Some of the important fossils which have been studied are those of organisms X, Y and Z. X were marine arthropods which were common between 400 to 600 million years ago. Y were the invertebrate animals (molluscs) with a flat, coiled, spiral shell which lived in the sea about 180 million years ago. Z are the extinct carnivorous or herbivorous reptiles which appeared on the earth about 250 million years ago and became extinct about 65 million years ago. What are X, Y and Z?
Marine arthropods whose fossils have been studies are known as Trilobites.
Ammonites are the extinct molluscs with flat, coiled spiral shape shell millions of years ago but are now extinct.
Carnivorous animals that appeared about 250mya and disappeared 65 mya are none other than Dinosaurs, as we all know dinosaur disappeared 65mya.
So, X- Trilobites; Y- Ammonites; Z- Dinosaurs.