The fossil record provides snapshots of the past that, when assembled, illustrate a panorama of evolutionary change over the past four billion years. Although the picture may be incomplete and may have bits of pieces missing – fossil evidence shows that life on Earth is old and has gone through many changes over time. Fossils contain important clues that increase our understanding of life's history and help us to answer questions about many organisms evolutionary stories.
THE FOSSIL RECORD
FUN FACT: Index fossils help paleontologists and geologists to determine the age of rock layers. This is called biostratigraphy. It works like this. If you find an ammonite from a genus known to be from the Triassic Period, then the rock layer it came from must be Triassic.
This ammonite fossil (on left) shows punctures that some scientists have interpreted as the bite mark of a mosasaur (a type of predatory marine reptile that lived around same time as the ammonite). This specific damage to the ammonite has been correlated to the shapes and capabilities of mosasaur teeth and jaws. Other scientists have argued that the holes may have been created by limpets that attached to the ammonite. Researchers examine ammonite fossils, as well as mosasaur fossils and the behaviors of limpets, in order to explore these hypotheses.
Modern scientists often use Ammonites as "index fossils". Their fossils are of fundamental importance in unraveling the ages of rocks, particularly for Mesozoic Era rocks. This is possible because these fossils are relatively abundant in rocks, are widely distributed, they belonged to a group that evolved rapidly, and they are easy to recognize. Because different species evolved and went extinct in rapid succession; the presence of a particular species is indicative of rocks being formed in a particular time period. The hard shell of the ammonite was also easily fossilized. This, combined with the sheer abundance of the species and its evolutionary duration through several geologic periods, make it a great index fossil.
AMMONITES
INDICATION OF INTERACTIONS
