Owner Comments:
In constructing this set, which has strived to illustrate the amazing fact that birds are living dinosaurs, it has occurred to me that there are many types of prehistoric animals that people think are dinosaurs, which actually are not. To address this problem and to fill in the story even more, I have decided to include some coins featuring prehistoric animals which are thought to be dinosaurs by many people, but which are not dinosaurs. This coin is the first of my “What is not a dinosaur” mini-set within this set featuring Tylosaurus pembinensis.
First, a little about Tylosaurus. Tylosaurus was a horrifying and huge marine predator that lived from 86.5–75 million years ago, during the late Cretaceous period. This aquatic monstrosity could reach up to 46 feet long and it was an undoubted apex predator in its ecosystem, the many seas and oceans of the late Mesozoic Era. Tylosaurus basically fed upon anything it could kill, including sharks and other fish, smaller mosasaurs, plesiosaurs and dinosaurs. It may not have been a dinosaur, but it definitely ate them. A skeleton of a Hesperornis was found in the abdominal region of one Tylosaurus fossil and a hadrosaur fossil shows evidence of scavenging by a Tylosaurus after it died and was washed out to sea. This thing was one of the most terrifying aquatic animals to ever live, and it was undoubtedly one of the most voracious and nasty predators of all time. It’s likely that Tylosaurus had no predators and that even large predatory Theropods such as Tyrannosaurs would try to stay out of its way when they were near the water.
So, if Tylosaurus isn’t a dinosaur, what is it? Tylosaurus is a mosasaur, which is actually a kind of lizard closely related to living monitor lizards and snakes. Skin impressions of Tylosaurus have been found which show scales very similar to these living reptiles. So, not only is Tylosaurus not a dinosaur, it’s not even a close relative of them. Tylosaurus was a Lepidosaur, which is the reptile clade that includes lizards, snakes and rhynchocephalians, a reptile order which was widespread in the Mesozoic but now only has one living group, the Tuatara of New Zealand. Dinosaurs are Archosaurs, the reptile group containing them, pterosaurs and crocodilians. Cladisitically speaking, a mosasaur is almost as far from being a dinosaur that an animal can be and still be considered a reptile. Only a rhynchocephalian is more distantly related to dinosaurs among the reptiles than lizards such as Tylosaurus are.
So, in spite of the fact that many people think that mosasaurs like Tylosaurus were aquatic dinosaurs, they were not. They were aquatic lizards. However, mosasaurs do have one thing in common with dinosaurs: they were likely endothermic as dinosaurs are. While there is no absolute proof of this, there is one thing that strongly suggests this was true: mosasaurs went extinct when that meteor hit 66 million years ago, and they did so rather quickly. Since some larger ectothermic reptiles survived such as crocodilians and sea turtles, there had to be some difference between them and the mosasaurs. The full annihilation of mosasaurs at the end of the Cretaceous indicates that the food shortages which caused the almost total extinction of the dinosaurs also took down the mosasaurs.
A 46-foot-long endothermic animal with a multi-ton weight requires a lot of food to keep going and unlike dinosaurs, mosasaurs do not appear to have evolved any small species. The surviving small dinosaurs could fly and had beaks, so they could change their feeding strategies quickly and look for scarce food resources over a large area. The surviving ectothermic crocodilians and sea turtles could go months without eating, regardless of their large sizes. Mosasaurs were toothy giants with flippers and flukes with huge food needs who were held prisoner in their marine habitats by their bodies. They were unable to undertake any of the survival strategies used by ectotherms and avian theropods to survive in the post-meteor holocaust that was the beginning of the Paleogene. Once superbly adapted for their ecological niche and some of the fiercest predators on Earth who had nothing to fear, mosasaurs were instantly doomed once the food chain fell apart. They likely were among the first animals to completely die off during the Cretaceous-Paleogene Extinction Event.
So, this one’s for you Tylosaurus. You may not be a dinosaur and I’m sorry your kind met such a rapid and bad end, but as I am already afraid of deep water, I think I’m happy that I never met you.