The Protungulatum donnae, a rodent-like mammal slightly larger than a mouse but smaller than a rat, is the earliest of the placental mammals, according to a study published in Science and authored by Maureen O'Leary. O'Leary is an associate professor in the Department of Anatomical Sciences in the School of Medicine at Stony Brook University , and a research associate at the American Museum of Natural Art.

The Protungulatum had a diet of insects, a fleshy nose, a light underbelly in its fur and a long tail, O'Leary said, though it lacked special bones near the pelvis and a small bone for hearing. Evidence of the creature was found in the North America, but it may have lived on other continents as well.

Over 5,100 living species, including Homo sapiens-- humans-- constitute the makeup of placental mammals, the largest in its class. Excluded are marsupials, such as kangaroos, and egg-laying mammals such as echidnas and the platypus.

The study shows that placental mammals arose 200,000 to 400,000 years after dinosaurs were decimated approximately 66 million years ago at the end of the Cretaceous period, Discovery reports. This is 35 million years later than the previously estimated time frame, which was based purely on genetic data. The Protungulatum was one of only a few species to have survived the asteroid crash that is believed to have caused the great dying, and it later went on to become the common ancestor to almost every mammal. Other survivors include some crocodiles, turtles and flowering plants.

This would place the genesis of placental mammals after Gondwana (one of two supercontinents that formerly belonged to Pangaea) fragmented 120 million years ago, according to the New York Times. Scientists previously believed that the fragmentation was the cause of mammalian diversification, but the new study rules out that line of reasoning, and implies that mammals could only flourish when the dinosaurs died out.

O'Leary used a program called MorphoBank to analyze data. The software is a web application that "enables teams of scientists who use anatomy to study phylogeny to work over the web" and is able to assess organisms on the basis of more than 4,500 possible traits.

Anne D. Yoder, an evolutionary biologist at Duke University, praised the sudy's meticulous analysis, but she took issue with how it disregarded the ticking of the molecular clocks of genetics, according to the Times.