The shells of aragonite are affected by various ongoing earth dynamic processes, such as physical and chemical weathering and erosion of the rock layers that contain these fossils, chemical reactions between fossils and acid water dissolving the materials of the shells.
The older Paleozoic and Mesozoic rocks have undergone significant changes with increasing time due to the effects of events such as tectonism, volcanism, erosion and so on. So the fossils present in these rocks were affected and were altered. But the rocks of the Cenozoic era are of recent age, about 65-60 million years old. During this era,<u> massive deposition of sediments has taken place throughout the globe which provides a suitable condition for the preservation of fossils like aragonite shells</u>. So these fossils found in these deposits are not much altered.
