Devonian park

Time after time you can hear the phrase like that “Scientists discovered the fossils of an unknown dinosaur, that became extinct 100 million years ago”. Of course, you take it seriously. Maybe you even admire the scientific progress. But I’ll surprise you: that phrase is out of all relation to palaeontologists.

If you are lucky enough to speak to a real fossil specialist – ask him when dinosaurs became extinct, and listen to the answer. You won’t hear any numbers, or words like “… a million years ago”, only something like “between the Cretaceous and Palaeogene periods”. It’s weird, but the last phrase is much more precise than the likely answer in the educational article “75 million years ago”.

Palaeontologists only use relative scale to date fossils. It is based on defining the layer of the Earth’s crust, where the fossils are found. The words “Jurassic”, “Devonian” or “Silurian” are nothing but the names of these layers. The higher the layer is, the younger the fossils are.

Defining the layers is very complicated. These can get mixed up after building of a house last spring or an earthquake two million years ago. Rocks from one layer can differ greatly in different parts of the world. It hardens the definition of the layers of one age. Besides, the animals can break the sequence of layers. But all these difficulties can easily be overcome.

There are several methods to define the absolute age of very old fossils. The most widespread one is based on measuring the speed of radioactive decay of certain types of isotopes. The main difficulty here is to assess the initial content of these isotopes in the fossils analyzed. Besides, the isotopes can vanish with the time. Scientists can also determine the age using the behaviour of certain minerals to keep the light energy and then to give it away under certain conditions.

The accuracy of absolute dating is considerably lower than the relative one. First of all it is connected with huge value that scientists deal with. History of a mankind dates 5 thousand years back; the last ice age was 12 thousand years ago. Palaeontologists deal with absolutely different values. For instance, the first birds appeared 100 million years ago (or, to be more exact, in early Jurassic period). Bias in this is usually no more than 5 million years.

Moreover, the methods themselves are far from being perfect, especially when the age is several million years. When the scientists tried to define the age of the Earth’s crust for the first time, in 1930’s, the different methods gave the value between 40 million and 7 billion years. That is why palaeontologists started to use the absolute age only about 40 years ago. But with the time the absolute dating methods are being improved more and more. Since 1980 the bias of dating the age of main geological layers decreased from 15 to 1 million years.

So, next time graciously smile when you read about the remains of a lizard that are 50 million years old. The phrase “Lizard from the late Palaeocene” would have been much more precise, but it would have definitely been less interesting for the general readers, like us.