Key Points:
•Sauropodomorpha is a clade of herbivorous dinosaurs characterized by small skulls, long necks, and (at least ancestry) leaf-shaped teeth.
•The Triassic and Early Jurassic taxa--basal sauropodomorphs, core prosauropods, and near-sauropods--represent a transitional series starting as meter-long obligate bipeds to 10 meter long obligate quadrupeds. These include the only successful Triassic dinosaurs.
•The remaining taxa--all obligate quadrupeds--form the clade Sauropoda. Sauropods were among the most successful of all dinosaur groups, and were a major part of the herbivorous niches in every region from the Middle Jurassic until the end of the Cretaceous. Sauropods include the largest animals ever to live on land.
•Various primitive sauropod branches lie outside the derived Neosauropoda. The latter include the long-skulled, pencil-toothed Diplodocoidea (divided into the enormous Diplodocidae, the tall-spined Dicraeosauridae, the gnawing Rebbachisauridae) and the large-nosed Macronaria (including the upwardly-built Brachiosauridae and the extraordinarily diverse Titanosauria).
•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.
•Therizinosauria had long arms with large clawed hands. The derived Therizinosauroidea within it have expanded guts, retroverted pubes, and shortened metatarsi, showing that they had evolved into slow-moving herbivores.
•Alvarezsauria were small-bodied insectivorous coelurosaurs. Derived forms (Alvarezsauridae) had highly transformed forelimbs that had become short but powerful picks.
•The remaining coelurosaurs formed the clade Pennaraptora, characterized by sideways-oriented shoulder joints, long arms with a specialized folding wrist, and broad pennaceous feathers on the arms and tail.
•Among the pennaraptorans were the omnivorous short-skulled Oviraptorosauria (including the toothless Caenagnathoidea) and the tiny gliding Scansoriopterygidae. (The Eumaniraptora, the remaining group, are covered in the next lecture).
MAJOR GROUPS OF SAUROPODOMORPHS
The derived sauropodomorphs are the Sauropoda (lizard feet), giant obligate quadrupeds that include the largest land animals of all time. Primitive sauropodomorphs (i.e., sauropodomorphs other than sauropods) were traditionally grouped together as "Prosauropoda". Modern studies differ as to whether all prosauropods form a paraphyletic grade (with no major prosauropod groups more closely related to each other than to sauropods), while others recognize a monophyletic "core Prosauropoda" composed of plateosaurids, riojasaurids, massospondylids, etc. This course follows the first (paraphyletic grade) hypothesis (see cladogram and phylogeny above).
BASAL SAUROPODOMORPHS
Specializations of the most primitive sauropodomorphs relative to other saurischians include:
These forms were small (~1-2.5 m long), bipedal plant eaters. Their long necks would allow them to feed on higher leaves than most of their contemporaries. Recent work suggest a paraphyletic series of basalmost sauropodomorphs. The most primitive seems to be the clade Saturnaliidae. The most completely known of these are Panphagia and Eoraptor of the Late Triassic of Argentina. (Eoraptor was long thought to be a basal theropod dinosaur, hence its name.) Newly discovered Chromogisaurus lived alongside them. The clade gets its name from Saturnalia of the Late Triassic of Brazil: its contemporary Guaibasaurus maybe a a saturnaliid, or a basal theropod, or a basal saurischian.
The teeth of saturnaliids are less phyllodont than the more derived taxa, and some teeth approach the ziphodont (blade-like) teeth of theropods in shapes: consequently, the earliest sauropodomorphs were almost certainly omnivores rather than strict herbivores. (In fact, that is the etymology for the name of Panphagia, "all eater" or "omnivore".) Intriguingly, the recently-discovered Buriolestes (initially described as the basalmost of all sauropodomorphs, but more recently found to be a saturnaliid) has honest-to-goodness ziphodont teeth.
Slightly more derived basal sauropodomorphs Thecodontosaurus, Asylosaurus and Pantydraco of Late Triassic England; Efraasia of Late Triassic Germany; and Plateosauravus of Late Triassic South Africa. Mussaurus of the Late Triassic of Argentina has often been considered one of these basal forms, but new evidence places it as a more advanced form (a near-sauropod).
