The earliest platypuses: bigger and toothier

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Platypuses are strange creatures, even to evolutionary biologists. They have hair, webbed feet, a beaver-like tail, and duck-like bills with which they can sense electrical currents, they lay eggs, and the males possess venom glands on their hind limbs. When a drawing and a pelt of the animal were first sent to British scientists at the end of the 18th century, the incredulous naturalists assumed it was a hoax. In college, I had a t-shirt that made light of their bizarre features:

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Given the idiosyncrasies of this animal, you might imagine that they’re a bit of an enigma to evolutionary biologists. In fact, I recall a creationist acquaintance of mine once declaring that they must be an “evolutionist’s worst nightmare”. To be sure, there are a lot of gaps in our knowledge regarding how they might have evolved. What the evidence points to pretty clearly is this: 1) they’re mammals, as they have hair, produce milk for their young, and their DNA is much more similar to other mammals than it is to, say, birds or lizards, and 2) they split from other mammals a long, long time ago, somewhere on the order of 220 million years. The fossil record of their relatives, known as monotremes, is extremely spotty, with the earliest dating to over one hundred million years ago. Unfortunately, the specimens are extremely fragmentary, frequently consisting of only part of a jaw or arm, preventing researchers from making many firm conclusions about their evolution.

However, at least one fossil animal gives some insights into their ancestry. Obdurodon includes several species of platypus-like monotremes that are estimated to have lived ~28.1 to 5.3 million years ago. Below you can see a comparison between Obdurodon dicksoni (left; 23-11.6 million years ago) and the modern platypus (right).

Obdurodon-and-Platypus-skulls

You’ll notice differences between the two, but overall they look extremely similar. Clearly the duck-like bill has been around for a while! However, at least one feature  points to Obdurodon being a transitional fossil, namely the fact that it has teeth. Modern platypuses have teeth when they are very young, but shed these by adulthood and replace them with horny pads (see below).

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Considering these horny pads are a very uncommon feature in mammals, and vertebrates as a whole, biologists interpret it as an evolutionary novelty. With Obdurodon, we have support for this hypothesis, with evidence that a toothed ancestor preceded the modern platypus dental condition.

Questions for creationists

Where did the other platypus species (i.e., Obdurodon) go? Could Noah not fit them or their eggs on his ark? Is it a coincidence that there are fossil platypuses that had teeth as adults, much like typical mammals, but modern platypuses replace them during development with horny pads?

References

1. Musser, A. M., & Archer, M. (1998). New information about the skull and dentary of the Miocene platypus Obdurodon dicksoni, and a discussion of ornithorhynchid relationships. Philosophical Transactions of the Royal Society of London B: Biological Sciences353(1372), 1063-1079.

Photo credit

platypus, t-shirt image, platypus Obdurodon comparison, horny pads

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