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LECTURE 2: WHAT ARE DINOSAURS?

Dinosaurs were the dominant vertebrate animals of terrestrial ecosystems for over 160 million years, from the late Triassic period (about 230 million years ago) to the end of the Cretaceous period (65 million years ago), when most of them became extinct in the Cretaceous–Tertiary extinction event.

The discovery in 1862 of Archaeopteryx first suggested a close relationship between dinosaurs and birds, as only the fossilized feathers of Archaeopteryx clearly distinguished it from contemporary small predatory dinosaurs such as Compsognathus. Research since the 1970s indicates that theropod dinosaurs are most likely the ancestors of birds; in fact, most paleontologists regard birds as the only surviving dinosaurs and some believe dinosaurs and birds should be put together under one biological class.^[1] Crocodilians are the other surviving close relatives of dinosaurs, and both groups are members of the Archosauria, a group of reptiles that first appeared in the very late Permian and became dominant in the mid-Triassic.

For about the first half of the 20th century, both scientists and the general public regarded dinosaurs as slow, unintelligent cold-blooded animals. However, the bulk of research since the 1970s has supported the view that they were active animals with elevated metabolisms, and often with adaptations for social interactions. This change of view was strongly influenced by evidence of the descent of birds from theropod dinosaurs.

Since the first dinosaur fossils were recognized in the early nineteenth century, mounted dinosaur skeletons have become major attractions at museums around the world. Dinosaurs have become a part of world culture and remain consistently popular among children and adults. They have been featured in best-selling books and films (notably Jurassic Park), and new discoveries are regularly covered by the media.

The term "dinosaur" was first coined 1842, by Sir Richard Owen and derives from Greek δεινός (deinos) "terrible, stiff, masterly, skillful" + σαῦρος (sauros) "lizard". It is sometimes used informally to describe other prehistoric reptiles, such as the pelycosaur Dimetrodon, the winged pterosaurs, and the aquatic ichthyosaurs, plesiosaurs and mosasaurs, although none of these were dinosaurs.

In colloquial English "dinosaur" is sometimes used to describe an obsolete or unsuccessful thing or person, despite the dinosaurs' 160 million year reign and the global abundance and diversity of their descendants, the birds. This usage became common while dinosaurs were regarded as cold-blooded and sluggish.

Under phylogenetic taxonomy, dinosaurs are usually defined as all descendants of the most recent common ancestor of Triceratops and modern birds.^[4] It has also been suggested that Dinosauria be defined as all of the descendants of the most recent common ancestor of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.^[5] Both definitions result in the same set of animals being defined as dinosaurs, including theropods (mostly bipedal carnivores), sauropodomorphs (mostly large herbivorous quadrupeds with long necks and tails), ankylosaurians (armored herbivorous quadrupeds), stegosaurians (plated herbivorous quadrupeds), ceratopsians (herbivorous quadrupeds with horns and frills), and ornithopods (bipedal or quadrupedal herbivores including "duck-bills"). These definitions are written to correspond with scientific conceptions of dinosaurs that predate the modern use of phylogenetics. The continuity of meaning is intended to prevent confusion about what the term "dinosaur" means.

There is an almost universal consensus among paleontologists that birds are the descendants of theropod dinosaurs. Using the strict cladistical definition that all descendants of a single common ancestor must be included in a group for that group to be natural, birds are dinosaurs and dinosaurs are, therefore, not extinct. Birds are classified by most paleontologists as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.^[6]

From the point of view of cladistics, birds are dinosaurs, but in ordinary speech the word "dinosaur" does not include birds. Additionally, referring to dinosaurs that are not birds as "non-avian dinosaurs" is cumbersome. For clarity, this article will use "dinosaur" as a synonym for "non-avian dinosaur". The term "non-avian dinosaur" will be used for emphasis as needed. It is also technically correct to refer to dinosaurs as a distinct group under the older Linnaean classification system, which accepts paraphyletic taxa that exclude some descendants of a single common ancestor.

Using one of the above definitions, dinosaurs (aside from birds) can be generally described as terrestrial archosaurian reptiles with limbs held erect beneath the body, that existed from the Late Triassic (first appearing in the Carnian faunal stage) to the Late Cretaceous (going extinct at the end of the Maastrichtian).^[7] Many prehistoric animals are popularly conceived of as dinosaurs, such as ichthyosaurs, mosasaurs, plesiosaurs, pterosaurs, and Dimetrodon, but are not classified scientifically as dinosaurs. Marine reptiles like ichthyosaurs, mosasaurs, and plesiosaurs were neither terrestrial nor archosaurs; pterosaurs were archosaurs but not terrestrial; and Dimetrodon was a Permian animal more closely related to mammals.^[8] Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic, especially the Jurassic and Cretaceous. Other groups of animals were restricted in size and niches; mammals, for example, rarely exceeded the size of a cat, and were generally rodent-sized carnivores of small prey.^[9] One notable exception is Repenomamus giganticus, a triconodont weighing between 12 kilograms (26 lb) and 14 kilograms (31 lb) that is known to have eaten small dinosaurs like young Psittacosaurus.^[10]