Like most saurischians, basal sauropodomorphs and core prosauropods had a "wrap around overbite": the toothrow of the mandible fit entirely within the tooth row of the upper jaw. Efraasia and later prosauropods (and basal sauropods) seem to have had a small fleshy cheek similar to (but smaller than) those interpreted for ornithischians. Less convincingly, some evidence suggests a small keratinous beak in front of the dentary (and possibly premaxilla) of basal sauropodomorphs and/or core prosauropods. The diets of basal sauropodomorphs would have included at least some plants, but these may have been more omnivorous than later, more derived sauropodomorphs.
CORE PROSAUROPODS
Plateosauridae (including Late Triassic European Plateosaurus), Riojasauridae (including Late Triassic Argentine Riojasaurus), and Massospondylidae (including Early Jurassic South African Massospondylus, Argentine Adeopapposaurus, Antarctic Glacialisaurus, North American Seitaad, and Chinese Lufengosaurus): considered by some to form a monophyletic "Prosauropoda" and by others a paraphyletic series relative to melanorosaurids and sauropods. Core prosauropods were the dominant group of herbivores worldwide in the later Late Triassic and Early Jurassic: in some communities they were up to 95% of the known biomass. They represent the first time dinosaurs dominated a major way of life. (They are the only common dinosaurs in Late Triassic Europe, for example.) Traditionally interpreted as facultative bipeds, new studies of their forelimbs (especially forearms and wrists) show that they were not used for locomotion in most of these and so these dinosaurs were obligate bipeds, although slowly built ones. (In Riojasaurus, however, limb proportions show that they were almost certainly primarily quadrupedal.) Like basal sauropodomorphs, the core prosauropods retained the ancestral dinosaurian grasping hand. Their distal hindlimb (tibia, fibula, and metatarsus) was shorter than in other saurischians (a trait they passed on to Sauropoda).
The expansion (in terms of both physical size and diversity) of core prosauropods may be a result of the Carnian Pluvial Event. Many new forms of tree-forming plant evolved or diversified in the wake of this wet climate episode, and likely provided the new resource that these larger dinosaurs could exploit.
Larger than the basalmost sauropodomorphs, most core prosauropods range from 3 to 8 m longer (although some grew larger, up to or more than 10 m). Their necks were proportionately even longer than in their ancestors. They have been found in many environments: from swamps to deserts.
Although very common in the Late Triassic and Early Jurassic, no core prosauropods are known after the end of the Early Jurassic. They may have "evolved themselves into extinction", by being outcompeted by their specialized descendants: the sauropods.
"NEAR-SAUROPODS" AND THE PROSAUROPOD-SAUROPOD TRANSITION
The most advanced prosauropods (Anchisaurus, Yunnanosaurus, Lamplughsaura of India, Leonerasaurus of Argentina, and Melanorosauridae (such as Late Triassic-Early Jurassic South African
Melanorosaurus) are considered basal sauropods by some workers. For the purpose of this course, we'll restrict "Sauropoda" to Saltasaurus and everything sharing a more recent common ancestor with it than with Melanorosaurus, and use the unofficial name "near-sauropods" for those sauropodomorphs phylogenetically intermediate between Massospondylus and Sauropoda.
Like sauropods, the near-sauropods were typically larger than the more primitive sauropodomorphs: in this case, some 10-13 m long or more as adults and perhaps 5 tons. (Anchisaurus [sometimes called Ammosaurus] is a tiny exception: only as big as the basal sauropodomorphs.) Recent work shows that it might have actually been a quadruped. Aardonyx of the Early Jurassic of South Africa is among the last branches of Sauropodomorpha which was an obligate biped. Most near-sauropods were at best facultative bipeds, and many may have been obligate quadrupeds. Most add another sacral vertebrae (total of four).
The evolution of sauropodomorphs shows a correlated progression towards larger body size, smaller heads, longer necks, and quadrupedality. This may have been related to two (not mutually exclusive) selective forces:
These transitional near-sauropod forms are Late Triassic-to-earliest Middle Jurassic in age.
