Although some ornithologists contend that the precursors of birds are arboreal creatures that glide from tree to tree, other believe that they were runners whose front limbs evolved into wings
(A) Although some ornithologists contend that the precursors of birds are arboreal creatures that glide from tree to tree ---
present tense 'are' and 'glide' are wrong; The main clause as may be seen is in past tense (B) However, it may be that some ornithologists contend that the precursors of birds were arboreal creatures that glide from tree to tree --
these prehistoric creatures are not there now to glide. Only a past tense verb is the correct form.
(C) Despite that the precursors of birds were, according to some ornithologists, arboreal creatures that glide from tree to tree. ----
that glided is the correct tense.
(D) Although some ornithologist content that the precursors of birds were arboreal creatures that glided from tree to tree. ----
is the correct choice avoiding the shift of tense. (E) According to some ornithologists gliding from tree to tree, the precursors of birds are arboreal creatures, that --
this says that the ornithologists are gliding from tree to tree.