It never occurred to me that what separates mammals from other types of animals is that we grow our fertilized eggs inside of us. Fish, amphibians, reptiles, birds, all lay eggs in nests or in the water. Insects and spiders etc…: eggs. Then there are sponges, corals, star fish, worms, snails, etc… reproduce in a variety of ways but only mammals evolved to grow their eggs inside of their bodies in order to improve chances of survival. Pretty brilliant!
Scientific American has an article on the discovery of an early “placental mammal”, known as Juramaia sinensis, a shrew-like creature discovered in China dating back to 160 million years ago.
“Until now, scientists believed that placental mammals first appeared some 125 million years ago. At that point, they branched off from the lineage that developed into modern marsupials, which nourish their young in their pouches instead of through placentas. Yet a recent fossil find backdates that divergence by about 35 million years, showing that mammals with placentas, known as eutherians, shared the earth with dinosaurs much longer than previously thought.”
According to an article from BBC News, scientists determined that it was a eutherian from its teeth!
“The teeth of Juramaia show all the typical eutherian dental features,” Dr Luo explained.
“Specifically, eutherians have three molars, and five premolars. This is in contrast to metatherians characterized by four molars and three premolars.
It’s pretty incredible to think that one creature would evolve over 160 million years into so many different types of mammals from kangaroos to lions and wolfs and ferrets and whales and armadillos and elk and people…
…more interesting info on the Juramaia…
“Today, 90% of all mammals, which include humans of course, are placentals. Knowing the timing of the split from marsupials is fundamental to understanding the full story of the evolution of mammals.
Another interesting aspect of the discovery is what the fossil can tell us about the lifestyles of the early placentals, and it seems they were pretty adept at climbing.
“Juramaia is an insectivorous mammal. It weighed about 15 -17 grams, more or less the size of a shrew,” Dr Luo said. “Its hand structure suggests that it was a capable climber. So we interpreted it to be a tree-climbing insectivorous mammal hunting insects for living,” the Carnegie Museum researcher told BBC News.”