The evolutionary split of opossums from other marsupials occurred about 65 million years ago. Today there are more than 60 species of possum, which live throughout the Americas, as well as Australia, New Zealand, Tasmania, and New Guinea.Ī 2009 study published in the journal PLOS ONE traces the opossum lineage back to a sister group of marsupials called the peradectids, which lived at the time of dinosaur extinction in the Cretaceous–Paleogene period. They don’t go back as far as the monotremes, but opossums are also quite ancient, 'with little change over the last few tens of millions of years,” Ibrahim says. Ibrahim adds that the oldest fossils that look like the modern platypus date to the Quaternary period, which is about 2.5 million years ago. (Also see 'Giant Platypus Found, Shakes Up Evolutionary Tree.') Their duck-like bill and unique jaws help scientists identify such fossils.

Skulls of platypus-like ancestors have been found dating back to the Cretaceous period (63 million to 138 million years ago), Hopkins says. Egg-laying mammals are called monotremes, and though once more diverse, today that group contains only the platypus and two species of echidna. Those include their leathery eggs and a lack of nipples, both traits that don't exist in mammals that evolved after platypuses.įebruary 6, 2009-National Geographic researchers are trying to collect DNA samples from these odd duck-billed mammals to determine whether there are separate subspecies.Įggs, you might be wondering? You heard right. The platypus has “a number of primitive features,” Ibrahim says, “both from what we know from fossils and from what we can see in their anatomy.' You could say the platypus is a survivor: It's one of the few living descendants of an ancestor that diverged from all the other mammals about 150 million years ago, Hopkins says. That said, two mammals that have undergone the fewest evolutionary shifts are the platypus and the opossum, says Samantha Hopkins, associate professor of geology at the University of Oregon. Physiology and DNA change somewhat over time, he says, both through the basic process of evolution as well as random genetic changes.

With only fossils to go by, scientists can examine an ancient animal's skeletal structure, but it's not the whole story. “'Unchanged' is a tricky word,” Nizar Ibrahim, a paleontologist at the University of Chicago and 2014 National Geographic Emerging Explorer, says via email. Luke asked Saturday’s Weird Animal Question of the Week via Facebook: “What is the oldest surviving mammal that has gone unchanged by evolution?” Don’t Go Changin' It’s nice when people lie about our Throwback Thursday pictures this way, but it’s also true that some of us stay better preserved than others.ĭavid Gohman Luke wondered which animals have maintained their look the longest-but he's talking millions of years, not decades.

Picture of platypus animal