in a placenta-like manner. Because multituberculates split from the rest of the mammalian lineage before placentas and marsupials evolved, these findings question the notion that marsupials were “less advanced” than their placental cousins.
“This study challenges the prevailing notion that the placental reproductive strategy is ‘advanced’ over a more ‘primitive’ marsupial strategy,” said lead author Lucas Weaver, a postdoctoral researcher at the University of Michigan who conducted this study as a UW. -PhD student. “Our findings suggest that placental-like reproduction is either the ancestral reproductive pathway for all mammals that give birth to live young, or that placental-like reproduction evolved independently in both multituberculates and placentas.”
Multituberculates arose about 170 million years ago in the Jurassic. Most were small creatures resembling rodents. For much of their history, multituberculates were the most common and diverse group of mammals. But scientists know very little about their life histories, including how they reproduced, because of their generally poor fossil record. The last multituberculates died out about 35 million years ago.
Weaver reasoned that the microscopic structure of fossilized bone tissues may contain useful information about the life histories of multituberculates, such as their growth rate. Working under co-author Gregory Wilson Mantilla, a UW professor of biology and curator of vertebrate paleontology at the Burke Museum, Weaver and his colleagues obtained cross-sections of 18 fossilized femurs — the femur — from multituberculates that lived about 66 million years ago. in Montana.
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 criss-crossed together. In organized bone, which reflects slower growth, layers are parallel to each other.
The researchers then examined femoral cross-sections from 35 small mammal species living today — 28 placentas and seven marsupials, all from the Burke Museum’s collections. Nearly all placental femurs showed the same “sandwich” organization as the multituberculates. But all the possums were made up almost entirely of organized bone, with only a bit of disorganized bone.
The team believes this large difference likely reflects their differing life histories.
“The amount of organized bone in the outer layer, or cortex, of the femur correlates strongly with the length of the lactation period,” Weaver said. “Marsupials have long lactation periods and a lot of organized bone in the outer cortex. The opposite is true for placenta: a short lactation period and much less organized bone in the outer cortex.”
The outer layer of organized bone was deposited after birth as the diameter of the femur increased. For small marsupial newborns, bones have to grow much more to reach adult size, so they deposit a greater amount of externally organized bone compared to placentas, according to Weaver.
“This is compelling evidence that multituberculates had long gestation and short lactation periods, similar to placental mammals, but very different from marsupials,” Weaver said.
Based on this correlation, the researchers estimated that multituberculates had a lactation period of about 30 days, comparable to today’s rodents.
These findings further cast doubt on an old view that marsupials have a “more primitive” and placenta a “more sophisticated” reproductive strategy. The common ancestor of multituberculates, placentas and marsupials may have had a placenta-like mode of reproduction that was preserved by placentas and multituberculates. Alternatively, multituberculates and placentas could have independently developed their reproductive methods for long gestation and short lactation.
Future studies of the life history of multituberculosis may clarify which statement is true, as well as other open questions about this and other ancient branches of our mammalian family tree.
“The real revelation here is that we can cut open fossil bones and examine their microscopic structures to reconstruct the intimate details of the life histories of long-extinct mammals,” Wilson Mantilla said. “That’s really unbelievable to me.”
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Lucas N. Weaver et al, Multituberculate mammals show evidence of a life history strategy similar to that of placentas, not marsupials, The American Naturalist (2022). DOI: 10.1086/720410
Provided by the University of Washington
Quote: New study challenges ancient notions of what is ‘primitive’ in mammalian reproduction (2022, July 25) retrieved July 26, 2022 from https://phys.org/news/2022-07-views-primitive-mammalian- reproduction.html
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