On June 24th, 2018 we learned about

Many dinosaurs’ hyoid bone left their tongues immobilized in their mouths

You’ve probably never worried about this, but paleontologists have finally confirmed that dinosaurs like Tyrannosaurus rex couldn’t french kiss. They couldn’t lick lollipops, nor could they do that trick where you tie a knot in a cherry stem in your mouth. However amazing their teeth or jaws may have been, analysis of a bone called the hyoid has found that T. rex, as well as most other dinosaurs, couldn’t do much with their tongues besides swallow. Lacking the articulation of many modern birds, snakes and lizards, this sheds light how tongues evolve to support different animals feeding habits.

Degrees of tongue articulation

The hyoid is a small bone that, millions of years ago, was a gill arch in our fish ancestors. As animals have evolved along different ecological paths, the hyoid has been adapted to support a variety of anatomical needs. In humans, it sits above the larnyx, connecting soft tissues to help us manage the passage of air and food in our throats. In many birds, the hyoid juts further into the mouth, often acting as a mobile attachment point for the tongue. This enables a variety of tongue movements, including extreme cases like the curling, tube-like tongue of a hummingbird. Beyond tongues, hyoids also enable modern reptiles like the Mata Mata turtle to stretch and open its throat to suck in prey underwater.

However, in most theropod and sauropod dinosaurs, hyoids weren’t quite so active. They were found to usually be a relatively simple pair of rods under the tongue, suggesting minimal possibility for movement. This arrangement most closely resembles the hyoids of modern crocodiles and alligators, which isn’t entirely surprising. These crocodilians can position their heads well enough to lop off hunks of food, only needing their tongue to help push that food back to be swallowed. Without other demands, their tongues can essentially be fixed along the bottom of the animal’s mouth by muscle and other soft tissues. Dinosaurs like T. rex could probably use this sort of feeding pattern as well, relying on their heads and teeth to get food into their mouths to be swallowed.

When to switch from simply swallowing

This doesn’t mean that hyoids only became complex in the last 65 million years though. Plant-eating ornithischian dinosaurs, like Stegosaurus or Triceratops, had more sophisticated hyoids than many of their compatriots. Pterosaurs were also found to have hyoids that would have facilitated more tongue waggling and lapping. Taken together, these ‘exceptions’ help prove what a mobilized tongue could be good for— if T. rex could simply aim its mouth at its food, Triceratops and pterosaurs likely had to use their tongues to grasp and manipulate their food before eating it. This could be necessary for grasping and chewing twiggy plants, or getting a hold of small prey with long, skinny beaks. In modern birds, this kind of tongue use is clearly demonstrated by parrots who use their tongues to pluck and hold nuts and seeds, almost like a finger built into their mouth.

To really make the point about what pressures push a species to develop a more complex hyoid, researcher noted the difference between tyrannosaurs and avian dinosaurs more directly related to modern birds. While both groups were theropods, the larger, bitier predators had the more crocodilian tongue arrangements. Flying theropods like microraptors had mouths that look much more like modern birds, showing that taking to the sky created a need for a more complex mouth.

Source: Tongue-tied: T rex couldn't stick out its tongue by Nicola Davis, The Guardian

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