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Dinosaurs extinction

Theories abound, but the mystery surrounding the disappearance of the dinosaurs at the end of the Cretaceous remains. Dinosaurs dominated the landscape for most of the Mesozoic era and are still in a sense very diverse today, as example with birds, their direct descendants. Animals commonly called dinosaurs, however, died out at the end of the Cretaceous and within a few million years, mammals and large birds occupied the place of theropods and duck-billed dinosaurs. Why did the dinosaurs disappear? The exact cause of the disappearance of non-flying dinosaurs is still the subject of passionate debate. Very few places today are on Earth where the transition from Cretaceous to Tertiary is preserved in sequences of rocky or freshwater sediments. We have a clear idea of ​​what happened in the marine environment, but only a few dinosaurs (waterfowl) lived at sea at that time. We are therefore forced to make a generalization from what these sites reveal to us and to apply it to the whole world.

Riddle of the past

Dinosaurs extinction - Mosasaurus and Plesiosaurs

Even recent events attended by thousands of people and videotaped are controversial. For example, the assassination of President John F. Kennedy. If we can not know with certainty what happened less than 50 years ago, how can we elucidate a mystery that is millions of years old, with the only clue being a fossil remnant that is probably very incomplete? In our opinion, 70% of the species of marine organisms died out in the late Cretaceous. It's mostly marine invertebrates that have been affected. The predominant groups such as ammonites (parents of the current nautilus and squid) have completely disappeared. Many species of marine reptiles, including mosasaurids and plesiosaurs, have become extinct in this way.

Sudden or progressive

We know less about what has happened on earth, but what is certain is that all non-flying dinosaurs disappeared in the late Cretaceous, as well as pterosaurs and many small groups of vertebrate animals. Some researchers have argued that this disappearance was very sudden and that several lineages became extinct within a few thousand years - a relatively short time compared to the history of our planet. Others, on the other hand, maintain that it has been more progressive and that some groups have died out well before the pivotal period between the Cretaceous and the Tertiary. The evidence we have, which boils down to a few sites, is not enough to solve the problem. If it is probable that the disappearance of marine invertebrates was quite sudden, that of terrestrial animals was probably more progressive.

Hypotheses

Many theories have been advanced to explain the extinction of non-flying dinosaurs: poisoning by flowers, the destruction of their eggs by the first mammals, the harmful radiation of a distant supernova, epidemics. While they may seem plausible, these explanations leave a large number of unanswered questions. Flowering plants, for example, developed tens of millions of years before the extinction of the dinosaurs. Mammals have also cohabited with them for more than 100 million years. These theories do not explain the massive extinctions that took place at the end of the Cretaceous, nor why so many groups of organisms were the victims.

Current theories

For paleontologists today, there are three major theories to explain the massive Cretaceous-Tertiary extinction. All are based on solid elements and all three may have been important factors that have been mutually reinforcing in the infernal cycle of an ecological disaster.

Climatic changes

The first theory involves climatic changes already initiated in the Cretaceous. The temperatures of the planet fell at the end of the Cretaceous and the seasonal variations increased. Temperature differences between summer and winter, especially at high latitudes, widened dramatically. This may have been due in part to significant changes in sea level, which certainly led to an increase in the area of ​​marine habitat. Rising sea level may have also contributed to the dislocation of terrestrial habitat. Large areas of forest, for example, may have become fragmented and large animal concentrations have in turn become fragmented, resulting in fewer and therefore more vulnerable populations. However, while changes in climate and sea level could clearly affect species survival or extinction, they may not have caused sudden death. If, as many researchers say, the dinosaurs died out suddenly, it is for other reasons.

Large scale volcanic eruptions

Volcanic eruptions

The second great theory invokes volcanic eruptions on a large scale. At the end of the Cretaceous, several regions of the world were undergoing a very intense volcanic activity, as evidenced by Ghats Indian Deccan formed by a huge basaltic flow.

Comet or asteroid impact

Comet impact

The third, the most tragic, states the principle of the impact of a comet or asteroid at the end of the Cretaceous. Several data would seem to support it. The most convincing is the existence of elements, such as iridium, which enrich themselves in the sedimentary deposits marking the passage from Cretaceous to Tertiary. These elements are rare on Earth, but common in extraterrestrial materials. Particles of metamorphic quartz (that is, subjected to intense pressure) are also found in these same sedimentary deposits. Geologists today believe they have found the crater dug by this impact: it is located in the Yucatan Peninsula, Mexico, far underground.

Disappearance of vegetation

Disappearance of vegetation

Volcanic activity and the impact of this asteroid would have had many similar effects on the environment. By raising significant amounts of dust and ash in the upper layers of the atmosphere, both phenomena would have prevented the sun's rays from reaching the Earth's surface and the plants from surviving. The disappearance of plant life would have had a cascading effect on other organisms, affecting of course the herbivores and consequently the carnivores who fed on them. Elements indicating point disruption in plant populations are indeed within the Cretaceous-Tertiary stratigraphic boundary of North America, ie a sudden spore increase in ferns. In general, ferns are very common when angiosperms and conifers are removed, so it suggests that large areas of North America near Yucatan where the impact would have occurred were suddenly deprived of the major part of their vegetation. Flowering plants and conifers reappeared in the Tertiary, but the large herbivorous dinosaurs were no longer there to eat them.

The mystery still remains

The key to the mystery of the massive extinction of dinosaurs will probably continue to elude us. It is possible that factors invoked by the great current theories have played an important role. There is good reason to argue that there is a link between climate change, volcanic activity, the impact of an asteroid and massive extinctions but we have no evidence yet. What we can say is that the world at the beginning of the Tertiary was very different from that of the Upper Cretaceous. Only feathered dinosaurs passed from Cretaceous to Tertiary.