Australia: The Land Where Time Began
Anatomy of theropods2
They were very variable, though all were obligatory bipeds. The shape and size of their heads were variable, and the bones of the skull were attached somewhat loosely to each other, in the middle of the lower jaw there was usually an extra joint. The large eyes were usually, and possibly always supported by an internal bony ring. In the jaws the teeth ranged from completely lacking to large, bladed and serrated. Their necks ranged from long to fairly short, usually, S-curved to some degree, and moderately flexible. They had a series of trunk vertebrae that were short and stiff
These were very different from the Sauropodomorpha that were closely related, being almost all carnivores that were agile and bipedal2. To counterbalance the front part of the body they had a long muscular tail and large teeth that have been described as knife-like, though the don't cut when fingers are pressed against them. In the group that is generally extraordinarily diverse there are some that are quite bizarre, though some of the group are well known. One of these groups that the author2 describes as bizarre are the therizinosaurs, that are believed to have been very large animals with huge bellies, claws on their hands that were long and scythe-like and very small heads, the jaws of which had teeth that were suited much more to herbivores than to conventional carnivores. Other groups of theropods, the ornithomimians and the oviraptorians, had beaks like birds and no teeth. The dromaeosaurians, a subgroup, are the most interest among the theropods. Their tails ranged from long and flexible to short and stiff. Their arms ranged from greatly reduced to very long, with fingers that were short to long and slender, ranging from 1 to 4 and the sharp claws on the fingers were from reduced to long. The pelvis ranged from moderate to very large, the leg was flexed at all sizes and long main toes from 4 to 3. The trackway gauge was very narrow as conformed by the footprints. Their brains ranged from of similar in size and complexity to those of reptiles to similar to those of birds.
These varied over a wide range from sealevel to highlands, from tropical to polar winters, and from arid to wet environments.
The diets of theropods covered the full range from herbivory to classic carnivory with opportunistic scavenging. The author2 suggests that small and juvenile theropods that had long arms and fingers with hook claws could probably climb. It has been found by studying trackways that many theropods of all sizes used shorelines to travel and possibly patroled them in the hope of finding carcasses washed up by the water.
This is the only group of dinosaurs that includes arch predators. When they arose they were already somewhat bird-like, a condition that increased over time, especially among some of the more advanced groups, including the direct ancestral group of birds.
Theropods ranged in size from small animals that appear rather delicate, similar to Compsognathus, usually called coelurosaurs, to the very large animals such as Tyrannosaurus, Gigantosaurus, Allosaurus, Baryonyx and Spinosaurus.
Included are several higher groups such as neoceratosaurians (Coelophysis, Dilophosaurus, Ceratosaurus, abelisaurids), allosaurids and carcharodontosaurids (Allosaurus, Gigantosaurus, carcharodontosaurus, acrocanthosaurus), coelosaurs and maniraptorans (dromeosaurids, archaeopteryx, oviraptorids, tyrannosaurids, ornithomimosaurids, etc.)
Several groups of primitive theropods had 4-fingered hands. More advanced forms such as the allosaurids had 3 fingers and the tyrannosaurids had 2 fingers. Other forms such as Mononykus had only 1 finger, these are now believed to have been very primitive birds, or highly derived forms of theropod dinosaurs.
Another specialisation of theropods is the ankle joint, the astragalus bone has a high ascending process that makes contact with the tibia (shin bone), and in advanced forms, stout, closely compacted tarsal bones of the foot. The tyrannosaurids had this 'arctometatarsalian' foot and ankle structure.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|