Dinosaurs
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[edit] Definition
The word dinosaurs refers to a superorder of reptiles distinguished by a common set of anatomical characteristics (see the section on dinosaur anatomy, below) of which the most important is a modification to the reptilian hip to allow the legs to be under the body rather than sprawled out to the sides.
Cladistically, dinosaurs may be defined as the descendants of the most recent common ancestor of Triceratops and the sparrow. This leaves us with the awkwardness of having to say "non-avian dinosaur" every time we want to exclude modern birds. Taxonomically, of course, the cladists are right: birds are dinosaurs (see the main article on dinosaur-bird evolution) but in practice the word "dinosaur" may be used either to include or exclude birds, according to who's talking and what they think of cladistics.
[edit] Dinosaur anatomy
Other anatomical characteristics of dinosaurs include a brevis fossa (a sort of bony shelf) on the ilium, a deltopectoral crest on the humerus for muscle attachment, a tibia with a transversely expanded subrectangular distal end and a caudolateral flange; and an ascending process on the astragalus to secure it to the tibia.
As you would expect, the earliest fossils demonstrate intermediate forms between archosaurs and dinosaurs, some of which will be discussed in the section on dinosaur evolution below. This means that there is a gray area where a given form may be classified either as an advanced archosaur or a primitive dinosaur, depending on which characteristics we take as being essential to the definition of a dinosaur, and on how much modification to the acetabulum we need to call a creature a dinosaur.
[edit] Types of dinosaur
[edit] Saurischia
The saurischian dinosaurs include theropods and sauropods. Theropods are bipedal and almost invariably carnivorous; the most famous theropods are Tyrannosaurus Rex and Archaeopteryx.
Sauropods are herbivorous; apart from early forms, they are quadrupedal. They include the largest dinosaurs, such as Diplodocus.
[edit] Ornithischia
Ornithischians are herbivores. They can be grouped into Ornithopoda, Thyreophora, and Marginocephala.
Ornithopods are usually medium-sized bipedal browsers. This suborder includes, amongst others, the crested and "duck-billed" dinosaurs.
The Thyreophora includes ankylosaurs (armoured dinosaurs) and the Stegosauria.
The Marginocephalia includes the ceratopians (such as Triceratops) and the pachycephalosaurs (literally "thick-headed dinosaurs").
[edit] Evolution of dinosaurs
As we have mentioned above, dinosaurs are modified archosaurs. As will become clear from the forms listed below, the general story of dinosaur evolution is of adaptive radiation from forms which were small and bipedal, had simple leaf-shaped teeth set along the jaw margins, had an archosaurian complement of digits, archosaurian phalanges on those digits, and typical archosaurian fenestrae in the skull, and perhaps had an acetabulum only partially perforated.
The number of features common to both the Saurischia and the Ornithischia, listed above have persuaded palentologists that these are common ancestral features of saurischians and ornithischians, rather than arising separately in two archosaur lineages.
The entire topic of dinosaur evolution is too large for the scope of this article: we shall content ourselves with pointing out some of the more interesting early forms.
[edit] Theropoda
The most interesting aspect of theropod evolution is the development of feathers, and, in coelurosaurs, flight. For more information, see the section on bird evolution in the article Intermediate Forms Between Classes.
[edit] Sauropoda
Saturnalia tupiniquim appears to be the most primitive known sauropod - like Herrerosaurus, its acetablum is not completely open, placing it anatomically between archosaurs and true dinosaurs. However, like Herrerasaurus, it has the simple hinged ankle characteristic of the saurischians.
Remains of this dinosaur are incomplete, so a comprehensive list of traits is not possible until better specimens are found. The rather better preserved Anchisaurus displays a number of primitive traits. It is the smallest known sauropod; it has five digits on pes and manus, like archosaur feet, and not resembling the elephant-like feet of later sauropods. It was capable of bipedalism, and it had leaf-shaped serrated teeth, poorly adapted to its vegetarian diet by contrast with later, more derived sauropods.
