Good things can come in small packages. Just ask the platypus: a one and a half foot long creature that, as a mammal, somehow manages to break all the rules. Or ask the paleontologists who just discovered the single tooth that changes everything we thought we knew about platypus evolution.
A study published this month in the Journal of Vertebrate Paleontology details the findings of authors Rebecca Pian, a graduate student at Columbia University and Mike Archer and Sue Hand, a professor and associate professor at the University of New South Wales. The tiny relic, an oblong brown molar, tells the story of the branching of the platypus family tree five to fifteen million years ago. Despite its insignificant size, it indicates to Pian and her co-authors that the ancient platypus who grew it, Obdurodon tharalkooschild, must have been huge – at least twice the size of our modern platypus. It also indicates something even more critical: the platypus family tree had branches.
Until this discovery, platypus evolution was thought to look more like bamboo than like a tree, evolving in more or less a straight line with only one species in existence at a time. Platypuses today are rare animals, found only in Australia. Fossil evidence of platypus relatives are even rarer, and groundbreaking discoveries often revolve around small, isolated fossils. In fact, three of the four ancient species of platypus that we know of are identified from their fossilized teeth. The genus name Obdurodon means “enduring tooth.” Interesting fact: platypuses don’t have teeth. Modern platypuses lose their few teeth by adulthood. They grind their prey using a bony pad in their beaks.
Toothlessness is hardly the strangest thing about the platypus. They use neither sight, nor hearing, nor smell to hunt in the muddy rivers of the outback, but can use their duck like bills to sense the electric pulses in the muscle contraction of the shrimp, frogs, and insects that they eat. The male platypus sports spurs on his webbed back feet that produce a venom strong enough to kill dogs and cause extreme pain in humans. Platypus mothers produce milk to feed their young but lack nipples, letting the babies lap the milk as it pools in their fur. They have more bones in their shoulders than the average mammal, which support a lizard-like muscle structure, and of course, adhere to one of the basic rules of mammal-hood by laying leathery, reptile-like eggs.
The monotremes are an order which includes only platypuses and echidnas. Monotremes broke away from the evolutionary line of mammals about million years ago, even before the invention of the placenta. Steropodon galmani, the earliest known platypus in Australia, was a contemporary of the dinosaurs in the Early Cretaceous. Monotrematum sudamericanum, the only platypus specimen to be found outside of Australia, lived 61 million years ago and provides evidence that platypuses must have lived in the parts of Gandwana that became Argentina and the rest of South America. Since their beginnings, monotremes have been living by their own rules – which is why understanding their development will tell us more about mammal evolution.
Obdurodon tharalkooschild’s adult tooth means that it could handle a broader diet of larger animals than the modern platypus, possibly even turtles and fish. It probably laid eggs and lived most its life in the fresh waters of Australia like its modern counterpart. It was almost certainly as strange, and scientists are hoping this newly discovered side branch of the platypus family tree will add to our knowledge of these fascinating animals.
Source: Plan, R. Archer, M. & Hand, S. (2013, Novemeber 05). A new, giant platypus, Obdurodon tharalkooschild, sp. nov. (Monotremata, Ornithorhynchidae), from the Riversleigh World Heritage Area, Australia. Journal of Vertebrate Paleontology, 32(1), 51–114; Retrieved from http://www.tandfonline.com/doi/abs/10.1080/02724634.2013.782876#preview
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