The Marine Reptile Timeline

The new Jurassic World trailer made me want to learn more about marine reptiles, so that’s what this post is all about.

When you hear the phrase “marine reptile,” your first thought is probably something along the lines of a marine iguana, sea snake, saltwater crocodile, or sea turtle. And you wouldn’t be wrong. Those are all reptiles that spend a good amount of time in the water.

A marine iguana doing as marine iguanas do. By Peter Wilton (Marine iguana  Uploaded by Magnus Manske) [CC-BY-2.0 (], via Wikimedia Commons

A marine iguana doing as marine iguanas do. (Photo by Peter Wilton.)

Those of you with a dab more prehistoric knowledge will also probably think of some extinct marine reptiles- your plesiosaurs, ichthyosaurs, mosasaurs, et cetera. Those of us who grew up with any knowledge of early 2000s viral videos will probably think of Liopleurodon thanks to Charlie the Unicorn.

But wait, some of you will now be saying, aren’t the liopleurodons and their long-necked brethren just, like, water dinosaurs?

This is a common misconception. There were no aquatic non-avian dinosaurs that we know of  (though the reconstructed Spinosaurus might be a contender). Liopleurodon was much more closely related to modern lizards than to any dinosaur. However, not all marine reptiles are lizards, obviously- they are not a monophyletic group (meaning, they are not all on the same branch of the family tree). The moniker ‘marine reptile’ is kind of misleading, as is, actually, the word ‘reptile.’

Let’s quickly look at a vertebrate phylogeny.


The green area highlights branches containing animals commonly known as reptiles. As you can see, it does not perfectly cover any one section of the phylogeny. (Pisces = fish, Synapsida = mostly mammals, Testudina = turtles, Lepidosauria = lizards, snakes, and tuataras, Crocodylia = crocodilians, Aves = birds. Dinosaurs are not shown because this is a living phylogeny, but they would be just under Aves.)

I already mentioned that marine reptiles include sea turtles, crocodiles, and marine iguanas, so that means that marine reptiles actually appear independently in three different branches of this phylogeny. This is similar to how marine mammals like seals and manatees and whales all evolved independently of each other, though turtles and crocodiles are much more distantly related than any pair of mammals.

If we ignore freshwater reptiles and only focus on their seagoing relatives, there are about forty living species of marine reptiles today, and surprisingly, most of them are sea snakes. (Sea snakes are not eels, before anyone asks. Eels are fish, sea snakes are snakes.)

Did you know that most sea snakes give live birth? (Photo by Craig D.)

Did you know that most sea snakes give live birth? (Photo by Craig D.)

However, this is really just the tip of the iceberg. Before marine mammals took over the water- and became some of the largest lifeforms ever to exist- there was a very long history of diverse and impressive marine reptiles.

Quick disclaimer: a lot of the taxonomic relationships of these extinct reptiles are hotly in dispute, and as such, there are many conflicting theories out there about their placement. I did the best I could with the info I found, but please take it all with a grain of salt.

Now, let’s go back to the mammal-free seas!

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