Simplified cladogram of Theropoda

More detailed phylogeny of Theropoda

MAJOR GROUPS OF THEROPODS

• Theropoda ("beast feet")
• First appear in the Late Triassic, persist ever since
• Global distribution (all continents)
• Known from diverse environments
• Major clade of carnivorous dinosaurs (although Coelurosauria contains numerous omnivorous & herbivorous clades)
• Remain obligate bipeds throughout their entire history

Herrerasauria and Eoraptor are considered by some paleontologists to be true theropods. However, many recent studies place them as basal saurischians instead of basal theropods, with the similarities between these groups due either to the retention of the primitive meat-eating habit and/or convergence due to their predatory adaptations.

The oldest and most primitive theropod seems to be Guaibasaurus of the Late Triassic of Brazil. Like Eoraptor it had 3 sacral vertebrae; like later theropods it had long slender pubes. New specimens have recently been discovered: hopefully they will reveal what the skull and the clavicles of this dinosaur were like.

NEOTHEROPODA
The unquestioned theropods (coelophysoids, dilophosaurids, ceratosaurs, and tetanurines) form a clade called Neotheropoda ("new theropods"), although some have called this group "Eutheropoda" ("true theropods") and restricted "Neotheropoda" to the group called "Averostra" below. Neotheropods become an important group of predators in the Late Triassic; are the dominant group of terrestrial carnivores throughout the entire Jurassic and Cretaceous; learned how to fly; had some members survive the great extinction; and are still with us today. In this lecture, though, we concentrate on the basal members of the neotheropod clade.

Neotheropods show a number of specializations relative to other saurischians (although some might eventually be recognized in Guaibasaurus):

• An intramandibular joint: a hinge between the tooth-bearing dentary and the postdentary bone, possibly functioning as a shock absorber for holding struggling prey
• The furcula ("wishbone"): clavicles are fused into a single mid-line bone that functioned as a spring/shock absorber (perhaps to deal with stresses on the forelimbs and shoulders from struggling prey)
• Manual digit V lost (so only four fingers or fewer)
• Five or more sacrals
• Pedal digits I and V reduced (making the foot functionally tridactyl ("three toed"): metatarsal V lacks a toe, while metatarsal I is reduced with a small digit I

Basal theropods all have blade-like serrated teeth, suggesting that they were flesh-eaters.

There were two major clades of advanced neotheropods: Ceratosauria and Tetanurae. Additionally, there are two primitive branches, the Coelophysoidea and Dilophosauridae. During the late 20th Century (and some early 21st Century studies), dilophosaurids were considered a type of coelophysoids, and this whole grouping was found to be closer to Ceratosauria than either were to tetanurines; however, this course follows newer analyses that place Ceratosauria and Tetanurae in a clade (Averostra) exclusive of coelophysoids, and follows very recent work suggesting that dilophosaurids were closer to averostrans than to Coelophysoidea proper.

BASAL NEOTHEROPODS: COELOPHYSOIDS AND DILOPHOSAURIDS
The coelophysoids were the first major theropod radiation. They appear (in the form of Coelophysis) in the middle Late Triassic, when they are mid-sized carnivores (2-4 m long). They are the first group of dinosaurs to show up in North America, where their footprints are one of the most common fossils.

Until recently, the coelophysoids and dilophosaurids were thought to form a clade united by a kink between the maxilla and the premaxilla (often with a corresponding large dentary fang below): this was likely a spot to hold onto narrow prey (modern crocodilians with a similar snout pattern do the same). However, more complete sampling of the taxa and characters shows that this is a trait shared by all early neotheropods and lost in averostrans. In other words, "coelophysoids" in the old (1990s) sense of the term were the paraphyletic ancestors of the averostrans.

True coelophysoids tended to be long and slender, with slender skulls. The first coelophysoids were middle-sized (2-4 m long) forms such as Coelophysis (first known from the Late Triassic of the American Southwest, but also found in the Early Jurassic of southern Africa under the synonyms "Syntarsus" (preoccupied by an insect!) and "Megapnosaurus"). However, there were small (~1-2 m long) coelophysoids: Late Triassic Procompsognathus of Europe and Segisaurus of the American Southwest.

Dilophosaurids were larger: 4-6 m long. They are definitely present in the Early Jurassic in the form of Dracovenator of South Africa, Dilophosaurus the American Southwest; "Dilophosaurus" sinensis of China, and Cryolophosaurus of Antarctica. (The latter was long thought to be a primitive tetanurine). As such, dilophosaurids represent the first large dinosaur predators and the first time dinosaurs were the top (apex) predators in their environment, since the big predatory crurotarsans that "ruled" the Triassic were extinct. Dilophosaurids have crests on their skulls, but since some coelophysoids and some other primitive neotheropods have similar crests, these simply might have been the "fashion" for carnivorous dinosaurs of the Triassic and Jurassic.

