fossils are the preserved remains or traces of animals , plants and other organisms from the past fossils range in age from 10 000 to 3.48 billion years old. the observation that certain rock strata led 19th centuary geologist to recorgnise a geological timescale.
Fossils provide solid evidence that organisms from the past are not the same as those found today; fossils show a progression of evolution. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record. By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. This approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. The resulting fossil record tells the story of the past and shows the evolution of form over millions of years.
There are several stages but the crossing over starts on the second stage. And the actual crossing sides happens on the following stage. Hope this helped :)
