Mosasaur Sails/Vertebrae

[Amtoine Grant <ajgrant@eastlink.ca>]

Hey all, what's the concensus on mosasaurs' often depicted 'elongate 
sail' of sorts? Being as they are often depicted with dorsal & ventral 
sails along the tail for added propulsion(ie. water snakes), I've 
noticed that the actual skeletons do not support this.
Off the top of my head, only Clidastes or Platecarpus(I forget which 
one) has even a bulge in the tail vertebrae to suggest this. I know 
that the outline or imprint of the unfossilized upper lobe on 
icthyosaurs is often preserved, but has it been for mosasaurs?
I'm modeling a 3D Tylosaurus so I need to know.