How are fossils evidence for evolution?
Layers of rock build up over time and the deeper the rock layer the older that rock layer tends to be. You can see the layers of sedimentary rock in the Grand Canyon 3D model below.
You won’t find dinosaur fossils in the sedimentary rocks of the Gran Canyon though as these rocks are much older than the oldest known dinosaurs, but you will find plenty of marine creatures such as crinoids and brachiopods.
Fossils are found buried in rocks and scientists can work out when the fossil formed by measuring the age of the rocks it is found in.
The oldest fossils that have been discovered are simple organisms such as cyanobacteria. Have a look at the 3D model below of a stromatolite, a layered rock made by micro-organisms such as cyanobacteria.
Fossils that have more recently formed (and can be found in younger rocks) are more complex. This supports the theory of evolution which states that more complex organisms have evolved from simpler ones.
Gaps in the fossil record
The formation of fossils is rare and many early forms of life were soft-bodied and so have not formed fossils, or the fossils have been destroyed by geological activity. This means that there are gaps in the fossil record and so scientists can not be certain how life began on Earth or what happened to organisms that lived in the past.
The crocodilian snout
Scientists can study lots of different types of fossils and build up a picture of how animals have evolved over time. An example of this is the crocodilian snout.
What is a crocodilian?
Crocodilians are large predatory reptiles such as crocodiles, alligators, caiman and gharials.
Crocodylia have roamed the Earth for millions of years. As with modern day crocodylia, prehistoric crocodiles and alligators lived and adapted to a variety of different environments. However, that doesn’t mean they have stayed the same to modern day. Crocodylia have changed significantly over Earth’s history and can show the variation in the climate and environment whilst also giving an insight into the evolution of reptiles over time.
Scientists are still working on uncovering how the crocodilian snouts have changed over time and how this might reveal their environment, diets and habits, so have a look at some examples below showcasing some of the different snout shapes!
66 million years ago: The Simosuchus clarki… A short snout
Smokeybjb, CC BY-SA 3.0, via Wikimedia Commons
125 million years ago: Thalattosuchia… A Long snout
Optional Activity
Sketching a Fossil
Try out your identification skills with this final activity.
Using the 3D model above, have a go at creating a sketch of the fossil and using the keywords in the worksheet (press the green button to open this worksheet) identify each of the key parts of the fossil.