SAUROPODA
Postcranially and in size (10-13 m length), basal sauropods do not differ much from melanorosaurids and other near-sauropods. But early sauropod skulls have been transformed:
The oldest and most primitive of the sauropods are the Late Triassic-to-Early Jurassic Lessemsauridae. Named after Argentine Late Triassic Argentine Lessemsaurus, other lessemsaurids include its contemporary Ingentia and Early Jurassic South African Antetonitrus and the 12-ton giant Ledumahadi. Other basal sauropods include Early Jurassic South African Pulanesaura, Thai Isanosaurus, Chinese Gongxianosaurus, and the Vulcanodontidae. Furthermore, in sauropods the fifth metatarsal is long and weight-bearing. (The fact that, unlike all other dinosaurs but like lizards (and, to be fair, the vast majority of tetrapods!) all five toes contacted the ground is what makes sauropods "lizard feet". Otherwise, the feet of a sauropod are about as UN-lizardlike as you can possibly imagine!)
Sauropods of the Triassic: Rare, but Not Absent: Up until recently, many South African and Thai basal sauropods were interpreted as coming from the Late Triassic, but new stratigraphic analyses show they are from the earliest Early Jurassic. However, Lessemsaurus and Ingentia are pretty securely Late Triassic, and some track ways do strongly support the existence of stump-footed quadrupedal sauropod-style sauropodomorphs in this epoch.
That said, there is some disagreement on where the term "Sauropoda" should be placed on the phylogeny. In this course we are using "Saltasaurus and all taxa sharing a more recent common ancestor with it than with Melanorosaurus": in this case, the animals mentioned in the last few paragraphs are sauropods. An alternative definition--also commonly used--is "Saltasaurus, Vulcanodon, their concestor, and all of its descendants." Under this definition, Lessemsaurus, Ingentia, Antetonitrus, Gongxianosaurus, Pulanesaura, Isanosaurus, and the like would NOT be sauropods, and there would be no definite evidence of pre-Jurassic Sauropoda. (For what it is worth, we are using this "Vulcanodon+Saltasaurus" definition for Gravisauria, the major subgroup of Sauropoda.)
In Gravisauria there are four or more sacrals, and there is a transformation of the jaws and teeth:
Early sauropods are known from the southern continents (Gondwana) in the Late Triassic, and only show up in northern continents in the Early Jurassic. They seem to have been only rare components in dinosaurian communities until the Middle Jurassic.
EUSAUROPODA
The sauropods of the Middle Jurassic onward comprise the Eusauropoda (true sauropod). Eusauropods differ from their ancestors in a number of ways:
There has been an ongoing debate over the position of the necks of eusauropods (well, long-necked sauropodomorphs in general, but especially for these guys). Computer studies of possible ranges of motion in sauropod necks suggested that they were basically horizontal. However, recent work on a diversity of living animals shows an upright curved neck in nearly all amniotes, so it is quite likely that a raised neck was the natural position for sauropods.
Recently discovered Spinophorosaurus of the Middle Jurassic of Niger is the most completely known primitive sauropod. It might be a eusauropod, or it might be the sister taxon to Eusauropoda. Its tail (like that of early Late Jurassic Chinese Shunosaurus) was interpreted as ending in a tail club: in fact, that of Spinophorosaurus was thought to be convergent in form on stegosaurian thagomizers. (New information indicates that the bones of the "tail club" may be misidentified.)
Eusauropods are the dominant herbivores worldwide in the Middle and Late Jurassic, and in at least some parts of the world all the way until the end of the Cretaceous. The best and most completely known early eusauropod is early Late Jurassic (long thought to be Middle Jurassic) Shunosaurus of China. Jobaria of the Middle Jurassic (once thought to be Early Cretaceous) of Africa has sometimes been considered a primitive member of Macronaria, but other analyses suggest it was a typical member of this early eusauropod radiation.
Several groups of primitive eusauropods are known. Recent studies suggest a paraphyletic series of "barapsaurs" like Barapasaurus of the Early Jurassic of India, Spinophorsaurus, Shunosaurus, and the Cetiosauridae, named after Cetiosaurus of the Middle Jurassic of England and the closely related Patagosaurus of the Middle Jurassic of South America.
Different clades of eusauropods show different specializations towards reaching higher in trees. Several Late Jurassic-Early Cretaceous Asian sauropods evolved extra cervical vertebrae which were proportionately very long. Some of these sauropods (including Late Jurassic Omeisaurus and Mamenchisaurus) seem to form a clade, Mamenchisauridae. Some mamenchisaurids seemed to have reached truly tremendous size. Tonganosaurus may be an Early Jurassic representative of this clade; Wamweracaudia of Late Jurassic Africa may be a non-Asian example.
