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Flying Dromaeosaurs A winged, but flightless, Deinonychus by Stephen A. Czerkas FLYING DROMAEOSAURS Stephen A. Czerkas, Dianshuang Zhang, Jinglu Li and Yinxian Li The Dinosaur Museum, 754 South 200 West, Blanding, Utah 84511, USA; Liaoning Provincial Bureau of Land Resources Management, Liaoning Fossil Administration Office, and Liaoning Museum of Paleontology, Left of Nanshan Park, Beipiao, Liaoning Province 122100, People’s Republic of China. The Dinosaur Museum © 2002 Abstract Dromaeosaurs have been regarded as theropod dinosaurs that were among the closest avian ancestors which were strictly terrestrial having not yet evolved the ability to fly. Consequently, phylogenetic analyses have resulted in the claims of birds having evolved from “the ground up” within a dinosaurian ancestry. Though widely accepted, the relationship between birds and dinosaurs has remained highly controversial and disputed by advocates of birds as having been derived from an arboreal, non-dinosaurian type of archosaur. The cladistical interpretation of the dinosaur/bird relationship hinges upon the presumption of the dromaeosaurs inability to fly. Recent discoveries of dromaeosaurs have revealed impressions of feathers and avian characters in the skeleton that nearly equal and even surpass that of Archaeopteryx. Yet despite this, the ability to fly has been discounted due to the shorter length of the forelimbs. Described below are two such dromaeosaurs, but preserved with impressions of primary flight feathers extending from the manus which demonstrate an undeniable correlation towards the ability to fly. This compelling evidence refutes the popular interpretation of birds evolving from dinosaurs by revealing that dromaeosaurs were already birds and not the non-avian theropod dinosaurs as previously believed. INTRODUCTION ALL BIRDS living today, including flightless feathers reveal that Archaeopteryx did fly (Feduccia species, are derived from flying ancestors. This and Tordoff, 1979). distinction further based on having feathers which Dromaeosaurs have been regarded as enabled flight is an essential prerequisite in the “non-avian dinosaurs” based on primitive skeletal definition of a bird. Archaeopteryx has long been characteristics which appear to be so distantly regarded as the most primitive true bird from which related to birds as to suggest that the ability to fly its descendants and possible ancestors can be had not yet evolved. In many respects similar compared. In order to not be considered as being a plesiomorphic avian characteristics are found in the true bird, the ancestral stock prior to Archaeopteryx skeleton of Archaeopteryx. However, the must present a clear inability to fly regardless of preservation of feather impressions, especially those however bird-like it otherwise may appear. The from the manus, clearly signify that Archaeopteryx skeletal anatomy of Archaeopteryx is so primitive was a true bird. It has been pointed out that without in its avian structure that how well it could have the preservation of feathers, the skeletons of flown is a matter of debate. But regardless of how Archaeopteryx may very well have been considered well it was capable of flying, fossilized impressions as dinosaurs and not birds (Ostrom, 1975). The of modern-looking asymmetrical, primary flight same maybe said of certain relatively small dromaeosaurs which appear to have remarkably significant that both sides have regarded bird-like skeletons more or less resembling that of dromaeosaurs as “dinosaurs”. Archaeopteryx. However, even with the discovery Phylogenetic analysis, more popularly of feather impressions on various specimens of known as cladistics, has gained tremendous support dromaeosaurs (Xu, et al., 1999; Xu, et al., 2000; over the past two decades, in part by generating a Ji, et al., 2001; Norell, 2001; Xu, et al., 2001), the broad consensus that birds are derived from interpretations have been highly controversial theropod dinosaurs called maniraptorans which due to highly charged scientifically political includes dromaeosaurs as among the closest philosophical differences which has basically divided non-volant avian ancestor to true volant birds. scientists into two divergent camps of thought. As Cladistics has been regarded as being the most with Archaeopteryx though, the determination of rigorous method for determining how animals are how avian dromaeosaurs actually are depends upon related. Still, cladistics has a vocal minority of critics the preservation of feathers even more so than the which claim that the method is unreliable as certain avian morphology of their skeleton. bird-like physical characteristics may be the result The reluctance to accept the concept of of convergence between unrelated forms rather than feathers on