MAJOR GROUPS OF THEROPODS
Over the last thirty years, there has been a debate as to whether Eoraptor and the Herrerasauria (such as Herrerasaurus and Staurikosaurus) were true theropods or just primitive carnivorous dinosaurs outside the Saurodomorpha-Theropoda clade ("Eusaurischia"). Similarly, the Late Triassic Brazilian dinosaur Guaibasaurus has been interpreted as a possible basal theropod (sharing long slender pubes). Some recent studies place Eoraptor and/or Guaibasaurus as basal sauropodomorphs. Herrerasaurs are typically recoverd as basal theropods, but some studies still indeed place them as outside Eusaurischia. Thus, the basal condition for Theropoda proper is somewhat problematic.
The traits uniting Theropoda seem to include:
Eodromaeus from the Late Triassic Ischigualasto Formation of Argentina is among the most primitive theropods known: in some ways it appears to be the closest model to an ancestral theropod. It is quite small (only a meter or so long) agile minor predator. Slightly more basal (but with their own specializations making them a little less like the ancestral state in terms of morphology) are the Herrerasauria, an exclusively Late Triassic group known from South and North America. These are larger: 3 m or so long, with deeper skulls than Eodromaeus. However, herrerasaurs were not the apex predators of their environments, as larger crurotarsan predators dwarfed (and presumably hunted) them. Little buck-toothed Daemonosaurus of the later Late Triassic of New Mexico is another newly discovered basal theropod.
Late Triassic New Mexican Tawa provides a convincing intermediate between the Neotheropoda ("new theropods", discussed below). It shares with basal neotheropods 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). Also, like those forms, it is a longer-necked, more slender animal than most other basal theropods of the Triassic. (Note: at least some phylogenetic analyses place Tawa within neotheropods as a coelophysoid.)
COELOPHYSOIDEA: BASAL NEOTHEROPODS OF THE TRIASSIC AND EARLY JURASSIC
The remaining theropods (coelophysids, "dilophosaurs", 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. Neotheropods 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 neotheropod clade.
Neotheropods show a number of specializations relative to other saurischians:
There were two major clades of advanced Middle Jurassic and younger neotheropods: 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, and follows very recent work suggesting that "dilophosaurs" are simply a polyphyletic assemblage of primitive crested neotheropods. We'll just call these "basal neotheropods" for now.
The oldest neotheropod known (in fact, currently the oldest known North American dinosaur) is Camposaurus of the middle Late Triassic. Much better known, however, is Coelophysis) in the late Late Triassic. These were mid-sized carnivores (2-4 m long), and representatitves 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. 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; Zupaysaurus of Argentina; and Liliensternus of Europe). The largest known coelophysoid is Early Jurassic double-crested Dilophosaurus the American Southwest. Dracovenator of South Africa may belong to this clade as well.
Some recent studies separated Dracovenator, Dilophosaurus, and possibly Zupaysaurus were not coelophysoids, but were instead closer to averostrans. A subset of these unite these taxa (as well as Sinosaurus and Cryolophosaurus) as the clade Dilophosauridae. The most recent comprehensive studies, however, do not find that position. On the other hand, these same studies show that the support for any particular structure in this part of the tree is not terrible secure, so Dilophosauridae may eventually be supported again.
"Dilophosaurs" (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 crurotarsans that "ruled" the Triassic were extinct. As with averostrans, the "dilophosaur"-grade theropods 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 neotheropods 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 "dilophosaur-grade" forms: such display structures are associated with within-species display behaviors, suggesting that at least on occasion they got together. Some coelophysoids (as well as 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 neotheropods 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".
Tachiraptor is a newly-described theropod from just after the Triassic/Jurassic extinction which appears to the be the sister taxon to 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 "dilophosaur-grade" coelophysoids. 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.
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:
Late Early Jurassic Berberosaurus of northern Africa may be the oldest and most primitive ceratosaur; however, some analyses place it within the "dilophosaur" grade (or clade). One clade of primitive ceratosaurs include some slender, somewhat coelophysoid-like forms such as early Late Jurassic toothless Chinese Limusaurus, Elaphrosaurus of Late Jurassic eastern Africa and western North America, and Spinostropheus of the Middle Jurassic or earliest Late Jurassic northern Africa. These seem to form a clade of "elaphrosaurs" (not yet formally named). Giant (8-10 m long) Deltadromeus of early Late Cretaceous northern Africa has previously been considered to be a member of the advanced ceratosaur group Noasauridae (and before that, a coelurosaur), but is regarded in the most recent studies as one of the last of the "primitive" ceratosaurs and arguably a late-surviving giant "elaphrosaur".
Other 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. Recently discovered, 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 later abelisaurids.
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. As mentioned previously, giant Deltadromeus seems in many studies to be a huge noasaurid.
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. 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 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. Among the most primitive are Early Jurassic Sinosaurus of China, and Cryolophosaurus of Antarctica; both of these have at least occasionally been found to be members of a dilophosaurid clade. However, the most recent analyses put these taxa at the base of Tetanurae. Slightly more derived 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"). One astonishing new discovery is that the orionide clades share a trait once thought to be restricted to coelurosaurs: protofeathers. 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!) 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 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 Eustreptospondylus of Late Jurassic Europe and the Megalosauridae. This last group contains Megalosaurus, Duriavenator, Poekilopleuron, and Dubreuillosaurus of Middle Jurassic Europe; Middle Jurassic Afrovenator of northern Africa; and massive Torvosaurus of Late Jurassic North America and Europe.
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 surpisingly) eastern Africa, known only by its teeth (and thus concievably 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.
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. A brand new (October 2009) study shows 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; and Carcharodontosaurus 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 rivalled the largest Spinosaurus specimens in mass (although the latter was probably longer, given the relatively long snout and neck of spinosaurids). These dinosaurs and the spinosauroids disappear around the same time, for reasons as yet uncertain.
The just-recently recognized Neovenatoridae includes primitive forms such as Neovenator of the Early Cretaceous of Europe and giant Chilantaisaurus of early Late Cretaceous China, as well as the specialized (and often quite slender) Megaraptora. The megaraptorans include forms that were once considered coelurosaurs (and are still considered tyrannosauroids by some researchers), carcharodontosaurids, basal tetanurines, and even ceratosaurs, but the new analyses by Roger Benson and colleagues unite these once-disparate forms. 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 of Argentina. The new discovery of the snout of a juvenile Megaraptor shows that this clade had relatively long and slender skulls.
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:
Gigantism and Miniturization:
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.
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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