T-Rex cousin Chilesaurus, a vegetarian, has scientists baffled

A seven-year old boy Diego Suárez discovered the bones of a turkey-sized cousin of T-Rex – Chilesaurus diegosuarezi – that was vegetarian and lived at the end of the Jurassic Period, about 145 million years ago.

Diego was walking with his geologist parents Manual Suárez and Riata de la Cruz, who were studying rocks in the Toqui Formation in Aysén, south of Chilean Patagonia, when he came across the fossils while searching for decorative stones with his sister Macarena.

Although closely related to the famous carnivore Tyrannosaurus rex, Chilesaurus diegosuarezi is proving to be an evolutionary jigsaw puzzle, because of its preference for plants.

Chilesaurus diegosuarezi, T-Rex’ little cousin that had a preference for plants. (Credit: Gabriel Lío)

Scientists are referring to Chilesaurus diegosuarezi, named after the boy who discovered it and the country where it was found, as a ‘platypus’ dinosaur because of its extremely unusual combination of characterstics that include a proportionally small skull, while its feet seem more akin to those of primitive long-neck dinosaurs.

Palaeontologists say Chilesaurus diegosuarezi is nested within the theropod group of dinosaurs, which included mainly carnivorous creatures such as the Carnotaurus, Velociraptor and Tyrannosaurus, from which modern birds evolved.

 

The presence of herbivorous theropods was known only in close relatives of birds, until now. However, the Chilean discovery shows us that a herbivorous diet was acquired much earlier than thought.

Bizarre combination of features

It was initially thought Diego had discovered several species, given Chilesaurus’ unusual combination of features. However, since his find, over a dozen Chilesaurus specimens have been excavated, including four complete skeletons, a first in Chile for the Jurassic Period.

The specimens demonstrate that this dinosaur certainly had a unique variety of anatomical traits.

While most of the unearthed fossils were the size of a turkey, some isolated bones indicate a maximum length of about three metres.

Palaeontologists from Argentina, Chile and the University of Birmingham in England, as well as Diego’s parents, had their findings published in the academic journal Nature.

Another of Chilesaurus’ unique features included robust forelimbs, similar to Jurassic theropods such as Allosaurus, although its hands consisted of two blunt fingers, unlike fellow theropod Velociraptor’s sharp claws.

The pelvic girdle of Chilesaurus resembles that of the ornithischian dinosaurs, even though it is actually classified in the other basic dinosaur division – Saurischia.

Chilesaurus diegosuarezi was named after the 7-year old kid who found the first fossil. (Image: University of Birmingham)

Evolutionary convergence

Chilesaurus’ body parts were adapted to a particular diet and way of life, which was similar to other dinosaur groups. As a result of these similar habits, several regions of its body evolved resembling those of other unrelated dinosaur groups, a phenomenon known as evolutionary convergence.

The authors say that Chilesaurus represents one of the most extreme cases of mosaic convergent evolution in the history of life. Chilesaurus’ teeth, for example, are very similar to those of primitive long-neck dinosaurs, because over millions of years they followed similar diets, even though they came from different dinosaur lineages.

Martin D. Ezcurra, a doctoral researcher at the School of Geography, Earth and Environmental Sciences, University of Birmingham, said:

“Chilesaurus can be considered a ‘platypus’ dinosaur because different parts of its body resemble those of other dinosaur groups due to mosaic convergent evolution. In this process, a region or regions of an organism resemble others of unrelated species because of a similar mode of life and evolutionary pressures.”

“Chilesaurus provides a good example of how evolution works in deep time and it is one of the most interesting cases of convergent evolution documented in the history of life.”

“Chilesaurus shows how much data is still completely unknown about the early diversification of major dinosaur groups. This study will force palaeontologists to take more care in the future in the identification of fragmentary or isolated dinosaur bones. It comes as false relationship evidence may arise because of cases of convergent evolution, such as that present in Chilesaurus.”

Team leader, Dr. Fernando Novas, from the Bernardino Rivadavia Natural Sciences Museum, Buenos Aires, Argentina, said:

“Chilesaurus is the first complete dinosaur from the Jurassic Period found in Chile and represents one of the most complete and anatomically correct documented theropod dinosaurs from the southern hemisphere.”

“Although plant-eating theropods have been recorded in North America and Asia, this is the first time a theropod with this characteristic has been found in a southern landmass.”

“Chilesaurus was an odd plant-eating dinosaur only to be found in Chile. However, the recurrent discovery in beds of the Toqui Formation of its bones and skeletons clearly demonstrates that Chilesaurus was, by far, the most abundant dinosaur in southwest Patagonia 145 million years ago.”

Citation: “An enigmatic plant-eating theropod from the Late Jurassic period of Chile,” Fernando E. Novas, Leonardo Salgado, Manuel Suárez, Federico L. Agnolín, Martín D. Ezcurra, Nicolás R. Chimento, Rita de la Cruz, Marcelo P. Isasi, Alexander O. Vargas & David Rubilar-Rogers. Nature. Published online 27 April, 2015. DOI: 10.1038/nature14307.