A recently discovered group of eusauropods is the Turiasauria. Some of these turiasaurs (such as Turiasaurus itself) evolved into tremendously large forms (rivaling various giant neosauropods). Initially known only from Late Jurassic European taxa, Turiasauria is now known to include taxa from the Middle Jurassic through the Early Cretaceous, and regions including North America (such as Moabosaurus) and Africa.
NEOSAUROPODA
The dominant group of post-Middle Jurassic sauropods is the Neosauropoda (new sauropods). Their transformations from primitive eusauropods are:
Neosauropods have two major subdivisions: Diplodocoidea and Macronaria.
DIPLODOCOIDEA
This group differs from their ancestors by a number of traits, including:
The pencil-shaped teeth were used for raking or scraping leaves off of trees, or grazing on low vegetation. Recent (2018) discoveries of a baby Diplodocus shows teeth more like basal eusauropods or macronarians, for cropping up whole plants. So there was likely an ecological switch as diplodocoids got older.
The first major branch of diplodocoids is the truly bizarre Rebbachisauridae, currently known from the Cretaceous, mostly from Gondwana. (However, a lost specimen which has been named "Maraapunisaurus fragillimus" is from the Late Jurassic of Colorado: it appears to be a primitive rebbachisaurid of tremendous size (~30-32 m long)). Rebbachisaurids are best known from Nigersaurus from the late Early Cretaceous of northern Africa. At least Nigersaurus was "little" (as sauropods go, that is: only the size of an Indian elephant!). Their skulls are highly transformed, most importantly with:
Unlike the slicing dental battery of ceratopsids or the grinding dental battery of hadrosaurids, the rebbachisaurid dental battery is at the front end of the jaw. The rate of replacement of the teeth was very high (one tooth/month; about twice as fast as hadrosaurids).
Other rebbachisaurids include Rebbachisaurus and Tatouinea of northern Africa and Limaysaurus, Rayososaurus, Cathartesaura, and Nopscaspondylus of South America, and Histriasaurus and Demandasaurus of Europe.
The remaining diplodocoids are the Flagellicaudata (whip tails). These form a major part of the sauropod communities of the Late Jurassic and early Early Cretaceous. Their specializations include:
There are two major divisions of the flagellicaudates. The Dicraeosauridae were smaller (only 10 m or less for most), and had:
More famous are the Diplodocidae. These are very large sauropods (over 30 m in the massively-built apatosaurines Apatosaurus and Brontosaurus, and more slender Diplodocus [the individual formerly considered Seismosaurus], and Supersaurus. They are characterized by the following specializations:
Diplodocids (and other diplodocoids?) may have been able to rear up on their hindlimbs to feed even higher in trees than they could using their long necks. Alternatively, they may have been low-browsing/grazing specialists: we'll come back to this question in a later lecture. At present, almost all diplodocids are known only from the Late Jurassic (Leinkupal of Early Cretaceous Argentina being the exception). In western North America they were tremendously diverse: at least six genera were present at one time.
Thunderstruck: Diplodocoid Whiptails: The tails of diplodocids (and possibly other diplodocoids, which are currently not well known) are muscular close to the hips, but are very, very, VERY long and slender for their posterior half. It has long been suggested that it was used defensively, to strike against an attacking theropod just like a modern monitor lizard or iguana does. (In fact, the Smithsonian's specimen of Allosaurus is damaged along its left dentary, scapula, and ribs, perhaps the result of a clobbering from a diplodocid!). Some have gone even further, and suggested that the whip was specifically used like a bullwhip: that is, that its tip went supersonic, forming a powerful crack sound. It remains debated to what degree either such use would damage the bones and (very limited) flesh of the posterior tail.
MACRONARIA
The other major branch of neosauropods is the Macronaria (big noses). They range from the Middle Jurassic to the very end of the Cretaceous. Their changes from ancestral sauropods include:
Primitive macronarians include Camarasaurus of the Late Jurassic of western North America (and possibly elsewhere); late Middle Jurassic Tehuelchesaurus of Argentina; Lourinhasaurus of Portugal; and others. The first three seem to form a clade Camarasauridae.
(However, a note for the future: some big on-going phylogenetic analyses threaten to shake up the family tree. It may be that Camarasaurus will fall outside of a clade comprised of diplodocoids, brachiosaurids and their kin, and titanosaurs. Stay tuned.)
