Hesperonychus elizabethae: Return of the Microraptorines.
Size: About 3 feet (90 centimeters) long.
Time Period: Hesperonychus lived about 75 million years ago, during the Campanian Stage of the Late Cretaceous period.
Locale: The Dinosaur Park Formation of Alberta, Canada.
Name: The generic name of Hesperonychus means “western claw,” in reference to the animal’s position on the map. The specific name refers to the specimen’s collector, who was (presumably) named Elizabeth.
The microraptorians are a very interesting group of dromaeosaurs. Not only were they constrained to the Early Cretaceous of China, but some of them could even glide and fly. These animals have been subjected to media hype because of the ‘four wings’ of Microraptor and the possibility of venomousness in Sinornithosaurus (disproved). Though what I’m about to discuss isn’t very ‘hypey,’ it is interesting and is the Microraptoria’s third major time in the spotlight.
Hesperonychus is the dinosaur of the day. It’s unusual because it is very, very atypical for a relative of Microraptor. This animal is from a very well-known North American formation dating to the Late Cretaceous. Both things are quite unusual. The distribution of Hesperonychus hints at a wider geographic range and temporal range (it succeeds other microraptorines for 45 million years) of distribution for the Microraptoria in general, as well as indicating possible ghost lineages for Hesperonychus. I expect that the Microraptorians attained a North American distribution during the Early Cretaceous period, and even when they became far less common in Asia, the American lineage stuck around for a while longer. Though Bambiraptor has also sometimes been thought to be a North American microraptorine, it has since been found to be most closely related to Saurornitholestes.
It’s been concluded that the famous Microraptor wasn’t too similar to Hesperonychus. Where the former had four wings and was a gliding animal, Hesperonychus was probably more similar to Sinornithosaurus, which was about the same size as it. This may indicate that the microraptorines didn’t vary in size or even behavior during their history.
Hesperonychus also fills a gap in the North American ecology of the time. At the time, European and Asian faunal assemblages were dominated by small animals. North American faunal assemblages of the time tended to have large ceratopsid and hadrosaurid animals, but fewer small species (the opposite of mammalian assemblages). Hesperonychus helps fill the gap in the Dinosaur Park ecosystem as a very common small carnivore.
After Hesperonychus, the next smallest Dinosaur Park carnivore was the primitive marsupial Eodelphis, which weighed only 600 grams (whereas Hesperonychus weighed roughly as much as a turkey). This indicates how much of a gap there was between the smallest dinosaurs and largest mammals in Cretaceous ecosystems. Though Hesperonychus is the smallest carnivore known from America, it was still far larger than its largest mammalian neighbor. It’s been hypothesized that mammals didn’t grow larger because of dinosaurs, or that dinosaurs didn’t grow any smaller because of mammals. I suspect that it might have even been both.
It’s no four-winged glider, but Hesperonychus is a very interesting animal. Despite how limited the range of any given dinosaur group is, we occasionally find fossils that aren’t only out of place, but out of time. Hesperonychus even implies things about the existence of ghost lineages, the filling of ecological niches, and the coexistence of mammals and dinosaurs that we never even knew about before.