by. Dr. Aaron Pan, Executive Director, Don Harrington Discovery Center
Around 160 million years ago in China, a small, furry, shrew-like mammal scurried around tree limbs and trunks in a subtropical, humid forest. This diminutive creature tipped the scales at around 15 – 17 grams (about the weight of 3 quarters) and feasted on insects. It had to constantly be mindful of its surroundings to avoid becoming a hors-d’oeuvre for feathered dinosaurs or winged pterosaurs. This mammal was Juramaia, and while it was of pint-sized dimensions, its discovery and scientific importance is quite hefty.
We are all familiar with mammals, animals with backbones that possess hair, are “warm-blooded” (can regulate their temperature), and produce milk to feed their young. A couple of other less well known characteristics of the group are that they also possess a four-chambered heart and three-middle ear bones (the malleus, incus, and stapes). However, a characteristic left off of the mammalian attributes above is that they give live birth. That is because, while most mammals do give live birth, a very small group of mammals actually produce eggs instead of live young – the monotremes, which include the platypus and echindas.
So why is Juramaia so important? Juramaia, meaning “Jurassic mother”, is the earliest known eutherian mammal. Eutherian mammals are mammals that bear live young, which have been nourished during a long gestation by a placenta. This differs from marsupials (opossums, koalas, kangaroos, numbats, wombats, sugar gliders, Tasmanian devils, etc.), with mommies that also possess a placenta when pregnant, but the organ is small, diminishes overtime (“yolk-like”), and can only nourish an embryo for a brief gestation period. In marsupials, most of an embryo’s development is done after birth, within the marsupial pouch. Interestingly, marsupials can concurrently be pregnant and raising an embryo in a pouch.
So what is so special about eutherian mammals? They happen to be the most diverse and dominant group of mammals around today with over 5,000 species (compared to a little over 330 for marsupials and 5 in the egg-laying monotremes). Except for the Virginia Opossum (a marsupial) that you sometimes find in yards on warm summer nights, all other species of mammals in North America are eutherians. That’s right…….every bear, bat, prairie dog, cat, deer, armadillo, Labradoodle, otter, and squirrels.
Eutherian mammal mothers protect their developing babies from the environment by carrying them for a long period of time. Babies that are born more mature have a greater survival rate and this can be seen in the global diversity and dominance of eutherian mammal species in comparison to the two other mammalian groups today.
Happy Mother’s Day and learn more about the amazing mothers of the natural world, modern and prehistoric!