Key Points:
•Theropods include the majority of carnivorous dinosaurs (although various subgroups evolved towards herbivory, omnivory, insectivory, etc.). They remained entirely obligate bipeds.
•Among the diagnostic characteristics are an intramandibular joint, a promaxillary fenestra, a furcula (wish bone), and a functionally tridactyl pes.
•Early theropods (coelophysoids) were only minor predators in their ecosystem; however, the Triassic-Jurassic extinction eliminated their competition, and from beginning of the Jurassic until the end of the Cretaceous theropods were the dominant group of terrestrial predators.
•Some major groups include: primitive Coelophysidae and Dilophosauridae; Ceratosauria, with reduced hand function; Megalosauroidea, including the fish-eating Spinosauridae; Carnosauria, which dominated the apex predator niche for most communities from the Middle Jurassic until the early Late Cretaceous; and the hugely diverse Coelurosauria.
•Coelurosaurs are the most diverse clade of theropods (or dinosaurs, for that matter). Coelurosaurs began as small-bodied agile hunters, but quickly diversified into many distinct subgroups. All groups are known to have some form of fuzzy or feathery body covering.
•Tyrannosauroids were a clade with enhanced bite and specialized teeth. Initially mid-sized predators, they culminated in the gigantic, short-armed, two-fingered, long legged, bone-crushing Tyrannosauridae.
•More derived forms typically had smaller skulls, long necks, and phyllodont dentition, showing a shift away from flesh-eating.
•Ornithomimosauria had long arms with clamping hands. This clade includes toothless forms, such as the large-to-gigantic Deinocheiridae and the slender-footed Ornithomimidae.
MAJOR GROUPS OF THEROPODS
BASAL THEROPODS
The traits uniting Theropoda seem to include:
As discussed before, there are number of Triassic dinosaurs which might or might not belong to Theropoda: herrerasaurs, Eodromaeus, Tawa, Chindesaurus, and others. The remaining (definite) theropod taxa (coelophysids, dilophosaurids, ceratosaurs, and tetanurines) form a clade called Neotheropoda, although some have called this group "Eutheropoda" ("true theropods") and restricted "Neotheropoda" to the group called "Averostra" below. Theropods are present 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 theropod clade.
Exclusive of the Triassic basal saurischians that might be theropods (which have some of the traits mentioned above), definite theropods ("neotheropods") show a number of specializations relative to other saurischians:
COELOPHYSOIDEA & DILOPHOSAURIDAE: BASAL NEOTHEROPODS OF THE TRIASSIC AND EARLY JURASSIC
There were two major clades of advanced Middle Jurassic and younger theropods: Ceratosauria and Tetanurae. Additionally, there are various primitive branches of the Late Triassic and Early Cretaceous. Some studies put these into two main clusters: Coelophysidae and Dilophosauridae. During the late 20th Century (and some early 21st Century studies), dilophosaurids, coelophysids, and the intermediate forms were collectively considered a clade "Coelophysoidea", 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 Coelophysoidea, with Dilophosauridae (and some other taxa) as intermediate between coelophysoids and avetrostrans. Collectively, we'll call celophysoids, dilophosaurids, and other non-averostran neotheropods "basal neotheropods" for now.
The oldest theropod known (in fact, currently the oldest known North American dinosaur) is Camposaurus of the middle Late Triassic. Much better known, however, is Coelophysis of the late Late Triassic. These were mid-sized carnivores (2-4 m long), and representatives of the true coelophysIDs (Coelophysidae): a clade characterized by long and slender bodies, with slender skulls. The best studied coelophysids are Coelophysis and Early Jurassic southern African Megapnosaurus (formerly called "Syntarsus", but that name is preoccupied by an insect!). (Note, some regard these as the same genus, with "Megapnosaurus" simply late-surviving species of Coelophysis). Camposaurus may be a close relative of Megapnosaurus. Other possible coelophysids were small (~1-2 m long) Late Triassic Procompsognathus of Europe and similar-sized Early Jurassic Segisaurus of the American Southwest and 3 m long Panguraptor of Early Jurassic Asia. Coelophysid footprints are some of the most common trace fossils of the terrestrial Triassic.
