In a paper this week in Science Advances, Masakazu Asahara and colleagues compared the skulls of the platypus we all know and love, Ornithorhynchus anatinus, with fossils of the extinct platypus genus Obdurodon. The most distinctive difference between our modern platypus and Obdurodon is that modern platypuses lose their teeth as they grow into adults, whereas Obdurodon retain their adult teeth. The modern platypus does eat crunchy things, such as crayfish, but they grind them up using the tough keratinized pads in their bills that you can see in the photo above.
So why did the modern platypus lose its teeth? Asahara's paper shows that it was due to the development of the electroreceptive organ in the platypus bill. Platypuses forage for crayfish in the mud of murky freshwater rivers and pools in eastern Australia. Visibility in these conditions is very low. Platypuses therefore use mechanoreceptors (touch sensors) and electroreceptors on their bill to find crayfish in the mud.
All animals (including us humans) generate weak electric fields due to the activities of nerves and muscles. The ability to detect those electric fields, called electrolocation, is much less common. It occurs in some sharks, fishes, dolphins, and bees (the bee paper is super cool). In biology there are two different types of electrolocation. Active electrolocation, such as in electric fish, is when the animal generates an electric field using an electric organ and then detects distortions in that electric field to localize obstacles or prey. Electric fish can also communicate through electrical signals and use them to paralyze their prey. In contrast, passive electrolocation, such as what occurs in the platypus, is when an animal uses an electroreceptive organ to detect electric fields, but is not generating its own electric fields with an electric organ. The platypus can use its electroreceptive bill to detect the weak electric fields generated by a crayfish in the mud.
Asahara's paper demonstrates that Obdurodon had a more upturned bill that the modern platypus which leads them to propose that Obdurodon spent more time foraging in open water than in the mud on the bottom, and may therefore have had less need for electro and mechanoreception in its bill. In accordance with this, the space in the skull for the nerves to the bill is smaller in Obdurodon than in the modern platypus. As the nerves enlarged in the modern platypus, however, they took up the space needed for the roots of adult teeth. Over evolutionary time the platypus therefore appear to have traded their teeth for improved prey finding in murky water.
If this tidbit peaks your interest in the platypus, this documentary has fantastic footage including foraging, courtship, and a cameo of a wombat in the snow.