WWD: Pliosaurus macromerus

And finally, the one I'm sure you've all been waiting for, probably the most controversial and infamous taxon depicted in the entire franchise. The history of this one is long, convoluted, and full of misinformation, but I'll just try and summarize it briefly. The episode depicts the taxa Liopleurodon as being a blue whale-sized colossus, the largest predator to ever live. However, there isn't, nor has there ever been, any physical evidence of any pliosaur ever getting even close to that scale, never mind the specific taxon Liopleurodon, which was only medium-sized for a pliosaur, reaching less than 7 metres long.

The main issue stems from a problem that plagues nearly all size estimates for large prehistoric animals: the very fragmentary nature of their fossils. This makes extrapolating size very vulnerable to large margins of error, especially when it's an animal with no close living relatives to compare to, as in this example. Reevaluation of many of the fragmentary remains that supposed pertained to gigantic, 15-20 metre-long pliosaur remains have since been found to represent large, but more reasonable animals of less than 10 metres in length. In one case, a supposed vertebrate belonging to a gigantic Oxford Clay pliosaur, from which the size of the WWD Liopleurodon is primarily inspired by, was later found to have more likely belonged to a cetiosaur sauropod. These estimates were further thrown off by the head-to-body length ratio originally thought to be 1:7, but more complete pliosaur remains found since indicate a general ratio of closer to 1:5. 

Another issue is one common to most animals of the episode, that it lives at the wrong time, living at the end of the Late Jurassic rather than the Mid Jurassic. The reason is for about the same reason as the others, Liopleurodon was a big wastebasket taxa which a large number of specimens were assigned to. This includes the species here, Pliosaurus macromerus, which was lumped into Liopleurodon for a long time, and the rather infamous "Monster of Aramberri", an undescribed Mexican pliosaur fossil (this does not seem to be one of the bases for  WWD's giant Liopleurodon, being only reported on afterwards). A reassessment of the genus in 2022 also found the status of the name itself may be in question, since it's based on a single tooth fragment of dubious distinction (as a general rule, if the prehistoric animal's name ends in -don, like Troodon, Steropodon, Trachodon, Deinodon, and Kollikodon, the holotype was probably terrible and mostly, or exclusively, consisted of teeth).

So anyway, this is the average size of most Pliosaurus species, around eight metres in length and about five metric tonnes in weight, although the very largest probably have exceeded nine metres and eight metric tonnes. Even if they weren't blue whale-sized animals, they were the largest known predators to roam the Earth during the Late Jurassic, far exceeding the mass of the largest dinosaur predators at the time.

Fun fact: Pliosaurus should actually be spelled Pleiosaurus, but that darn Sir Richard Owen forgot how he spelled the name he invented after only using it once, and Pliosaurus has now taken precedence since it's been used far, far more since then. The name doesn't make any sense either way, ("more lizard") because it was only given under the impression that Pliosaurus was more closely related to crocodilians than Plesiosaurus, which it isn't.