One the one hand, the Spinosaurus back in 1937 (when Impossible Creatures takes place), would perhaps be more like this (https://www.tumblr.com/dinosaur-discourse/141350689457/honestly-the-discovery-that-spinosaurus-was ). But on the other hand, most people are familiar with the crocodile-like depiction, thanks to it's more recent discoveries and films and shows like Jurassic Park 3 and Planet Dinosaur. But on a 3rd hand, if it's based on the more modern version, how could would it be to see it swim in IC.

I'd say it's look should be a mix of retro and modern like with the following artwork: https://www.deviantart.com/emilystepp/art/JP3-Spinosaurus-DNA-Variants-1156828785
https://www.deviantart.com/emilystepp/art/Jack-Dragonfly-s-Dinopedia-Set-2-807099948

Or this early design by Luis V. Rey from this 90's Usborne Book: https://chasmosaurs.blogspot.com/2014/02/vintage-dinosaur-art-usborne-book-of.html