About 40 percent of all known dinosaur taxa are theropods. The most basal members of the clade were among the first dinosaurs to walk the Earth. Nonavian theropods were also among the last dinosaurs to perish, their success spanning the entire age of dinosaurs. They are survived today by avian theropods, the birds.
Theropod remains have been found on every continent. These animals ranged in size from the tiniest of dinosaurs, the chicken-sized Compsognathus, to hulking monsters such as Tyrannosaurus,
Giganotosaurus ("gigantic southern lizard"), and Carcharodontosau-rus ("great white shark-toothed lizard"). In between these extremes were many wonderful varieties of the basic two-legged carnivorous dinosaur.
Theropods are united by a suite of shared anatomical features. Many of these traits can be linked to the driving force behind their existence: finding, attacking, and consuming other animals.
Some of the best clues to the predatory lifestyle of theropods are found in their skulls. The theropod skull was lightly built and had a somewhat loosely joined assemblage of bones. This structure provided flexibility to absorb the physical shock that came with biting and subduing wriggling prey. The jaw itself was constructed more firmly, especially the lower jaw, made up of several bones including the tooth-bearing dentary. A primitive feature of the jaw found in the basal saurischian Herrerasaurus was an intramandibular joint that allowed the jaw to slide back and forth. This allowed the animal to maintain a firm grip with its teeth. A similar sliding joint was present in the lower jaw of many later theropods, including such giants as Allosaurus.
Theropod skulls came in a variety of shapes and sizes that were adapted to the choice of carnivory of a given kind of prey. A large head with strong biting jaws was the customary arrangement of the meat eaters that probably fed on other large animals. Predators that probably fed on insects and smaller animals had tinier, more birdlike skulls. The theropod skull had large fenestrae that lightened its weight, and large orbits, or eye sockets, as well. The presence of these indicates superior eyesight for tracking and following prey. Good eyesight is, in fact, a natural adaptation for predators, and some theropods had exceptionally large eyes. Whereas herbivorous dinosaurs had smaller eyes on the sides of their heads—better for scanning wide areas all at once to watch for predators—some theropods, including the troodontids, had forward-looking eyes that provided overlapping, stereoscopic vision, like that of humans. The ability to focus both eyes on the prey target greatly improves the ability of a predator to chase down and capture a fast-moving, evasive prey animal.
Most theropods were equipped with a mouthful of pointed, bladelike, serrated teeth. The deeply-rooted teeth of the largest tyrannosaurs were banana-sized and sturdy, capable of crunching through bone. Such teeth represent the extreme end of dinosaur tooth design for a highly derived theropod, the last of its line. For tyrannosaurs, whose arms were quite short, the head and jaws became its primary tool for grappling with prey. The oversized head, stout teeth, and highly muscular neck and jaw design suggest that tyrannosaurs probably captured their prey by lunging at it with open jaws, possibly in ambush.
Most other theropod teeth were less robust and more knifelike than those of tyrannosaurs. Such teeth provided a means to slice through the flesh of the prey animal once it was subdued. This is evident in the dromaeosaurs—dog-sized, swiftly moving predators such as Deinonychus (Late Cretaceous, Montana) and Velociraptor (Late Cretaceous, central Asia). The teeth of the dromaeosaurs were small and serrated but could be yanked loose if the prey resisted strenuously. Instead of relying only on their teeth, the dromaeosaurs used a combination of sickle-like foot claws and hand claws to slash their victims into submission while chasing them down. They probably also used the claws as hooks to get them in close enough to bite out big chunks of the prey! Once the prey was adequately weakened and unable to resist, the teeth of the dromaeosaur took care of the business of eating.
There were also a few theropod taxa that had no teeth at all; instead, these theropods had only a horn-covered beak. It has been presumed that dinosaurs such as Oviraptor (Late Cretaceous, Mongolia) and the ostrich-like Gallimimus (Late Cretaceous, Mongolia), two kinds of toothless theropods, fed on small vertebrates, insects, or perhaps even small, freshwater mollusks and crustaceans. There is growing evidence that some forms of toothless theropods were herbivorous. Oviraptorosaurs such as Caudipteryx and ornithomi-mosaurs such as Sinornithomimus had gizzards full of gastroliths, indicating that they were at least largely, if not entirely, herbivorous, like modern birds with the same structures. Carnivorous birds lack such gizzards.