The most striking feature of the evolution of sauropods is their development of giganticism - this is the suborder containing Apatosaurus, Brachiosaurus and Diplodocus.
[edit] Ornithopoda
Lesothosaurus is a good candidate for a basal ornithopod. It has the horn-covered beak typical of the group, but it still has six teeth on the premaxilla of the beak. Simple leaf-shaped teeth go along the outside of its jaw, whereas later forms have teeth deeply inset to allow for capacious cheeks. Unlike the higher ornithopods, it lacks the hinge joints along the maxilla which help them grind their food. In short, it was less well-adapted to browsing than later forms.
Other primitive characteristics include a small prepubic process compared to later forms, a well-formed digit I of the manus, and the fact that like archosaurs and early dinosaurs it was small (three feet long, one foot high) and bipedal.
The Hadrosaurinae and Lambeosaurinae appear to be derived from the Iguanodonta. The earliest known lambeosaurine is Eolambos, a crestless lambeosaurine.
- Ornithopods
- Early ornithischian dinosaurs: the Triassic record
- Lesothosaurus diagnosticus
- Iguanodontia
- Early Jurassic forms
- Early and Middle Jurassic dinosaurs
[edit] Marginocephalia: Pachycephalosauria and Ceratopia
![From top to bottom: Psittacosaurus neimongoliensis, Archaeoceratops oshimai, Leptoceratops gracilis, and Protoceratops andrewsi. Not to scale: scale bars (blue) represent 10 cm. Adapted from The Dinosauria, ed. Weishampel, Dodson, and Osmólska[1]](/index.php/Image:Ceratopia.jpg)
The most primitive known ceratopians are the various members of genus Psittacosaurus. These have the bony "beak" which we associate with ceratopians, no nasal horn and a parietal shelf which could scarcely be described as a frill; they are small and bipedal. Their post-cranial anatomy is typical of a generic ornithischian. The skeletons of Archaeoceratops oshimai, Leptoceratops gracilis, and Protoceratops andrewsi, pictured opposite, show further developments: as the lineage becomes larger and heavier, a quadrupedal gait is adopted. Archaeoceratops has a tiny incipient neck frill, more developed in Leptoceratops and substantial in Protoceratops. Neither Archaeoceratops nor Leptoceratops has a nasal horn, but Protoceratops has an incipient horn core.
As so often among dinosaurs, we see the lineage changing from bipedal, undifferentiated and small to obligate quadrupedal, distinctive and enormous - a full grown Triceratops was the size of an elephant.
Remains of early pachycephalosaurs are extremely scanty and fragmentary. Stenopelix valdensis is thought to be a basal form, based on the characteristics of its hips, but it is known only from a partial skeleton.
[edit] Thyreophora: Ankylosauria and Stegosauria
The early Jurassic Scutellosaurus is a basal thyreophore. It is one-a-half meters long, and was basically bipedal, although it may have been capable of getting on all fours. It had simple leaf-shaped teeth, which were sited on the jaw margins, rather than inset as in later forms. It has rows of unconnected scutes along its back, like basal ankylosaurs such as Scelidosaurus and basal stegosaurs such as Huayangosaurus (see below).
Scelidosaurus is a little further on: it is sometimes placed among the basal thyreophores, and sometimes as a primitive ankylosaur. It was four meters long and basicially quadrupedal, although it may have been able to rear up on its hind legs. Its teeth were simple and leaf-shaped, but they are inset from the cheek margins, unlike Scutellosaurus. It still has premaxilliary teeth. Its scutes, like those of Scutellosaurus were disconnected, and its skull is unarmoured. The skull is also higher than its wide, and still has five fenestrae (openings) in the skull, where later Ankylosauria have the antorbital and supratemporal fenestrae fused shut.
