The reign of the reptiles: Meet the monsters

From the depths of the ocean to Queensland Museum, discover the secrets of the monsters from the deep with Sea Monsters: Prehistoric Ocean Predators. Presenting the profiles of the three giant marine reptiles that ruled the sea.


Ichthyosaur (pronounced ick-thee-o-sore) remains the original marine reptile or “sea lizard” as they’re often referred to, marking their reign of the ocean for nearly 150 million years.

Identified as an ocean shape-shifter, their evolution saw changes in their structure. At the beginning, their appearance resembled that of a dolphin, with long bendy bodies and crescent shaped tails. Over time their bodies became more rigid and their tails developed extra bones, allowing them to manoeuvre through the water at great speeds of 40 kilometres per hour.

They were speedy, rapidly propelled throughout the water by moving their tails from side to side, much like the movement of tuna and shark. They had big eyes, with the ability to see clearly into the dark depths of the ocean, making them serious ocean predators and attending to their prey with a significant number of teeth.


Taking the form of a lizard-like sea reptile, the plesiosaur (pronounced  plea-zee-o-sore) were the second sea monster to evolve after the ichthyosaur.

Together, they shared the ocean for 100 million years, however the plesiosaurs outlived the ichthyosaur, spanning the sea, seen in the discovery of their fossils found on each and every continent.

Varied in size from as small as 1.5 metres to 15 metres long, the plesiosaur body is described as an almond shape, consisting of a short tail, long flippers and flattened bodies, with differences seen in the shape and size of the head and neck.

Their movement through the ocean saw them flapping their four flippers as birds flap their wings.


The mosasaurs (pronounced: mow-sa-sores) were the last of the sea monsters to dive into the ocean, joining the plesiosaurs five million years after the extinction of the ichthyosaurs.

Just like the ichthyosaurs, their presentation morphed over time from a small lizard-like species to large ocean predators, ruling as the top of the food chain.

Mosasaurs swam like crocodiles, swinging their long tails in a back and forth motion. They had goanna-like bodies, pronounced snouts and forked tongues and were covered in a scaly skin.

Their diet was unbiased covering anything from ammonites, fish, turtles, plesiosaurs, sea birds and even smaller mosasaurs. 

The Tylosaurus (pronounced: Tie-low-sore-us) is the most well-known mosasaur since its starring role in the recent Jurassic World movie and is often referred to as the ‘T.rex of the sea’. That’s a cooler name than its real one – Tylosaurus means ‘knob lizard’!

Their extinction came at the end of the Cretaceous period, ending the sea monsters reign of the sea.

Find out more about these ancient marine reptiles and the mysteries of the deep in Sea Monsters: Prehistoric Ocean Predators.

An Australian National Maritime Museum touring exhibition, developed in partnership with Queensland Museum Network, presented in Brisbane with support from BHP as part of Project DIG.

Images courtesy of Australian National Maritime Museum ©