Dinosaurs (useful notes): Difference between revisions

[[Blatant Lies|A brief summary]] of [[Tropesaurus Index|dinosaurs]], for those of us who wish to address a few problems where [[Somewhere a Palaeontologist Is Crying]].

 

* [http://albertonykus.deviantart.com/art/Why-Birds-are-Dinosaurs-193639479 Birds are dinosaurs.] Because there are still birds alive today, dinosaurs are [[Science Marches On|technically not exinct]]!

 

 

== The Dinosaur Family Tree ==

Dinosaurs are divided into two different groups, and those two groups are ''not'' "meat-eaters" and "plant-eaters". In fact, some plant-eaters were more closely related to the meat-eaters than they were to other plant-eaters. In any case, this kind of grouping fails to account for those dinosaurs that were omnivorous, insectivorous, piscivorous etc., and is largely frowned upon within phylogenetic biology in any case, since in evolution a species can switch from one form of diet to another at the drop of a geological hat.

 

 

The second group is known as the Ornithischia, or “bird-hipped” dinosaurs, and are mostly herbivorous dinosaurs, very different in body-shape both when compared with saurischians and when compared with each other. This group includes the ornithopods, such as the duck-billed hadrosaurs and ''[[Stock Dinosaurs True Dinosaurs|Iguanodon]]'', the thyreophorans, such as the stegosaurs and ankylosaurs, and the marginocephalians, such as the thick-skulled pachycephalosaurs and horned and frilled ceratopsians.

When the names of the Saurischia and the Ornithischia were first coined, they were differentiated chiefly by their pelvic arrangement, with the saurischian hip bones arranged more like a lizard hip, the pubis bone pointing forwards (but this is the case in crocs, mammals and turtles as well, so it is not something unique about lizards), and the ornithischian hip bones more like an avian hip, the pubis bone pointing backwards. This latter was possibly an adaptation designed to accomodate longer guts for the digestion of vegetation, which is notoriously hard to digest.

 

 

At first, you might think this rather puts a hole through our lovely little classification. How can a saurischian be a saurischian if it's got bird-like hips, a characteristic of ornithischians? The pelvic arrangement is not the only way to differentiate between the two groups. Luckily, ornithischians can still be kept separate from saurischians by another diagnostic: all ornithischians have a U-shaped protruding bone at the front of the lower jaw called a predentary bone, often ending with a beak, and most of them have a row of chewing teeth lined up either side of the face, giving it a certain "hollow cheek" look like that of a horse. This suggests that many ornithischians chewed their food, unlike saurischians. Also, saurischians have air sacs in the vertebrae that ornithischians lacked, air sacs which the birds inherited, and which help to keep birds lightweight when flying.

Birds, considered as a distinct class of vertebrate in traditional systematics, are actually theropod dinosaurs, a hypothesis already proposed in the eighteenth century after the discovery of the famous ''[[Stock Dinosaurs True Dinosaurs|Archaeopteryx]]'' (an animal with dinosaurian skeleton ''and'' feathered wings and tail) but rejected by most scientists for a long amount of time. The link between dinosaurs and birds through ''Archaeopteryx'' was resurrected again in the 1960s, and has been definitively proven only in the 1990's by the long list of feathered dinosaurs and early birds recently found in the fossil record, which show strong anatomical similarities.

 

 

 

Main points:

And now, the history. We've got a lot to cover - ninety thousand times more dinosaur history than human history, for a start.

 

 

 

The mammal-like "reptiles", which had ruled in waves during the Permian period, were the first to spread on the planet, and it looked like they were set to rule it all again. There were two groups of these, mostly the herbivorous dicynodonts and the cynodonts. Mammals evolved from the latter set; the former died out without leaving any descendants.

 

 

 

 

 

Most ornithischians were still small plant-eaters, though even here, specialized new forms were emerging - such as the heterodontosaurids, creatures with large canine teeth that may have been used in mating disputes. As an aside, ''Heterodontosaurus'' is often speculated to be the male form of another heterodontosaurid species, and its tusks were thought to be weapons used against other males in mating disputes, rather like the tusks used by musk deer today. They are almost never talked about in pop culture, though the scientific interest for them sky-rocketed with the discovery of proto-feathers on the fossils of ''Tianyulong''. No one knows what became of the heterodontosaurids after their heyday in the Triassic and Early Jurassic periods, though the species ''Echinodon'', which lived during the Early Cretaceous period, implies that they may have survived quietly for millions of years long after the Jurassic.