Huayangosaurus is a nice example of a primitive stegosaur: it has a short high skull compared to later types, and retains the primitive antorbital fenestra. Most significantly, as well as stegosaurian spines along the ridge of its back, it has rows of armoured scutes down its sides, in common with basal Thyreophora such as Scutellosaurus, but unlike later stegosaurs.
- Scelidosaurus and Ankylosaurs
- Thyreophora
- Scutellosaurus lawleri
- Scutellosaurus and others
- Huayangosaurus
[edit] History of discovery
The earliest description we know of a dinosaur bone dates from 1677, when the lower part of a femur was discovered in Oxfordshire. It was identified as the femur of a giant by the antiquarian Robert Plot[3], then Professor of Chemistry at Oxford University. Although the fragment has now been lost, Plot's illustration is sufficiently detailed to identify it as probably belonging to a Megalosaurus.
It is not impossible that people had looked at dinosaur fossils previously, and made conjectures as to their appearance in life, but this is the first known instance of a dinosaur bone being described in print by a Western scientist.
In 1822 the fossil collector Gideon Mantell discovered the teeth of an Iguanodon. After much puzzlement, he decided that the teeth were somewhat like those of an iguana, only much bigger; further finds confirmed the diagnosis of a giant reptile. In 1824 William Buckland described fragments of Megalosaurus, again correctly diagnosing it as a giant reptile. In 1842, Richard Owen coined the term "dinosaur" to include the species then known: Iguanodon, Megalosaurus and Hylaeosaurus.
Britain is, however, a poor locale for dinosaur hunting, because although it has rocks spanning the entire age of dinosaurs, nature has inconsiderately covered them with a lot of topsoil and grass. It was with the discovery of dinosaur remains in the deserts of the American Midwest, in the second half of the nineteenth century, that dinosaur hunting entered its first golden age, driven by the insatiable ambition and bitter rivalry of Othniel Marsh and Edward Cope. Many of the most familiar dinosaurs, such as Stegosaurus and Triceratops, were discovered and named during this period. [4]
The second golden age of dinosaur studies is the present. The early dinosaur hunters were restricted, by birth and by financial constraints, to European and North American fossil beds. Today paleontologists have access to deposits in China, Argentina, and Mongolia which are currently supplying a constant flow of interesting finds.[5]
[edit] Dinosaur Misconceptions
There are a number of misconceptions about dinosaurs: we shall point out some of the most common.
[edit] Dinosaurs and other extinct animals
Sometimes people, often children, use the phrase "dinosaur" to refer to anything impressive, extinct, and reptilian. This is not right. Pterosaurs weren't dinosaurs, ichthyosaurs were not dinosaurs, and Dimetrodon was not a dinosaur. Woolly mammoths are especially not dinosaurs.
[edit] Dinosaurs in swamps
The idea that dinosaurs, and large sauropods in particular, lived in swamps, or partially immersed in water, has been discredited by modern research. Their feet were not adapted for such a habitat, and the idea that large dinosaurs would need their buoyancy in water to support their weight is considered erroneous. The idea that they lived in this fashion became engrained in public consciousness as a result of the work of artists such as the brilliant, but occasionally misinformed, Charles Knight.
[edit] Dinosaurs dragging their tails
Again, this misconception has been put into the public mind by early illustrations of dinosaurs. Anatomical studies show that their tails didn't drag, as do fossilised dinosaur trackways, which show no trace of a dragging tail.
[edit] Alverez' comet
There is a common misconception that the question of the extinction of (non-avian) dinosaurs has been laid to rest by the hypothesis of a comet or meteor strike around the Cretaceous-Tertiary boundary. This hypothesis is in fact still controversial, and is not accepted by a majority of paleontologists.
[edit] Creationist misconceptions
There are a number of creationist misconceptions about dinosaurs, most of which revolve around the idea that there are, or were until relatively recently living non-avian dinosaurs. A partial list of their arguments can be found in the Creationists and dinosaurs section of our list of creationist arguments.
