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Skulls of Dinosaurs Give Clues About Their Diets Beak specializations let paleontologists know how changes in diets affect skull shape and influence evolution in oviraptors

The term “Darwin’s finches” was first used in 1936 to describe a group of birds from the Galápagos Islands. One of the key differences between each species of finch was the drastic difference in beak size and its effect on what the bird eats. For example, a bird with a very long beak can access seeds and nectar that a short-beaked bird cannot.

It turns out that the same trends used to categorize the diets of Darwin’s finches can be applied to the fossil record as well. It is especially true in dinosaurs whose skulls reveal changes in dietary preference. . According to a recent paper, an oviraptor called Gigantoraptor shows evidence of a beak that has not only adapted to eating plant matter but has also evolved to prefer a specific type of plant-based diet.

Oviraptors are an extinct group of theropod dinosaurs that were closely related to birds. Tyrannosaurus and Velociraptor belong to this group.  These dinosaurs shifted away from meat-eating, a trait often associated with theropods, sometime during the Early Cretaceous (about 125 million years ago). Oviraptors are divided into families based on their how they look and fall into two main groups: Oviraptorsauridae (found only in Asia) and Caenagnaethidae (found in North America and Asia). Gigantoraptor, the dinosaur that the authors focused on, belonged to the family Caenagnathidae and it’s slower jaw had a very deep beak.

When a non-avian (non-birdlike) dinosaur has a beak, it usually means it was a plant-eater or herbivore. Oviraptors from the Early Cretaceous have a few teeth but also have the beginnings of a beak, thus hinting that they were in the process of shifting their food preferences to a more plant friendly diet. The mouth of Gigantoraptor was completely lacking teeth and had a large pronounced beak along the maxillaries (upper jaws) and dentaries (lower jaws). When theropods are found without teeth, it is generally associated with herbivory. By fitting the mouth with a toothless beak, a large grinding surface is created where slicing teeth once were. Modifications to the beaks of theropods – such as getting larger or bulkier in the case of Gigantoraptor – show that the diet was being specialized within herbivory. This is indicative of a clear separation from carnivory to a strictly herbivorous diet according to the paper’s primary author, Waisum Ma. Herbivory (or at least omnivory) has been assumed for many oviraptors so an enlarged beak on Gigantoraptor would be a modification that would help with the processing of plant material and not a weapon used to catch and kill prey items.

Since Gigantoraptor belonged to the family Caenagnaethidae, it was easy to compare its beak to other known oviraptors to see how beak size and shape changed through time. Based on comparative research that measured the depth of the beak, angle of the mandible, and how prominently the back of the mandible connected with the rest of the skull, they found that the beak of Gigantoraptor was the deepest of the any in its respective family. This means that the ancestral beak size and shape of Gigantoraptor was large, robust, and deep, and trended towards shallower, thinner beaks as the Cretaceous progressed.

What does this mean and how does it tie into the diet of Gigantoraptor? Much like the finches, oviraptors would have had specialized beaks that were adapted to their preferred food source. The large, powerful beak in Caenagnaethidae could very well be an adaptation for crushing seeds or hard plant material (i.e. a parrot’s beak) whereas the shallower, flatter beak in Oviraptoridae would have been better suited for grinding or raking in large amounts of plants at one time (i.e. a duck’s bill). This indicates that the feeding styles between Caenagnaethidae and Oviraptoridae had diverged and were very different from each other.

Knowing that the beak of Gigantoraptor is more useful as a crushing tool clues us in to two conclusions. First off, it let’s us know that some types of primitive oviraptors had a specialized diet. While this is not surprising, it does set a precedence for beak size and shape that scientists can use to measure all other beaks of oviraptors. Secondly, knowing the use of Gigantoraptor’s beak shows that the usage of beaks in oviraptors was not static and dramatically changed through time. And, just like Darwin’s finches, we can observe that food preference was a driver in changes of skull anatomy in dinosaurs and was a contributor to speciation within a group of related extinct organisms – just like it is today!

Article Information

Edited By: Namrata Dave and Gina Riggio
Source: Functional anatomy of a giant toothless mandible from a bird-like dinosaur: Gigantoraptor and the evolution of the oviraptorosaurian jaw
Publication Date: 24 November, 2017

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