CBSE Class 10 Science 2017 · Paper 531 · Question 13

Fossils are the preserved remains or traces of organisms that lived in the remote past - bones, teeth, shells, footprints, or the impressions of leaves and bodies - usually found in rocks. Formation: usually a dead organism decomposes and is lost. But if the body, or part of it, is buried where it cannot decompose completely - for example, a dead insect caught in hot mud - the mud hardens and keeps the impression of the body. Over long periods more layers settle on top and harden into rock; in some fossils the hard parts are slowly replaced by minerals. Two roles in tracing evolutionary relationships: (1) Fossils can be dated - fossils in layers nearer the surface are more recent than those in deeper layers, and the ratios of isotopes in the fossil material give their age - so arranging them by age shows the order in which different forms of life appeared and how they changed over time. (2) Some fossils have features of two groups and so show how the groups are related: Archaeopteryx has feathers like a bird but teeth, clawed fingers and a long bony tail like a reptile, and fossil dinosaurs with feathers show that birds are closely related to reptiles.