Wrong flood. This flood is described in Genesis 1:2, Jeremiah 4 and IIPeter 3. IF these dinos were still plodding God's green earth in the days of Noah, we would still be finding live ones trampling this earth. OR Noah's flood was 'geography' specific where those son's of God cohabited with the daughters of the Adam.
The Adam and his offspring was very carefully traced for us in this generation to know evolution was NOT the vehicle the Heavenly Father elected to bring forth HIS only begotten son.
Perhaps they traced back to Adam and for lack of anymore 'hard' history, decided to begin with Adam. Makes life simple!