WWD: Kimmerosaurus langhami

The Cryptoclidus equivalent, a closely related cryptoclidine plesiosaur that actually lived at the time of the episode. There were two suboptimal choices from the Kimmeridge Clay, either one with basically only a skull or the one with only post-cranial remains (Colymbosaurus), and I picked the one with the skull, although it's possible they're the same animal, in which case the latter name takes priority. You may recognize Kimmerosaurus from the other BBC prehistory documentary series, Planet Dinosaur, although it was portrayed a little bit speculatively there, being much larger than it really was.

In WWD, Cryptoclidus is also portrayed as being much larger than it really was, at a staggering eight tonnes, which is close to nine times heavier than the real animal is thought to be. Cryptoclidus is about the same size as this Kimmerosaurus, so your guess is as good as mine how anyone looked at this and thought it somehow weighed more than an adult bull African elephant.

More significant than its size was its portrayal as a pinniped-like semi-aquatic animal. WWD is possibly the last real depiction of plesiosaurs as being able to crawl on land. Skeletal anatomy indicates the likely impossibility of this, as their bodies and flippers were too stiff to do a seal-like shuffle or bend to push themselves about, and the muscles supporting the neck would not have been strong enough to hold its head up out of the water. The larger species were also almost certainly way too heavy to survive on land. All this suggests they were no more terrestrial than dolphins or manatees.

The episode heavily implies that Cryptoclidus laid eggs on land due to its semi-aquatic nature, but we know now for certain that plesiosaurs gave birth to live young. Unlike ichthyosaurs, which gave birth to a relatively large number of small young, plesiosaurs had birthing patterns similar to modern marine mammals, giving birth to just one large infant at a time. Due to this, it's believed that they must've had parental care. If this had been known at the time, I think they might've made Cryptoclidus the main character instead of Ophthalmosaurus, since the situation would've been more behaviourally interesting.

Another minor addition is the tail fluke. It wasn't widely known at the time, but closer examination of plesiosaur tail anatomy and a fossil of a Seeleyosaurus which seems to preserve the soft tissue of a fin strongly suggests its presence. The exact shape is not known for certain however; it's possible that the fin was horizontal, like in cetaceans, rather than vertical.