Sometimes all it takes is one new discovery to send scientists back to the drawing board.
Of course, as astrophysicist Neil deGrasse Tyson correctly points out, scientists are always at the drawing board, or should be. "If you're not at the drawing board," he says, "you're not doing science." But still, it does seem sometimes like things are pretty well figured out, and then...
There was a "boom" moment in the field of mammalian evolution this week, delivered by a paper in the journal Alcheringa: The Australasian Journal of Palaeontology. The authors -- led by the brilliant paleontologist and polymath Timothy Flannery, of the University of Melbourne -- describe a fossil find that would seemingly be of interest only to people fascinated by minutiae of paleontology; a jawbone of a tribosphene, a proto-mammal with distinctive triangular, three-pointed molars, from the early Jurassic Period in Australia.
The problem is, it kind of shouldn't have been there. Tribosphenes, which are in a group that is ancestral to both marsupial and placental mammals, were thought to originate in Laurasia, the northern half of the (at that point, split) supercontinent Pangaea. (Laurasia comprised land that is now found in North America, Europe, and Asia.) Australia, on the other hand was part of the southern half of Pangaea, called Gondwana, along with Africa, Antarctica, and South America.
This origin for the tribosphenes was considered so certain that they used to be called boreosphenes -- from the Greek word Βορέας, which was the name of the god of the north wind.
Guess it's a good thing they changed the name.
There's no doubt that there were tribosphenes in Laurasia, too; one of the earliest, Tribactonodon, can be found in the Lower Cretaceous Durlston Formation in England. (Others have been found in Mongolia and in Portugal.) The idea was that they started in Laurasia and only later spread southward to Gondwana -- so Australia's iconic marsupials originally started out much farther north.
The discovery of a tribosphene in Australia sixty million years earlier than that indicates that some rethinking may be in order."I was re-analyzing these fossils that turned up in Victoria from the age of dinosaurs," Flannery said, in an interview with Australian Geographic. "And then I started looking more widely for similar sorts of fossils found elsewhere and it turned out all of them were in the southern hemisphere and all are Jurassic or Cretaceous in age [from 199–66 million years ago]... And we realized the thing that unites all these Southern Hemisphere fossils is they have these very strange, complicated molars that let the animals puncture shear and crush, all at the same time, what they were eating. I resisted the conclusion as long as I could, but the evidence is compelling. These shrew-like animals from Australian are actually the ancestors of both the earliest placentals and the earliest marsupials."