Dromaeosaurus

A 1922 diagram of the holotype skull

Despite receiving widespread attention through popular books, TV shows, documentaries, etc. on dinosaurs, along with the usage of a complete mounted skeletal cast in museums throughout the world, Dromaeosaurus is somewhat poorly known from actual fossils.[4] The preparation of the popular cast by the Tyrrell Museum in Alberta was only made possible through the knowledge gained from other dromaeosaurids that were discovered more recently.

The first known Dromaeosaurus remains were discovered by paleontologist Barnum Brown during a 1914 expedition to the Red Deer River in Alberta on behalf of the American Museum of Natural History.[5] The area where the bones were collected is now a part of Dinosaur Provincial Park in Alberta.[6] The holotype specimen, AMNH 5356, consists of a partial skull 24 cm (9+1⁄2 in) long, a mandible, two hyoids, a first metacarpal, and various foot bones; the skull lacked most of the top of the snout.[6][5] Several other skull fragments, along with around thirty isolated teeth, are known from subsequent discoveries in Alberta and Montana.[6]

In 1922, William Diller Matthew and Brown named and described the type species of Dromaeosaurus as Dromaeosaurus albertensis. The generic name is derived from the Greek words δρομεύς (dromeus), meaning 'runner', and σαύρος (sauros), meaning 'lizard'. The specific name, "albertensis", refers to it first being found in Alberta.

Another seven species of Dromaeosaurus were named: Dromaeosaurus laevifrons (Cope 1876) Matthew & Brown 1922; Dromaeosaurus cristatus (Cope 1876) Matthew & Brown 1922 (Troodon); Dromaeosaurus? gracilis (Marsh 1888) Matthew & Brown 1922; Dromaeosaurus explanatus (Cope 1876) Kuhn 1939; Dromaeosaurus minutus (Marsh 1892) Russell 1972 (an alvarezsaurid); Dromaeosaurus falculus (Cope 1876) Olshevsky 1979 and Dromaeosaurus mongoliensis (Barsbold 1983) Paul 1988 (Adasaurus). Most of them were based on fragmentary material, some belonging to other genera, and were much less complete than that of Dromaeosaurus albertensis. Those that haven't been reclassified are considered to be nomina dubia today.[6] Nevertheless, it has grown apparent that Dromaeosaurus albertensis is even rarer in its habitat than other small theropods, despite it being the first dromaeosaurid of which reasonably good cranial material was described.[6]The caenagnathid oviraptorosaurian Chirostenotes was once considered to be synonymous with Dromaeosaurus at one point in time.[7]

An artistic restoration of D. albertensis

Dromaeosaurus was a medium-sized dromaeosaurid, within the range of a coyote, at 2 m (6 ft 7 in) long and weighing 16 kg (35 lb).[8][9] Its mouth was full of sharp teeth and it probably had a large, sharply curved "sickle claw" on the second toe of each foot. It lived during the mid Campanian to the mid Maastrichtian ages of the Late Cretaceous. However, some fragmentary remains that may belong to this genus, such as teeth, have been found from the late Maastrichthian age Hell Creek and Lance Formations of Montana and Wyoming, respectively, dating to roughly 66 million years ago.[10] Teeth attributed to Dromaeosaurus have also been found in the Aguja Formation of Texas.

Dromaeosaurus had a relatively robust skull with a fairly deep snout. Its teeth were rather large for its size and shaped like a curved cone with a coat of enamel covering the crown.[11] It had only nine teeth in each maxilla.[6] Dromaeosaurus also had a vein at the back of its head, the vena capitis dorsalis, which drained the front neck muscles through two long canals running to the posterior surface of the brain.[12] The Meckelian groove of Dromaeosaurus is rather shallow and does not have much depth to it.[13]

A D. albertensis skull cast exhibited at the Geological museum in Copenhagen, Denmark

Matthew and Brown originally placed Dromaeosaurus in its own subfamily, Dromaeosaurinae, within "Deinodontidae" (which is now known as Tyrannosauridae), based on some similarities in the general proportions of its skull.[5] In 1969, John Ostrom recognized that Dromaeosaurus shared many features with Velociraptor and the newly discovered Deinonychus, and so assigned them to a new family: Dromaeosauridae. Since then, many newer relatives of Dromaeosaurus have been found worldwide.