The theropod skull was lightweight, but it also was mobile. It had a flexible connection to the vertebrae of the neck. This connection allowed the dinosaur to rotate its head from side to side, gave it reach, and allowed it to move its head quickly and precisely as it thrust its jaws to grab at moving prey. A similar joint connects the skull and neck of modern birds. In most theropods, this mobility was aided by the relatively long and flexible S-shaped neck.
Other parts of the theropod skeleton were equally optimized for the animal's carnivorous lifestyle. The forelimbs of theropods were always shorter than the hind limbs. The proportions of the fore-limbs to the hind limbs, the supporting bony structure of the foot, and evidence of theropod trackways clearly indicate that theropods were bipedal. This posture freed the forelimbs for other matters, such as grasping prey, and theropods evolved some effective adaptations for doing just that. The evolution of theropod forelimbs began with the development of longer digits in the first dinosaurs. The second digit, equivalent to the index finger in humans, was the longest of the fingers. Theropods in fact had the equivalents of the human thumb, index finger, and middle finger. Later theropods developed longer arms with flexible shoulders that could be twisted enough to allow these carnivores to lunge forward to snag prey. The most deadly forelimb innovation of all was the addition of large, curved claws to the fingers—a trait seen in most later theropods, regardless of body size. Interestingly, in some smaller theropods, the ability to sweep the forelimbs forward and backward with a circular motion would later become the adaptation of wings for flapping, an important stage before the appearance of powered flight in their descendants, the birds.
The hind limbs of theropods provided excellent balance on three clawed toes, the longest of which was the center one. Most thero-pods actually had four toes, all with claws, but one (the first toe)
was small and not typically in contact with the ground. The hind limbs provided two advantages to these hunting animals: the ability to move swiftly to chase down prey, and the use of the feet as an aid while hunting and subduing prey. Theropod toe claws were robust but not generally sharp because they continually rubbed against the ground when the animal walked. Except in dromaeosaurs, the claws were not used as weapons so much as for holding down prey or anchoring a carcass. In dromaeosaurs, the second toe of each foot was sharp and retractable so that it could be raised off the ground when the animal walked. These claws were certainly used as weapons, although recent experiments show that the toe claw would have made a poor weapon for slashing and disemboweling prey. Instead, dromaeosaurs probably used this foot claw as a puncturing weapon, wounding prey by jabbing at such vulnerable spots as the jugular or carotid artery in the neck. Even though the claws of most theropods were not sharp like those of the dromaeosaurs, theropod legs were undoubtedly strong and capable of delivering powerful kicking blows, not unlike the kicks delivered by extant flightless birds such as the ostrich and emu. The upper leg bone, or femur, was bowed and shorter than the rest of the leg. These traits improved the running speed of theropods.
Dromaeosaurs may have had the most elegant of theropod weaponry. Their forelimbs and hind limbs combined to form an impressive arsenal. The t hree-clawed hand was equipped with sharply curved claws. The hand was attached to a flexible wrist, allowing the animal to grab and twist with the movements of the prey. It was the foot of the dromaeosaur, however, that bore the most effective device of all, a large, pointed, retractable claw on the second toe of each foot. The toe claw retracted up out of the way when the animal walked but could be flipped down, like an open switchblade knife, when the dromaeosaur was on the attack. Such an arsenal leaves little to the imagination. One of the larger dromaeosaurs was Deinonychus, named "terrible claw" after its formidable foot claw. Everything about its anatomy suggests that this dromaeosaur
was an energetic, swift- sunning animal. After closing in on its prey—perhaps a moderately sized plant-eater such as Tenontosaurus (Early Cretaceous, Montana), which is also found in the same areas where the fossils of Deinonychus are found—the predator probably launched its attack by leaping at the victim feet first, with toe claws extended, gained a foothold on the prey, and punctured it with its claws. Thus did the bloodbath begin. The prey, weakened considerably, soon would be too weak to struggle. Using its forelimbs to hold the prey, Deinonychus could finish it off by biting and continuing to inflict puncture wounds with its feet.
