H is for Hesperonychus

Next up in our Alberta dinosaurian alphabet is Hesperonychus, a small theropod. Its name means 'western claw', in reference to the sickle-shaped claw it has on both hindlimbs. This claw tells us that it was a dromaeosaur, commonly referred to in popular media as the 'raptors' like Velociraptor or Deinonychus. It lived during the Late Cretaceous period of southern Alberta, approximately 75 million years ago. These fossils were collected in 1982, and were left undescribed and virtually ignored until they were re-discovered by Nick Longrich and Phil Currie in 2009 [1].

Artists impression of Hesperonychus by Nobu Tamura

Although known from only a few bones, the estimated total length of Hesperonychus is less than 1 m, and it weighed as little as 1.9 kg [1]. Due to the degree of fusion in the bones, it's likely this is in fact a full-grown adult individual, making it one of the smallest theropod dinosaurs found in North America, second only to Albertonykus. 

Hesperonychus is the first microraptorine dinosaur found in North America, and it is also one of the few small theropods, which is significant ecologically. Previously, the ecosystems of the Late Cretaceous of North America lacked small theropods (we couldn't find their fossils), while similarly aged environments in Europe and Asia had several. This find helped to fill this gap rather than having to develop an explanation for why these small animals didn't exist in North America. We now understand that carnivorous dinosaurs in North America ranged from very tiny (Hesperonychus) to very large (e.g. Tyrannosaurus rex) [1]. It is even likely that this small theropod was very common throughout Cretaceous Alberta. Although few bones are easily distinguishable as Hesperonychus, there are several small claws within collections of museums that likely represent claws from this animal. Unfortunately, claws are not very distinct between species and it can be hard to determine exactly what animal it comes from. For a bit more about this find, you can view articles from CBC and BBC. 

That's really all that is known from this small carnivorous theropod. Next week, we're going to talk about something a little bit different: dinosaur trackways!

Other dinosaurs from Alberta that start with 'H':
"Hadrosaurus" - although this isn't necessarily considered to be a valid name by all palaeontologists
"Hanssuesia" - synonym of the pachycephalosaur Stegoceras
Hypacrosaurus

References:
1 Longrich, NR, and Currie, PJ. 2009. A microraptorine (Dinosauria-Dromaeosauridae) from the Late Cretaceous of North America. PNAS 106: 5002-5007. Not open access but can be viewed here.