Atrilobite is considered on index fossils because this organism existed during the limited in earths history . that means any fossil located in the layer of rock beneath a layer containing trilobite fossils is then a trilobite.

It's considered an index Fossil before it would be a trilobite. Hope this helps :)

explanation:

the universal gravitation equation. since the gravitational force is directly proportional to the mass of both interacting objects, more massive objects will attract each other with a greater gravitational force. if the mass of one of the objects is doubled, then the force of gravity between them is doubled.