J.R.R. Tolkien in a letter to Rhona Beare - October, 14 1958
"I did not intend the steed of the Witch-King to be what is now called a
'pterodactyl', and often is drawn (with rather less shadowy evidence than lies
behind many monsters of the new and fascinating semi-scientific mythology of the
'Prehistoric'). But obviously it is pterodactylic and owes much to the new
mythology, and its description even provides a sort of way in which it could be a
last survivor of older geologic eras."
( courtesy tolkienonline.com)