To be an index fossil, a fossil must (1) be present in rocks that are scattered over a
large geographic area
(2) be the remains of an organism that lived for a
long period of geologic time
(3) occur in small numbers within the rock layers
(4) consist of bones from an extinct vertebrate

Density because it’s holyyy

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).