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|Although there a number of groups of dinosaurs, all known carnivorous dinosaurs are members of a group called the theropods. This group not only includes some of the largest terrestrial carnivores in history, it also includes the smallest known dinosaurs. The earliest known dinosaurs were theropods and a number of new theropods evolved in the Late Cretaceous, just prior to the end of the dinosaurs. There is a great deal of evidence which suggests that birds evolved from theropods and, if true, this would mean that theropods are the only group of dinosaurs which did not become extinct.|
The name theropod literally means “beast foot” and was used by Nathaniel
Marsh in 1881. Is there a mistake?
Theropods have rather birdlike feet while the “ornithopods” (“bird
feet”), which were named by Marsh in 1871, have feet which can resemble
those of mammals. (Marsh also named
the “sauropods”–lizard feet.) Theropods include the ancestors of birds and
a number of dinosaurs which are so similar to birds that their names literally
translate as “bird mimic”, “ostrich mimic”, “chicken mimic”, and “pelican
mimic”, among others. Why do theropods
have birdlike feet and ornithopods have beast-like feet? It may be that Marsh originally noted that two
groups of dinosaurs possessed feet which resembled those of birds and
mammals and later simply forgot which group was which. Afterwards this classification system was simply
accepted on the authority of Marsh’s name (Fastovsky,
Theropods are known from all continents. Their legs and feet were well adapted for bipedal locomotion. The first toe is separated from the rest of the foot and did not touch the ground. Three elongated toes (digits 2-4) bear the body’s weight. In most theropods, the number of phalanges (toe bones) on the five toes had the following formula: 2-3-4-5-0 (Weishampel, p. 212) Because the hip bore all of the body’s weight, at least 5 vertebrae fused to the hip to form the sacrum in later theropods. The pelvis was large and provided a great deal of room for muscle attachment. The back portion of the tail was rigid and may have functioned in maintaining balance. Modern birds still have such a structural organization in their tails: the first portion of the tail is flexible and the rear section is rigid.
Not only did the feet bear the body’s weight, the claws of the feet could be used as weapons. Dromaeosaurs and troodontids had especially large claws on their second toes which were retracted while walking. The joint between the claw and the rest of the toe would have allowed a great arc of movement and such a weapon could have been used to disembowel prey.
Since the hands were not used in locomotion, they became powerful, grasping structures. The first carpal (wrist bone) of the thumb was on the palm side of hand and thus the other fingers were semiopposable. The arms were capable of great extension and the wrist was modified for grasping. In later theropods, wrist modifications allowed even better grasping. Although most theropods probably relied on their hands, the hands of tyrannosaurs and carnotaurs were much reduced. This may have been an adaptation for balance in an attempt to compensate for the increased size of the skull in these animals (Fastovsky, 1996; Lucas, 2004).
Theropods also possessed modifications of the skull and neck. The large fenestrae (openings) made the skull lighter and most theropods possessed a second antorbital fenestra. Where the vertebral column meets the skull, a rounded occipital process (called a condyle) and its articulation with the atlas (the first vertebra) was both mobile and resisted dislocation. This would be important for animals which grabbed onto moving prey with their teeth. Many theropods also possessed supraoccipital crests in the back of their skulls which allowed for more attachment of neck muscles (Weishampel, p. 212). A highly mobile skull may also indicate that they were visual hunters. Birds still have this type of joint between the skull and atlas. The bones of the lower jaws were not firmly joined; to each other and perhaps this resisted some of the stress involved in biting moving prey (Fastovsky, 1996).
Most theropods had large, curved, serrated teeth that were adapted to cutting through flesh. Some lineages of theropods lost their teeth such as ornithomimids and oviraptors perhaps to specialize for feeding on eggs, insects, shellfish, or even plant material. Masaikasaurus possessed teeth which projected from the mouth, perhaps as an adaptation to eat insects. Segnosaurs possessed highly modified teeth which might have adapted them for a varied diet.
Were theropods carrion eaters? Some feel that the absence of wear on the teeth of some species indicated that they ate carrion although this interpretation is questionable. Some teeth do show wear, even wear after having been broken. Tyrannosaurus possessed more bulbous teeth and some feel this indicates a need for bone-breaking important to scavengers. Other observations (such as tyrannosaur bitemarks on the sacrum of a Triceratops and the adaptations of their legs for stamina rather than speed) have also been interpreted as possible evidence for scavenging in tyrannosaurs. It may be unwise to attempt to classify dinosaurs as being solely predatory or scavenging since no large modern predator can maintain itself solely through scavenging and many modern top predators can resort to some scavenging.