Coelophysids seem to be united with a set of larger Late Triassic and Early Jurassic theropods, collectively the Coelophysoidea. These larger coelophysoids include (4-6 m long) primitive theropods of the Late Triassic (Gojirasaurus of the the American Southwest and Liliensternus of Europe). Zupaysaurus of Late Triassic Argentina might be a coelophysoid, or may be intermediate between them and the Dilophosauridae. Another, slightly later Late Triassic Swiss transitional theropod is Notatesseraeraptor.
Long considered the largest coelophysoid is Early Jurassic double-crested Dilophosaurus the American Southwest. However, other analyses place it, Dracovenator of South Africa, Sinosaurus of Asia, and Cryolophosaurus) of Antarctica as the clade Dilophosauridae. Shuangbaisaurus of Early Jurassic China is another possible dilophosaurids. Other studies suggest some or all of these are coelophysoids or a paraphyletic grade running from Coelophysoidea up into the base of Tetanurae. In this class we'll take the simple solution of a monophyletic Dilophosauridae, recognizing that this remains a problematic part of the tree.
Dilophosaurids (either as a clade or grade) represent the first large dinosaur predators and the first time dinosaurs were the top (apex) predators in their environment, since the big predatory pseudosuchians that "ruled" the Triassic were extinct. As with averostrans, the dilophosaurids have a reduced total number of maxillary teeth. This seems to represent an ecological change from being minor predators feeding on small animals to being predators on other big dinosaurs.
There is evidence that in basal theropods that there was significant sexual dimorphism. The bones of some members of the population were generally more robust, and had more pronounced muscle attachments, while others of the same length were more gracile. Based on observations of modern predatory birds, the robust forms are typically interpreted as female, but this is not certain. Additionally, in Coelophysis and Megapnosaurus there are sites where dozens or hundreds of individuals were found dead together, strongly implying that they were at least on occasion gregarious. This is further supported by the presence of display structures in the dilophosaurids and some basal tetanurines: such display structures are associated with within-species display behaviors, suggesting that at least on occasion they got together. Some coelophysoids, dilophosaurids, basal tetanurines, and even basal coelurosaurs had crests on their skull: these probably served as display structures.
Although very common in the Late Triassic and Early Jurassic, no basal theropods are known after the end of the Early Jurassic. They seem to have been displaced by the averostrans. Curiously, the range of these primitive theropods is very similar to that of "core prosauropods".
Tachiraptor is a newly-described theropod from just after the Triassic/Jurassic extinction which appears to the be the sister taxon to Averostra.
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 Coelophysidae and Dilophosauridae. The ceratosaur-tetanurine clade, then, is now Averostra ("bird snouts").
Averostrans can be recognized by the following transformations:
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 particularly strong if shaken back-and-forth, and so these dinosaurs probably did not hold onto their prey for very long with their jaws. This made primitive averostrans "bite-and-slice" feeders: they could carve chunks out of victims, or wound them, but could not hold onto them with their jaws. (We will see later examples of averostrans that evolved alternative forms of feeding.)
Many basal averostrans run in the 6-8 m range, like the dilophosaurids. However, these more derived taxa are typically more robustly built, and likely tackled bigger prey. Indeed, their rise coincides with the rise of more advanced and larger herbivorous dinosaurs (thyreophorans, iguanodontians, eusauropods), and the more powerful build of averostrans may be a co-evolutionary "arms race" with the new-style herbivores.
