Since the 1990s, some scientists have concluded from family trees based largely on molecular evidence that at least some lineages of modern placental mammals originated as early as 100 million years ago, during the Cretaceous period.
To build the family tree, paleontologist Maureen O'Leary of Stony Brook University in New York and colleagues began with 46 species of living mammals, including placental, marsupial and egg-laying mammals.
The groundbreaking six-year research collaboration also produced the most complete picture yet of the evolution of placental mammals, the group that includes humans.
Placental mammals are the largest branch of the mammalian family tree, with more than 5,100 living species.
This also means that marsupials have lower food and water requirements than placentals.
Charles Darwin and others have assumed that the diversity of marsupial species in Australia reflects the absence of competition from placental mammals during the early Tertiary period.
London, August 25 (ANI): A remarkably well-preserved fossil of a small shrew-like mammal has been discovered in the Liaoning Province in northeast China that provides new information about the earliest ancestors of most of today's mammal species-the placental mammals.
Juramaia is the earliest known fossil of eutherians-the group that evolved to include all placental mammals, which provide nourishment to unborn young via a placenta.
Prior to discovery of the tooth, the oldest Australian placentals
(other than bats) dated to 5 million years ago -- a time when the northward-moving continent had drifted close enough to allow the passage of Asian rodents across the Malay Archipelago.
This isolation is providing modern scientists with a remarkable opportunity to study an unparalleled experiment in mammalian evolution: Biologists can compare the development of marsupials "down under" with that of their counterparts in the rest of the world -- where they have had to compete with the now-dominant placental
mammals, such as lions and deer.
School of Medicine) and Archibald (biology, San Diego State U.), this volume examines the evolutionary success of placental
mammals (which range from bats to hippos to whales).