CBSE Class 10 Science 2017 · Paper 31/2/1 · Question 21

Fossils are the preserved traces of organisms that lived long ago: the remains of hard parts such as bones, shells and teeth, or impressions of the body, usually found in rocks. Formation: when an organism dies its body usually decomposes, but sometimes the body or some part of it is buried quickly, for example in mud or sand, where it cannot decompose completely. Soft parts usually decay while hard parts or an impression remain. More sediment settles on top, and over long periods, often millions of years, the layers harden into rock with the remains or their impression preserved inside; erosion or digging may later expose them. Age: (i) by the depth of the layer: in undisturbed rock, fossils found nearer the surface are more recent than those found in deeper layers; (ii) by fossil dating: detecting the ratios of different isotopes of the same element in the fossil material (for young remains, the ratio of carbon-14 to carbon-12), since such isotopes change at a steady rate. Importance: fossils are direct evidence that the organisms of the past were different from those of today, so living things have evolved; arranged by age, they show the order in which groups appeared and help trace evolutionary relationships; and some show links between groups, for example fossil dinosaurs with feathers show that birds are closely related to reptiles (Archaeopteryx, with features of both, is often cited as such a link).