Temporal range: Late Jurassic–Holocene, 165–0Ma Possible Early Jurassic record
|Reconstructed coelurosaur skeleton, Wyoming Dinosaur Center|
von Huene, 1914
Coelurosauria // (from Greek, meaning "hollow tailed lizards") is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. Coelurosauria is a subgroup of theropod dinosaurs that includes compsognathids, tyrannosaurs, ornithomimosaurs, and maniraptorans; Maniraptora includes birds, the only dinosaur group alive today. Most feathered dinosaurs discovered so far have been coelurosaurs; Philip J. Currie considers it probable that all coelurosaurs were feathered. In the past, Coelurosauria was used to refer to all small theropods, although this classification has been abolished.
The studying of anatomical traits in coelurosaurs indicates that the last common ancestor had evolved the ability to eat and digest plant matter, adapting to an omnivorous diet, an ability that could be a major contributor to the clade's success. Later groups would hold on to the omnivory, while others specialized in various directions, becoming insectivorous (Alvarezsauridae), herbivorous (Therizinosauridae) and carnivorous (Tyrannosauroidea and Dromaeosauridae). The group includes some of the largest (Tyrannosaurus) and smallest (Microraptor, Parvicursor) carnivorous dinosaurs ever discovered. Characteristics that distinguish coelurosaurs include:
- a sacrum (series of vertebrae that attach to the hips) longer than in other dinosaurs
- a tail stiffened towards the tip
- a bowed ulna (lower arm bone).
- a tibia (lower leg bone) that is longer than the femur (upper leg bone)
To date, fossilized traces of feathers have been identified among most coelurosaurian lineages. Feathers of some type, or morphological features suggesting feathers, have been found in fossils of at least one species in all coelurosaur subgroups. The arrangements of feathers currently observed on Coelurosauria other than modern birds are without exception more primitive, and some coelurosaurian species are known to have had bare or scaly skin rather than feathers on at least some parts of their bodies. These include large tyrannosaurids, some compsognathids such as Juravenator, and Scansoriopteryx. To date, all examples of scale impressions on these dinosaurs have been found near the hind legs and tail; fossils of at least some of these animals (Scansoriopteryx and possibly Juravenator) also preserve feathers elsewhere on the body. Most coelurosaurs, including modern birds, retained scales on the feet, though in a few (such as Anchiornis and the modern Rock Ptarmigan) the feet and toes are also entirely covered in feathers.
Though once thought to be a feature exclusive to coelurosaurs, feathers or feather-like structures are also known in some ornithischian dinosaurs (like Tianyulong), though whether these are related to true feathers or novel structures that evolved independently is unknown.
Nervous system and senses
Although rare, complete casts of theropod endocrania are known from fossils. Theropod endocrania can also be reconstructed from preserved braincases without damaging valuable specimens by using a computed tomography scan and 3D reconstruction software. These finds are of evolutionary significance because they help document the emergence of the neurology of modern birds from that of earlier reptiles. An increase in the proportion of the brain occupied by the cerebrum seems to have occurred with the advent of the Coelurosauria and "continued throughout the evolution of maniraptorans and early birds."
Fossil evidence and age
A few fossil traces tentatively associated with the Coelurosauria date back as far as the late Triassic. What has been found between then and the start of the late Jurassic is fragmentary. A typical example is Iliosuchus, known only from two ilia bones in the mid Jurassic. It was a 1.5 m long carnivore from about 165 Ma (million years ago) in Oxfordshire and is tentatively assigned to the Tyrannosauroidea.
Many nearly complete fossil coelurosaurians are known from the late Jurassic. Archaeopteryx (incl. Wellnhoferia) is known from Bavaria at 155-150 Ma. Ornitholestes, the troodontid WDC DML 110, Coelurus fragilis and Tanycolagreus topwilsoni are all known from the Morrison Formation in Wyoming at about 150 Ma. Epidendrosaurus and Pedopenna are known from the Daohugou Beds in China, whose age is still being debated, but may be about 160 Ma or 145 Ma.
The wide range of fossils in the late Jurassic and morphological evidence suggests that coelurosaurian differentiation was virtually complete before the end of the Jurassic.
