The mighty (tiny, but mighty) primate ancestor… waaay bigger than expected.

New Scientist tries to keep some perspective about our great-great-etc. grandfather, Ursolestes, a prehistoric primate who might seem to us, a squirrel monkey. To dinosaurs, a giant:

New fossil finds from Montana, US, reveal a species so different from others that some scientists now think the first primates evolved when dinosaurs still roamed.

It weighed between 500 and 1500 grams, the size of a large squirrel, but it would have dwarfed other early primates living at the time about 66 million years ago.

“The big surprise is a primate of such large body size that early in primate evolution,” says Craig Scott of the Royal Tyrrell Museum in Alberta, Canada, who describes the find in the journal Palaeontology.

Its body mass was some 4 to 10 times that of a typical Purgatorius, making Ursolestes a giant among early primates, but not exceptionally large among post-impact mammals.

So much primate diversity in the first million years of the Palaeocene also casts new light on a big question in mammalian palaeontology: whether major modern groups originated before or after the dinosaurs left the stage.

Most palaeontologists hold that the evolutionary breakthrough came at the start of the Palaeocene. But co-author Richard Fox of the University of Alberta says the early primate diversity means primates must have originated near the end of the Cretaceous, shortly before the impact.