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 specializations:
Late Early Jurassic Berberosaurus of northern Africa is likely one the oldest and most primitive ceratosaur; however, some analyses place it within the "dilophosaur" grade (or clade). Unfortunately the specimen is too fragmentary to get much sense of the proportions of this dinosaur. Even older is early Early Jurassic Saltriovenator. Also only partially known, it is among the oldest large-bodied theropod. Interestingly, the manual digits of Saltriovenator are not reduced as in more derived ceratosaurs (and indeed was long considered the oldest tetanurine before the detailed analysis of this specimen.)
Most of the ceratosaurs belong to the clade Neoceratosauria. These include the larger ceratosaurs, and are typically 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.
Middle Jurassic Eoabelisaurus of Argentina was first thought to be a true abelisaurid. However, it appears to be in the same general region of the phylogeny as the ceratosaurids. Its arms are not as strongly reduced as the later true abelisaurids.
The most successful group of ceratosaurs is the Abelisauroidea, 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 Abelisauridae, as well as the smaller Noasauridae.
Early abelisaurids such as (such as early Late Cretaceous Rugops) and Kryptops of early Late Cretaceous northern Africa 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:
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, Skorpiovenator, 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.
The remaining group of ceratosaurs (the latest analyses put them alterantively as the sister taxon to Neoceratosauria, to Abelisauridae [as shown here], or even as a paraphyletic grade outside Abelisauridae) is the Noasauridae. Noasaurids are generally small-to-medium sized slender dinosaurs. There are two major subdivisions. The Jurassic clade Elaphrosaurinae includes somewhat coelophysoid-like forms such as early Late Jurassic toothless Chinese Limusaurus, 6 m long Elaphrosaurus of Late Jurassic eastern Africa, and similar unnamed forms from the same age in Africa and western North America. The smaller Noasaurinae range 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. Newly described Vespersaurus of Late Cretaceous Brazil was a desert-dwelling noasaurine: it is described as being functionally monodactyl (using only its pedal digit III to support its weight), with digits II and IV raised as cat-like retractable claws. Early Cretaceous Afromimus from Niger was originally interpreted as an ornithomimosaur, but re-analysis shows that it is a noasaurine.
(At least a few phylogenetic analyses had Elaphrosaurinae as not being close to Noasaurinae, with a branching order of Elaphrosaurinae, Ceratosauridae, Abelisauridae, and Noasaurinae. This pattern is actually more congruent with the stratigraphic distribution of these clades, so keep your eye out in future studies to see if this is confirmed.)
The Bizarre Bahariasaurids: A particularly problematic group of theropods is the "Bahariasauridae", a group exclusively known so far from the early Late Cretaceous of Argentina and Africa. The most completely known are Argentine Gualicho and African Deltadromeus. They show that these are medium-sized slender long-legged theropods. The arms are greatly reduced, and (in the case of Gualicho at least) end in only two small fingers (convergent with tyrannosaurid coelurosaurs). Argentine Aoniraptor may be the same species as Gualicho; similarly, Bahariasaurus may be the same species as Deltadromeus. Bahariasaurus shows that these dinosaurs reached extremely large size, as it is almost as large as Giganotosaurus or Tyrannosaurus! Bahariasaurids show a mixture of different traits, making them extremely difficult to pin down phylogenetically. They might be gigantic noasaurids (possibly derived elaphrosaurines); they might be related to the neovenatorid allosauroids; or they might be basal coelurosaurs. They may be part of the similarly mysterious Megaraptora. At present we don't have any skull bones from them, so we know nothing of significance about their feeding ecology: were they armed with sharp teeth? Did they have toothless beaks? They are one of the big mysteries of theropod paleontology at the moment.
TETANURAE
The remaining theropods form the Tetanurae ("stiff tails"). Tetanurines (some prefer the form "tetanurans") are specialized from earlier theropods in possessing:
Basal tetanurines tended to be large (5-8 m long) hatchet-headed carnivores. Some analyses place Early Jurassic Sinosaurus of China and Cryolophosaurus at the base of Tetanurae. A basal tetanurine is Middle Jurassic Monolophosaurus of the Middle Jurassic of China (once considered one of the oldest and most basal carnosaurs, or alternatively as a primitive megalosauroid). Note that all of these have some form of crest on the head: this was apparently the "fashion" for Early and Middle Jurassic big theropods.
