Jurassic Jigsaw: Is T-Rex’s new vegetarian relative a confusing evolutionary puzzle?

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With an average discovery rate of one dinosaur per week, a new species may not seem so special anymore – however, scientists are convinced they’ve described one of the weirdest ancient reptiles yet. Following the initial discovery of the fossils by a seven year old boy, the researchers report discovering a vegetarian chimera related to both T-Rex and velociraptors, which may reshape our view of dinosaur evolution.

Herbivorous Hoax?

Whilst geologist couple Manuel Suárez and Rita de Cruz were conducting research in Black Hill, Chile, their son, Diego, found some dinosaur fossils. In recognition, Manuel and a team of palaeontologists led by Fernando Novas, aptly named the dinosaur Chilesaurus diegosuarezi. It lived about 145 million years ago during the Jurassic period, measuring around 3.2 metres from tip to tail. However, the fossils posed a problem to the researchers during the years of analysis, with Novas calling the skeletons an “evolutionary jigsaw puzzle”. Looking in reconstructions like a cross between a diplodocus and a velociraptor, the Chilesaurus fossils had conflicting characteristics. The herbivorous leaf-shaped teeth, large, strong legs and space for an enlarged gut to digest delicious plants all paint the picture of a prehistoric picky-eater. Though its appearance, much like that of a bipedal brontosaurus, belies its green diet, its overall skeletal structure is too similar to its purported carnivorous cousins.

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Chilesaurus seems a chimera of characteristics shared with multiple other dinosaurs, which begs the question of whether Novas and his team have made some kind of Frankenstein’s mistake – assembling the bones of multiple different dinos. The situation resembles that of the platypus during the 18th century, when specimens brought from Australasia to Europe were considered a hoax by many naturalists due to their adorable mosaic of beaver, otter and duck qualities. But in both cases the scientists were not hoaxers; Novas and his crew recovered four whole, well-preserved skeletons, dismissing this possibility.

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Shared traits between animals that may initially seem weird can be explained by an evolutionary process called ‘convergent evolution’, where similar behaviours or body parts can evolve in different animals in a similar environment. The platypus shares duck-like qualities since it lives in an aquatic environment, and bats fly with wings functionally similar to birds; these mammals have evolved similar solutions to our feathered friends because they are the most efficient strategies. But why does Chilesaurus, the supposed ‘veg-loceraptor’ and relative of the carnivorous T-rex, have a convergently evolved herbivorous body? The answer is most likely its environment. Novas argues it’s easy to imagine a carnivorous ancestor evolving plant-eating traits to take advantage of the abundant, lush vegetation thought to have existed around the river system in Black Hill where the fossils were found.

Cretaceous controversy

What set the media ablaze about Chilesaurus’ odd appearance is the reported link between both carnivore and herbivore. The mosaic mixture of meat eater and vegetarian, resembling multiple different dinosaurs, made initial identification of Chilesaurus’ position in the prehistoric family tree difficult. Each different characteristic pointed to a different lineage. Ultimately, the analysis of Novas’ team determined the bipedal beast to be a primitive ‘theropod’ – a group of dinosaurs including large carnivores such as T-Rex and in the same lineage as the ancestors of modern birds. The largely meat-loving nature of its relatives is what made the Chilesaurus so puzzling to Novas and his team, as the prevailing theory was that carnivorous theropods evolved into plant-eaters much later.

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Chilesaurus would be remarkable for being the earliest known vegetarian member of the group, pushing the evolution of herbivory back further than before. Previously discovered plant-eating theropods exist such as Nothronychus, a bizarre creature which looks much more transitional between birds and dinosaurs and was thought to exist much later than Chilesaurus – though the feathery coat in reconstructions is a guess based on other similarly feathered skeletons. This suggests that vegetarian diets may have evolved in T-Rex’s family multiple times to exploit lush environments, showing how adaptive the evolutionary process can be. The endangered Giant Panda is a modern example illustrating the principle of carnivorous bears evolving into plant specialists, but also highlights the sad fact that evolution often cannot keep pace with human habitat destruction.

The focus on the supposed weirdness of Chilesaurus is, however, controversial among palaeontologists. Some question the very placement of the new dinosaur within the theropod group, treating it as a miscategorisation. The original analysis by Novas’ team may have ignored certain family trees which would instead place Chilesaurus in its own novel plant-eating group which was merely transitioning from carnivory to herbivory and in no way related to later meat-eaters. This would make Chilesaurus’ story the norm rather than a bizarre evolutionary offshoot, but these debates give us glimpses into the headache inducing detective work which palaeontologists are passionate about.

The story is not over for Novas and friends, with the researchers planning to return to Black Hill to dig up more fossils to try to build a more complete picture of what ecosystem Chilesaurus may have lived in.

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