| Dromaeosaurus | |
|---|---|
 | |
| Reconstructed skeleton,Canadian Museum of Nature | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Dromaeosauridae |
| Clade: | †Eudromaeosauria |
| Subfamily: | †Dromaeosaurinae |
| Genus: | †Dromaeosaurus Matthew & Brown, 1922 |
| Type species | |
| †Dromaeosaurus albertensis Matthew & Brown, 1922 | |
Dromaeosaurus (/ˌdroʊmiəˈsɔːrəs,-mi.oʊ-/;[3]lit. 'running lizard') is agenus ofdromaeosauridtheropoddinosaur that lived during theLate Cretaceous period (middle lateCampanian andMaastrichtian), sometime between 80 and 69 million years ago, inAlberta,Canada and the western United States. Thetype species isDromaeosaurus albertensis, which was described byWilliam Diller Matthew andBarnum Brown in1922. Its fossils were unearthed in theHell Creek Formation,Horseshoe Canyon Formation andDinosaur Park Formation. Teeth attributed to this genus have been found in thePrince Creek Formation.Dromaeosaurus is thetype genus of bothDromaeosauridae andDromaeosaurinae, which include many genera with similar characteristics toDromaeosaurus such as possibly its closest relativeDakotaraptor.Dromaeosaurus was heavily built, more so than other dromaeosaurs that are similar in size, likeVelociraptor.
Despite receiving widespread attention in popular books on dinosaurs, and the usage of a complete mounted skeleton cast in museums throughout the world,Dromaeosaurus is poorly known from actual fossils.[4] The preparation of the popular cast by theTyrrell Museum was only made possible by knowledge gained from other dromaeosaurids that have been discovered more recently.
The first knownDromaeosaurus remains were discovered by paleontologistBarnum Brown during a 1914 expedition toRed Deer River on behalf of the American Museum of Natural History.[5] The area where these bones were collected is now part ofDinosaur Provincial Park in Alberta, Canada.[6] The find,holotype AMNH 5356, consisted of a partial skull 24 cm (9+1⁄2 in) in length, amandible, twohyoids, a firstmetacarpal and some foot bones. The skull lacked most of the top of the snout.[6][5] Several other skull fragments, and about thirty isolated teeth, are known from subsequent discoveries in Alberta andMontana.[6]
In 1922William Diller Matthew and Brown named and described thetype species ofDromaeosaurus:Dromaeosaurus albertensis. The generic name is derived from the Greekδρομεύς (dromeus) meaning 'runner' andσαύρος (sauros) meaning 'lizard'. Thespecific name,"albertensis", refers toAlberta.
Another seven species ofDromaeosaurus 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 (analvarezsaurid);Dromaeosaurus falculus (Cope 1876) Olshevsky 1979 andDromaeosaurus mongoliensis (Barsbold 1983) Paul 1988 (Adasaurus). Most of them were based on fragmentary material, some belonging to other genera, and far less complete than that ofDromaeosaurus albertensis, and those that haven't been reclassified are considerednomina dubia today.[6] Nevertheless, it has grown apparent thatDromaeosaurus albertensis is even rarer in itshabitat than other small theropods, although it was the first dromaeosaurid of which reasonably good cranial material was described.[6]The genusChirostenotes was considered to be synonymous withDromaeosaurus at one point in time.[7]
Dromaeosaurus was a medium-sizedcarnivore, about 2 m (6 ft 7 in) in length and 16 kg (35 lb) in body mass.[8][9] Its mouth was full of sharp teeth, and it probably would have had a sharply curved "sickle claw" on each foot. It lived during theCampanian stage of theLate Cretaceous. However, some fragmentary remains such as teeth that may belong to this genus have been found from the lateMaastrichtian ageHell Creek andLance Formations, dating to 66 million years ago.[10] Teeth have also been found in theAguja Formation.
Dromaeosaurus had a relatively robust skull with a deep snout. Its teeth were rather large and were shaped like a curved cone with a coat of enamel covering the crown.[11] It had only nine teeth in eachmaxilla.[6]Dromaeosaurus also had avein at the back of the head, thevena capitis dorsalis, that drained the front neck muscles through two long canals running to theposterior surface of the brain.[12]TheMeckelian groove of Dromaeosaurus is rather shallow and does not have much depth.[13]
Matthew and Brown originally placedDromaeosaurus to its own subfamily, theDromaeosaurinae, within the "Deinodontidae" (now known asTyrannosauridae) based on some similarities in the general proportions of the skull.[5] In 1969,John H. Ostrom recognized thatDromaeosaurus shared many features withVelociraptor and the newly discoveredDeinonychus, and assigned these forms to a new family:Dromaeosauridae.[14] Since then, many new relatives ofDromaeosaurus have been found.
The exact relationships ofDromaeosaurus are somewhat unclear. Although its rugged build gives it a primitive appearance, it was actually a very specialized animal.[15] In an analysis of theclade Dromaeosaurinae, species such asUtahraptor,Achillobator andYurgovuchia have been recovered.[16] The genusDakotaraptor has been classified as thesister taxon toDromaeosaurus,[17] but more recent analysis do not recover such a close relationship.
Below is a cladogram by Senteret al. in 2012.Dromaeosaurus is recovered as the sister taxon toYurgovuchia,Utahraptor andAchillobator.[16]
The cladogram below follows a 2015 analysis by paleontologists Robert DePalma, David Burnham,Larry Martin,Peter Larson, andRobert Bakker, using updated data from the Theropod Working Group. In this analysis,Dromaeosaurus is classified as the sister taxon ofDakotaraptor.[17]
Dromaeosaurus differs from most of its relatives in having a short, massive skull, a deep mandible, and robust teeth. The teeth tend to be more heavily worn than those of its relativeSaurornitholestes, suggesting that its jaws were used for crushing and tearing rather than simply slicing through flesh. Therrienet al. (2005) estimated thatDromaeosaurus had a bite nearly three times as powerful as that ofVelociraptor and suggested it relied more on its jaws than on the sickle claw to kill its prey.[18] In a study predominantly centered aroundShuvuuia,Dromaeosaurus was compared to the former and also toAlioramus, in which bothDromaeosaurus andAlioramus were discovered to be diurnal predators.[19][20]
Dromaeosaurus' feeding habits were also discovered to be typical of coelurosaurian theropods, with a characteristic "puncture and pull" feeding method. Studies of wear patterns on the teeth of this animal by Angelica Toriceset al. in a study regarding theropod feeding habits indicate that dromaeosaurid teeth share similar wear patterns to those seen in the Tyrannosauridae andTroodontidae, respectively. However, micro-wear on the teeth indicated thatDromaeosaurus likely preferred larger prey items than the troodontids it shared their environment with. Such differentiations in its diet likely allowed the theropod to inhabit the same environment as its more distant maniraptoran relatives. The same study also indicated that bothDromaeosaurus andSaurornitholestes (also analyzed in the study) likely included bone in their diet and were better adapted to handle the stresses associated with attacking struggling prey while troodontids, equipped with weaker jaws, preyed on softer animals and prey items such as invertebrates and carrion. This feeding strategy and ability to handle struggling prey was also a feature that the theropod also shared with tyrannosaurids such asGorgosaurus, which was also analyzed in said study alongside these smaller theropods.[21][22]
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