The three major clades within Tetanurae (Megalosauroidea (also known as Spinosauroidea), Carnosauria, and Coelurosauria) are united into the clade Orionides ("hunters"). Both megalosauroids and coelurosaurs are confirmed to have protofeathers (not yet demonstrated in carnosaurs). Protofeathers are simple, apparently hollow, down-like tufts on the body. They represent the evolutionary precursors to true feathers. In this primitive state, may have helped to insulate; for display; for brooding; or some other function. Note that if these do prove to be homologous with the fuzz of the heterodontosaurid Tianyulong than protofeathers would be shared derived features of Dinosauria (at least!) in the Saurischia model, or Ornithoscelida if that model is correct, and not just Orionides. At present, though, the lack of positive evidence of any such structure in non-tetanurine theropods or in sauropodomorphs means that this is not the simplest explanation.
Until 2012 the only definite protofeathers known in theropods were in primitive coelurosaurs. However, the discovery of Sciurumimus (a possible megalosauroid known only from a juvenile specimen) shows that at least small megalosauroids were fuzzy. (It is true that some think that Sciurumimus may turn out to be a primitive coelurosaur rather than a megalosaur.)
It is not yet certain if the protofeathers found in Sciurumimus are all simple strands or tufts (plumulose, or downy, feathers) or if some might not have a central shaft (pennaceous feathers).
One of the first major clades of tetanurines are the Megalosauroidea (often also known as Spinosauroidea). Megalosauroids share elongate skulls. Primitive megalosauroids include the Piatnitzkysauridae (such as Piatnitzkysaurus of Middle Jurassic Argentina and Marshosaurus of the Late Jurassic of the western US). More specialized were the Megalosauridae. This group contains Megalosaurus, Duriavenator, Poekilopleuron, and Dubreuillosaurus of Middle Jurassic Europe; Middle Jurassic Afrovenator of northern Africa; Eustreptospondylus of Late Jurassic Europe; and massive Wiehenvenator of Middle Jurassic Germany and its even larger close relative Torvosaurus of Late Jurassic North America and Europe. One trait that unites the megalosauroids is that the maxillary fenestra becomes a fossa (that is, the sinus no longer punches all the way through the maxilla bone, but forms a 'divot' on the lateral surface).
Giant Torvosaurus shares its enormous size and powerfully-developed forelimbs with the Spinosauridae. The spinosaurids are a group of Late Jurassic to mid-Late Cretaceous giant (8-14 m long) theropods characterized by:
Additionally, spinosaurids share with at least some of the megalosaurids an enormously enlarged thumb claw (even by saurischian 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, and chemical analyses of their bones show that they ate substantial amounts of food from the water. All spinosaurids have been discovered in environments in which large fish are common.
The oldest known spinosaurid is Ostafrikasaurus of the Late Jurassic of (not surprisingly) eastern Africa, known only by its teeth (and thus conceivably NOT a spinosaurid). More complete 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; Ichthyovenator of Early Cretaceous Thailand; 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. It was also the youngest named spinosaurid (and indeed megalosauroid), although limited spinosaurid material is known from middle Late Cretaceous of China. However, none are yet known from the later Late Cretaceous, and thus this large clade is long gone before the end of the Cretaceous. Curiously, although this clade has been found on nearly every continent, at present there are no known North American spinosaurids.
New skeletal material shows that Spinosaurus was even more bizarre than previously thought. The hind limbs (at least) were solid (in other theropods, even giants, they are hollow). The hind limbs and pelves are proportionately shorter than expected in a dinosaur of this size. As a consequence, the center of mass is forward of the hips. This suggests that perhaps Spinosaurus may have been aquatic: more like a crocodile than a heron in terms of the way it approached food. If these discoveries are correct, it may have actually spent relatively little time on land. (By the way, the super-large forelimbs in these reconstruction is almost certainly wrong, as the humerus used to scale the rest of the arm is almost certainly that of the diplodocoid sauropod Rebbachisaurus, and not Spinosaurus at all! Thus, claims that Spinosaurus was a quadruped are not yet supported.)
