T-Rex’s Weird Looking Vegetarian Cousin

By: Helen Beilinson

Eleven years ago, seven-year-old Diego Suarez found dinosaur bones while hiking with his parents in the Toqui Formation in southern Chile. Fortunately, his parents, Manuel Suarez and Rita de la Cruz, are geologists. They were instantly able to identify these bones as fossils and continued to search of the rest of this beast. Little did the family know, but they had unearthed a previously unknown species—T-Rex’s funny looking, vegetarian cousin. A study published this week in Nature describes this interesting beast, named Chilesaurus diegosuarezi (as the name suggests, this dinosaur was named after the lucky kid who discovered it).

Velociraptors and T-Rexes are some of the best known meat-eating dinosaurs. They both, along with many of their cousins, belong to the group of dinosaurs known as theropods. Theropods are bipeds, meaning they walk on two legs. They first appeared 230 million years ago and although the ‘dinosaurs’ have since gone extinct, common day birds are modern dinosaurs evolved from the theropod family.

For a long time, it was thought that theropods were strictly carnivores. The last decade, however, has brought forth new data showing that a variety of different diets were consumed. Of course, as these organisms are no longer around, it’s difficult to know for sure what they ate, but paleontologists have many tools at their disposal to understand dinosaur diets. First, instead of looking what goes into the dinosaurs, they can study what came out of them by looking at their fossilized poop, called coprolites. Meat-eating dinosaurs tend to have crushed up bones in their poop, whereas coprolites from vegetarian dinosaurs contain more traces of plants. Sometimes, paleontologists are even able to find another animal's bones inside the stomach of a bigger predatory dinosaur! Second, a dinosaur’s teeth give many clues about its diet. Sharp teeth, as you can imagine, are good for killing prey and biting through skin. Large, flat teeth, or leaf teeth, are better for chewing up plants. Skeletons also give scientists an idea of what the animal’s body type was, which gives hints to how it hunted, and thus, what it ate. For example, if you compare the body of an elephant to that of a panther, you might be able to predict which one would be better at chasing after prey. So based on all of these sources of evidence, paleontologists have been able to discern that most theropods are carnivores, except for a few anomalies, likes C. diegosuarezi.

The 3-meter long C. diegosuarezi was found to have flat teeth and a horny beak, characteristic of a vegetarian dinosaur. Although most theropods have sharp teeth characteristic of carnivores, it isn’t entirely surprising that they would have a plant-eating cousin. When in competition with other organisms, it’s good to have something that sets you apart; this make it easier for you to survive. When you are surrounded by meat eaters, if you eat plants, you’ll have less competition for your food source. This leads to many forms of convergent evolution, where you see the same type of feature (such as short, fatter leaf teeth) in different organisms that are unrelated, which can give the appearance they are related. This is why it took so long for this dinosaur to be announced, even though it was discovered over a decade ago—its features were convergently evolved with other vegetarian dinosaurs. Importantly, the discovery of C. diegosuarezi shows that vegetarianism in theropods appeared much earlier than previously thought.

You might be thinking that if only this dinosaur’s teeth were slightly different than other theropods, why did it take so long to classify it? That’s because C. diegosuarezi is special not just because of its teeth, but because it has other non-theropod features, like the leaf teeth.

Like other theropods, C. diegosuarezi ran on its two hind legs with its shorter forearms (about half the size of its hind legs) in the air, much like a raptor. Their hands have two, stump-like fingers with short claws at the end, much like other theropods such as  the T-Rex. Unlike the T-Rex, however, which had a characteristic massive head with a large mouth and thick neck, C. diegosuarezi had a longer neck and a small, rounded head with a small neck. These physical features provide even more evidence that it was a vegetarian.

The femur (a large bone in the leg) of C. diegosuarezi is somewhat different that the classic femurs of theropods, looking a lot more like those of another group dinosaurs called sauropodomorpha. These dinosaurs were long-necked, bipedal herbivores, like C. diegosuarezi, but usually a bit larger. This is another example of convergent evolution contained within C. diegosuarezi’s body. The pelvic girdle, which connects the lower limbs to the spine and upper body, of C. diegosuarezi is also distinct from other theropods.

I can honestly say I’m not a paleontologist, so I cannot give you the details of the new dinosaur with much confidence, but I can say this: it is always fascinating to me when such ‘platypus’ animals come about (as Martín Ezcurra at University of Birmingham calls them). These are animals that at first glance look like they’re made up of different parts of different animals (like the duck-like bill, otter-like body, and beaver-like tail of the platypus), but actually they’re merely very demonstrative examples of convergent evolution. If something works for one animal, proof of how highly adapted it is for a particular function or environment is much stronger if it independently evolves in separate families of organisms. And, hey, they might look slightly different, but if it works, it works.