With the 6 types of Dragons we see in Skyrim they all have distinct anatomical differences (it always melts away to the same basic skeleton). I was wondering, if these are all Jill - Black Dragons - What causes these differences? Age?

-Basic - Pretty basic, not very horny.
-Frost - White and very horny
-Blood - Green and finned. The Argonian of Dragonkind
-Elder - Bronze, not very horny
-Ancient - Red, not very horny

Are these Subspecies of the Jill? If so, are there Elder Frost, Ancient Frost, Elder Blood, and Ancient Blood that were just left out of the game because they were unnecessary?

Having Elder and Ancient Frost Dragons wouldnt make sense because Elder and Ancients use both elements.