Theropoda

Introduction

Theropods, meaning “beast-footed,” are a group of bipedal, predominately carnivorous saurischian (“lizard-hipped”) dinosaurs noted for their hollow bones and backward curving teeth set along their entire jaw. The bipedal theropods were an extremely diverse and widely distributed group that included massive carnivorous dinosaurs such as the North American Tyrannosaurus, Mongolian Velociraptor and Oviraptor, North African Spinosaurus, and the Argentinean Giganotosaurus, as well as the small birdlike troodontids and Microraptor from the Cretaceous period.

Evidence of the earliest theropods can be found in the Late Triassic, about 235 million years ago, with the youngest nonavian theropods surviving until the end of the Cretaceous period 66 million years ago. Theropods still exist today, in fact, and are represented by all bird species.

Fast Facts

Pronunciation: Theropoda (pronounced thee-ROP-o-da) meaning “beast footed”

Time Period: Nonavian theropods — Late Triassic to Late Cretaceous (235–66 million years ago); avian theropods (birds) — Late Jurassic to Current (150 million years ago–today).

Size: Nonavian theropods — 4.7 cm to 18 m (1.85 in–59.05 ft)

Weight: Nonavian theropods — 110 g to 21 metric tons (0.24 lbs–46,297 lbs)

Diet: Primarily carnivorous, some omnivorous, herbivorous, or insectivorous

Location: All continents including Antarctica

Lifespan: Nonavian theropods — up to 30 years or more

Homologous Traits

Beast-footed theropods are a group of predominately carnivorous obligate bipedal dinosaurs—meaning they are meat eaters who walked upright on two legs. Theropods were characterized by their hollow, thin-walled bones, adapted for strength and reducing overall body weight.

All the lizard-hipped theropods possessed a backward facing pubis, reduced forearms, and long, strong hind limbs to aid their bipedal nature. Modified birdlike, clawed feet, ending with three weight-bearing toes, were common among theropods, as were their three-fingered hands and reduced fourth and fifth digits.

Many nonavian theropods are also known for their impressive dentition, possessing extraordinarily sharp, backward-curving teeth along their entire jaw. These teeth were an evolutionary advance for the meat-eating theropods that, together with their well-developed jaw muscles, greatly improved feeding efficiency and survival. The later evolution of herbivory in some species of theropod also saw a change in dentition to leaf-shaped teeth used for feeding on vegetation.

It is thought that all dinosaurs, including theropods, were oviparous (egg-laying) animals that laid eggs in clutches. Despite theropods falling at the higher end of dinosaur intelligence, paleontologists believe that most dinosaur species, including the nonavian theropods, exhibited very little pre- or postparental care. Certainly, to date, no irrefutable evidence has been discovered to support sophisticated parental care among dinosaurs. However, recent fossil finds from dinosaur nests have hinted at advanced parenting behavior. Of particular interest are fossilized egg sites of the theropod species Oviraptor, which appear to provide strong evidence of brooding behavior.

Evolutionary Divergences

Together with crocodiles, pterosaurs, and birds, dinosaurs are classified as subgroups of Archosauria. Regardless of which taxonomic branch an archosaur belongs, it is classified as an archosaur by its ankle structure and the presence of two openings in the skull. Archosaurs are monophyletic, with all species descended from a sole common ancestor and possessing certain traits. Archosaurs, which consist of the Crurotarsi and Avemetatarsalia clades, evolved from even more primitive reptiles.

Dinosaur species are divided into one of two orders, Ornithischia (bird-hipped dinosaurs) and Saurischia (lizard-hipped dinosaurs). The Saurischia order, in turn, comprises two major groups, the large, herbivorous Sauropoda suborder and the predominately carnivorous Theropoda suborder. Theropoda comprises several clades: the early ceratosaurs and the tetanurans and its two main divisions, the Carnosauria and the Coelurosauria. Theropods are not, however, a dead linage. Modern theropods are still represented by all bird species. Paradoxically, it is from the lizard-hipped dinosaurs (not the bird-hipped dinosaurs) that modern-day birds evolved.

