Cigal1000 dating rock
Geologists get a wide range of information from fossils.
Flowering plants evolved in the late Jurassic or early Cretaceous.Organisms that lived for relatively short time periods are particularly useful for dating rocks, especially if they were distributed over a wide geographic area and so can be used to compare rocks from different regions. There is no specific limit on how short the time span has to be to qualify as an index fossil.Some lived for millions of years, and others for much less than a million years.The K-Pg extinction is thought to have been caused by the impact of a large extraterrestrial body (10 km to 15 km across), but it is generally agreed that the other four Phanerozoic extinctions had other causes, although their exact nature is not clearly understood.
As already stated, it is no coincidence that the major extinctions all coincide with boundaries of geological periods and even eras.
The major groups of organisms that we are familiar with evolved between the late Proterozoic and the Cambrian (~600 Ma to ~520 Ma).