There are a few other large (4-6 m long) primitive neotheropods of the Late Triassic (Gojirasaurus of the the American Southwest; Zupaysaurus of Argentina) that might be giant coelophysoids, early dilophosaurids, or intermediate between the two.

Dilophosaurids share several derived features with later theropods (averostrans), including:

• A promaxillary fenestra: an extra skull opening between the naris and the antorbital fenestra (although this might be lacking in coelophysoids, and limited to dilophosaurids, ceratosaurs, and tetanurines)
• A lacrimal fenestra (airsac opening into the upper corner of the lacrimal)
• Reduced total number of maxillary teeth
• Modified jaw joint

Although very common in the Late Triassic and Early Jurassic, no coelophysoids or dilophosaurids are known after the end of the Early Jurassic. They seem to have been displaced by the averostrans. Curiously, the range of these primitive neotheropods is very similar to that of "core prosauropods".

AVEROSTRA
The clade comprised of Ceratosauria and Tetanurae has sometimes been called "Neotheropoda"; however, that name has come to mean the more inclusive group that also contains Coelophysoidea and Dilophosauridae. The ceratosaur-tetanurine clade, then, is now Averostra ("bird snouts").

Averostrans can be recognzied by the following transformations:

• Asymmetrical premaxillary teeth (not simple cones)
• Exapanded anterior end of the ilium, with a hook-like blade

The basal members of Ceratosauria and Tetanurae typically have mediolaterally narrow, dorsoventrally deep skulls: sometimes nicknamed "hatchet heads". This skull patterns is good for striking hard against a victim and slicing it up. However, it is not particulary strong if shaken back-and-forth, and so these dinosaurs probably did not hold onto their prey for very long with their jaws.

CERATOSAURIA
The ceratosaurs begin as a minor part of the theropod community, but in the Late Cretaceous dominate most of the world (particularly the southern continents and Europe). Ceratosaurs share the following specialziations:

• Reduced size of manual phalanges (so their had relatively weak, or even functionless, hands)
• 7 or more sacral vertebrae

Some primitive ceratosaurs include some slender, somewhat coelophysoid-like forms such as Elaphrosaurus of Late Jurassic eastern Africa and western North America and Spinostropheus of Early Cretaceous northern Africa. Other early forms, such as the oldest ceratosaur known (late Early Jurassic Berberosaurus of northern Africa) were more solidly built. Until recently 8-10 m long Deltadromeus of early Late Cretaceous northern Africa was thought to be a member of the advanced ceratosaur group Noasauridae (and before that, a coelurosaur), but is no regarded as one of the last of the "primitive" ceratosaurs.

Later ceratosaurs seem to have been relatively short-necked forms. Among these are the Ceratosauridae proper. These are best known from 6-8 m long Ceratosaurus of the Late Jurassic western North America and Europe.

The dominant group of Cretaceous ceratosaurs are grouped into Abelisauroidea. Abelisauroids (characterized by special prongs on their vertebrae and a flange on their femur) include two major divisions. The slender Noasauridae range are relatively small, ranging from (< 1 m long) Ligabueino and Velocisaurus of South America; to 2-3 m long Noasaurus of Late Cretaceous South America and Masiakasaurus of Late Cretaceous Madagascar.

The sister group to Noasauridae is Abelisauridae, a clade that includes the top predators of South America, India, Madagascar, and Europe (and for all we know continental Africa and Australasia/Antarctica) during the Late Cretaceous. The first abelisaurids (such as early Late Cretaceous Rugops) were minor predators compared to their neighbors the spinosaurids and carcharodontosaurids (about whom see below). With the extinction of those two groups, however, the abelisaurids came into their own.

Abelisaurids are further specialized from other abelisauroids by:

• Very rugose (horny) texture of the bones of the face: this supported some sort of softer tissue, but whether keratin or some thickened skin is not ceratin
• Short rounded snouts
• Thickened skull roof
• Squat thickened teeth
• Forearms highly reduced: the ulnae and radii are practically no more than carpals
• Relatively short and stocky hindlimbs

The particulars of their forelimbs show that they were useless in grappling; their tough skulls and stout teeth suggest that they may have used their skulls to hold onto prey with their jaws in order to kill it.

Notable Late Cretaceous abelisaurids include Rajasaurus of India; Majungasaurus (formerly "Majungatholus") of Madagascar; and Abelisaurus, Aucasaurus, and Carnotaurus of South America.