NEOTETANURAE and AVETHEROPODA
The remaining groups of theropods comprise the Avetheropoda ("bird theropods") (sometimes "Neotetanurae", or "new tetanurines"). Avetheropods share the following transformations from the ancestral tetanurine condition:
Note: in pre-1990s literature, these terms were often used as synonyms for "big theropod" and "little theropods", respectively. So larger coelophysoids, ceratosaurids, abelisaurids, spinosauroids, and tyrannosaurid coelurosaurs were considered by many to be "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, although a few persist until the middle part of the Late Cretaceous. Carnosaurs (sometimes called Allosauroidea) are best known in the form of Late Jurassic North American and European Allosaurus. Carnosaurs are characterized by:
(However, some of these traits are also shared with Monolophosaurus, and may actually be basal tetanurine rather than carnosaur traits!)
One primitive clade of carnosaurs the Metriacanthosauridae (formerly Sinraptoridae) of the Middle Jurassic to Early Cretaceous of Asia (such as Jurassic Sinraptor and Yangchuanosaurus of China and Cretaceous Siamotyrannus of Thailand) and Europe (such as Metriacanthosaurus of early Late Jurassic England, and possibly Lourinhanosaurus of the Late Jurassic of Portugal (which some studies suggest is a basal coelurosaur). Larger are 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 Carcharodontosauria. Most studies show that the carcharodontosaurs contain two major groups: the more massive, powerfully built Carcharodontosauridae and the Neovenatoridae. (A Late Jurassic genus Veterupristisaurus from the Late Jurassic of eastern Africa may be an early primitive carcharodontosaur, or even cacharodontosaurid.) Primitive carcharodontosaurids include giant (12-13 m long) Acrocanthosaurus of the later Early Cretaceous of North America, Concavenator of Early Cretaceous Spain; Eocarcharia of the late Early Cretaceous of northern Africa and Shaochilong of China. 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; andCarcharodontosaurus of the early Late Cretaceous of Africa (youngest of the carcharodontosaurids proper.) Carcharodontosaurids may have been sauropod-eating specialists, and indeed many co-occur with particularly large titanosaur or brachiosaurid sauropods. Carcharodontosaurids are among the largest theropods known: in particular, Mapusaurus and Giganotosaurus just about equal the largest individuals of Tyrannosaurus rex in size, and rivaled the largest Spinosaurus specimens in mass (although the latter was probably longer, given the relatively long snout and neck of spinosaurids). It seems that these dinosaurs and the spinosauroids disappear around the same time, for reasons as yet uncertain. (Some recently described teeth and jaws from the end of Cretaceous in Brazil were initially thought to be from carcharodontosaurids, but new study shows these are more likely abelisaurid fossils.)
The just-recently recognized Neovenatoridae includes forms such as Neovenator of the Early Cretaceous of Europe and giant Chilantaisaurus of early Late Cretaceous China. At least some models include the Megaraptora (see next lecture) in this clade.
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:
Locomotory adaptations:
Gigantism and Miniaturization:
Co-Evolution:
Increased mass and "fire power" of basal averostrans occurs in time and space with the rise of advanced larger herbivorous dinosaurs: ankylosaurs, stegosaurs, iguanodontians, eusauropods. It may be that there was an co-evolutionary arms race between predators and prey: new types of offensive weaponry in the former, new types of defenses (increased body armor, social behaviors, and size) in the latter.
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.
