June 25, 2012
Hadrosaurs have often been called “duck-billed dinosaurs.” You don’t have to look at their skulls for very long to see this analogy is wide of the mark. Not only did hadrosaurs such as Edmontosaurus have shovel-shaped, grooved beaks, but their jaws were lined with rows of cropping, crushing teeth. These dinosaurs didn’t dabble in Cretaceous swamps – they grazed the prehistoric plains. And, up until recently, it was thought that these huge herbivores possessed an evolutionary innovation that made them the dinosaurian equivalent to cows.
In general, dinosaur jaws and teeth were for cutting, plucking, and tearing. Dinosaurs didn’t chew their food, but instead ripped or clipped their morsels, which were then swallowed whole. (Strange as it may seem, this style of eating might have had a role to play in why sauropods were able to maintain such large body sizes.) But hardosaurs were thought to be different.
The idea I encountered as a kid was that when hadrosaurs such as Edmontosaurus opened their jaws, the tooth-bearing bones of their upper jaws – the maxillae – swung inwards. Then, when the lower jaws came back up, the lower teeth met the upper teeth and ground the plant food across the tooth surfaces. This wasn’t chewing like mammalian herbivores do it, but it was an evolutionary alternative that allowed hadrosaurs to better break down their food before swallowing. You can see a visualization of this hypothesis in action in this YouTube video.
But this model of hadrosaur chewing required a great deal of flexibility in the skull to create a complex chewing motion. As a video uploaded by the Canadian Museum of Nature – posted above – shows, hadrosaur jaw movements were probably a great deal simpler. The key to the puzzle is the interlocking group of small bones at the back of the skull. When the virtual Edmontosaurus drops its lower jaw, the movement compresses some of these bones at the back of the skull, which moves the upper tooth rows slightly inward. But the lower jaw doesn’t just drop – a joint at the back of the mandible also allows the lower jaw to extend forward. When the jaws close, the lower jaw moves back in a diagonal motion, and the contact of the upper and lower teeth gently push the maxilla slightly outwards. There is still a lot of movement in the skull, but it’s not quite as dramatic as the swingin’ maxilla version. And this goes to show just how much we still have to learn about dinosaurs. Even though we know more about Edmontosaurus and its kin than ever before, the basics of dinosaur biology remain rich grounds for investigation and debate.
[Hat-tip to Thomas Holtz for sharing this video on Twitter.]
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