I think the fins were originally a turtle for the new sea-elves; same as the "howdah" piece on the things back. Edit: yup looks like it. Whale conversion is a preparation for upcoming Age of Sigmar: Apocalypse (or maybe Ragnarok? or RAGNARORK??)
Indeed they seemed to use the Leviadon as the basis. Or using the awful AoS names, RAGNARORRUK! In any case, an AoS apocalypse would be fun to see especially as 40K has gone down the drain - now all the Fyreslayers players who collected and painted several hordes of 30 Vulkite Berzerkers for First Edition will be able to use them again without them being a burden on the rest of their army (I'm aiming for smaller units myself but there are those who have done this).