Dinosaurs were an extremely varied group of animals; according to a 2006 study, over 500 dinosaur genera have been identified with certainty so far, and the total number of genera preserved in the fossil record has been estimated at around 1,850, nearly 75% of which remain to be discovered.^[11] An earlier study predicted that about 3,400 dinosaur genera existed, including many which would not have been preserved in the fossil record.^[12] Some were herbivorous, others carnivorous. Some dinosaurs were bipeds, some were quadrupeds, and others, such as Ammosaurus and Iguanodon, could walk just as easily on two or four legs. Many had bony armor, or cranial modifications like horns and crests. Although known for large size, many dinosaurs were human-sized or smaller. Dinosaur remains have been found on every continent on Earth, including Antarctica.^[13] Despite their diversity and dominance, however, dinosaurs (other than birds) did not spread into aquatic or aerial niches.

While recent discoveries have made it more difficult to present a universally agreed-upon list of dinosaurs' distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral archosaurian skeleton. Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical across Dinosauria; the earliest dinosaurs had them and passed them on to all their descendants. Such common features across a taxonomic group are called synapomorphies.

Dinosaur synapomorphies include an elongated crest on the humerus, or upper arm bone, to accommodate the attachment of deltopectoral muscles; a shelf at the rear of the ilium, or main hip bone; a tibia, or shin bone, featuring a broad lower edge and a flange pointing out and to the rear; and an ascending projection on the astragalus, one of the ankle bones, which secures it to the tibia.^[14]

A variety of other skeletal features were shared by many dinosaurs. However, because they were either common to other groups of archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies. For example, as diapsid reptiles, dinosaurs ancestrally had two pairs of temporal fenestrae (openings in the skull behind the eyes), and as members of the diapsid group Archosauria, had additional openings in the snout and lower jaw.^[15] Additionally, several characteristics once thought to be synapomorphies are now known to have appeared before dinosaurs, or were absent in the earliest dinosaurs and independently evolved by different dinosaur groups. These include an elongated scapula, or shoulder blade; a sacrum composed of three or more fused vertebrae (three are found in some other archosaurs, but only two are found in Herrerasaurus);^[14] and an acetabulum, or hip socket, with a hole at the center of its inside surface (closed in Saturnalia, for example).^[16] Another difficulty of determining distinctly dinosaurian features is that early dinosaurs and other archosaurs from the Late Triassic are often poorly known and were similar in many ways; these animals have sometimes been misidentified in the literature.^[17]

Dinosaurs stood erect in a manner similar to most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side.^[18] Their posture was due to the development of a laterally-facing recess in the pelvis (usually an open socket) and a corresponding inwardly-facing distinct head on the femur.^[19] Their erect posture enabled dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of "sprawling" reptiles.^[20] Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs.^[21] Some non-dinosaurian archosaurs, including rauisuchians, also had erect limbs but achieved this by a "pillar erect" configuration of the hip joint, where instead of having a projection from the femur insert on a socket on the hip, the upper pelvic bone was rotated to form an overhanging shelf.^[21]

For a long time many scientists thought dinosaurs were polyphyletic with multiple groups of unrelated "dinosaurs" evolving due to similar pressures,^[22][23][24] but dinosaurs are now known to have formed a single group.^[25][14]

Dinosaurs diverged from their archosaur ancestors approximately 230 million years ago during the Middle to Late Triassic period, roughly 20 million years after the Permian-Triassic extinction event wiped out an estimated 95% of all life on Earth.^{[26] [27]} Radiometric dating of the rock formation that contained fossils from the early dinosaur genus Eoraptor establishes its presence in the fossil record at this time. Paleontologists believe Eoraptor resembles the common ancestor of all dinosaurs;^[28] if this is true, its traits suggest that the first dinosaurs were small, bipedal predators.^[29] The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators.

When dinosaurs appeared, terrestrial habitats were occupied by various types of basal archosaurs and therapsids, such as aetosaurs, cynodonts, dicynodonts, ornithosuchids, rauisuchias, and rhynchosaurs. Most of these other animals became extinct in the Triassic, in one of two events. First, at about the boundary between the Carnian and Norian faunal stages (about 215 million years ago), dicynodonts and a variety of basal archosauromorphs, including the prolacertiforms and rhynchosaurs, became extinct. This was followed by the Triassic-Jurassic extinction event (about 200 million years ago), that saw the end of most of the other groups of early archosaurs, like aetosaurs, ornithosuchids, phytosaurs, and rauisuchians. These losses left behind a land fauna of crocodylomorphs, dinosaurs, mammals, pterosaurians, and turtles.^[14]

The first few lines of primitive dinosaurs diversified through the Carnian and Norian stages of the Triassic, most likely by occupying the niches of groups that became extinct. Traditionally, dinosaurs were thought to have replaced the variety of other Triassic land animals by proving superior through a long period of competition. This now appears unlikely, for several reasons. Dinosaurs do not show a pattern of steadily increasing in diversity and numbers, as would be predicted if they were competitively replacing other groups; instead, they were very rare through the Carnian, making up only 1-2% of individuals present in faunas. In the Norian, however, after the extinction of several other groups, they became significant components of faunas, representing 50-90% of individuals. Also, what had been viewed as a key adaptation of dinosaurs, their erect stance, is now known to have present in several contemporaneous groups that were not as successful (aetosaurs, ornithosuchids, rauisuchians, and some groups of crocodylomorphs). Finally, the Late Triassic itself was a time of great upheaval in life, with shifts in plant life, marine life, and climate.^[14]

