Dinosaurs
Over 350 genera of dinosaurs have been discovered and named so far.
Dinosaurs were land-dwelling reptiles that walked with an erect stance. Their unique hip structure caused their legs to stick out under their bodies, and not sprawl out from the side (as with other reptiles).
When dinosaurs first evolved from more primitive archosaurs, they were bipedal (they walked on two legs). Much later, some dinosaur groups returned to a four-legged stance, most having rear legs much larger than their front legs . There were many different types of dinosaurs, differing in size and weight, locomotion (two- or four- legged walk, fast or slow speed), diet (meat or plants), and just about every other physiological and anatomical measure
Most dinosaur species perished in background extinctions that occurred throughout the Mesozoic Era. For example, Maiasaura, Styracosaurus, Protoceratops, Dilophosaurus, Cetiosaurus, Iguanodon, Allosaurus, Ouranosaurus, Deinocheirus, and Edmontosaurus were probably decimated in background extinctions. The flowering plants (angiosperms) appeared during the Cretaceous period, displacing most conifers and many other plants. This had major effects on dinosaurs. Some herbivorous dinosaurs, like Edmontosaurus, ate only conifers. As flowering plants came to dominate the landscape, Edmontosaurus died out, unable to find enough conifers to sustain itself
The nemesis hypothesis of Raup and Sepkoski theorizes that there is a periodicity of 26 million years to mass extinctions which is caused by collisions with comets from the Oort cloud as they are perturbed in their orbits by a dark star (a companion star to the sun). The search for the Oort cloud continues.
Most dinosaur species perished in background extinctions that occurred throughout their Mesozoic Era. The other dinosaur species died out during the several mass extinctions that occurred in the Mesozoic: at the end of the Triassic (213 million years ago), during and at the end of the...
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