Dinosaur

There are many theories about what caused the extinction of the last of the dinosaurs, which occurred at the end of the Cretaceous period, about 66 million years ago. Some of the more interesting ideas include: the intolerance of these animals to rapid climate change, the emergence of new species of dominant plants that contained toxic chemicals the herbivorous dinosaurs could not tolerate, an inability to compete successfully with the rapidly evolving mammals, insatiable destruction of dinosaur nests and eggs by mammalian predators, and widespread disease to which dinosaurs were not able to develop immunity. All of these hypotheses are interesting, but the supporting evidence for any one of them is not enough to convince most paleontologists.

Interestingly, at the time of the extinction of the last of the dinosaurs, there were also apparently mass extinctions of other groups of organisms. These included the reptilian order Pterosauria, along with many groups of plants and invertebrates. In total, perhaps three quarters of all species and one half of all genera may have become extinct at the end of the Cretaceous. A popular hypothesis for the cause of this catastrophic, biological event was the impact of a meteor hitting Earth. The impact of an estimated 6 mi-wide (10 km-wide) meteorite could have spewed an enormous quantity of fine dust into the atmosphere, which could have caused climate changes that most large animals and other organisms could not tolerate. As with the other theories about the end of the dinosaurs, this one is controversial. Many scientists believe the extinctions of the last dinosaurs were more gradual and were not caused by the shorter-term effects of a rogue meteorite.

Another interesting concept concerns the fact that dinosaurs share many anatomical characteristics with Aves, the birds, a group that clearly evolved from a dinosaur ancestor. In fact, there are excellent fossil remains of an evolutionary link between birds and dinosaurs. The 3 ft-long (1 m-long), Late Jurassic fossil organism Archaeopteryx looked remarkably like Compsognathus but had a feathered body and could fly or glide. Moreover, some of the living, flightless birds such as emus and ostriches and recently extinct birds such as elephant birds and moas bear a remarkable resemblance to certain types of dinosaurs. Because of the apparent continuity of anatomical characteristics between dinosaurs and birds, some paleontologists believe that the dinosaurs did not actually become extinct. Instead, the dinosaur lineage survives today in a substantially modified form, as the group Aves, the birds.