Dinosaurs (including birds) are archosaurs, like modern crocodilians. Archosaurs' diapsid skulls have two holes, called temporal fenestrae, located where the jaw muscles attach, and an additional antorbital fenestra in front of the eyes. Most reptiles (including birds) are diapsids; mammals, with only one temporal fenestra, are called synapsids; and turtles, with no temporal fenestra, are anapsids. Anatomically, dinosaurs share many other archosaur characteristics, including teeth that grow from sockets rather than as direct extensions of the jawbones. Within the archosaur group, dinosaurs are differentiated most noticeably by their gait. Dinosaur legs extend directly beneath the body, whereas the legs of lizards and crocodylians sprawl out to either side.

Collectively, dinosaurs are usually regarded as a superorder or an unranked clade. They are divided into two orders, Saurischia and Ornithischia, depending upon pelvic structure. Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia, while Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia. Saurischians ('lizard-hipped', from the Greek sauros (σαυρος) meaning 'lizard' and ischion (ισχιον) meaning 'hip joint') retained the hip structure of their ancestors, with a pubis bone directed cranially, or forward.^[19] This basic form was modified by rotating the pubis backward to varying degrees in several groups (Herrerasaurus,^[30] therizinosauroids,^[31] dromaeosaurids,^[32] and birds^[6]). Saurischia includes the theropods (bipedal and mostly carnivores, except for birds) and sauropodomorphs (long-necked quadrupedal herbivores).

By contrast, ornithischians ('bird-hipped', from the Greek ornitheios (ορνιθειος) meaning 'of a bird' and ischion (ισχιον) meaning 'hip joint') had a pelvis that superficially resembled a bird's pelvis: the pubis bone was oriented caudally (rear-pointing). Unlike birds, the ornithischian pubis also usually had an additional forward-pointing process. Ornithischia includes a variety of herbivores. (NB: the terms "lizard hip" and "bird-hip" are misnomers — birds evolved from dinosaurs with "lizard hips".)

The following is a simplified classification of dinosaur families. A more detailed version can be found at List of dinosaur classifications.

• Dinosauria

 

• Saurischia (theropods and sauropods)

 

• Herrerasaurians (early bipedal predators)
• Theropods (all bipedal; most were carnivores)

 

• Coelophysoids (Coelophysis and close relatives)
• Ceratosaurians (Ceratosaurus and abelisaurids - the latter were important Late Cretaceous predators in southern continents)
• Spinosauroids (long bodies, short arms, some with crocodile-like skulls and bony "sails" in their backs)
• Carnosaurians (Allosaurus and close relatives, like Carcharodontosaurus)
• Coelurosaurians (diverse, with a range of body sizes and niches)

 

• Tyrannosauroids (small to gigantic, often with reduced forelimbs)
• Ornithomimosaurians ("ostrich-mimics", toothless, carnivores to herbivores)
• Therizinosauroids (bipedal herbivores with long arms and small heads)
• Oviraptorosaurians (toothless; their diet and lifestyle are uncertain)
• Dromaeosaurids (like the "raptors" in Jurassic Park)
• Troodontids (similar to dromaeosaurids, but more lightly built, and possibly omnivorous)
• Birds (the only living dinosaurs)

 

 

• Sauropodomorphs (quadrupedal herbivores with small heads and long necks and tails, and elephant-like bodies)

 

• "Prosauropods" (early relatives of sauropods; small to quite large; some possibly omnivorous; bipeds and quadrupeds)
• Sauropods (very large, usually over 15 meters long [49 ft])

 

• Diplodocoids (skulls and tails elongated; teeth typically narrow and pencil-like)
• Macronarians (boxy skulls; spoon-shaped or pencil-shaped teeth)

 

• Brachiosaurids (very long necks; forelimbs longer than hindlimbs)
• Titanosaurians (diverse; stocky, with wide hips; most common in the Late Cretaceous of southern continents)

 

• Ornithischians (diverse bipedal and quadrupedal herbivores)

 

• Heterodontosaurids (meter- or yard-scale herbivores or omnivores with prominent canine teeth)
• Thyreophorans (armored dinosaurs, mostly quadrupeds)

 

• Ankylosaurians (scutes as primary armor; some had club-like tails)
• Stegosaurians (spikes and plates as primary armor)

• Ornithopods (diverse, from meter- or yard-scale bipeds to 12 meter (39 ft) animals that could moves as both bipeds and quadrupeds, evolved a method of chewing using skull flexibility and large numbers of teeth)

 

• Hadrosaurids ("duckbilled dinosaurs")

• Pachycephalosaurians ("bone-heads", bipeds with domed or knobby growth on skulls)
• Ceratopsians (dinosaurs with horns and frills, although most early forms had only the rudiments of these features)

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