The exact relationships of Dromaeosaurus are somewhat unclear. Although its rugged build gives it a somewhat primitive appearance, it was actually a highly specialized animal.[14] In an analysis of the clade Dromaeosaurinae, species such as Utahraptor, Achillobator, and Yurgovuchia have been recovered.[15] The genus Dakotaraptor has been classified as the sister taxon to Dromaeosaurus,[16] but some more recent analyses do not recover such a close relationship.

Below is a cladogram by Senter et al. from 2012. Dromaeosaurus is recovered as the sister taxon to Yurgovuchia, Utahraptor and Achillobator.[15]

Size of Dromaeosaurus (4) compared with other dromaeosaurids

The cladogram below follows a 2015 analysis by paleontologists Robert DePalma, David Burnham, Larry Martin, Peter Larson, and Robert Bakker, using updated data from the Theropod Working Group. In this analysis, Dromaeosaurus is classified as the sister taxon of Dakotaraptor.[16]

A cast of a reconstructed D. albertensis foot

Dromaeosaurus differs from most of its relatives in having a short, massive skull, a deep mandible, and robust teeth. Its teeth tend to be more heavily worn down than those of its relative Saurornitholestes, suggesting that its jaws were used for crushing and tearing, rather than simply slicing through flesh. Therrien et al. (2005) estimated that Dromaeosaurus had a somewhat powerful bite force, nearly three times as powerful as that of Velociraptor, and suggested it relied more on its jaws than on the sickle claw of its second toe to kill its prey.[17] In a study predominantly centered around Shuvuuia, Dromaeosaurus was compared to the former and also to Tyrannosaurus, in which both Dromaeosaurus and Tyrannosaurus were discovered to be diurnal predators.[18][19]

• Timeline of dromaeosaurid research

• Currie, Philip J. (1995). "New information on the anatomy and relationships of Dromaeosaurus albertensis (Dinosauria: Theropoda)". Journal of Vertebrate Paleontology. 15 (3): 576–591. doi:10.1080/02724634.1995.10011250.
• Currie, P. J.; Rigby, K. J.; Sloan, Robert E. (1990). "Theropod teeth from the Judith River Formation of southern Alberta, Canada". In Currie, P. J.; Carpenter, K (eds.). Dinosaur Systematics: Perspectives and Approaches. Cambridge: Cambridge University Press. pp. 107–125. ISBN 0-521-43810-1.
• Dixon, Dougal (2006). The Illustrated Encyclopedia of Dinosaurs. Lorenz Books. pp. 200–201. ISBN 0-7548-1573-0.
• Larsson, H.C.E. 2001. Endocranial anatomy of Carcharodontosaurus saharicus (Theropoda: Allosauroidea) and its implications for theropod brain evolution. pp. 19–33. In: Mesozoic Vertebrate Life. Ed.s Tanke, D. H., Carpenter, K., Skrepnick, M. W. Indiana University Press.
• Matthew, William D.; Brown, Barnum (1922). "The family Deinodontidae, with notice of a new genus from the Cretaceous of Alberta". Bulletin of the American Museum of Natural History. 46: 367–385. hdl:2246/1300.
• Ostrom, John H. (1969). "Osteology of Deinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana". Peabody Museum of Natural History Bulletin. 30: 1–165.
• Paul, Gregory S. (1988). "The Wonderful and Spectacular Dromaeosaurian Sickle-Claws". Predatory Dinosaurs of the World. Simon & Schuster. pp. 357–370. ISBN 0-671-61946-2.
• Colbert, E.; Russell, D. A. (1969). "The small Cretaceous dinosaur Dromaeosaurus". American Museum Novitates (2380): 1–49. hdl:2246/2590.
• Therrien, Francois; Henderson, Donald M.; Ruff, Christopher B. (2005). "Bite Me: Biomechanical models of theropod mandibles and implications for feeding". In Carpenter, Kenneth (ed.). The Carnivorous Dinosaurs. Life of the Past. Indiana University Press. pp. 179–237. ISBN 0-253-34539-1.
• Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. 861 pp. ISBN 0-520-24209-2.

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