Which type of change would be more likely to have se­vere consequences—substitution or deletion? why?

While both will equate changes in a genetic chain, deletion would have more severe consequences.  A substitution mutation usually only causes minor disorders and diseases, like sickle cell anemia.  Whereas deletion causes much more serious diseases, like cystic fibrosis, and turner syndrome which is  where a female is born with only one X chromosome.  

i think the miocene epoch spans the time from 23 million to 12 million years ago.

the sphincter muscles at the end of the digestive tract is the anus.