MAJOR GROUPS OF COELUROSAURS
Coelurosaurs are the sister group to Carnosauria within the avetheropods. The coelurosaurs differ from other theropods by possessing:
Additionally, where known, all coelurosaurs preserved in the appropriate-style sediment show at least some protofeathers or true feathers. However, as discussed previously, the discovery of the megalosauroid Sciurumimus pulls the origin of protofeathers much deeper into theropod (or dinosaur, or ornithodiran) history. At least some of the protofeathers of primitive coelurosaurs seem to be pennaceous (having a central shaft), rather than simply plumulose tufts.
Among the most primitive and oldest known coelurosaurs are the basal tyrannosauroids Proceratosaurus of the Middle Jurassic of England and Kileskus of Russia. Only the skull of the former, and skull, hand, and foot bones of the latter, are known at present. However, the most primitive known coelurosaur is actual a relatively late one: Bicentenaria of the mid-Cretaceous of Argentina. It shares with basal tyrannosauroids and basal maniraptoriforms the same general body plan: relatively small (2-4 m) slender animals with skulls full of small ziphodont teeth. Their narrow grasping hands suggest they adapted to catching small prey; their light build, slender limbs, and narrow dynamic stabilizing tail suggests relatively agile animals (useful both in chasing prey and in avoiding predators). Additional basal coelurosaurs include the late Middle Jurassic Aorun and the early Late Jurassic Zuolong of China.
Recent studies find the Late Jurassic western North American Coeluridae (Coelurus and Tanycolagreus) are basal members of Tyrannosauroidea and their neighbor Ornitholestes is a basal member of Maniraptoriformes. However, these do not show the derived features of their relative clades which will be discussed below (their position based on other skeletal traits not discussed in this course), so we won't address them there.
The megaraptorans fall out as basal coelurosaurs in some analyses, and even as tyrannosauroids in others.
Another important group of small primitive theropods is the Compsognathidae. This group ranges from the 1 m long Compsognathus of the Late Jurassic of Europe and Sinosauropteryx of the Early Cretaceous of China to 1.75 m long Huaxiagnathus of the Early Cretaceous of China to the "giant" Sinocalliopteryx of the Early Cretaceous of China at 2.5 m long. Compsognathids are also known from Early Cretaceous Europe and South America, and represented a minor radiation of small-bodied dinosaurs. Gut contents show that they ate lizards and small mammals. Being primitive and of generalized form, these dinosaurs show up in the phylogeny sometimes as basal coelurosaurs outside Tyrannoraptora (the tyrannosauroid-maniraptoriform clade); sometimes as basal maniraptoriforms (as shown here); and sometimes as basal maniraptorans.
TYRANNOSAUROIDEA
The most long-lived and ecologically significant group of primitive coelurosaurs was Tyrannosauroidea, the tyrant dinosaurs. Best known from the later Late Cretaceous Asia and North American Tyrannosauridae, recent discoveries reveal a long history of tyrant dinosaurs going back into the Middle Jurassic.
Basal tyrannosauroid specializations include:
The oldest and most primitive (other than the coelurids) are the Proceratosauridae. The oldest known are Middle Jurassic English Proceratosaurus and equally old Siberian Kileskus. Slightly younger (and far more completely known) is Guanlong of the early Late Jurassic of China: a 3 m or longer crested proceratosaurid. Like other early coelurosaurs, the arms were fairly long. Long arms with tridactyl manus were likely found in all proceratosaurids, but we do not yet have the arms for Proceratosaurus, Kileskus, or Early Cretaceous (and possibly 8 m long!) Sinotyrannus, is one of the youngest known proceratosaurids. Of similar age (and possibly the same genus) is Yutyrannus of Early Cretaceous of China. Known from three nearly-complete fossils, this is a 9 m long predator. It still retained the tridactyl manus with long claws of typical tetanurines. Because they were preserved in fine-grained ash-based sediments, the remains of long protofeathers were found over its body, indicating that even giant theropods had a fuzzy coat!
Dilong of the Early Cretaceous of China represents the next phase of tyrannosauroid evolution, the Pantyrannosauria. It was the first tyrannosauroid found with protofeathers. At 1.5 m length, it still indicated that some basal tyrannosauroids were small members of the predatory community.
