Dinosaurs hadadaptations that helped make them successful. The first known dinosaurs were smallpredators thatwalked on two legs.[9][10] All theirdescendants had anuprightposture, with the legs underneath the body. This transformed their whole life-style. Most of the smaller dinosaurs hadfeathers, and were probablywarm-blooded. This would make them active, with a highermetabolism than modern reptiles.Social interaction, with living inherds and co-operation, seems certain for some types. The existence of communal egg-laying sites is best understood if the adults travelled in herds, asherbivores do today.
The first fossils were recognised as dinosaurs in the early 19th century. Some of their bones were found much earlier, but were not understood.William Buckland,Gideon Mantell andRichard Owen saw these bones were a special group of animals.Georges Cuvier was also important in explaining what dinosaurs were. Dinosaurs are now major attractions atmuseums around the world. They have become part ofpopular culture. There have been best-sellng books and movies about dinosaurs. New discoveries are reported in themedia.
Labeled diagram of a typical archosaur skull, the skull ofDromaeosaurus
Dinosaurs are so varied that it is not easy to find what they all share. A reasonable list would include many features of theskeleton which are not familiar to the general reader.[11]
Dinosaurs were, at the start, small and bipedal; they walked on their hind legs. They laid eggs in nests. From the start of their fossil record, there were bothcarnivores and herbivores.
Changes in the basic set-up of dinosaurs happened because ofadaptations to differentlifestyles. We now know that birds are the living descendants of dinosaurs, and later adapted to different lifestyles as well.
Adaptive radiation let dinosaurs live in manyecological niches.Paleontologists have identified over 500 differentgenera and 1,000species of non-avian dinosaurs.[12] Their descendants, the birds, number 9,000 living species, and are the most diverse group of land vertebrates.
The largest dinosaurs wereherbivores (plant-eaters), such asApatosaurus andBrachiosaurus. They were the largestanimals to ever walk on dry land. Other plant-eaters, such asIguanodon, had special weapons to help them fight off the meat-eaters. For example,Triceratops had three horns on its head shield,Ankylosaurus was covered in boney plates, andStegosaurus had spikes on its tail.
Dinosaurs are united by at least 21traits in their skulls and skeletons.[13] These common characters (called 'synapomorphies') are the reasonpalaeontologists are sure dinosaurs had a common origin.
However, when definite dinosaurfossils appear (early in the Upper Triassic), the group had already split into two greatorders: theSaurischia, and theOrnithischia. The Saurischia keep the ancestral hip arrangement inherited from theirarchosaur ancestors, and the Ornithischia have a modified hip structure.
The following is a simplified list of dinosaur groups based on theirevolution.[8] Groups with a dagger (†) next to them do not have any living members.
Dinosauria
Saurischia ("lizard-hipped"; includes Theropoda and Sauropodomorpha)
Among the earliestornithischian ('bird-hipped') dinosaurs isPisanosaurus 230–220mya. AlthoughLesothosaurus comes from 199 to 189mya, skeletal features suggest that it branched from the main Ornithischia line at least as early asPisanosaurus.
Early saurischians were similar to early ornithischians, but different from moderncrocodiles. Saurischians differ from ornithischians by keeping the ancestral configuration of bones in thepelvis (shown in a diagram above). Another difference is in the skull: the upper skull of the Ornithischia is more solid, and the joint connecting the lower jaw is more flexible. These features are adaptations toherbivory; in other words, it helped them grind vegetable food.
Dinosaurs were primitivelybipedal: their probable ancestors were small bipedal Archosaurs. The date of the early dinosaurgenusEoraptor at 231.4 million years ago is important.Eoraptor probably resembles thecommon ancestor of all dinosaurs;[16] itstraits suggest that the first dinosaurs were small, bipedalpredators.[17] The discovery of primitive, pre-dinosaur,[18] types in MiddleTriassicstrata supports this view. Analysis of their fossils suggests that the animals were indeed small, bipedal predators.
Hip joints and hindlimb postures
Those dinosaurs which returned to four-legged stance kept all four legs under their body. This is much more efficient than the sprawling legs of alizard.
The big sauropods could never have reached so large a size without their pillar-like legs. A review surveys what we know about the mechanics of dinosaur movement.[19]
A major change in outlook came in the 1960s, when it was realised that small theropods were probably warm-blooded.[20] The question of whether all theropods or even all dinosaurs were warm blooded is still undecided.
It is now certain (from fossils discovered inChina: seeJehol biota) that small theropods hadfeathers. This fits well with the idea that they were warm-blooded, and that theorigin of birds can be traced to a line of small theropods.
