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From massive meat-eaters like Tyrannosaurus to plant-eaters like Argentinosaurus, one trait that helped the dinosaurs thrive for 165 million years was how quickly they could grow.

But when did this characteristic first appear? A new study indicates it was present in the earliest dinosaurs, as revealed by microscopic features in bone fossils from Argentina.

“Growing quickly allows organisms to escape the risky business of being small for a long part of their life history. It also allows them more time to reproduce throughout their lifespan,” said palaeontologist Kristi Curry Rogers, lead author of the study published this month in the journal PLOS ONE.

The researchers examined patterns preserved in the bones of dinosaurs that inhabited northwestern Argentina between about 231 and 229 million years ago.