Abelisaurids make it all the way until the end of the Cretaceous. Interestingly, their stratigraphic range and geographic distribution closely matches that of lithostrotian titanosaurs.

TETANURAE
The remaining theropods form the Tetanurae ("stiff tails"). Tetanurines (some prefer the form "tetanurans") are specialized from earlier theropods in possessing:

• Teeth restricted to the front of the jaws
• Proportionately larger hands
• Interlocking tail vertebrae in at least the distal half of the tail: this would help serve as a dynamic stabilizer, helping them to turn faster

Basal tetanurines tended to be large (5-8 m long) hatchet-headed carnivores. There are three major clades within Tetanurae: Spinosauroidea, Carnosauria, and Coelurosauria. Piatnitzkysaurus of Middle Jurassic Argentina was once considered a primitive spinosauroid, but now seems to have diverged before the other tetanurines diversified.

One of the first major clades of tetanurines are the Spinosauroidea. Spinosauroids share elongate skulls. Megalosaurus of the Middle Jurassic of Europe seems to be a spinosauroid (although it is uncertain, since the material of this taxon is actually fairly limited). A series of primitive spinosauroids (once considered a clade termed either "Megalosauridae" or "Torvosauridae") include Eustreptospondylus and Dubreuillosaurus of Late and Middle Jurassic Europe (respectively); massive Torvosaurus of Late Jurassic North America and Europe; and Early Cretaceous Afrovenator of northern Africa. Lourinhanosaurus of the Late Jurassic of Portugal may be a megalosaurid, but it might instead be a primitive carnosaur.

Giant Torvosaurus shares its enormous size and powerfully-developed forelimbs with the remaining spinosauroids. Most distinctive and diverse of these latter dinosaurs are the clade Spinosauridae. The spinosaurids are a group of mid-Cretaceous (late Early and early Late Cretaceous) giant (8-14 m long) theropods characterized by:

• Elongate snouts
• Conical teeth
• Tall dorsal neural spines, in the extreme forming a sail along the back

Additionally, spinosaurids share with at least some of the megalosaurid/torvosaurids an enormously enlarged thumb claw (even by eusaurischian standards).

The adaptations of the crocodile-like spinosaurid jaws and teeth (as well as their gut contents) suggest that they added large fish as well as dinosaurs to their diet. All spinosaurids have been discovered in environments in which large fish are common.

Spinosaurids include Baryonyx of Early Cretaceous Europe; Suchomimus (which may simply be a species of Baryonyx) of the Early Cretaceous of northern Africa; Irritator of the Early Cretaceous of Brazil; and giant (14 m long) Spinosaurus of the early Late Cretaceous of northern Africa. Spinosaurus is one contender for the largest known theropod of all time.

Megaraptor from the Late Cretaceous of Argentina (and a closely related form from the late Early Cretaceous of Australia) seems have been the last of the spinosauroids. Originally thought to be a possible coelurosaur, and popularly (on the Internet, although not in the scientific literature!) considered a giant dromaeosaurid raptor, new specimens show that it was more primitive. It shares some traits with carcharodontosaurid carnosaurs and others with spinosaurids and Torvosaurus. (Ah, for a skull, which might settle this!) Otherwise, the youngest spinosaurid is Spinosaurus, and this clade is long gone before the end of the Cretaceous.

NEOTETANURAE and AVETHEROPODA
The remaining groups of theropods comprise the Neotetanurae (new tetanurines). share the following transformations from the ancestral tetanurine condition:

• Maxillary fenestra: an opening in the maxilla posterior to the promaxillary fenestra but anterior to the antorbital fenestra
• Extremely complex air sac chambers in vertebrae (convergently evolved in abelisaurids and in some sauropods)

Primitive Monolophosaurus of the Middle Jurassic of China was once considered the oldest and most basal carnosaurs; new analyses show that it is a more primitive neotetanurine and might be the sister taxon to the carnosaur-coelurosaur clade Avetheropoda ("bird theropods"). Avetheropods have lost of manual digit IV making the hand (like the foot) functionally tridactyl. Avetheropods generally fall in two clades: Carnosauria and Coelurosauria. The latter are so diverse we'll spend two separate lectures on them.

Note: in pre-1990s literature, these terms were often used as synonyms for "big theropod" and "little theropods", respectively. So larger coelophysoids, ceratosaurs, spinosauroids, and tyrannosaurid coelurosaurs were considered "carnosaurs", while small coelophysoids were included with the "coelurosaurs". Since the rise of cladistic studies, however, these names are restricted to two branches of the derived tetanurines.

