Natural History And Fossils

The Story Of Life On Planet Earth

What are Seamonsters?

SeaMonsters are large swimming reptiles that lived in the Mesozoic era (Triassic, Jurassic and Cretaceous). Some were only a few feet long but some grew to imense proportions! At the start of the Triassic, Seamonsters were small in size but as time went on they got bigger and bigger and bigger! Soon they were the biggest and most dangerous things in the ocean. 

Nothosaurs, Placodonts and Early Icthyosaurs- Triassic Period

 During the Triassic, A wide variety of Seamonsters evolved, you will read about each one in this  paragraph:

 Nothosaurs- Nothosaurs were streamline, speedy swimmers similar to Plesiosaurs. Some were less  than 30cm long while others grew to 4m. A Nothosaur flapped or rowed with its powerful front legs  and used its back legs for stearin. It could dart its long, flexible neck to grab slippery prey with its  rows of sharp, peg like teeth. One of the largest Nothosaurs was Nothosaurus which was 3m long  and hunted fish and squid.

 Placodonts-  Before dinosaurs lived on land, Placodonts lived in the sea. Some resembled big armour  plated newts, others looked like crocodiles or turtles. Placodonts flat teeth were designed to crush  shellfish.

 Early Icthyosaurs- The first icthyosaurs were quite large. Shonisaurus was the largest growing to  15m long! Shonisaurus lived in the late Triassic. The other large Icthyosaur was Cymbospondylus  which also grew quite large. Later Icthyosaurs would reduce in size due to the other large predators  that were evolving.


Plesiosaurs, Pliosaurs and Icthyosaurs- Jurassic and Cretaceous

Plesiosaurs were large swimming Seamonsters with incredibly long necks. Some had necks that made up more than ½ of the animals body length. There were advantages in having long necks. One is that a Plesiosaurs head and neck could veer the animal to one side by angling the head and neck to the left or right. Here are some Plesiosaur species:

Plesiosaurus: Perhaps the most well known of the Plesiosaurs, Plesiosaurus grew to an estimated 3-5m in length. It hunted Fish and Squid. Plesiosaurus’s flippers could propel it along at incredible speeds and the flippers also acted as turning devices that could easily change the Plesiosaurus’s direction instantly.

Woolungasaurus: Woolungasaurus was an 8m long Plesiosaur from Australia. It also hunted Fish. Woolungasaurus’s biggest enemy was a Pliosaur called, Kronosaurus. Woolungasaurus was much more agile and quick than Kronosaurus so, Woolungasaurus might have been able to escape the Pliosaurs jaws most of the time.

Elasmosaurus: Elasmosaurus was the largest of all the Plesiosaurs. It grew to 14m in length and hunted fish and other small Seamonsters. Its size was its best protection but it was still hunted by the giant Mosasaur, Tylosaurus.

Pliosaurs are a subgroup of Plesiosaurs with short necks instead of long ones. Here are a couple of Pliosaur species:

Liopleurodon: Liopleurodon was quite possibly the largest killing machine ever! It grew to 23m in length, that’s bigger than the sperm whale!
It hunted every other Seamonster. It was 20 times heavier than t-rex and had teeth twice as big.

Kronosaurus: Kronosaurus wasn’t quite as big as Liopleurodon but it still packed a punch! Kronosaurus lived in Australia during the Cretaceous period along with the Plesiosaur, Woolungasaurus, which it liked to prey upon.

As you read above the Icthyosaurs started off big and got smaller as the time went by. Here is quite possibly the last species of Icthyosaur.

Icthyosaurus: Icthyosaurus was only about 2m long compared to the Late Triassic, Shonisaurus (See Nothosaurs, Placodonts and Early Icthyosaurs) that was 15m in length! Icthyosaurus had enormous eyes compared to its body size. These were used to spot Fish and Squid in the murky water or at night. Icthyosaurus never made it to the Late Cretaceous and all other Icthyosaurs were extinct by then as well.

Mosasaurs- Cretaceous Period

Mosasaurs were giant swimming sea lizards with large heads and powerful jaws. They were the top predators throughout the Cretaceous. Some were only a few feet but others were bigger than buses, and all Mosasaurs had one thing in common. They were cannibals. Here are a few species of Mosasaur:

Platecarpus: Platecarpus lived in the Triassic period and was about 16ft in length. Platecarpus hunted fish, crabs and other small Mosasaurs. Eventually, Platecarpus died out due to larger species of Mosasaurs that were evolving.

Mosasaurus: Mosasaurus was much larger than Platecarpus. Mosasaurus was the top predator in the early Cretaceous and Late Jurassic. It hunted Plesiosaurs, Icthyosaurs and just about any other living creature during its time.

Tylosaurus: Tylosaurus was the largest of the Mosasaurs, it grew to 14m in length and stalked the Late Cretaceous seas. It preyed on others of its kind and even the giant Plesiosaur, Elasmosaurus. Big Fish skeletons have been found in the fossilized stomach of this monster. 

Image- Kronosaurus

Image- Mosasaurus And Plesiosaurus