In the early Cretaceous, a superb range of coelurosaurian fossils (including avians) are known from the Yixian Formation in Liaoning. All known theropod dinosaurs from the Yixian Formation are coelurosaurs. Many of the coelurosaurian lineages survived to the end of the Cretaceous period (about 65 Ma) and fossils of some lineages, such as the Tyrannosauroidea, are best known from the late Cretaceous. A majority of coelurosaur groups became extinct in the Cretaceous–Paleogene extinction event, including the Tyrannosauroidea, Ornithomimosauria, Oviraptorosauria, Deinonychosauria, Enantiornithes, and Hesperornithes. Only the Neornithes (modern birds) survived, and continued to diversify after the extinction of the other dinosaurs into the numerous forms found today.
There is consensus among paleontologists that birds are the descendants of coelurosaurs. Under modern cladistic definitions, birds are considered the only living lineage of coelurosaurs. Birds are classified by most paleontologists as belonging to the subgroup Maniraptora.
The phylogeny and taxonomy of Coelurosauria has been subject to intensive research and revision. For many years, Coelurosauria was a 'dumping ground' for all small theropods. In the 1960s several distinctive lineages of coelurosaurs were recognized, and a number of new infraorders were erected, including the Ornithomimosauria, Deinonychosauria, and Oviraptorosauria. During the 1980s and 1990s, paleontologists began to give Coelurosauria a formal definition, usually as all animals closer to birds than to Allosaurus, or equivalent specifiers. Under this modern definition, many small theropods are not classified as coelurosaurs at all and some large theropods, such as the tyrannosaurids, were actually more advanced than allosaurs and therefore were reclassified as giant coelurosaurs. Even more drastically, the segnosaurs, once not even regarded as theropods, have turned out to be non-carnivorous coelurosaurs related to Therizinosaurus. Senter (2007) listed 59 different published phylogenies since 1984. Those since 2005 have followed almost the same pattern, and differ significantly from many older phylogenies.
The relationships among coelurosaurs shown below were found in a phylogenetic analysis by Godefroit and colleagues in 2013.
"Coelurosaurus" is an informal generic name, attributed to Friedrich von Huene, 1929, that is sometimes seen in lists of dinosaurs. It probably arose as a typographical error; von Huene intended to assign indeterminate remains to Coelurosauria incertae sedis, but at some point in the process of publication a revision to the text made it appear that he was creating a new generic name "Coelurosaurus" (as described by George Olshevsky in a 1999 post to the Dinosaur Mailing List). The name is undescribed and has not been used seriously, although it has appeared in works of fiction.
- Godefroit, Pascal; Cau, Andrea; Hu, Dong-Yu; Escuillié, François; Wu, Wenhao; Dyke, Gareth (2013). "A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds". Nature 498 (7454): 359–362. Bibcode:2013Natur.498..359G. doi:10.1038/nature12168. PMID 23719374.
- Holtz, Thomas R. Jr. (2012) Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages, Winter 2011 Appendix.
- Turner, A.H., Makovicky, P.J., and Norell, M.A. 2012. A review of dromaeosaurid systematics and paravian phylogeny. Bulletin of the American Museum of Natural History 371: 1–206.
- Currie (2005) Page 368.
- "Meat-eating dinosaurs not so carnivorous after all". ScienceDaily.
- "Abstract," Larsson (2001). Page 19.
- Dinodata: Coelurosauria.
- Padian (2004). Basal Avialae. Pages 210–231.
- Currie, Philip J. (2005). Dinosaur Provincial Park: A Spectacular Ancient Ecosystem Revealed. Indiana University Press. p. 368. ISBN 0-253-34595-2.
- Dinodata: Coelurosauria.
- 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.
- Mayr, G., B. Pohl & D.S. Peters (2005). "A well-preserved Archaeopteryx specimen with theropod features". Science, 310(5753): 1483-1486.
- George Olshevsky. "Re: What are these dinosaurs". Retrieved 2007-01-29. (on "Coelurosaurus")
- Padian, K. (2004). Basal Avialae. In: Weishampel, D.B., Dodson, P., and Osmólska, H. (eds.). The Dinosauria (second edition). University of California Press, Berkeley, 210–231. ISBN 0-520-24209-2.
- Senter, P. (2007). "A new look at the phylogeny of Coelurosauria (Dinosauria: Theropoda)." Journal of Systematic Palaeontology, (doi:10.1017/S1477201907002143).
- Zanno, L.E., Gillette, D.D., Albright, L.B., and Titus, A.L. (2009). "A new North American therizinosaurid and the role of herbivory in 'predatory' dinosaur evolution." Proceedings of the Royal Society B, Published online before print July 15, 2009, doi:10.1098/rspb.2009.1029.
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