In theropods, the senses were well developed, as would be expected for predators. Their eyes were large compared to those of other dinosaurs. In Tyrannosaurus, the eyes were forward facing and probably allowed binocular vision. The middle ears were large (especially in ornithomimids and troodontids). In troodontids, the 2 ears communicated through a canal at the base of the braincase; in birds a similar setup allows them to pinpoint the direction of a sound. Theropod brains were typically large. In general, the ratio of their brain size to body size fall within or above those observed for crocodiles and lizards and were the greatest among dinosaurs. Some of these brain to body ratios were within the bird and even primitive mammal range with that of Troodon was the largest.
What did theropods eat? Only a few examples are known with certainty from fossils. There are fossils which link Velociraptor and Triceratops; Compsognathus and a lizard, and Coelophysis and its own young. Three skeletons of Deinonychus individuals have been found with the iguanodontid Tenontosaurus, suggesting that they hunted large prey in packs. One Apatosaurus skeleton has Allosaurus bite marks in vertebrae and tooth fragments (Fastovsky, 1996; Lucas, 2004).
Some theropods possessed crests, some had horns, and sexual dimorphism of these structures is known in some species.
|The elongated nasal bones of the Antarctic theropod Cryolophosaurus formed a crest. (Hammer, 1994).|
The theropod Megalosaurus was one of the first dinosaurs named by Owen. As more were discovered, they were classified in two groups depending on size. All large theropods were grouped into the group “Carnosauria” and the small theropods into the group “Coelurosauria”. More careful examination shows that size is not an adequate classification criterion. Tyrannosaurus is one of the largest theropods but it evolved from smaller coelurosaurs. The modern classification of theropods identifies the most primitive and earliest theropods as the basal Triassic theropods with two more advanced group appearing later, the Ceratosaurs and Tetanurans. The tetanurans are the larger of the two groups and within this group is another group known as the maniraptorans.
communal? A few of the smaller
species seem to have been pack hunters, enabling them to take down larger
prey. There are several mass accumulations of bones
of an individual species which might indicate communal feeding sites. One Coelophysis
site produced hundreds of individuals and one site in
Theropods may have been endothermic, especially the smaller ones. A number of theropods are known to have possessed feathers.
TRIASSIC BASAL THEROPODS
There are several theropods which are recognized as the most primitive dinosaurs known. Although they are now classified as primitive theropods, they were once considered to be the most primitive dinosaurs, predating the split between the saurischians and ornithischians. This change in classification occurred because the first fossils were not complete enough to allow the most accurate interpretation.
Eoraptor is a basal theropod that is more primitive than the other basal theropods Herrerasaurus and Staurikosaurus. It measured about 1 meter long and lacked the specializations of the skull and teeth observed in other dinosaur groups. Its back teeth were curved as in theropods but its front teeth were leaf-shaped as in the early prosauropods and ornithopods (which were plant eaters). It possessed a 5th finger although it was a functionless stub. It is a theropod, but it is the earliest known theropod, apparently existing just after the two saurischian lineages (theropods and sauropods) split.
|Staurikosaurus or “southern cross lizard” measured about 2 meters and lived in the Mid to Late Triassic of South America. Unlike later dinosaurs, only 2 vertebrae join vertebral column to pelvis at the sacrum (Lambert, 1990).|
| Herrerasaurus was given the name “Herrera lizard” because it
was found by an Andean goat farmer named Victorino Herrera. At first, some concluded from its leg and foot
that it preceded the split in the dinosaur clades but a later skull (found
in 1992) showed that its jaw joint placed it among the theropods. The hip sloped at an angle similar to the ornithischians
but the pubis ended in an expansion (or “boot”) which was typical of later
saurischians. The back portion of
its tail was stiffened (the tail vertebrae possessed processes which were
inter-locking) as in later theropods. It
was larger, measuring 4 meters and lived in the Late Triassic to Early Jurassic
There are some who feel that Herrerasaurus is close to the ancestry of all dinosaurs and should not be included in the Saurischians (Dingus, 1998).
|If Aliwalia is a herrasaur, then some kinds could grow to 11 meters. Other fossils, such as Ischisaurus, may also be basal theropods but they are more likely to be additional specimens of Herrerasaurus. Their sacrum also contains only 2 vertebrae. The basal theropod Frenguellisaurus may have measured up to 5 meters in length (Lambert, 1990; Fastovsky, 1996).|