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Theropod

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iTheropods
Fossil range: Triassic - Cretaceous (non-avian)
TrexFoot
Conservation status
Extinct (fossil)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Superorder: Dinosauria
Order: Saurischia
Suborder: Theropoda
Marsh, 1881
Infraorders & Superfamilies

Theropods ('beast foot') are a group of bipedal saurischian dinosaurs. Although they were primarily carnivorous, a number of theropod families evolved herbivory, during the Cretaceous Period. Theropods first appear during the Carnian age of the Late Triassic about 220 million years ago (mya) and were the sole large terrestrial carnivores from the Early Jurassic until the close of the Cretaceous, about 65 million years ago. Today, they are represented by the 9,300 living species of birds, which evolved in the Late Jurassic from small specialized coelurosaurian dinosaurs.

Among the features linking theropods to birds are the three-toed foot, a furcula (wishbone), air-filled bones and (in some cases) feathers and brooding of the eggs.

Evolutionary history

During the late Triassic, a number of primitive proto-theropod and theropod dinosaurs existed and evolved alongside each other.

The earliest and most primitive of the carnivorous dinosaurs were Eoraptor of Argentina and the herrerasaurs. The herrerasaurs existed from the early late Triassic (Late Carnian to Early Norian). They were found in North America and South America and possibly also India and Southern Africa. The herrerasaurs were characterised by a mosaic of primitive and advanced features. Some paleontologists have in the past considered the herrerasaurians to be members of Theropoda, though they are now thought to be basal saurischians, and may even have evolved prior to the saurischian-ornithischian split.

The earliest and most primitive unambiguous theropods (or alternatively, Eutheropods - 'True Theropods') are the Coelophysidae. The Coelophysidae (Coelophysis, Megapnosaurus) were a group of widely distributed, lightly built and apparently gregarious animals. They included small hunters like Coelophysis and larger (6 meters) predators like Dilophosaurus. These successful animals continued from the Late Carnian (early Late Triassic) through to the Toarcian (late Early Jurassic). Although in the early cladistic classifications they were included under the Ceratosauria and considered a side-branch of more advanced theropods (e.g. Rowe & Gauthier 1990), they may have been ancestral to all other theropods (which would make them a paraphyletic assemblage (e.g. Mortimor 2001, Carrano et al 2002).

The somewhat more advanced true Ceratosauria (including Ceratosaurus and Carnotaurus) appeared during the Early Jurassic and continued through to the Late Jurassic in Laurasia. They competed quite well alongside their more advanced tetanuran relatives and - in the form of the abelisaur lineage - lasted to the end of the Cretaceous in Gondwana.

The Tetanurae are more specialised again than the Ceratosaurs. They are subdivided into Megalosauroidea (alternately Spinosauroidea or Torvosauroidea) and the Avetheropoda. They were most common during the Middle Jurassic but continued to the Middle Cretaceous. The latter clade - as their name indicates - were more closely related to birds and are again divided into the Carnosauria (including Allosaurus) and the Coelurosauria, a very large and diverse dinosaur group that was especially common during the Cretaceous.

Thus, during the late Jurassic, there were no fewer than four distinct lineages of theropods - ceratosaurs, megalosaurs, carnosaurs, and coelurosaurs - preying on the abundance of small and large herbivorous dinosaurs. All four groups survived into the Cretaceous, although only two - the abelisaurs and the coelurosaurs - seem to have made it to end of the period, where they were geographically separate, the abelisaurs in Gondwana, and the coelurosaurs in Asiamerica.