The purpose of these plates is unclear, but the purpose of their tail spikes and shoulder spikes was arguably for self-defence against any animal that tried to hurt or kill the animal. Their cousins, the ankylosaurs, took the scutes and developed them into stronger armour all along their backs and even, in some cases, along their undersides. The species ''Ankylosaurus'' even had armour-plated eyelids, and a thickened block of bony tissue at the end of their tails to act as clubs in case the armour wasn't a good enough hint for some carnivores. They came into greater prominence during the Cretaceous period, when the stegosaurs died out.

 

 

Main points:

Dinosaurs continued to diversify further in the Cetaceous, but times were changing and things weren't what they used to be.

 

 

Strictly speaking, the split between pachycephalosaurs and ceratopsians is largely believed to have occurred in the Jurassic period, but fossils of Jurassic pachycephalosaurs have not yet been found. It is a bit of a mystery what the thick skulls of the pachycephalosaurs were used for. The impulse to say that they were used to defend the animal from predatory attack should not be indulged - the smaller creatures tend to have flatter skulls, admittedly, but the larger creatures have heads like bowling balls, which means that they have a very small surface area available for actually hitting anything. Try running into somebody while holding a bowling ball out in front of you at arm's length and you'll get some idea of how tricky this actually would be to pull off. Paleobiologists thesedays tend to believe that the skulls were used to butt the sides of rival pachycephalosaurs, perhaps over territorial or mating rights. Some pachycephalosaur skulls were surrounded by spikes and knobs of bone, and probably would have looked frightening in the eyes of a small predator or a rival, and an interesting little debate is going on over whether the spikier forms, like ''Stygimoloch'' and ''Dracorex'', are really different forms of the less spikier species - they may be child forms, or male-female forms, for example.

 

 

Hadrosaurs were the more successful of the styracosternans, and came with a broad variety of crests and nasal passages decorating the skulls, which were probably used to tell each other apart in case one hadrosaur made the mistake of trying to mate with another hadrosaur from a different species. The styracosternans as a whole were successful, perhaps because of their chewing teeth, which could pulp up even the toughest plant material. Fossil nests and eggs show that hadrosaurs were devoted mothers, like many modern birds are, and looked after their babies until they were old enough to fend for themselves. ''Maiasaura'' is probably the best known hadrosaur whose fossils include evidence of maternal care - indeed, ''Maiasaura'' means "good mother lizard".

During their time on Earth, the dinosaurs thrived in diverse terrestrial habitats, from swampy terrain and dense forests to open prairies and the driest of deserts. Some even weathered the harsh winter conditions of Antarctica and Australia (which were near the South Pole at the time). However, the dinosaurs did not survive the Cretaceous-Paleogene (formerly known as the Cretaceous-Tertiary) mass extinction event which wiped out roughly 65% of all living things, at least according to the fossil record. This is that eponymous event which underscores the trope [[Phlebotinum Killed the Dinosaurs]], and if you read some of the hypotheses about how they died, they often run a lot like that trope.

 

 

Here is the most likely hypothesis currently available. The extinction began with an increase in volcanic activity during the last few million years of the Cretaceous period, which would have introduced toxic gases and ash clouds into the atmosphere. This interfered with the relatively stable weather conditions the dinosaurs had enjoyed all over the globe (most of the Mesozoic era was comparatively stable, at least when compared with the turbulent climate changes of most of the Cenozoic era). Certainly, the fossil evidence suggests they were already in a state of decline (at least in North America) when the fateful meteor, about 65.5 million years ago, [[Rocks Fall, Everyone Dies|collided with the Gulf of Mexico]], producing the Chicxulub crater.

* Not all the surviving lineages are still alive today. Multituberculate mammals and champsosaurs survived the extinction but died later in the Cenozoic.

 

 

 

{{reflist}}

[[Category:Tropesaurus Index]]

[[Category:Dinosaurs]]