More robustly-built are the Stokesosauridae, including Jurassic tyrannosauroids such as Stokesaurus of North America, and Juratyrant (formerly considered a species of Stokesosaurus) of Europe, and Aviatyrannis of both. A later stokesosaurid is Early Cretaceous Eotyrannus of Europe, with an adult size of possibly 4.5 m or more. It was dwarfed by other theropods in its community: the carnosaur Neovenator and the spinosaurid Baryonyx.
Even larger is 6 m or more longer Dryptosaurus, a late-suriving primitive tyrannosauroid of eastern North America. The arms of Eotyrannus are primitively long; in Dryptosaurus the arm is very short but has a very large claw. It is confirmed as having an arctometatarsus. As with more derived tyrannosauroids, the distal hindlimbs (tibia, metatarsi) are elongated: an indication of cursorial (running) ability. Similar but older is Xiongguanlong of late Early Cretaceous China and Timurlengia of early Late Cretaceous Uzbekistan.
Two small-bodied tyrannosauroids with arctometatarsi are known from the early Late Cretaceous of western North America: incompletely known Moros and more completely known Suskityrannus. Both lived in environments when other forms of theropods were the dominant apex predators.
The more derived Eutyrannosauria increase size again with the 6 m or longer Appalachiosaurus of the Late Cretaceous of eastern North America and deep-skulled Bistahieversor of the Late Cretaceous of the American Southwest, and again with the Tyrannosauridae proper.
TYRANNOSAURIDAE
Tyrannosaurids proper are one of the last groups of large bodied theropods to evolve, showing up only in the last 20 million years or so of the Late Cretaceous of North America and Asia. (Consequently they have a similar range distribution to coronosaur ceratopsians, pachycephalosaurs, corythosaur-line lambeosaurines, and club-tailed ankylosaurines). Although for most of their history tyrannosauroids were minor predators in their habitats, tyrannosaurids were by far the largest flesh-eaters in their environments. Small tyrannosaurids were about 8 m long; most reached at least 10 m; and at least one genus reached 13 m.
Tyrannosaurids were specialized relative to their ancestors by possessing:
Tyrannosaurids include the relatively slender Albertosaurus and Gorgosaurus of western North America; slender long-snouted Alioramus and Qianzhousaurus of Asia; and more heavily built Lythronax, Daspletosaurus, and Teratophoneus of western North America and Tarbosaurus and Zhuchengtyrannus of Asia; and giant 13 m long, 8-10 ton Tyrannosaurus of western North America. Tyrannosaurus rivals the biggest carcharodontosaurs and spinosaurids in mass.
Tyrannosaurids seem to have relied solely on their jaws to kill their food. Their long legs meant that they were faster than their potential prey (hadrosaurids, ceratopsids), although adults of the 2 ton or greater size range may not have been fast runners. (Juvenile tyrannosaurids, though, would have been among the fastest dinosaurs). At least some tyrannosaurids have been found in groups of different ages: possibly family associations.
The Mysterious Megaraptorans: A newly-discovered clade of specialized (and often quite slender) theropods is Megaraptora. Their phylogenetic position is extremely difficult to pin down: initially considered megalosauroids close to spinosaurids, they were then found as carcharodontosaurians (specially neovenatorids), but are now best placed as either primitive coelurosaurs in general OR tyrannosauroids closer to tyrannosaurids than are proceratosaurids. Megaraptorans get their name from Megaraptor from the Late Cretaceous of Argentina (originally thought to be a possible coelurosaur, and popularly (on the Internet, although not in the scientific literature!) considered a giant dromaeosaurid raptor). Others include Aerosteon of the mid-Late Cretaceous of South America; Fukuiraptor of Early Cretaceous Japan; Australovenator of the late Early Cretaceous of Australia; and mid-Late Cretaceous Orkoraptor and Murusraptor of Argentina. The new discovery of the snout of a juvenile Megaraptor shows that this clade had relatively long and slender skulls.
