You may also view a large-screen picture of a dinosaur skeleton for a lesson in anatomy. Some basic dinosaurian modifications to the ancestral archosaurian skeleton: Reduced fourth and fifth digits on the manus hand ; pes foot reduced to three main toes; three or more vertebrae composing the sacrum region of the vertebral column which attaches to the pelvis ; and an open acetabulum hip socket; see below. Some of these features were modified during the evolution of later groups, but these features are considered to be synapomorphies, or shared derived features, for the Dinosauria; the first dinosaurs had these features, and passed them on to their descendants.
One important dinosaurian synapomorphy is the perforate acetabulum , simply a "hip bone" actually three connected bones, together called the pelvis with a hole in the center where the head of the femur "thigh bone" sits. Dinosaurs are unique among all tetrapods in having this perforate acetabulum. One anatomical division proposed by British paleontologist Harry Govier Seeley in remains one of the most important organizing concepts for understanding dinosaurs.
Early dinosaur finds were scrappy. Extremely so. Bits of jaw, spinal columns and limbs were often all that remained, and some dinosaurs, such as Megalosaurus , were reassembled from isolated parts of different animals found in the same strata. By the s, however, paleontologists had uncovered more complete material. The American Bone Rush and a Belgian coal mine full of complete Iguanodon skeletons spurred a major image change.
Dinosaurs went from the strange, pseudo-mammalian creatures envisioned by Richard Owen to roughly bird-like animals which were closer in form to dinosaurs as we know them today.
The influx of new dinosaur varieties during the late 19th century required a classification system to organize all the strange beings. There were more kinds of dinosaur than anyone expected. Seeley reviewed three previously proposed arrangements in an presentation before the Royal Society of London. Saurischians, whose name means "lizard-hipped," had a pelvic structure more similar to that of modern lizards, while the ornithischians "bird-hipped" had a pelvic structure more like modern birds.
Confusingly, many scientists believe that modern birds are actually evolved from a group of lizard-hipped dinosaurs called coelurosaurs, which evolved the bird hip in an example of parallel evolution. All non-avian dinosaurs went extinct around 65 million years ago in an event called the Cretaceous-Tertiary extinction. Thus, saurischians left no living descendants.
Ornithischian dinosaurs have a hip structure where the pubis points backwards and runs parallell to the ischium. These dinosaurs also frequently have a forward pointing process as well which gives them the appearance of having a four pronged pelvis.
Those four prongs come from the two rear facing processes and the two forward facing processes. Superficially the hip structure of bird hipped dinosaurs is similar to that of birds, which is how these dinosaurs got their name. So when it comes to Saurischia Versus Ornithischia, which is your favorite dinosaur clade?
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