March 17, 2009
Hesperonychus: A Tiny Killer
A restoration of the tiny theropod Hesperonychus by Nicholas Longrich.
If you visited what is now Alberta, Canada 75 million years ago, you would have to beware of some formidable predators. The large tyrannosaurids Daspletosaurus and Gorgosaurus prowled the landscape while the smaller sickle-clawed killers Dromaeosaurus and Saurornitholestes stalked their prey in the forest. You might be excused, then, if you missed a smaller feathered predator that weighed about as much as a domestic chicken and was named Hesperonychus.
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.
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The artist, of course, thought it would be neat to give them the coloration of a fox.
Before I moved from Edmonton, Currie and co. were working on an excavation somewhere in southeast Edmonton (although they couldn’t say where, because of looting problems). I wonder if this is what came from that site?
Alan; I love the restoration. I think if they made a plush version based upon Longrich’s drawing they would make quite a bit of money (I’d buy one!).
Melanie; Currie & Longrich said that they expect to find more Hesperonychus, but the “original” specimen was actually found in the early 1980′s. Yet another treasure that was dug up in a museum long after it had been taken from the field.
Gone are the “raptors” of Jurassic Park – those scaly, featherless dromaeosaurids that the Velociraptors represented.
They don’t exist.
Kids may be shocked to know that nearly every small theropod looks like birds. In particular, robins. God, will all the childhood fascination in dinosaurs perish!
Those Hesperonychus may be a Therizinosaurid.