Warm blooded animals have a high metabolic rate (use up food faster). They can be more active, and for longer, than animals who depend on theenvironment for heating. Therefore, the idea of warm-blooded dinosaurs insulated by feathers led to the idea that they were more active, intelligent and faster runners than previously thought.[20]
Main-stream palaeontologists have followed this view for small theropods, but not for larger herbivores.[21] Since we know that the size of aStegosaur's brain was about the size of awalnut, there is good reason to think itsintelligence was limited.
Despite their great success over a long period, there were lifestyles which the dinosaurs never evolved. None ever evolved to live entirely in water, as many mammals do, thoughSpinosaurus was semi-aquatic. They never entirely dominated the small terrestrialniche. All through theMesozoic most small vertebrates were mammals andlizards.[22] Some small dinosaurs developed into early birds. In general, we have much still to learn about the smaller fauna of the Mesozoic.
Theextinctions at the end of the Cretaceous were caused by a catastrophic event: a massivemeteorite hit the Earth (theChicxulub impact). We now know where it hit: in theYucantan peninsula in what is now Mexico.
Several other impact craters, and massive volcanic activity in theDeccan Traps inIndia, have been dated to about the time of the extinction event. These geological events may have reduced sunlight and hinderedphotosynthesis, leading to a massive disruption in Earth'secology.[23]
Did any terrestrial dinosaurs survive the great extinction event? Yes they did, because we now know that birds are descended from dinosaurs. But dinosaurs as generally understood were eliminated. Several fossils have been found in theHell Creek Formation about 40,000 years later than theK/T extinction event. Many scientists dismiss the "Paleocene dinosaurs" as re-worked, that is, washed out of their original places and then re-buried in much later sediments.[24] An associated skeleton (e.g. more than one bone from the same individual) found above the K/T boundary would be convincing, but no such finds have been reported.
"...Dragons of the prime, that tare each other in their slime".Tennyson,In Memoriam,1849.
Books about dinosaurs have been popular, especially with children, but adults have also enjoyed these kinds of books. InEdwardian times,Arthur Conan Doyle wrote anovel about aplateau filled with dinosaurs which he calledThe Lost World.
Jurassic Park in 1990 started a new phase in dinosaur popular culture.
↑The word 'dinosaur' comes fromGreek, meaning 'terrible lizard, ["Dinosaurs - What's in a name?". Children's BBC. 26 October 2001. Retrieved2009-10-03.] and was coined by English biologist Richard Owen in 1842. ["Richard Owen". Natural History Museum. Retrieved2009-10-05.]
12Palaeos lists "probably habitually bipedal" among the characteristics of theDinosauromorpha (that is, early proto-dinosaurs).Archived 2015-03-15 at theWayback Machine
↑Nesbitt, Sterling J. 2011. The early evolution of Archosaurs: relationships and the origin of majorclades.Bulletin of the American Museum of Natural History. New York: American Museum of Natural History. 2011 (352): 1–292. doi:10.1206/352.1. hdl:2246/6112. ISSN 0003-0090. S2CID 83493714.
↑Nesbitt S.J. 2011. The early evolution of archosaurs: relationships and the origin of major clades.Bulletin of the American Museum of Natural History352: 1–292.
↑Allen, Vivian; Bates, Karl T; Li, Zhiheng and Hutchinson John R. 2013. Linking the evolution of body shape and locomotor biomechanics in bird-line archosaurs.Nature497, 104–107.; popular summary
↑Alcober O.A & Martinez R.N. 2010. A new herrerasaurid (Dinosauria, Saurischia) from the Upper Triassic Ischigualasto Formation of northwestern Argentina.Zookeys.63, 55–81.
Bakker, Robert T. 1986.The Dinosaur Heresies: new theories unlocking the mystery of the dinosaurs and their extinction. New York: Morrow.ISBN0-688-04287-2
Farlow J.O. and Brett-Surman M.K. (eds) 1997.The Complete Dinosaur. Indiana University Press.ISBN0-253-33349-0
Holtz, Thomas R. Jr. 2007.Dinosaurs: the most complete, up-to-date encyclopedia for dinosaur lovers of all ages. New York: Random House.ISBN978-0-375-82419-7
Iggulden, Hal; Iggulden, Conn (2007). "Dinosaurs".The Dangerous Book for Boys. New York: HarperCollins. pp.30–34.ISBN978-0061243585.
Paul, Gregory S. 2000.The Scientific American book of dinosaurs. New York: St. Martin's Press.ISBN0-312-26226-4
Weishampel, David B; Dodson, Peter and Osmólska, Halszka (eds) 2004.The Dinosauria. 2nd ed. Berkeley: University of California Press.ISBN0-520-24209-2
National Geographic:Dinosaurs: a new look at the prehistoric icons. Good account of discoveries at American sites.
