Styxosaurus is, or was, an elasmosaur – an undersea predator with a long neck and sharp teeth, all the better for grabbing ammonites and prehistoric fish for a quick snack. It takes a lot of smaller, slower creatures to fuel a 17-foot-long swimming reptile. The other fish in this image are Xiphactinus, though, which are (or were) just as predatory and grew about as large. The Wikipedia article describes them thusly: “The genus grew up to 5–6 metres (16–20 ft) in length, and superficially resembled a gargantuan, fanged tarpon.”
Glad I’m not swimming with them.
The image originally came from https://abelov2014.deviantart.com/art/Styxosaurus-Xiphactinus-audax-var-1-658020267, but I found it on Wikimedia Commons.