The more advanced macronarians (the Titanosauriformes) are specialized by:
Astrodon (State Dinosaur of Maryland; Early Cretaceous of Maryland, and possibly Texas) is a titanosauriform, as is as is gigantic Sauroposeidon of Early Cretaceous western North America. Three major clades of titanosauriform include the Brachiosauridae, the Euhelopodidae, and Titanosauria.
Brachiosauridae (Middle Jurassic to Early Cretaceous) are very distinctive. They are characterized by:
The result is that brachiosaurids were built "up hill", making them some of the tallest animals that ever lived. Best known are Late Jurassic Brachiosaurus of North America, Giraffatitan of Africa, Lusotitan and Europasaurus of the Late Jurassic of Europe, and Early Cretaceous Abydosaurus, Venenosaurus, and Cedarosaurus of western North America. Early Late Jurassic Vouivria appears to be the oldest and most primitive brachiosaurid.
Astrodon and Sauroposeidon are sometimes considered to be brachiosaurids. Alternatively, some studies find the "brachiosaurids" as a paraphyletic series with regards to euhelopodids + titanosaurs.
The Euhelopodidae is a clade of ultra-long-necked Cretaceous titanosauriforms. Most known taxa are from Asia, but Late Jurassic African Australodocus may be a primitive euhelopodid (alternatively, it may be a brachiosaurid, more consistent with its age and location). Euhelopodids include (among others) Early Cretaceous Euhelopus, Qiaowanlong, Phuwiangosaurus, and early Late Cretaceous Erketu. Daxiatitan and Ruyangosaurus of the early Late Cretaceous of China may both be euhelopodids: the latter is one of the contenders of "largest dinosaurs of all time", with a 2.35 m (7.7 foot) tall femur. (However, some traits of the vertebrae of Ruyangosaurus suggest it is a titanosaur, perhaps an andesauroid, perhaps a colossosaur.) Huabeisaurus is a later Late Cretaceous euhelopodid. Gannansaurus (from near the end of the Cretaceous) seems to be the youngest known euhelopodid.
Titanosauria (Cretaceous) were the dominant group of Cretaceous sauropods in most parts of the world. They were very diverse. The different groups share:
Our understanding of titanosaur phylogeny is, quite frankly, a mess. There are a lot of phylogenetic analysis using quite a number of taxa (titanosaurs represent nearly 1/2 of all sauropod genera!), but very little in the way of consensus between them. We are using a grossly simplified version here. One basal clades is the "chubutisaurus" of Gondwana, including Wintonotitan of late Early Cretaceous Australia, Chubutisaurus of late Early Cretaceous Argentina, and a few early Late Cretaceous African and Argentine forms. Another clade of early titanosaurs is Andesauroidea, which contains Andesaurus, among others.
Malawisaurus of the Early Cretaceous of eastern Africa is the oldest known member of Lithostrotia ("stone skins"), the clade of titanosaurs known to have osteoderms. (However, it might be that osteoderms are found even more basally within the clade).
In addition to these primitive taxa, the titanosaurs include a more derived clade Eutitanosauria, which is further modified by the evolution of:
Relationships among the eutitanosaurs are still being worked out with no strong resolution at present. There might be a split between Colossosauria and Saltasauridae (the version shown here), but the memberships vary and the arrangement of other eutitanosaurs come out differently in varkljs analyses. Colossosauria is a robust clade that seems to show up in many recent studies. It includes two strong subclades: Rinconsauria (Rinconsaurus and Muyelensaurus) and the Lognkosauria (including Futalognkosaurus, the recently named Patagotitan, and its sister taxon, the even more gigantic 90-100 ton Argentinosaurus (perhaps the largest dinosaur of all)). Nearly all the very largest titanosaurs (indeed, dinosaurs) are lognkosaur colossosaurs, but not all. For instance, Dreadnoughtus seems to be a close relative of the saltasaurids, and Alamosaurus (the last North American sauropod) seems to be a saltasaurid itself. The position of tremendous Argyrosaurus (and many other titanosaurs) remains unresolved.
