Not all fossils are skeletons of marine animals or the bones of terrestrial forms.  During the Cretaceous, Paleogene, and Neogene certain trees produced so much sap that it entombed insects in the sticky substance. Because the sap was water insoluble, the insects so trapped were preserved for millions of years. Below is a photograph of an ant that lived in Europe 40 million years ago.  Such fossils show the ants were well established by the late Paleogene, and that they split from their wasp ancestors much earlier.  Indeed, such a fossil of a wasp/ant ancestor was found in Cretaceous amber from New Jersey.

These fossils provide considerable information because they preserve details not usually seen on mineralized fossils.