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Dromaeosaurs

Audrea Westfall, Auburn University
Dromaeosaur Fossil
Dromaeosaurs are dinosaurs of the family Dromaeosauridae, coming from the Greek dromeus meaning "runner" and sauros meaning "lizard." Dromaeosaurs lived from the middle of the Jurassic (164 million years ago) to the end of the Cretaceous (66 million years ago), disappearing during the Cretaceous-Paleogene extinction event, formerly known as the K-T extinction. Dromaeosaur fossils have been found on all continents except Australia. They are popularly and informally known as raptors, a term that emphasizes their birdlike traits and was largely popularized by the Jurassic Park franchise. Interestingly, one species of dromaeosaur, Utahraptor ostrommaysorum, was intended to be U. spielbergi after Jurassic Park director Steven Spielberg in exchange for research funding, but no agreement could be reached.
Dromaeosaurs ranged widely in length, with smaller species being just over two feet (0.7 meters) in length and larger species averaging 20 feet (6 meters); a few were even larger, with the Utahraptors of up to 36 feet (11 meters) being found. The most primitive species, such as those in genus Microraptor, were very small, with large size evolving later into multiple lineages. Dromaeosaurs had distinctive S-curved necks and short birdlike trunks. They had large skulls with a long snout, serrated teeth, and forward-facing eyes as well as long, muscular rear legs—all characteristics of their adaptations to life as bipedal pursuit hunters.
Dromaeosaur Foot
Their front limbs were shortened and had three long, clawed fingers, and their rear legs also had three clawed digits, with the second claw being hyperextended, deeply curved, and held higher than the rest of the foot. Paleontologists speculate that this claw was used for either climbing trees or snagging prey, or potentially both. One hypothesis is that smaller dinosaurs used it for climbing, and larger species too big to live in trees adapted it for hunting. Paleontologist Phil Senter has suggested that smaller dromaeosaurs may have been insectivores, with this "killing claw" being used to get into insect nests. All dromaeosaurs had long, highly flexible tails used as a stabilizer and counterweight in locomotion. Researchers believe that all members of the family were covered in feathers, including the types of feathers associated with flight in modern birds, with the first confirmed discovery being that of a Sinornithosaurus found in 1999 with fully feathered wings. Interestingly, Sinornithosaurus may have been venomous; it had grooved teeth and evidence of venom pouches.
Microraptor Reconstruction
With the evolution of feathers came flight, and evidence indicates that two Dromaeosaur species, Rahonavis ostromi and Microraptor gui. R. ostromi, were capable of flight, or at least gliding. With prominent quill knobs for strong feather attachment and comparatively large ulnas, R. ostromi would have been a more powerful flyer than the famed Archaeopteryx, although paleontologist Lui M. Chiappe notes that R. ostromi would have been much less coordinated. M. gui had four wings, one on each of its limbs, with long true flight feathers, but because their shoulder joints were too primitive for flapping flight, they likely glided between trees and to the ground because their hind wings would have prevented efficient running. Rahonavis and Microraptor were both very early, small Dromaeosaur species, and it is very unlikely that any of the later, larger dromaeosaurs were capable of flight. Rather, they were terrestrial, bipedal runners and climbers. The presence of feathers on these primitive species suggest that all later dromaeosaurs also possessed feathers. Fossils show that dromaeosaurs lived both solitary and gregarious lives, depending on the species. Fossil remains of multiple individuals have been found surrounding the remains of larger prey species, and this suggests possible pack hunting behavior. Rows of tracks found together also support hypotheses about pack or family group behavior. This kind of social grouping has evolved throughout history in the animal kingdom, in the form of family groups and pack hunting, so it would not be surprising to find that Dromaeosaurs lived, traveled, and hunted in groups. Some paleontologists, however, suggest that instances in which many dromaeosaurs have been found around a single, much larger prey animal may be evidence of scavenging or a rare feeding frenzy.
The Dromaeosaur family includes a diverse group of dinosaurs that spans 35 genera divided over five subfamilies. The most primitive is Microraptor, sometimes called Cryptovolans. Most of its fossils have been found in either Argentina or China. Six genera have been found in the United States, including an almost complete Bambiraptor in Montana, named for the Walt Disney character Bambi because it was believed for several years to be a juvenile of another species. Dromaeosaurs likely lived in many parts of what is now Alabama, but only one fossil, a single tooth, has been found in the state. It was discovered by paleontologist David Schwimmer in the Mooreville Chalk Formation of the Black Belt region of west-central Alabama in 2004.
Deinonychus, an 11-foot-tall predator whose name means "terrible claw," has been found in Montana, Wyoming, and possibly Maryland. Based on comparisons with crocodiles and birds, it is estimated that this animal had a second claw nearly 5 inches (~ 13 centimeters) long. Long feathers on their upper limbs could have been used for stability when immobilizing prey or in a behavior called "mantling," in which a predator uses its wings to encircle captured prey; this practice has been observed in modern birds of prey. Yurgovuchia is represented by a single specimen found in Utah in 2005, and Utahraptor, also discovered in Utah, is estimated at 23 feet (~7 meters) long and 1,100 pounds (500 kilograms) and had a 9.4-inch (~23 centimeter) recurved claw. Archeoraptor from Montana is the most geologically recent Dromaeosaur species, having been dated to approximately 66 million years ago. The genus Saurornitholestes has two species: S. langstoni from Montana and S. sullivani from New Mexico, and it is most similar to Velociraptor.
Additional Resources
Chiappe, Luis M. Glorified Dinosaurs: The Origin and Early Evolution of Birds. Hoboken, N.J.: Wiley-Blackwell, 2007.
Fowler, Denver W., et al. "The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds." PLoS One 6 (December 2011); http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0028964.
Gong, Enpu, et al. "The Birdlike Raptor Sinornithosaurus Was Venomous." Proceedings of the National Academy of Sciences of the United States of America 107 (January 2010): 766-68.
Manning, Phillip L., et al. "Dinosaur Killer Claws or Climbing Crampons?" Biology Letters 2 (March 2006): 110-12.
Kiernan, Caitlin R., and David R. Schwimmer. "First Record of a Velociraptorine Therapod (Tetanurae, Dromaeosauridae) from the Eastern Gulf Coastal United States." Journal of the Delaware Valley Paleontological Society 7 (May 2004): 89-93.
Xu, X., et al. "Four-winged dinosaurs from China." Nature 421 (January 2003): 335-40.
Published:  June 22, 2015   |   Last updated:  January 25, 2016