A little bit on Hesperonychus...
I managed to get a hold the paper on PNAS (DOI: 10.1073/pnas.0811664106), and I thought I'd flesh out what I thought was the most interesting report yesterday. First, a bit of clarification: I mentioned that it is the smallest, non-avian North American dinosaur yet found, but "small" is relative. Most people, thinking of carnivorous non-avian theropods, think of superpredators such as Allosaurus fragilis or tyrannosaurids.
Now, for many reasons, smaller specimens are not likely to be fossilized, despite being much more numerous than their superpredator bretheren. Being light of frame, they are more prone to scavanging, or direct physical damage to their remains. Thus, the fossil evidence becomes much more scant at lower mass ranges, and Hesperonychus is the first animal found in North America under 10 kg, under-massing Sauronitholestes langstoni.
Hesperonychus is further an interesting find in that it is a microraptorine dromaeosaurid (first one in North America), unlike other Dinosaur Park Formation dromaeosaurids found to date. The holotype pelvic girdle was discovered in 1982, but remained unidentified for a number of years (UALVP 48778, figure at right), but like many specimens, remained unstudied for a long time (25 years in this instance!). In addition to the holotype pelvis, Longrich et al. discovered a number of toes, including the typical sickle claw of basal dromaeosaurids. While superficially similar to the more infamous ones of Deinonychus, Utahraptor, and Velciraptor, the claw is less blade-like and resembles on close inspection (including on cross-section) that of microraptorines such as Rahonavis. The claw was quite small, only about a 1.5 cm long (projection, not along curve), but well-preserved. The precise cladistic placement of Hesperonychus is not particularly well defined, residing in a polytomy of various members of Microraptorinae. The number of phalaxes found (particularly phalanx II-3) suggest that there at least ten speciments in the TMP collections — when recognizing species by that element, for comparison, only 2 Dromaeosaurus specimens have been found, suggesting that Hesperonychus might have in fact been quite common in its day. This is especially true in light of how difficult preservation would have been!
It is finally worth noting that even though this is a tiny dinosaur at 1900 g, it outmasses the largest metatherian (early marsupial mammal) Eodelphis by 150% (600 g). Yep, mammals — still puny!
Longrich, N., & Currie, P. (2009). A microraptorine (Dinosauria-Dromaeosauridae) from the Late Cretaceous of North America Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0811664106
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