Extinction

Some 65 million years ago at the end of the Mesozoic Era the dinosaurs disappeared from the earth. This has always been one of the great mysteries in science. What caused this mass extinction is unknown and still the subject of a fierce debate.

Dinosaurs appeared throughout the Triassic and Jurassic periods and were the dominant form of life for around 140 million years. We know they were here because we can see their bones in the geological record laid down over time.

The lower strata of rock contain the earliest and most primitive species of dinosaur, and the upper strata contain the newer species. Then, suddenly, around the Cretaceous period, the dinosaurs are no more.

But what was the reason for this mass extinction? Many theories have been put forward including: cosmic radiation from a nearby exploding supernova; acid rain from volcano activity; a change in the climate because of continental drift; disease ravaged dinosaur populations or the appearance of small mammals that fed on dinosaur eggs.

One of the most intriguing theories suggested for dinosaur extinction is the asteroid theory. In the 1980s the father-son team of Luis and Walter Alvarez discovered a layer of Iridium in the Cretaceous layer of the earth.

Although Iridium is rare on Earth, it's abundant in meteorites, suggesting that a huge asteroid or comet, perhaps miles in diameter, hit the Earth at that time. The resulting dust clouds would have been thrown into the sky, darkening the planet and blocking out the sun.

Other scientists point out the fossil record shows the dinosaurs were already in decline before this time . perhaps the asteroid was the final blow that finished off a population already weakened by disease.

There have been several other less drastic waves of extinction since that time. Indeed, over the last 2.5 million years there have been many occurrences, both of large terrestrial organisms like mammals, and of marine fauna like molluscs. It is also unlikely we have seen the last of the mass extinctions. We may in fact be witnessing a mass extinction right now, caused not by asteroids, volcanoes or an ice-age, but by what might be an even more destructive force -humankind.

Part of the mystery of extinction is why certain species died while others survive. Monosaurs became extinct while other marine reptiles, like crocodiles are still around. If climate change is responsible why did the dinosaurs, hearty creatures that lived in all kinds of conditions all over the planet, die when frogs, who are much more sensitive to temperature, survive till today?

What is a dinosaur?

In 1842 Sir Richard Owen used the name dinosaur to describe the creatures whose bones were being discovered within the outer layers of the earth. The name dinosaur means "terrible lizard" in Greek, but are they just big lizards?

The first dinosaur bones, those of an herbivore Iguanodon, were discovered in 1822 in Sussex, England, by Gideon Mantell. Shortly after this the first carnivorous dinosaur, a Megalosaurus, was found.

Unlike most reptiles, it's believed that all dinosaurs walked with their legs directly below their hips. This is different from a lizard or a crocodile where the top section of the limb is splayed out horizontally from the body and the lower section goes from the "knee" joint straight down to the ground.

Because of this, lizards and crocodiles move low to the ground and almost appear to be dragging themselves around on their bellies. In contrast, dinosaurs walked in an upright manner with a few notable exceptions.

Everything we know about these ancient creatures is based on the few fossilised skeletons discovered over the last 150 years. None of the soft tissue of the dinosaur's bodies have survived, although we do have a few, rare skin and footprint impressions left in rock.

Unlike other reptiles, many scientists believe that dinosaurs were at least partly, "warm-blooded" - they used their metabolism to control their temperatures, instead of depending on the environment. Snakes and other reptiles adjust their body heat by moving into the sun or moving into the shade as need be. If a reptile gets too cold he will slow down or even go into a state of hibernation.

Warm blooded animals, like mammals and birds, maintain a constant temperature no matter what they are doing This allows mammals and birds to be active and quick moving in even cold climates.

Overall, it seems that dinosaurs were not simply giant lizards and that might hold some clues to why they disappeared when other reptiles survived. 

This article was originally published in 1999.