Theropoda

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{{Taxobox | color = pink | name = Theropoda | status = Conservation status: Fossil | image = TrexFoot.jpg | image_width = 200px | image_caption = T. rex foot
Picture taken at Field Museum of Natural History, Chicago | regnum = Animalia | phylum = Chordata | classis = Sauropsida | superordo = Dinosauria | ordo = Saurischia | subordo = Theropoda | subordo_authority = Marsh, 1881 | subdivision_ranks = Subdivisions | subdivision = ?Eoraptor
Herrerasauria
Coelophysoidea
Ceratosauria
Cryolophosaurus
Spinosauridae
Carnosauria
Coelurosauria
}}

Theropods ("beast foot") are a group of bipedal saurischian dinosaurs. Although they were primarily carnivorous, it is believed a number of theropod families evolved herbivory during the Cretaceous. Theropods first appear during the Carnian age of the Late Triassic (about 220 million years ago), and were the sole large terrestrial carnivores from the Early Jurassic until the close of the Cretaceous (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 wishbone, air-filled bones, and (in some cases) feathers and brooding of the eggs.

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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 are Eoraptor of Argentina, and the Herrerasaurs known so far from the early late Triassic (Late Carnian to Early Norian) of North and South America (and possibly also India and Southern Africa), and characterised by a mosaic of primitive and advanced features. Experts disagree over whether these animals are basal theropods, basal saurischians, or prior-to the saurischian-ornithischian split.

The earliest and most primitive unambiguous theropods (or alternatively, Eutheropods - "True Theropods") are the Coelophysidae, a group of widely distributed, lightly built and apparently gregarious (Coelophysis, Syntarsus) animals that included not only smallish hunters like Coelophysis, but 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) appear during the Early Jurassic, and continued through to the Late Jurassic in Laurasia, competing quite well alongside their more advanced tetanuran relatives, and - in the form of the abelisaur lineage - the end of the Cretaceous in Gondwana

The Tetanurae are more specialised again than the Ceratosaurs. They are subdivided into Spinosauroidea or Torvosauroidea (originally called "Megalosaurs") which were most common during the Middle Jurassic but continue to the Middle Cretaceous, and the Avetheropoda. The Latter clade - as their name indicates - are 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 less than four distinct lineages of theropods - Ceratosaurs, Torvosaurs, Allosaurs (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 superficially 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, has 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, Paul's theory has not caught on among most vertebrate paleontologists.

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Classification

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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 0671619462)
  • ----- (2002) Dinosaurs of the Air (ISBN 0801867630):
  • 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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