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Unit 340:Theropoda |
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The Vertebrates |
Overview |
Abbreviated CladogramDINSAUROMORPHA |--ORNITHISCHIA `--+--SAUROPODOMORPHA | Theropoda |--Herrerasauridae `--+--Ceratosauria `--Tetanurae |--Torvosauroidea `--Avetheropoda |--Carnosauria `--Coelurosauria |--Tyrannosauroidea `--Maniraptora |--Dromaeosauridae | `--AVES |
Contents340.000 Overview |
Saurischia is split into two large lineages, the giant, plant-eating sauropodomorphs and the medium to small-sized theropods. Theropods are the line of mainly carnivorous, bipedal dinosaurs from which birds evolved. They may be defined as all saurischians whose last common ancestor with birds is closer than their last common ancestor with Apatosaurus. Or, to use the usual shorthand, birds > brontosaurs. This unit covers only the non-avian theropods. Birds are taken up in later units.
Like the prosauropods and the ornithischians, theropods evolved from lagosuchid ornithodires, perhaps some time in the early or middle Carnian. The most primitive type are the herrerasaurs. From some similar ancestral form, theropods diverged into ceratosaurs and carnosaurs during the later Triassic, allosaurians during the Early Jurassic, and finally more advanced types during the earliest Cretaceous, following the Tithonian mass-extinction. The direct ancestorsof these advanced Cretaceous meat-eating dinosaurs presumably appeared during the Jurassic, but the Jurasisc remains attributed to them are mostly isolated teeth, and their actual forms are largely unknown
Most theropods retained the basic bipedal carnivorore form, but still they evolved into a huge diversity of different types, in different shapes and sizes, the most spectacular of all being the birds and bird-dinosaurs. However, as Dr. Tom Holtz points out, "Theropod dinosaurs (bipedal, primarily carnivorous forms) have received widespread attention in recent years owing to their importance in understanding the origin of birds. However, the evolution of theropods was more than a 'bird factory'. Indeed, these dinosaurs represent one of the most successful radiations of terrestrial predators in Earth history."
Until the 1980's, the theropods were divided according to size. The small, agile forms were called Coelurosaurs ("hollow- tailed saurians"), ancestral to the birds. The large to gigantic predators, such as Allosaurus, and Tyrannosaurus, were called Carnosaurs ("flesh(-eating) saurians"), which group evolved from early Coelurosaurs.
This clear-cut distinction was upset in the 1960s and 70s by the discovery of unusual medium-sized theropod dinosaurs such as Deinonychus. A growing number of paleontologists rediscovered the observation of vonHuene in the 1920's that some carnosaurs, notably Tyrannosaurus, were more similar in structure, and hence more closely related to, certain coelurosaurs than they were to other carnosaurs. The term "Carnosaur" is now somewhat disfavored, although it is retained here as meaning the stem group consisting of everything more closely related to Allosaurus than to birds. Another, and deeper split in the theropod dinosaurs is the division between primitive forms (Ceratosauria), and more advanced and bird-like forms (Tetanurae). This clasisfication may also be simplistic, for there are a number of forms, sometimes referred to as "Megalosaurs", which do not fit easily into one or the other of these camps.
The coelurosaurs can still not be arranged with much confidence. The tyrannosauroids and Compsognathus are undoubtedly more basal than the other forms, but the exact relationships are not clear. Bear in mind that the Late Jurassic Archaeopteryx is a bird. Therefore, the origin of all of the other coelurosaurian groups must have occurred earlier. However, they are little known until the middle of the Cretaceous, so we are dealing with some fairly long "ghost lineages" -- groups of dinosaurs which must have existed, but which have little or no known fossil record.
Big is beautiful, at least in the dinosaur world. Time and again, little ancestors evolve into great big descendants. Every line of theropods has its giants; in the late Jurassic period (the time of the great sauropods) no less than four groups - the Ceratosauridae, Megalosauridae, Eustreptospondylidae, and Allosauridae - all included great predators; 10 to 14 metres in length and 3 to 6 tonnes or so in weight (the size of a fully-grown elephant) Packs of these giant meat-eaters were the only creatures big enough and fierce enough to bring down a full-grown sauropod.
During the early to mid Cretaceous there were other similarly sized giant types - the slender Spinosaurs - and several parallel lines of allosauroids - acrocanthosaurs, chilantosaurs, cacharodontosaurs, and bahariasaurs.
And during the late Cretaceous there appeared the bigger tyrannosaurs, the last, among the largest, and certainly most formidable of all the theropods. Yet these evolved not from the giant allosaurs but from small coelurosaur-sized forms. So it seems that the small ancestor evolves into small and large descendants; the large forms rule the Earth for a while and then die out, while the small ones give rise in turn to large and small descendants.
Since dinosaurs are popular -- almost as popular with scientists as with the public -- they have generated a good many questions. Just a few of the current issues in the literature include:
1. A good deal of attention is being given to the large bird-like forms such as therizinosaurs and oviraptorosaurs because they are hard to fit into the typical cladograms. Are they early birds which have lost the ability to fly? Aberrant dromeosaurs? Something else?
2. When did feathers get started among dinosaurs? Why? But mostly, how?
3. How about flight? Did it evolve from the ground up? From the trees down? How was this related to feathers, and in what order did the adaptations for flight occur?
4. The great cold-blooded vs. warm-blooded debate is still unresolved. Less has been published on the issue in the last few years, but the problem of dinosaur metabolism is still very controversial.
5. Despite the popularity of Tyrannosaurus, the evolution of the Tyrannosauridae is just beginning to make sense. A number of other theropod groups are also still rather mysterious, and the theropod cladogram still gets significantly rearranged every year or so. It looks (as of 2002) as though things are beginning to settle down a bit, but many of the Gondwanan dinosaur lineages, particularly torvosauroids, are still rather poorly understood.
6. Although the family tree is beginning to make sense, the biogeography is still confused. Less than ten years ago, most paleontologists believed there was a fairly strict separation between Laurasian and Gondwanan theropods after the Early Cretaceous. The picture now is rather more disorganized. It seems that at least some interchanges occurred between Europe and Africa, and between the Americas.
7. The cause of the end-Cretaceous extinction still unclear. All dinosaurs except birds disappeared in a space of time which may have been practically overnight -- or at least very short compared to the 100 million year history of the dinosaurs as a whole. Almost everyone now admits that a significant factor was a an extraterrestrial object, at least several km in diameter, which hit the Yucatan Penninsula in Mexico about 65 million years ago. However, the selectivity with which the extinctions occurred is puzzling, and other factors may have played a major role.
8. Much attention is being given to theropod biomechanics, particularly walking and jaw movements.
-- ATW & MAK 020308
checked ATW050601
