These are placental mammals (Wisconsin Historical Images; http://tinyurl.com/6ekftw7)
Biologists obsessed with mammalian evolution have been having a debate that sometimes gets nasty. They’re fighting over when placental mammals, that includes you and I, diverged from the marsupials, those cute and cuddly kangaroos, koalas and Tasmanian devils. But a new fossil unearthed in China [http://tinyurl.com/6dh5poy] by Zhe-Xi Luo in the USA and researchers in Beijing might force them to put their gloves down.
This debate has been raging on for quite a while, in March 2002 a small, 125 million year old, tree-climbing mammalian fossil was found. This pushed back mammalian evolution by around 50 years. Placental mammals were much older than anyone expected.
By 2007, even this date was being challenged. DNA sequences from several different placental mammals, marsupials, monotremes (platypus and short-beaked echidna) were compared to each other, which pushed back the marsupial-placental split another 20 million years.
This challenged the dogma that marsupials and placentals diverged recently and that the death of the dinosaurs allowed them to flourish. It’s a logical conclusion, but this is another case of truth being stranger than fiction.
This is a marsupial mammal (OZinOH; http://tinyurl.com/5scay8r)
The recent find by Zhe-Xi Luo and co-workers puts the nail in the coffin on this idea. They unearthed a fossil called Juramaia sinensis (Pictured), a tiny tree-climbing mammal that weighs in at around 12 to 15 g. It’s really tiny. Its teeth, jaw and heel bones are similar enough to ours to be grouped with us. On the other hand, it is different enough from other placental fossils to be stamped an ancestor. With enough fossils evolution can become very clear. Remarkably its also 160 million years old.
Juramaia Sinensis a 160 million year old placental mammal (from: http://tinyurl.com/6dh5poy)
Such an old time frame matches the molecular data perfectly, and it also pushes back our own ancestors by another 35 million years. Placental mammals diverged from marsupials in the Middle to Late Jurassic before flowering plants existed; when Allosaurus roamed, the big meat-eating daddy preceding T. rex; when the 40 ton long-necked giant sauropod Supersaurus roamed America; and, when Archaeopteryx was testing its wings (As a side note: Archeropteryx is no longer the ancestor of all birds, just closely related — instead other vegetarian dinosaurs are their ancestors). J. sinensis must had had a tough time competing with these dinosaurs and was even smaller than some of the insects they might have been trying to eat. But, was a mammals life really that tough?
The evidence suggests differently. It seems that in the early and middle Jurassic, a few million years leading up to J. sinensis, mammalian evolution was exploding. Several different mammalian forms developed, though in true Darwinian fashion, only a few survived. By around 100 million years ago, they were petered down into three classes: the monotremes, placentals and marsupials. After 100 million years ago, the dominance of mammals might just have been inevitable. The asteroid that wiped out the dinosaurs was simply a lucky break.
Sources
A Jurassic eutherian mammal and divergence of marsupials and placentals
The delayed rise of present-day mammals
Robust Time Estimation Reconciles Views of the Antiquity of Placental Mammals
An Unexpected Science Nugget 