Fossils

Fossil 'X' can be considered to have evolved earlier than fossil 'Y' if fossil
(a) 'Y' has vestigial structures that are homologous to functional structures in fossil 'X'.
(b) 'Y' is structurally more complex than fossil 'X'.
(c) 'Y' is in better state of preservation than 'X'.
(d) 'X' is found in lower stratum of undisturbed sedimentary rock than 'Y'.