Name Meaning: Acheron Thief
Geologic Era: Late Cretaceous
Location Discovered: Hell Creek Formation (all but first pictured vertebrae are from Carter County, MT)
Size: 3 meters long
Estimated Range: Modern Northwest United States
Extinction Date: Late Cretaceous (K-T extinction)
I make the identification of Acheroraptor with some degree of uncertainty. The only officially describe Acheroraptor fossils are pictured below. The ones above do have theropod characeristics, and fit the size that I would expect from a 2-3 meter long animal. Fossil identification does carry a certain amount of guesswork, especially if the fossils in question are found isolated. All of the fossils above come from Carter County, MT, with the exception of the topmost caudal vertebra. The seller listed it as being from the Hell Creek Formation, which includes parts of Montana, North Dakota, South Dakota, and Wyoming.
The metacarpal is of particular interest to me. The interior is partially hollow, which is a characteristic of dromaeosaur bones. This bone would correspond to the bones of the human hand, and is the distal portion (closer to the finger than the wrist). I have to assume that all of the bones I have were found isolated, as I have no indication that they weren’t.
Acheroraptor was formally describe only recently, when paleontologists at the Royal Ontario Museum purchased the above fossils from a private collector. They were found relatively close together, so they are considered associated, though they could possibly be from the same animal. Acheroraptor was identified as a velicoraptorine dromaeosaur, and has the distinction of being the youngest dromaeosaur yet described. It does have striking similarities to Asian dromaeosaurs, which implies that North America and Asia may have had land connections more recently than previously thought.
Dromaeosaurs are often pictured as pack hunters, with a group taking on prey animals many times their size. This is based primarily on Deinonychus, an earlier dromaeosaur species. Several specimens were found associated with Tenontosaurus, and this was thought to suggest coordinated hunting behavior. That idea has been called into question more recently, with a less coordinated, mobbing style of attack being proposed.
This behavior is seen with modern Komodo dragons, and often results in smaller animals being injured, killed, and eaten by the larger ones. This is consistent with some of the Deinonychus finds. On the other hand, dromaeosaur trackways have been found that show an ordered, linear pattern of movement among individuals, which would argue for pack behavior, or at least family groups.
Acheroraptor would have coexisted with well known animals such as Tyrannosaurus rex and Triceratops, though probably would have avoided the adult animals. The below animals are all well represented in the Hell Creek Formation, and several are represented in our classroom collection. Acheroraptor probably fed on smaller animals such as lizards and juvenile dinosaurs. Many dromaeosaurs have recently been found to be feathered, so in both reconstructions, Acheroraptor is shown with a fine coat of feathers.
Dinosaurs have come a long way in terms of illustration, from the lumbering, iguana-like animals shown in the early 20th century to the agile, feathered creatures now shown. I will admit, I am used to seeing dinosaurs unfeathered, and still find the newer illustrations going against my childhood memories. I remembered dinosaurs as being fast reptiles; now the notion is dinosaurs were more avian than reptilian. Such is the nature of science…changing with each new discovery.
Holotype: By Jonathan Chen – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=72906692
Head/Neck Reconstruction:”Acheroraptor reconstruction” by Emily Willoughby, (email@example.com, emilywilloughby.com) – Own work. Licensed under CC BY 3.0 via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:Acheroraptor_reconstruction.jpg#/media/File:Acheroraptor_reconstruction.jpg
Acheroraptor and Other Hell Creek Dinosaurs
“Hell Creek dinosaurs and pterosaurs by durbed” by Durbed – http://durbed.deviantart.com/art/Happy-New-Year-from-Hell-Creek-345906630. Licensed under CC BY-SA 3.0 via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:Hell_Creek_dinosaurs_and_pterosaurs_by_durbed.jpg#/media/File:Hell_Creek_dinosaurs_and_pterosaurs_by_durbed.jpg