Of all the theropod groups, the coelurosaurs were by far the most diverse. Some coelurosaur clades that flourished during the Cretaceous are: tyrannosaurs, including the famous Tyrannosaurus rex, the dromaeosaurs, including Velociraptor and Deinonychus, which are remarkably similar in form to the Archaeopteryx (Ostrom 1969, Paul 1988, Dingus & Rowe 1998), the dromaeosaur-like Troodontidae, the omnivorous oviraptorosaurs, the herbivorous ornithomimids ("ostrich dinosaurs") and Therizinosauridae (giant-clawed herbivores) and the birds (the only dinosaur lineage to survive the end Cretaceous mass-extinction). While the roots of these various groups must have been in the Late or possibly even the Middle Jurassic, they only became abundant during the early Cretaceous. A few paleontologists, such as Gregory S. Paul, have suggested (Paul 1988, 2002) that some or all of these advanced theropods were actually descended from flying dinosaurs or proto-birds like Archaeopteryx that lost the ability to fly and returned to a terrestrial habitat. While this hypothesis can explain why coelurosaurs are so rare during the Jurassic, more fossil evidence is needed before the exact relationships of advanced theropods can be accurately tested.

Taxonomy

Classification

Phylogeny

Theropoda
  |--Agnostiphys
  |--Guaibasaurus
  |--Chindesaurus
  `--Neotheropoda
         |--Coelophysoidea
         |--Ceratosauria
         |      `--Neoceratosauria
         |            |--Ceratosauridae
         |            `--Abelisauroidea
         `--Tetanurae
               |--Condorraptor
               |-?Cryolophosaurus
               |-?Sinosaurus
               `--+--Xuanhanosaurus
                     |--Megalosauroidea
                     |      |--Megalosauridae
                     |      `--Spinosauridae
                     `--Avetheropoda
                             |--Carnosauria
                             |      `--Allosauroidea
                             |              |--Sinraptoridae
                             |              |--Allosauridae
                             |              `--Carcharodontosauridae
                             `--Coelurosauria
                                    |--Compsognathidae
                                    |--Tyrannosauroidea
                                    `--Maniraptoriformes
                                           |--Ornithomimoidea
                                           |      |--Ornithomimosauria
                                           |      `--Alvarezsauridae
                                           `--Maniraptora
                                                   |--Oviraptoriformes
                                                   |      |--Therizinosauria
                                                   |      `--Oviraptorosauria
                                                   `--Paraves
                                                           |--Deinonychosauria
                                                           `--Aves

The largest theropods

Tyrannosaurus was the largest and most popular theropod known to the general public for many decades. Since its discovery, however, a number of other giant carnivorous dinosaurs have been described, including Spinosaurus, Carcharodontosaurus, Giganotosaurus, Tyrannotitan and Mapusaurus. In the film Jurassic Park 3, Spinosaurus is depicted as being larger than Tyrannosaurus and the original Spinosaurus specimens (as well as new fossils described in 2006) support this, showing that Spinosaurus was about 4 meters longer and 4 tons heavier than Tyrannosaurus (a size comparison of the largest theropods can be found in the article Dinosaur size). There is still no clear scientific explanation for exactly why these animals grew so much larger than the predators that came before and after them.

References

  • Carrano, M. T., Sampson, S. D. & Forster, C. A., (2002), The osteology of Masiakasaurus knopfleri, a small abelisauroid (Dinosauria: Theropoda) from the Late Cretaceous of Madagascar. Journal of Vertebrate Paleontology Vol. 22, #3, pp. 510-534
  • Dingus, L. & Rowe, T. (1998), The Mistaken Extinction: Dinosaur Evolution and the Origin of Birds, Freeman
  • Kirkland, J. I., Zanno, L. E., Sampson, S. D., Clark, J. M. & DeBlieux, D. D., (2005) A primitive therizinosauroid dinosaurs from the Early Cretaceous of Utah, Nature: Vol. 435, pp. 84-87
  • Mortimer, M., (2001) "Rauhut's Thesis", Dinosaur Mailing List Archives, 4 Jul 2001
  • Ostrom, J.H. (1969). Osteology of Deinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana, Peabody Museum Nat. History Bull., 30, 1-165
  • Paul, G.S., (1988) Predatory Dinosaurs of the World Simon and Schuster Co., New York (ISBN 0-671-61946-2)
  • ----- (2002) Dinosaurs of the Air (ISBN 0-8018-6763-0):
  • Rowe, T., & Gauthier, J., (1990) Ceratosauria. 151-168 in Weishampel, D. B., Dodson, P., & Osmólska, H. (eds.), The Dinosauria, University of California Press, Berkley, Los Angeles, Oxford.

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