dromaeosaurs stems from the well direct relationships. established belief of dinosaurs as being all scaly A major criticism of dromaeosaurs as being reptilian giants. This admittedly broad generality a precursor to birds is that they existed too late in of what a “dinosaur” is created an obstacle that time to represent an ancestral form. Also, the size has, at least in part, obfuscated the objectivity in of the first dromaeosaurs to be known, such as determining the relationship between birds and Deinonychus and Velociraptor, were all considered dinosaurs. Adding to the confusion, was the fact too great for an ancestor of birds. But recent that the first well known specimens of discoveries have revealed that much smaller dromaeosaurs, notably Deinonychus, were dromaeosaurs did exist much earlier in time. Some identified as having avian characteristics while also of these dromaeosaurs are well within the size range being obviously too large to have been capable of of Archaeopteryx and even smaller (Xu, et al., flight. So since the discovery of Deinonychus, the 2000). These smaller forms also had forelimbs that initial life restoration of this dromaeosaur had much longer proportions than the larger established the conceptual imagery of these dromaeosaurs from later in time. Looking all the dromaeosaurs as being scaly and looking more more bird-like, the small dromaeosaurs appear to reptilian, or dinosaurian, than avian (see the be equal or even more capable of flight than frontispiece by Robert Bakker in Ostrom, 1969). Archaeopteryx except in one detail. The total length Certainly the size of Deinonychus precluded of their arms is not quite as long as those on any idea of it having been a flying animal, but over Archaeopteryx. As a result, even the most rigorous time Ostrom recognized avian characteristics within phylogenetic analyses have regarded dromaeosaurs this dromaeosaur that implied an evolutionary as being incapable of flight and as strictly terrestrial relationship between dinosaurs and birds (Ostrom, precursors of birds. 1976). However, just exactly what the relationship The most significant evidence to support between dinosaurs and birds really was has remained the cladistical analysis of birds having been derived even more controversial ever since. Scientists from dinosaurs has come from the discovery of studying the origin of birds have for the most part various fossils of theropods which have had fallen into two camps with diametrically opposing impressions of feathers preserved (Chen, et al., views of the ancestral avian forms either: evolving 1998; Ji, et al., 1998; Xu, et al., 1999; Xu, et al., within dinosaurs “from the ground up”; or from a 2000; Ji, et al., 2001; Norell, 2001). These feather non-dinosaurian arboreal archosaur “from the trees impressions have stimulated new heated debates down”. While both camps have vigorously as to the authenticity of the feathers and what it all disagreed with each other and found little common meant towards the dinosaur/bird relationship. But ground in their divergent opinions, it is highly the number of specimens now present an overwhelming display of examples demonstrating that different kinds of bird-like dinosaurs were DIAGNOSIS covered in feathers. Specimens, such as those of Caudipteryx, have even had symmetrical primary Cryptovolans pauli is characterized by the presence feathers preserved extending from the manus. But of primary flight feathers as being a bird. It differs otherwise, none of the feathered dinosaurs have from other known dromaeosaurs in having revealed the presence of asymmetrical primary completely co-ossified sternals, forming an avian feathers which would provide a clear association sternum; no less than 28 or more than 30 caudal with the ability flight. In particular, the fossil of one vertebrae; and does not have the theropod dromaeosaur, NGMC 91, appears to be ideally proportions in the third manal digit in which phalanx suited to represent what the dinosaur/bird III-3 should be the longest. Instead, phalanx III-1 proponents have maintained from the beginning: is longer than III-3. that the ancestral precursors of birds included small feathered dromaeosaurs which could not fly. In that specimen, the body, tail, legs and arms were all covered with feathers, but what is conspicuously DISCUSSION missing are indications of primary feathers coming off from the hands. Since the primary flight feathers emanate from the second digit and metacarpal of the hand in birds, the absence of such feathers would Since the days of Thomas Huxley, when he suggest an inability to fly. However, the specimens first proposed an evolutionary relationship between described below are of dromaeosaurs possibly birds and dinosaurs (Huxley, 1868), the subject of co-generic
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