Nonavian theropods were one of the most successful groups of dinosaurs to have ever lived. They appeared very early on in the fossil record and were among the last to disappear at the end of the Late Cretaceous (66 million years ago). Theropod evolution can be traced as far back are the Late Triassic (235 million years ago), with even the earliest of species exhibiting characteristics typical of this group. It is believed that all theropods originally descended from a much more primitive bipedal dinosaur, whose feet also consisted of just three toes. It is from these ancient dinosaurs that the earliest theropods evolved, alternatively thought to be either Herrerasaurus or Eodromaeus, both relatively small bipedal carnivores from Argentina.

Creatures in This Group

There is some debate regarding whether or not herrerasaurs are theropods, with some paleontologists classifying them as basal (early) theropods, and some classifying them as either very primitive saurischians or as genera outside of Dinosauria. Based on cladistics, the suborder Theropoda consists of several nodes: Ceratosauria, Tetanurae, Spinosauridae, Avetheropoda, Carnosauria, and Coelurosauria. These nodes differ in bone structure, particularly in the wrist, teeth, and spine, as well as diet.

An early division of the suborder Theropoda, the superfamily Coelophysoidea contains the North American Coeleophysis from the Late Triassic. Ceratosauria is considered an infraorder of relatively primitive theropods within Coelophysoidea. Ceratosaurs are characterized by larger air space in their neck bones, fused ankles, and changes to the spine, ribs, and knee joints. These theropods were carnivorous. Neoceratosauria, a division of Ceratosauria, includes North American Ceratosaurus from the Late Jurassic and South American Abelisaurus from the Late Cretaceous. Tetanurae, an unranked clade within the Coelophysoidea superfamily, includes all of the more advanced theropods. Tetanurans developed interlocking connections in their vertebrae, resulting in stiffer tails that provided more balance and less flexibility, fewer fingers on the hands, prominent jaws set in front of the eye socket, and ankle extensions.

A superfamily of Tetanurae is Megalosauroidea, which includes the families Spinosauridae and Megalosauridae. Spinosauridae, a family of Tetanurae, includes the North African Spinosaurus from the Late Cretaceous. The family Megalosauridae, a sister group to Spinosauridae, includes Xuanhanosaurus, Piatnitzkysaurus, Eustreptospondylus, Afrovenator, and its name-sake genus, the European Megalosaurus from the Middle Jurassic. The information regarding megalosaurs is incomplete; new discoveries may provide greater insight into their specific differentiations. The spinosaurs had longer skulls, with finely serrated teeth that may have been used to consume fish.

Avetheropoda, the other major division of Tetanurae, includes Gasosaurus, infraorder Carnosauria, and the unranked clade Coelurosauria. Carnosaurs, including the superfamily Allosauroidea, were the largest theropods. Allosaurus, Charcharodontosaurus, and Yangchuanosaurus (now classified within the genus Sinraptor) are representative genera for three of the families within Carnosauria. Coelurosauria includes the families Tyrannosauridae, Oviraptoridae, Ornithomimidae, Troodontidae, Dromaeosauridae, Therizinosauridae, the unranked clade Avialae, and Aves (modern-day birds). Coelurosaurs are noted for their increased manual dexterity due to changes in the wrist. Ornithomimids and oviraptorids developed toothless jaws, possibly indicating diets of shellfish or plant matter.

Ecology

Although the very earliest theropods were certainly carnivores, research has indicated some of the later species evolved more varied diets, such as probable herbivory in the therizinosaurs and ornithomimosaurs. For much of their history, however, theropods were at the top of the food chain and, for more than 100 million years, were the only large meat eaters in existence.

All the lizard-hipped theropods possessed long, strong hind limbs, the evolution of which aided the move to bipedal locomotion and increased speed, a definite advantage for a predator. The reduced forearms of many theropod species likely had a significant impact on hunting ability and influenced how they captured and consumed prey.

