If you visited what is now Alberta, Canada 75 million years ago, you would have to beware of some formidable predators. The large tyrannosaurids
Announced by paleontologists Nicholas Longrich and Philip Currie this week in the journal PNAS, Hesperonychus is the smallest predatory dinosaur yet known from North America (even smaller than the termite-eating Albertonykus, which Currie and Longrich described last year). It still would have been quite large compared to the mammals of its day, however, and it may have been the scourge of our ancient relatives. This fits with the hypothesis that dinosaur predation on mammals kept mammals small, but as Longrich and Currie point out, it could also mean that the occupation of niches by mammals kept dinosaurs from becoming much smaller.
During the Mesozoic, the time when non-avian dinosaurs flourished, there were no large mammals. One of the biggest was Repenomamus, which was about the size of a small dog and lived during the Cretaceous. It was large enough to eat some baby dinosaurs (which fossil evidence has shown it did) but this was unusual. Most mammals were smaller and ate seeds, insects, and fruit. This means that if there were dinosaurs smaller than Hesperonychus they may have come into competition with mammals for food and places to live in the forest. Rather than coming into such direct competition for resources with mammals it seems that the smallest of theropod dinosaurs were just large enough to see mammals as food.
What is even more surprising is that Hesperonychus does not fit in with any other maniraptoran dinosaurs from North America. When Longrich and Currie studied its bones to determine what kind of dinosaur it was, they found that it was most closely related to the microraptorine dinosaurs from China. This group of feathered dinosaurs, which includes Microraptor and Sinornithosaurus, had not been found in North America before. Not only that, but Hesperonychus is about 45 million years younger than the oldest members of this group in Asia. Therefore it extends the range of the microraptorine dinosaurs over both time and geography, hinting at other tantalizing finds yet to be disinterred from the rock.