Before we begin, I should note that recently a jawbone of an even LARGER ichthyosaur was discovered in the UK. However, being just a jaw, it's hard to say what it would have looked like.
Now, this is going to veer on the more controversial side, as there are a LOT of conflicting ideas of how this animal was shaped- so I'll jump straight into my rationale for reconstructing this thing.
As we know, there is a huge debate over what, exactly S. sikanniensis is even supposed to be; whether the "s" is for Shonisaurus, or Shastasaurus; two related, but anatomically very dissimilar animals.
Which brings me to the next point- anatomy. Fortunately, Shastasaurus sikanniensis is known from an impressively complete skeleton, rather than a few meagre fragments, so reconstructing this thing wasn't a complete stab in the dark.
Well, sort of. Most of the visuals of this animal were tourist photos taken at obnoxiously bad angles, and after some exhaustive searching for images of the skeletal anatomy, I managed to find a few sources.
-A very small thumbnail of the top of the animal I managed to swipe from the Nicholls and Manabe study into this animal via google image search.
-A series of skeletal reconstructions at various angles by David Peters (for those who don't know who he is, he does excellent skeletals, but is known to make some very, very, very bold, abstract and outspoken top-down anatomical assumptions)
-Some well preserved skeletons of OTHER species of Shastasaurus, shown at various angles.
-Scott Hartman's Shonisaurus skeletal, which looks very dissimilar to any of the Shastasaurus skeletons, but still good as they're supposedly close relatives and Scott knows his anatomy.
So, what was I to do?
Simple- I was going to compare the top-view skeletals and see a pattern- and one stands out clearly- using Nicholls and Manabe as the 'true' model to build from.
While Shonisaurus appears to have a short, rigid neck and a dorsal-ventral 'tall' thorax; S. sikanniensis along with all of the Shastasaurus species have a long bendy neck, elongated bendy body with a laterally-protruding thorax- which reflected in other Ichthyosaurs, suggests it was an elongated, snaky animal rather than a dense, porpoise-shaped creature. All of the specimens were basically the same and extremely difficult to tell apart. Therefore, I could entertain the assumption that this animal was in fact a Shastasaurus. It is interesting to note that David Peters' reconstruction looks very similar to the Nicholls and Manabe one- merely the latter (which I erred to most) had a much wider skull, and was actually set to a much larger scale than Peters version. Otherwise, I felt confident that Peters' skeletal reconstruction was a reliable enough substitute for most of the missing pieces; which I quickly realized he was filling in with features of other Shastasauruses, which.... well.... I was going to do anyway.
Two noticeable departures from the 'popular' depiction of S. sikanniensis were the elongated snout- an unusual feature of Shastasauruses, but due to S. sikanniensis' skull structure tapering suddenly forward before the front of the skull was destroyed by time shows clear signs of an elongated nose (unless the skull were to suddenly taper inwards... which is unlikely. Peters clearly made the same assumption. Secondly, Peters inferred the flippers (particularly the front flippers) were tiny. However, I disagree; the sheer brawn of the humerus, radius and ulna could only mean one thing, as far as the typical ichthyosaur anatomy appears to be concerned.... giant pointy flippers.
So all that remained was the soft-tissue anatomy.
Peters inferred this animal would have a slim neck, an elongated tail and a tiny fluke. But given literally every ichthyosaur fossil showing clues to soft-tissue anatomy show a fatty creature with a sleek, projectile-shaped body and a massive sharklike tailfin, I opted for that, noting the tiny bones at the tip of the tail seem to consistently curve sharply downwards.
Skin colour, I opted for something a little darker (as the only known ichthyosaur skin appears to be jet-black) but diffused and slightly different (to infer separate species, and of course size).
Naturally, these changes has resulted in an animal whose 'reduced' tail length (along a horizontal plane) gives it a length of 19 meters (from snout to the tailfluke) and 19.5 meters to the very tips of the tailfins.