What is a plesiosaur?
I've always assumed plesiosaurs were dinosaurs that lived in water. I've found out I was wrong (It's OK, I'm used to that.) A very intelligent 7 year old asked me why some dinosaurs lived on land and some in water. While trying to get the answer to her question I found that my assumption, that there were aquatic or marine dinosaurs, was incorrect. I tried to research plesiosaurs, because they were the first animals that came to my mind when I thought of "marine dinosaurs". Everything I've read says that they were not dinosaurs, but doesn't say why.

I guess my bottom line is: Please tell me the criteria used to determine that a plesiosaur (or any other water-dwelling creature) that looked liked a dinosaur and had "saur" at the end of it's name was not a dinosaur. I'd much appreciate it if your answer is understandable to a bright 7 year old.....and to a dumb 50 year old.
First thing. The -saur at the end of the name means 'lizard'. Its' use dates from a time when dinosaurs (which means 'terrible lizards') were first described. At that time only parts of skeletons had been found, and the first reconstructions of dinosaurs were as giant lizards. There are some wonderful life-sized models at the Crystal Palace at Sydenham, South London which show dinosaurs in this way. Since then we have learned that dinosaurs were very different from modern reptiles. They stood upright, with their legs under the body as modern mammals do (think of a lizard with its' legs stuck out sideways). It is possible that they were warm-blooded (though there is a lot of argument on that point). They had much more sophisticated feeding and digestive mechanisms than modern reptiles. We are now discovering that many of them had feathers. It's highly likely that one the most famous dinosaurs, the dreaded Tyrannosaurs rex, had feathers! Dinosaurs were the ancestors of modern birds, and shared many of their characteristics. There are other details, such as the structure of their skulls and so on which show the similarity to birds, and their difference from reptiles.

Plesiosaurs were much more like modern reptiles than dinosaurs. They had reptile-like skulls, and were reptilian in other details of their skeleton. We don't know about the soft tissues - the muscles and internal organs - because these are very very rarely preserved in any animal. Plesiosaurs were the second type of extinct animal known to science, well before the dinosaurs. The name mean 'almost lizard', as their anatomy is very lizard-like. It was the discovery of plesiosaurs in the 1820's that gave a boost to thoughts on evolution which led to Darwin's great work in 1853. The first extinct animal known to science was the ichthyosaur - the name means 'fish-lizard'. This is rather similar to modern dolphins and tuna, and its' discovery in the 1810's didn't challenge too many beliefs. But plesiosaurs, with their long necks, short bodies and flippers. are so unlike any modern animal that they couldn't be dismissed as varieties of known existing animals.

It is a bit of a puzzle that we haven't found any marine dinosaurs. After all, birds are quite capable of becoming aquatic - just think of penguins. One explanation could be that the plesiosaurs and other marine reptiles, the ichthyosaurs and the mosasaurs, were so successful that they didn't leave any ecological niches free for the dinosaurs to exploit. We know that the oldest of these groups, the ichthyosaurs, were already well-established in the Triassic period, around 220 million years ago, when the dinosaurs were just starting to emerge. They were very big, some of them as big as modern whales. An ichthyosaur has recently discovered in Canada which is 23 meters long (75 feet). There was also a range of ichthyosaurs of all sizes, from 1 meter (3 feet) long up to the size of the giants. There was simply no opportunity for dinosaurs to move into the sea.

I hope this answers your questions, and satisfies your bright 7 year old! If you need any more answers, email me again