Saltasaurids include the last of the titanosaurs. Some (such as Alamosaurus) are true giants; others are smaller, such as Saltasaurus itself (the first titanosaur known with osteoderms) and Rapetosaurus; broad-snouted forms such as Nemegtosaurus (known only from a head); broad-bodied Opisthocoelicaudia (known only from a body, and quite possibly the same animal as Nemegtosaurus); and many others. The phylogenetic position of Early Cretaceous Brazilian Tapuiasaurus is uncertain: it was originally considered in the saltasaurid lineage, but may be an earlier branch of titanosaur. Tapuiasaurus's skull is very similar to that known from the saltasaurid Nemegetosaurus and what is known of the skulls of aeolosaurids, and to that of the very well preserved basal lithostrotian Sarmientosaurus (see below); this gives us a sense of what all titanosaur heads looked like:
Titanosaurs are the main group of herbivorous dinosaur in the southern continents (and parts of Europe) until the end of the Cretaceous, and were found on all continents. Titanosaurs include some truly tremendous forms: Futalognkosaurus, Argentinosaurus, Puertasaurus, Alamosaurus, and Patagotitan all seem to reach just about the same size: masses of perhaps 100 tons or so (depending on the methodology used; possibly around half that under other estimates), and lengths much more than 30 m. Other clades of eusauropod have multiple examples nearly as large. These dinosaurs are still smaller (less massive) than the modern blue whale, but otherwise exceed any living species of animal in mass. We will explore later on HOW it was that dinosaurs could be able to reach this supremely gigantic sizes.
Diets for Giants: A question many have pondered is how sauropods managed to get enough food for their sustenance in the course of a day. A very recent study examined the nutritional value of extant plants of Mesozoic style when grown under range of possible Mesozoic atmospheric conditions and compared them to likely metabolic rates of sauropods. They found that even though the nutritional value of plants doesn't vary under these different conditions as much as was once thought, but that sauropods could easily acquire enough food every day to grow and survive.
Sauropods: the Aquatic Dinosaurs?: In the late 19th through the mid-20th Century, sauropods were often portrayed in both scientific and popular literature as "the aquatic dinosaurs." Some imagined that because they reached sizes far surpassing the largest land animal today, they simply could not have been land animals at all! Instead, sauropods were envisioned to have been swamp- and lake-dwellers, using the water to buoy up their enormous masses. One school of thought suggested that their long necks and dorsally-placed nares was used as snorkels, with just the tops of their heads occasionally breaking the surface of the water to breath, like a dinosaurian blowhole.
However, others pointed out the flaws with this model. For one thing, the narrow feet of sauropods were the opposite of what you would want as an adaptation for wet mud. The snorkel hypothesis wouldn't work: the water pressure on the lower neck and lungs would be too great to allow them to expand to let air in: in fact, it would be a great way for a dinosaur to try to kill itself! As we now recognize, the nostrils themselves are NOT dorsally placed, but are instead at the anterior end of the snout. And sauropod fossils are known from a wide variety of habitats: dry uplands, forests, river systems, and yes, even lake shores, sea shores, and swamps. Surely some of them lived at least part of their time in wet environments, but the group as a whole was not adapted to it.
EVOLUTIONARY PATTERNS IN SAUROPODOMORPHA
Feeding adaptation transformations:
Gigantism:
Locomotory changes:
Heterochrony and the origin of Sauropoda:
Niche partitioning:
Many dinosaur communities had multiple sauropod species at the same time. How could they support this diversity? Niche partitioning: each clade
(and even each species) specialized in feeding in a slightly different manner. For example, in Late Jurassic communities there were ground-feeding
dicraeosaurids, mid-level feeding primitive eusauropods and primitive macronarians, high feeding brachiosaurids, mamenchisaurids, and diplodocoids
(which could feed exceptionally high if they could actually rear on their hindlegs)
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.
MANIRAPTORA
The remaining theropods form the clade Maniraptora ("hand grabbers"). Maniraptorans show numerous specializations:
One possible problematic shared derived feature of Maniraptora is a backwards-pointing pubis. Most coelurosaurs (and saurischians in generally) have a vertically-oriented or anteriorly-oriented pubis. In therizinosauroids, alvarezsaurids, the basal troodontid Sinovenator and the derived troodontid Latenivenatrix, dromaeosaurids, Archaeopteryx, and avialians the pubis points backwards; in the basal therizinosaur Falcarius, the basal alvarezsauroid Haplocheirus, oviraptorosaurs, most troodontids, and the basalmost avialian Anchiornis it points vertically or anteriorly. So it is difficult to say which condition is found in the concestor of Maniraptora. (Regardless of the answer, there is a LOT of convergence going on!!)