Paleontology News

Like evolution itself, the story of dinosaurs is constantly evolving as new fossil discoveries shed light on what life was like many millions of years ago. Despite the long-standing fame of Tyrannosaurus, only recently has fossil research demonstrated that this species actually belong to the coelurosaurs, which contain those theropod species that have more in common with birds than carnosaurs.

Perhaps the most controversial aspect of theropod research is the growing body of evidence challenging the long-held belief that birds descended from theropod dinosaurs. Research has suggested that not only have birds evolved certain anatomical characteristics not found in dinosaurs, particularly the fused thigh bones required by birds during flight, but also that birds actually existed before the dinosaurs from which they are thought to be descended. If this were found to be true, it would have a significant impact on the current classification of avian and theropod species.

Bibliography

Books

Carpenter, Kenneth. Carnivorous Dinosaurs. Bloomington: Indiana University Press, 2005.

• Examines the carnivorous theropod dinosaurs, with scientific papers covering topics on classification, anatomy, morphology, and ecology.

Fastovsky, David E., and David B. Weishampel. Dinosaurs: A Concise Natural History. New York: Cambridge University Press, 2009.

• Focuses on many aspects of the natural sciences and how they relate to dinosaur biology, evolution, life history, and classification.

—. Evolution and Extinction of the Dinosaurs. New York: Cambridge University Press, 2005.

• An illustrated text that covers complex scientific processes and dinosaur species in an informative and comprehensible way.

Lucas, Spencer George. Dinosaurs: The Textbook. 5th ed. Boston: McGraw-Hill, 2007.

• Provides an overview of important aspects of dinosaur discovery, behavior, biology, and classification.

Martin, Anthony J. Introduction to the Study of Dinosaurs. Malden, MA: Blackwell, 2006.

• Provides comprehensive scientific-based chapters on the major dinosaur clades, as well as anatomical, physiological, and behavioral information.

Parker, Steve. Dinosaurus: The Complete Guide to Dinosaurs. Richmond hill, on: Firefly Books, 2009.

• Provides useful information on more than 500 dinosaurs, such as discovery, location, anatomy, and diet.

Weishampel, David B., Peter Dodson, and Halszka Osmólska. Dinosauria. Berkeley: University of California Press, 2007.

• An in-depth text providing resources and scientific papers on dinosaurs, especially looking at the Saurischia and Ornithischia orders in regards to their evolution, distribution, and ecology.

Journals

Brusatte, Stephen L., Roger B. J. Benson, and Xu Xing. “The Evolution of Large-bodied Theropod Dinosaurs during the Mesozoic in Asia.” Journal of Iberian Geology 36.2 (2010): 275–96.

• Provides information on the discovery, classifications, and estimated lengths and weights, and distribution of theropods found in Asia. Includes related photographs, diagrams, and tables.

Ősi, Attila, Sebastián Apesteguía, and Michał Kowalewski. “Non-avian Theropod Dinosaurs from the Early Late Cretaceous of Central Europe.” Cretaceous Research 31.3 (2010): 304–20.

• Using teeth, digits, and leg bones discovered in Hungary, the authors identify Gondwanan and Euramerican therapods that inhabited central Europe during the Late Cretaceous. Includes supporting photographs, plots, and maps.

Additional Works Used

“Bird-from-dinosaur theory of evolution challenged: Was it the other way around?” Science Daily. 10 Feb. 2010. http://www.sciencedaily.com/releases/2010/02/100209183335.htm.

Jacobsen, Aase Roland. “Feeding Behaviour of Carnivorous Dinosaurs as Determined by Tooth Marks on Dinosaur Bones.” Historical Biology: An International Journal of Paleobiology 13.1 (1998): 17–26.

Norell, Mark, et al. Discovering Dinosaurs: Evolution, Extinction, and the Lessons of Prehistory. Berkeley: University of California Press, 1995.

“Theropoda.” Wikipedia. Web. May 2011. <http://en.wikipedia.org>.