Life on Earth is hard to imagine without mammals swimming in the depths of the ocean, jumping through deserts and amazingly diverse.
Research says this diversity can be deceiving, at least when it comes to how mammals shape the next generation. Based on how they reproduce, almost all modern mammals fall into two categories: placental mammals and marsupials.
Placental mammals, including humans, whales, and rodents, have long gestation periods, give birth to well-developed young with all major organs and structures, and have relatively short weaning or nursing periods during which the young are suckled by their mother’s milk. their mothers.
In marsupials such as kangaroos and opossums, the opposite is true: they are characterized by short periods of pregnancy, the birth of almost embryos, and long periods of lactation, during which the offspring spend weeks or months feeding and developing in the mother’s pouch, or marsupials.
For decades, biologists viewed the marsupial method of reproduction as the most “primitive” state, and assumed that placentas developed their “most advanced” method after the two groups diverged from each other, but new research is testing that view in a paper published July 18. in The Journal of The American Naturalist, a research team at the University of Washington and its Burke Museum of Natural History and Culture, has presented evidence that another group of mammals, the extinct polytubers, likely reproduced in a placental-like fashion.
Given that several tubercles diverged from the rest of the mammalian lineages prior to the development of the placenta and marsupials, these results cast doubt on the notion that the marsupials were “less developed” than their placental relatives.
Fossil evidence indicates that these creatures were the most abundant mammals in western North America before and after the mass extinction 66 million years ago that killed the dinosaurs.
“This study challenges the prevailing notion that the placental reproductive strategy is more advanced than the more primitive follicular strategy, and our results indicate that placental-like reproductive is either the ancestral reproductive pathway of all giving birth mammals, or placental-like reproduction has evolved. independently in both multispecies and placentas.
Polytubers arose about 170 million years ago during the Jurassic period. Most of them were small-bodied, rodent-like creatures. Over a long period of their history, polytubers were the most numerous and diverse group of mammals.
But scientists know very little about its life history, including how it reproduced. Due to its generally meager fossil record, the last multiple tubercles died about 35 million years ago.
Working under co-author Gregory Wilson Mantella, professor of biology at the University of Washington and curator of vertebrate paleontology at the Burke Museum, Weaver concluded that the microscopic structure of fossilized bone tissue could contain useful information about the life history of many tubers, such as their growth rate. Weaver and his colleagues discovered cross sections of 18 fossilized femurs, the femur, from several tubercles that lived about 66 million years ago in Montana.
All of the 18 samples showed the same structural organization: a layer of disorganized bone “stuck” between the inner and outer layers of organized bone.
Irregular bone or woven bone refers to rapid growth and is so named because, under the microscope, the layers of bone are criss-crossed, in organized bones, reflecting slower growth, the layers are parallel. each other.
The researchers then examined femoral sections taken from 35 species of small mammals living today, 28 placentals and seven marsupials, all from the Burke Museum’s collections.
All choroidal femurs showed almost the same “sandwich” organization as seen in several participants, but all follicular femurs are composed almost entirely of organized bones, with only a small fragment of irregularly shaped bone present, and the team believes a striking difference is likely . reflects their disparate life history.