The creature that gave rise to all the placental mammals – a huge group that includes whales, elephants, dogs, bats and us – has at last been pinpointed. An international effort mapped out thousands of physical traits and genetic clues to trace the lineage. Their results indicate that all placental mammals arose from a small, furry, insect-eating animal.
A report in Science resolves the debate as to when the creature lived; it came about after the demise of dinosaurs. That had been a hotly debated question over years of research.
Placental mammals – as opposed to the kind that lay eggs, such as the platypus, or carry young in pouches, such as the kangaroo – are an extraordinarily diverse group of animals with more than 5,000 species today. They include examples that fly, swim and run, and range in weight from a couple of grams to hundreds of tonnes.
A wealth of fossil evidence had pointed to the notion that the group, or clade, grew in an “explosion” of species shortly after the dinosaurs’ end about 65 million years ago.
But a range of genetic studies that look for fairly regular changes in genetic makeup suggested that the group arose as long as 100 million years ago, with mammals such as early rodents sharing the Earth with the dinosaurs. Via Earliest placental mammal ancestor pinpointed.
Puzzled by this report, take a look at wiki. http://en.wikipedia.org/wiki/Juramaia
It looks like placental mammals date to about 160 mya rather than 65 mya. The Britanica website tells a similar story.
Found this on Wiki which may explain:
The oldest known fossil among the Eutheria (“true beasts”) is the small shrewlike Juramaia sinensis, or “Jurassic mother from China,” dated to 160 million years ago in the Upper Jurassic. A later eutherian, Eomaia, dated to 125 million years ago in the Lower Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage. In particular:
Epipubic bones extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, and nontherian mammals like the multituberculates as well as in Ukhaatherium, an Upper Cretaceous animal in the eutherian order Asioryctitheria. They are apparently an ancestral feature which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles of these animals during locomotion, reducing the amount of space being presented, which placentals require to contain their fetus during gestation periods.
A narrow pelvic outlet indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.
When true placental mammals evolved is uncertain – the earliest undisputed fossils of placentals come from the early Paleocene, after the extinction of the dinosaurs.