Changes in the muscle attachments in the hindlimbs of maniraptorans show a switch from the femur-and-tail power stroke found in other dinosaurs (inherited from the early diapsids) to one where the flexion of the knee is more important.
Maniraptorans are the most diverse clade of dinosaurs. None retain a basal theropod form: indeed, very few retain the ancestral carnivorous condition. Major groups include the Therizinosauria, Alvarezsauria, Oviraptorosauria, and Eumaniraptora (which get their own lecture).
The oldest maniraptorans are some possibly Middle Jurassic eumaniraptorans, and definitely this clade is present by the Late Jurassic. A therizinosaur dentary possibly from the Early Jurassic (but may be as young as the Early Cretaceous!) of China is considered by some authors to be a therizinosaur: however, it might simply be a derived sauropodomorph.
THERIZINOSAURIA
This group and the Oviraptorosauria were once thought to form their own clade (Oviraptoriformes). However, discovery of the primitive members of both Therizinosauria and Oviraptorosauria shows that many of the similarities between the derived members of these clades are convergences. More recent studies typically place therizinosaurs as the most basal branch of Maniraptora.
Therizinosauria ("scythe reptiles") have been considered sauropodomorphs and late surviving proto-ornithischians, but are in fact coelurosaurian theropods. Prior to the 1990s, they were often called the "segnosaurs." Similar to the ornithomimosaurs, this group is characterized by:
Other than the jaw Eshanosaurus, this group is known only from the Cretaceous, and only from Asia and North America at present. The basalmost form is Early Cretaceous Falcarius of western North America. It retains a relatively elongate metatarsus and a vertically-oriented pubis. The derived therizinosaurs form the clade Therizinosauroidea, and are characterized by shortened metatarsi in which all four toes touch the ground and backwards-pointing pubes. (In this case, like the ornithischians, this is almost certainly to accommodate a large gut for digesting plants.
Therizinosaurs seem to have been primarily, if not strictly, herbivores. Their stumpy feet and short legs show them to have been among the slowest theropods. To defend themselves (and possibly to help them feed) they had huge claws. They ranged from bear-sized taxa such as Erlikosaurus and Beipiaosaurus through Nothronychus to Tyrannosaurus-sized Therizinosaurus with 1 m long claws.
ALVAREZSAURA
Alvarezsauria (sometimes called "Alvarezsauroidea") is a recently discovered, highly specialized group of maniraptoran theropods. Haplocheirus is the oldest well-known form, from the early part of the Late Jurassic of China. (Poorly-known Aorun from the latest Middle Jurassic might actually be an even more primitive alvarezsaur.) Until 2018, all the remaining alvarezsaurs (collectively the Alvarezsauridae) are from the Late Cretaceous. However, recently described are Early Cretaceous Xiyunykus and Bannykus, which document two different stages in transition relative to their Late Cretaceous relatives. Alvarezsaurids are known from South and North America, Europe, and Asia. They have numerous bird-like features, and were once thought to have been specialized flightless birds. Alvarezsaurids range in the chicken-to-rhea sizes.
Alvarezsaurids have small beaky skulls with tiny teeth and hands in which the thumb is much more powerful than the other fingers.
In the Cretaceous Alvarezsauridae, the forelimbs are further transformed into bizarrely powerful arms with a huge thumb claw and exceedingly small digits II and III. The alvarezsaurids have a backwards pointing pubis. Unlike the therizinosauroid and ornithischian situation, this backwards position of the pubis is more likely associated with changes in the locomotory muscles towards knee-driven power from the ancestral tail-and-femur driven power.
Only a little is known of Alvarezsaurus itself (the basalmost form); somewhat more is known for the more derived Patagonykus and Achillesaurus (all from South America). Only a foot is known of rhea-sized Kol of Asia.
The highly derived Parvicursorinae (also called "Mononykinae"), in contrast, are known from many excellent specimens. The best studied are the Asian taxa Mononykus, Parvicursor, and Shuvuuia. More fragmentary Asian parvicursorines include tiny Albinykus, Linhenykus (in which digits II and III had entirely vanished), and Xixianykus. (However, North American forms such as Albertonykus are known). Parvicursorines have an extreme version of the arctometatarsus, in which the upper portion of metatarsal III is entirely missing.
