General questions about plesiosaurs

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


Were they dinosaurs?


I need to know if the plesiosaur was a reptile hip dino or a bird hipped dino.


The plesiosaur was neither - it was not a dinosaur. Dinosaurs all lived on the land, and were related to birds and crocodiles. Plesiosaurs lived in the sea, and were related to lizards and turtles

What was the largest pliosaur?


I'm working on a learning fair project with my son titled "Giant Predators of the Ancient Seas". We are having trouble finding a definitive answer to the question of the which pliosaur was the largest. Resources tend to name either Liopleurodon or Kronosaurus. Please let us know what the current thinking on the matter is.?


The identity of the largest pliosaur has been a matter for some debate recently, and most popular books are quite simply wrong. I think it comes from the BBC series, 'Walking with Dinosaurs', which featured 'Liopleurodon' as a 25m, 150 ton monster. Other researchers for popular books have taken these numbers at face value, and changed them slightly to make it look as if they've done their own research (they make it 23 or 24 m long, rather than 25).

I've worked with a number of specimens of Liopleurodon, and it was not 25 meters long. At the most, it was around 10m, weighing around 4 tons - a formidable predator, but not baleen whale size. However, there are a few odd bones which show that there was another, much larger predator around at the same time. All we have are a couple of teeth about 400mm long (the largest liopleurodon teeth measure about 200mm from root to tip - much the size of T-rex teeth), three vertebrae the diameter of dinner plates - well over twice the diameter of those of Liopleurodon, and 40% bigger than those of Kronosaurus. There is also a lower jaw in Oxford Museum which is over 3m long, and about 10 million years younger than Liopleurodon. The skull of Liopleurodon is about one seventh of its' overall length, which would make this animal 21 meters long, weighing around 20 tons - around the size of a Sperm Whale, the largest modern carnivore.

The problem with these few enormous bones is that we don't have enough material, or material of the right kind, to work out if they came from a new species of mega-predator, or from a very large individual of a known species. The jury is still out, and every time I go down one of the clay pits, I hope.

Kronosaurus was a very large animal, though not as large as this British pliosaur. It is well known from the skeleton in Harvard Museum of Natural History. Unfortunately, this specimen is largely reconstructed from fragmentary material, and is much longer than more recent finds have shown the animal to have been. One of the best preserved comes form Columbia, and shows that it was a strange beast, with a huge head about one quarter of its' overall length.

When talking about marine monsters, I have to mention the latest and most extraordinary find of all, a giant ichthyosaur from British Columbia, Canada. This monster is 23 meters long, and is a complete, articulated skeleton. The skull alone is 5.5 meters long. We don't know much about it yet. It is being excavated bit by bit, but as it is in a very remote place all the bones have to be taken out by helicopter. The block containing half the skull weighed 3 tons!

Hope this helps your son with his science project

Were there any fresh water plesiosaurs?


I am a senior at Pennsylvania State University doing my thesis on the paleoclimate of southeastern Australia during the early Cretaceous. Sediments in the area are interpreted as being fluvial/lacustrine while my thesis advisor and I believe them to be marine.

Plesiosaur teeth have been found by Patricia and Thomas Rich in these "freshwater" sediments and they have interpreted theses plesiosaurs to be freshwater. Plesiosaur bones have also been found in the Judith River Group in Montana/Alberta in fluvial sediments.

A friend of my thesis advisor mentioned that large marine whales swim up rivers at times to free themselves of a number of parasites that depend on normal marine salinities.

My question to you is: Do you think that plesiosaurs could have inhabited freshwater? If not, could you offer any explanation to this "freshwater" plesiosaur phenomena?


Cruickshank (1997) reports that several plesiosaurs, especially those from southern continents, have originated from non-marine sediments, and quotes Bartholomai (1966), Molnar (1982, 1984) and Rich et. al. (1989 (note that Cruickshank mistakenly gives the year as 1991)) in support. The remains of Leptocleidus are known from fresh water deposits from the Isle of Wight, though nothing has been published. I have recently submitted for publication an paper on a fauna including small plesiosaurs from lagoonal or estuarine deposits from north Lincolnshire which straddle the Jurassic/Cretaceous boundary.

I can see no reason why plesiosaurs could not have inhabited fresh water - after all (and following a quick mental review) all modern secondary marine adapted animals have freshwater as well as saltwater forms. I suspect that many collections contain plesiosaur bones from freshwater deposits which have been misidentified. I've come across them labeled as dinosaur and pterosaur, as well as the classic 'reptilia indet'.

Why did plesiosaurs become extinct


Why and when did plesiosaurs become extinct?


Plesiosaurs became extinct at the end of the Cretaceous period, about 65 million years ago.

This was one of the great extinction events in the history of the Earth, when many different kinds of living things died out at the same time. The most well-known of these are of course the dinosaurs.

There have been many theories about why this event happened. At the moment, there are two main theories being researched by geologists and palaeontologists.

The best-known of these is that the Earth was hit by a giant asteroid. There is a huge crater buried under the Yucatan peninsular in Mexico. There are deposits all around the globe which have a thin layer which contains an unexpectedly high proportion of an element called iridium. Iridium is rare in normal rocks, but much more common in meteorites, and which gives added support for the asteroid impact theory. The devastation caused by this catastrophe wrecked ecological systems all over the earth, including those in the seas.

Another possible cause is massive volcanic activity. In the southern states of India, an area of 500,000 square kilometers is covered in volcanic deposits over 2,000 meters thick. This was deposited in a series of massive eruptions around 60-65 million years ago, and may have contributed to the mass extinction event. Volcanic eruptions produce large amounts of toxic gasses, and these can also have a devastating effect on living organisms.

It is likely that both these event contributed to the wide-scale destruction of habitats both on land and in the water. It might be that there was already much destruction from volcanic fallout when the asteroid hit.

However it happened, the consequences were massive and far-reaching. On the land, only the birds survived from the dinosaur lineages. Some mammals survived to become the ancestors of all modern mammals, including humans. In the sea, ichthyosaurs, mosasaurs, ammmonites and many other organisms including the plesiosaurs became extinct.