CARNOSAURIA
The dominant group of large theropods from the Middle Jurassic through the Early Cretaceous. Carnosaurs are best known in the form of Late Jurassic North American and European Allosaurus. Carnosaurs are characterized by:

• Extra openings on the maxillae and nasals
• Very large naris size
• And some other cranial, vertebral, and other features

(However, some of these traits are also shared with Monolophosaurus, and are more likely neotetanurine rather than carnosaur traits!)

Definite carnosaurs include the Sinraptoridae of the Middle and Late Jurassic of China (such as Sinraptor and Yangchuanosaurus) and possibly of the Middle Jurassic of Europe; and the Allosauridae of Late Jurassic North America and Europe, including Allosaurus and giant 13 m long Saurophaganax (largest known Jurassic theropod).

The Cretaceous carnosaurs mainly consist of the clade Carcharodontosauridae. Neovenator of the Early Cretaceous of Europe is a primitive form of this group; more derived is giant (12-13 m long) Acrocanthosaurus of the later Early Cretaceous of North America. The most specialized carcharodontosaurids are those of the late Early Cretaceous and early Late Cretaceous of South America and Africa. These include Tyrannotitan, Giganotosaurus, and Mapusaurus of the late Early Cretaceous of South America and Carcharodontosaurus of the early Late Cretaceous of Africa. These are among the largest theropods known: in particular, Mapusaurus and Giganotosaurus exceed Tyrannosaurus rex in size, and rivalled the largest Spinosaurus specimens in mass (although the latter was probably longer, given the relatively long snout and neck of spinosaurids).

There is an unnamed carcharodontosaurid from the middle Late Cretaceous of South America, but as far as we know this group (and Carnosauria as a whole) was extinct well before the end of the Cretaceous. Their role as top predators were replaced by abelisaurids in much of the world, and by tyrannosaurids in North America and Asia.

EVOLUTIONARY PATTERNS IN BASAL THEROPODA
Feeding adaptation transformations:
Like the Jabberwock, the theropod predatory armament consisted of "jaws that bite" and "claws that catch". Each of these were modified in different ways among the Theropoda:

• The ancestral theropods (and herrerasaurs) combined blade-like serrated teeth (ancestral for Archosauria) and grasping hands (a dinosaur shared derived character) with the intramandibular joint and powerful claws. In addition, neotheropods at least had the furcula as a possible aid for holding onto prey.
• Coelophysoids and dilophosaurids retained these ancestral traits and evolved the premaxillary-maxillary kink to hold onto victims
• Ceratosaurs reduced their manual grasp to concentrate on jaw-based attacks.
  • This is taken to an extreme in the stumpy armed rounded skulled abelisaurids.
• Tetanurines, in constrast, increased the size of the hands and the power of the arms.
• Within the tetanurines, the spinosaurids evolved numerous additional adaptations associated with the ability to capture large fish.

Locomotory adaptations:

• In general basal theropods retain the ancestral dinosaurian striding habit
• Overall, theropods tended to be more slender than similar-sized herbivorous dinosaurs, and consequently were most likely faster and/or more agile
• This agility was aided in the tetanurines by evolution of the tighly-locked distal tail as a dynamic stabilizer
• In some ceratosaurs (Elaphrosaurus and some noasaurids) there is evolution of cursorial (fast-running) limb proportions: this evolved many times independantly in coelurosaurs
• In contrast, the limbs of at least some abelisaurids are proportionately short and stocky, suggesting that they were slower than other theropods of their body size. However, if sauropods were their main prey, they might not need to be particularly fast.

Gigantism and Miniturization:

• Early theropods (e.g., Guaibasaurus, coelophysoids) are in the 2-4 m range, and so not much larger than the first dinosaurs
• Dilophosaurids and basal averostrans grew larger: about 6-7 m long (after the extinction of the crurotarsan predators)
• In each of the averostran groups (ceratosaurids, abelisaurids, advanced spinosauroids, carnosaurs) there are examples of gigantism well beyond this 6-7 m size
• Miniturization also happened, however: in some coelophysoids and in noasaurids
• Both patterns (gigantism and miniturization) are also seen in coelurosaurs

Niche partitioning:
In many environments several different theropods shared the same habitat. In some cases they may have partitioned the resources by body size (although the juveniles would still overlap). But in the case of the spinosaurids there seems to have been evolution of the ability to access meat that other theropods couldn't: fish. Similarly, spinosaurids could travel more easily from lake to lake and also capture food more easily on land than the giant crocodyliforms that were their main competitor for fish.

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Last modified: 14 August 2008