Most theropods had long and somewhat stiff tails, a trait that improved their balance while running. The most reasonable way to envision a running theropod is to view it leaning forward, using its hip area as a fulcrum, with its head stretched out in front and its tail extended behind to maintain a counterbalanced posture and efficient forward motion. The posture of a running theropod may be compared to that of a racing cyclist who leans forward on dropped handlebars to improve the leverage and energy that can be applied to the pedals.
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THINK ABOUT IT
The Telltale Anatomy of Theropod Teeth
Is it possible to identify a dinosaur from its teeth alone? Can teeth be considered the "fingerprints" of an extinct species of dinosaur?
Vertebrate anatomy features two kinds of hard parts that make vertebrates unique among other organisms: bone and cartilage. Teeth are enamel-coated bone. Cartilage rarely fossilizes, so it does not help the paleontologist. Bone, of sufficient quality and quantity, can provide a highly accurate picture of an extinct animal and its lifestyle. When left with only fossil teeth, however, the paleontologist is much less able to ascertain the exact nature of an extinct animal, except at a most general level: whether it was a predator or a herbivore, for example.
For some kinds of fossil vertebrates, such as mammals, teeth were so highly specialized that they can serve as a means for identifying extinct taxa down to the species level. Mammals have heterodont dentition: different kinds of teeth in different zones of the jaws. Characteristics of the teeth are key to the adaptive strategy and success of a vertebrate and are passed along to its descendants through natural selection. The teeth of mammals are so informative that many clades of extinct mammals are first classified based on their dentition. A well-trained paleontologist, presented with only the teeth of an extinct mammal, will be able to identify which specific kind of mammal those teeth came from.
Most dinosaurs do not exhibit dramatic heterodonty in their dentition, which limits what can be discerned from a single tooth or part of a jaw bone. Being able to identify a dinosaur from little more than its teeth, however, would be a valuable tool for studying dinosaurs. Take the case of predatory dinosaurs. The teeth of theropods fossilized well and frequently are found in Mesozoic fossil beds that contain terrestrial specimens. Such teeth are often found to the exclusion of any other remains, suggesting that the teeth were shed by a living predator—not an uncommon occurrence because dinosaurs, like most reptiles, were able to replace a tooth whenever they lost one. Being able to identify the kind of dinosaur that shed such a tooth would enable a paleontologist to understand more about the range and interaction of a given taxon. Joshua Smith is an American paleontologist who believes that theropod teeth have more to tell science than what is obvious to the naked eye.
To find out whether the teeth of theropods are distinguishable from one another, Smith conducted an exhaustive, groundbreaking analysis of one well-known taxon, Tyrannosaurus. Smith suggested that if he could find traits in T. rex teeth that could distinguish them from the teeth of other theropods, then the same could be done for the teeth of other kinds of dinosaurs.
An experienced paleontologist can use the naked eye to gauge the relative girth, shape, and curvature of a tooth and then surmise with some accuracy the general size of the theropod whose tooth it is and the position of the tooth in the dental battery. The drawbacks to this approach are that eyeballing a specimen is a procedure that is prone to error, and the outcome is entirely dependent on the knowledge of the person doing the examination. To avoid such biases, Smith applied precise, electronically aided measurements and numerical analysis to measure such T. rex tooth data as size and spacing as well as morphological characteristics that included the curvature of the leading and trailing edges of a tooth, its length, its width, its breadth, its crown size and shape, the density of its serrated denticles, and other nearly microscopic traits of tooth structure. Once collected, such data can be objectively interpreted.
Smith examined and measured all the teeth of known T. rex specimens. Using methods that have been widely applied to the diverse morphology of mammalian teeth, Smith sought to determine whether he could extract data from the analysis of theropod teeth that would allow him to distinguish one taxon from another. Despite the superficially simple structure of theropod teeth, Smith's exhaustive research with T. rex
dentition demonstrated that this taxon of dinosaur could be singled out of the fossil record on the basis of its teeth alone. His work is of tremendous benefit to theropod researchers and to all dinosaur paleontologists who desire to create a gold standard for examining the teeth of any kind of dinosaur.
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