It is animals and plants which lived in or near water whose remains are most likely to be preserved, for one of the necessary conditions of preservation is quick burial, and it is only in the seas and rivers, and sometimes lakes, where mud and silt have been continuously deposited, that bodies and the like can be rapidly covered over and preserved.
But even in the most favourable circumstances only a small fraction of the creatures that die are preserved in this way before decay sets in or, even more likely, before scavengers eat them. After all, all living creatures live by feeding on something else, whether it be plant or animal, dead or alive, and it is only by chance that such a fate is avoided. The remains of plants and animals that lived on land are much more rarely preserved, for there is seldom anything to cover them over. When you think of the innumerable birds that one sees flying about, not to mention the equally numerous small animals like field mice and voles which you do not see, it is very rarely that one comes across a dead body, except, of course, on the roads. They decompose and are quickly destroyed by the weather or eaten by some other creature.
It is almost always due to some very special circumstances that traces of land animals survive, as by falling into inaccessible caves, or into an ice - crevasse, like the Siberian mammoths, when the whole animal is sometimes preserved, as in a refrigerator. This is what happened to the famous Beresovka mammoth which was found preserved and in good condition. In his mouth were the remains of fir - trees - the last meal that he had before he fell into the crevasvas some 10,000 years ago. The mammoth has now been restored in the Museum of Zoology in St. Petersburg.
Other animals were trapped in tar pits, like the elephants, sabre - toothed cats, and numerous other creatures that are found at Rancho La Brea, near Los Angeles. Apparently what happened was that water collected on these tar pits, and the animals came to drink. Once they were in the tar, they were stuck fast, and they eventually died there. As the tar pits were gradually filled up by debris, the remains of the animals were buried deeper and deeper, and so were preserved.
There is one fairly common event that sometimes leads to the preservation of land - animal remains, and that is when the animals die in a river or a lake. This may seem a strange statement, but in fact the water in a river or a lake is more likely to be alkaline than acid, and so the bones are more likely to be preserved in such a water. But, even so, the bones of the animals are very likely to be scattered by scavengers and other agents of decay, and only a small fraction of them are likely to be preserved.
One of the most remarkable preservations of land - animal remains has been discovered on the island of Flores in Indonesia. There, on a ledge of rock about 200 metres above sea - level, an almost complete skeleton of a Homo floresiensis was found. The skeleton was so small that it was first thought to be that of a child. But when the bones were analysed, it was discovered that they belonged to an adult. The Homo floresiensis was a very small human - like creature that lived about 18,000 years ago. It was only about one metre tall, and its brain was only about 400 cubic centimetres, which is about one - third the size of the brain of a modern human. The discovery of the Homo floresiensis was a great surprise, for it was thought that all humans had evolved into the modern form long ago. But the discovery of this small human - like creature shows that evolution can take different paths in different parts of the world.