Magazine article UNESCO Courier

The Disappearance of the Dinosaurs, a Scientific Enigma

Magazine article UNESCO Courier

The Disappearance of the Dinosaurs, a Scientific Enigma

Article excerpt

Dinosaurs lived on Earth for 150 million years, from the Triassic to the Cretaceous Periods, and then became extinct. Reptiles with elongated legs, they are a source of interest and astonishment because of the gigantic size of some of them, because of their strange shape and their longevity, and because they disappeared simultaneously from the face of the Earth 65 million years ago.

Study of their skeletons has made it possible to reconstitute the physical appearance of dinosaurs, but the reason for their disappearance has proved more difficult to establish. Specialists in several scientific disciplines including biology, physiology, palaeoclimatology, palaeography and astronomy have tried to solve the mystery. Many hypotheses have been proposed, most of them short-lived. At present there are two conflicting theories based on diametrically opposed premises and modes of reasoning. According to the first or "catastrophist" thesis, dinosaurs were exterminated by extra-terrestrial objects. The second, or "gradualist" thesis is centred on the evolution of geological phenomena.

The killer comet theory Large amounts of iridium, palladium and platinum have been found near Gubbio in central Italy in sediments deposited during the transition between the Cretaceous and the Tertiary Periods-what is known as the K/T boundary. Layers rich in iridium from the same geological period have also been found in Denmark, Spain, France and New Zealand. The American geologist Walter Alvarez believes that the fact that the sedimentary deposits date from the same period as the extinction of the dinosaurs cannot be a mere matter of chance, and that there is a cause-and-effect relationship between the two phenomena.

The scenario proposed by Alvarez in 1980 is as follows: iridium is a very rare element and its concentration in a sedimentary deposit must result, not from the erosion of ancient rocks nor from biochemical phenomena, but from the impact made by an extraterrestrial object that collided with the Earth. Meteorites are rich in iridium and Alvarez suggested that a meteorite with a diameter of between 6 and 1 0 kilometres crashed into the Earth and raised an immense cloud of terrestrial dust which mingled with the dust released by the break-up of the meteorite itself, blotting out the sunlight and preventing photosynthesis from taking place. This night may have lasted between six and ten years, during which the dinosaurs died of hunger. The much smaller mammals may have survived by eating seeds and undecomposed plant residues.

Since it was first put forward, this theory has undergone several modifications. In the iridium-rich layers it has been observed that the mass of matter Of terrestrial origin is much less than that which the impact of a meteorite could have produced. Thus the meteorite was replaced by comets which would have disintegrated as they approached the Earth and unleashed a hail of debris. After that point the scenario is unchanged. Failing sea levels and glaciation Another theory, which I first put forward in 1964, is based on the fact that what are now known as the mass extinctions coincided with regressions of the sea. Each major division in the geological time-scale begins with a marine transgression (the spread of the sea over land areas) accompanied by the appearance of a new fauna, and ends with a marine regression (the retreat of the sea) which is contemporary with a mass extinction. It is hard to believe that the simultaneous occurrence of marine regressions and mass extinctions over a period of 600 million years could have been fortuitous. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.