The parvicursorines show numerous cursorial adaptations, but these were almost certainly defensive. They seem to have been insectivores, and their forelimbs may have been used to batter into ant and termite nests. They have been found from deserts to well-watered environments.
PENNARAPTORA: Fully Feathered Dinosaus
The remaining maniraptorans form the clade Pennaraptora ("feathered raptors"). These comprise the oviraptorosaurs, the scansoriopterygids, and the eumaniraptorans. These groups are united by several important characteristics:
OVIRAPTOROSAURIA
Oviraptorosauria is characterized by
The basalmost oviraptorosaurs are toothy Incisivosaurus, Protarchaeopteryx, and the Caudipteridae (sometimes spelled "Caudipterygidae": Similicaudipteryx, and Caudipteryx), all from the Early Cretaceous of China. (If the scansoriopterygids turn out to be oviraptorosaurs, they represent the Jurassic members of this clade).
Other early branches of the oviraptorosaurs are Microvenator (probably a caenagnathid) of the Early Cretaceous of western North America and specialized Avimimus (with an arctometatarsus) of the Late Cretaceous of Asia. (The latter seems to have dwelt in herds/flocks/whatever).
These basal branches of Oviraptorosauria are relatively small (chicken-to-turkey sized). The more derived Caenagnathoidea contain forms that range from turkey to human to tyrannosaur size. Caenagnathoids are derived by loss of all teeth as well as other specializations. Most (but not all) phylogenetic studies divide the Late Cretaceous caenagnathoids into two branches: the Caenagnathidae (sometimes called the "Elmisauridae" or "Elmisaurinae": taxonomy on this remains in flux) which had elongate hindlimbs and sometimes even an arctometatarsus, and the stout-footed Oviraptoridae. However, other studies mix and match the components of these groups. Oviraptoridae proper seems to be limited to Asia, while Caenagnathidae is found in both Asia and North America. Examples of oviraptorids include crested Oviraptor, Citipati and Rinchenia, crestless Khaan, and headless (okay, we don't have the head yet...) Nomingia. Caenagnathids include tiny Asian Elmisaurus and Caenagnathasia, and larger North American forms such as Chirostenotes, Leptorhynchos, Epichirostenotes, Apatoraptor, Hagryphus, and Anzu. By far the largest oviraptorosaur is the recently discovered caenagnathid Gigantoraptor of Asia: as large as an Albertosaurus or other smaller tyrannosaurid. Egg evidence points to the existence of Gigantoraptor-sized oviraptorosaurs in early Late Cretaceous North America.
The life habits of oviraptorosaurs are confusing. While the ancestral ones seem to be convincingly herbivorous, there are lizards in the gut contents of some oviraptorids: perhaps they were omnivorous? Many oviraptorosaurs have been found in desert environments, but others in forested regions.
SCANSORIOPTERYGIDAE
An unusual clade of small pennaraptorans are Scansoriopterygidae. Known from Middle-early Late Jurassic
Epidendrosaurus (also known as "Scansoriopteryx") and Epidexipteryx (which may just be the adult form of Epidendrosaurus!), and spectacular Yi and Ambopteryx these are among the smallest Mesozoic dinosaurs (pigeon-sized). (Early Cretaceous Zhongornis was once thought to be scansoripterygid, but is most likely a basal avialian.) As they are small enough, and have the distally-placed pedal digit I, it may be that they may have spent some time up in the trees. The skull shape and tiny teeth of the scansoriopterygids suggest that they might have been insect eaters or omnivores. Many recent studies place these dinosaurs as the basalmost members of Avialae, but newer analyses place them outside Eumaniraptora proper, and some even place them as basal members of Oviraptorosauria (which would solve the problem of the lack of pre-mid-Cretaceous oviraptorosaurs.) Most specimens are juveniles, but Yi seems to be from an adult.
Very bizarrely, Yi shows an extra bone projecting from its wrist, and a membrane attached to this. Such accessory bones attached to membranes have evolved conversantly in some gliding and flying mammals. These were confirmed in Ambopteryx. This suggests that Yi (and maybe other scansoriopterygids) were flying squirrel-like gliders.
EVOLUTIONARY PATTERNS IN BASAL COELUROSAURS
Feeding adaptation transformations:
Locomotory adaptations:
Gigantism and Miniaturization:
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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