MANIRAPTORIFORMES: Plant-Eating "Carnivorous" Dinosaurs
The remaining coelurosaurs (Maniraptoriformes) all have brains that are twice again as large or larger (based on skull size) as the more basal coelurosaurs. They also share a suite of unusual features that strongly suggest a move away from the strictly carnivorous diet of their ancestors and relatives. In particular, they typically:
A new discovery of broad pennaceous feathers (i.e., feathers with a shaft, branches off of that, subbranches off of that, etc.) in ornithomimosaurs seem to place this trait at the same part of the tree as this shift to non-carnivory. However, at present they are only known on the arms of ornithomimosaurs: the spread to other parts of the body seems to be further up the tree.
ORNITHOMIMOSAURIA
With the possible exceptions of Ornitholestes and Compsognathidae, the basalmost lineage of the maniraptoriforms are the Ornithomimosauria, the ostrich dinosaurs. Ornithomimosaurs differ from the ancestral state by:
Their adaptations suggest a move away from predation towards a more omnivorous or even herbivorous lifestyle.
Primitive ornithomimosaurs are known from the Early Cretaceous of Africa (Nqwebasaurus: currently the oldest and most primitive known ornithomimosaur, and the only one from the Southern Continents), Europe (Pelecanimimus), and Asia (Harpymimus, Shenzhousaurus, and Hexing). The remaining ornithomimosaurs fall into two clades: Deinocheiridae and Ornithomimidae. Both of these are dinosaurs larger than the basal members, and possess toothless beaks.
Deinocheirids are currently only known from Asia. They include large of the Early Cretaceous Beishanlong), Late Cretaceous of Asia (Garudimimus, and truly gigantic Deinocheirus, long known only from its arms and a few isolated bones (but new discoveries give us a better sense of the animal), is a Tyrannosaurus-sized primitive ornithomimosaur (lacking an arctometatarsus). The new discoveries show that Deinocheirus had a spinosaur-like sail over the hip region, an expanded blunt snout, a deep jaw, highly reduced supratemporal fenestrae, and oddly blunt toes. Deinocheirids retain the ancestral limb proportions of most theropods. The belly contents of Deinocheirus include numerous fish bones and scales, so it was probably omnivorous. At 6.4 tonnes, it was among the very largest theropods.
The Ornithomimidae did not produce any forms this large (although the largest rival Beishanlong). At present they are only known from the Late Cretaceous of Asia and North America. Ornithomimids are characterized by arctometatarsus (convergently evolved with Tyrannosauridae). Early Cretaceous Kinnareemimus of Thailand shows an incipient arctometatarsus form and may turn out to be the basalmost ornithomimid. Otherwise, the lodes and most primitive is Sinornithomimus of the early Late Cretaceous of China. Other ornithomimids include western North American Struthiomimus, Dromiceiomimus, and Ornithomimus, and Asian Gallimimus and Anserimimus. These dinosaurs were among the most cursorial of all theropods.
At least some ornithomimosaurs lived in herds/flocks. Recent discoveries reveal that adult ornithomimosaurs had pennaceous feathers on the arms and tail, but juveniles do not seem to have this.
EVOLUTIONARY PATTERNS IN BASAL COELUROSAURS
Feeding adaptation transformations:
Locomotory adaptations:
Gigantism:
Niche partitioning:
Basal coelurosaurs represented the minor predators of many Jurassic and Early Cretaceous environments. Diversification into non-predatory modes allowed coelurosaurs to diversify into niches previously unoccupied by theropods. (In Late Cretaceous Asia, small non-predatory coelurosaurs are very common, while small ornithopods are absent.) In contrast, tyrannosauroids evolve into giant top predators in the Late Cretaceous of Asia and North America after the disappearance of carcharodontosaurids and spinosaurids.
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