THE MESOZOIC SEAS

INTRODUCTION

No dinosaurs took to the Mesozoic seas. Many of the plants and animals that were to be found in the Mesozoic seas would be quite familiar to you. Fishes,turtles and sharks were plentiful and many shelly animals which exist today had relatives that lived in the seas. The notable shelly creatures that did not survive are the belemnites and the ammonites. However the big difference was in the big predators. Instead of dolphins, seals and killer whales, the marine mammal predators of the Cenozoic, great marine reptiles patrolled the oceans.

AMMONITES AND BELEMNITES

These two groups were numerous during the Mesozoic. The ammonites are very useful to geologists today, because they help to work out the relative age of the rocks they are found in and check whether two groups of rocks far apart from each other today were laid down at the same time. This is because they evolved quite quickly and were distributed all over the Earth's oceans because they swam freely in the water. This technique is called biostratigraphy The ammonites and belmenites are both types of molluscs. They are part of a group called cephalopods. Today only the nautaloids survive as examples of what the ammonites looked like in life. Squids and octopi are related to belemnites, but do not possess external shells. This groups is known as the coleoids. The belemnites did not make it past the K-T boundary. Cephalopods have adapted to swim freely in the water and use their tentacles to grab prey and drag it to their mouths. Although many people think of the ammonites as being coiled in a spiral they achieved many different forms,as you can see in the image below. Their size varied too.

The collection of ammonities above shows the amazing range of sizes and shapes that they grew to. This is what helps geologists to define the various time zones in biostratigraphy. The scale bar is 5cm long. Note the ammonite in the centre right of the picture that has coiled in a vertical rather than horizontal fashion.

PREDATOR VERSUS PREY

The struggle to eat and to avoid being eaten is a constant battle. Today you can watch as cheetahs try to run down gazelle on the African plains. As one animal becomes better at what it does, then the other must improve or suffer hunger or death. The invertebrates in the seas have exactly the same struggles going on. Ammonites were preyed upon by the 'sea dragons' and thus animals thicker shells and improved their jet systems to allow them to escape the jaws of the predator were favoured. One ammonite shell that had been bitten sixteen times and survived is known.

Predators come up with new tricks too. The gastropods started to bore into the shells of their bivalve prey during the Cretaceous. Bivalve shells are thinner and more vulnerable to boring than gastropods and a combination of a rasping tongue and the use of acid, which dissolved the shell of the victim allowed gastropods to become more efficient feeders. The final step the process was to suck out the flesh of the bivalve by inserting a mouth tube.

These 'arms races', as they are sometimes called, have gone on since Cambrian times. The meat-eating dinosaurs and their prey would have had similar struggles, showing that very different types of animals are governed by similar rules.

TURTLES

Turtles first appeared in the Triassic. They are reptiles but not part of the archosaur group. They lived in much the same way as turtles do today. The main difference is that the Triassic turtles still had some teeth They would have spent a lot of time in the water, possibly to escape the dinosaur predators. They went on to land to lay thei eggs as turtles still do today. They would have feed on shellfish, using their powerful beaks to crush their prey.

During the Cretaceous the turtles, along with many other groups of reptiles underwent a period of 'giantism'. This meant that some turtles like Archelon became up to 4m long and weighed up to 1 tonne. That's the size of many cars. Large modern turtles are only about half this size.

SEA DRAGONS-ICHTHYOSAURS, PLEISIOSAURS AND MOSASAURS

ORIGINS-FROM LAND TO SEA

The marine reptiles are just that. Reptiles which evolved from being land dwellers to marine animals. The mosasours of the Late Cretaceous are closely related to a group called monitor lizards which survive today. The plesiosaurs and icthyosaurs had other ancestors and are reptiles too. This means that they had to breath air like all other lizards, so they returned to the surface periodically. The marine mammals that we see today, whales, seals and dolphins have followed the same pattern of moving from the land into the sea. So, although people often think that mammals have lots of new ways of life, the reptiles were there first.

ICTHYOSAURS ('fish reptiles')

These reptiles appeared in the Late Triassic. They had the same fish-like body shape as dolphins. This is known as convergent evolution. Like dolphins they were predators, using their streamlined shape and their small stabbing teeth to catch fish and ammonities. The early icthyosaurs were quite small but by the Jurassic some of these animals grew up to 9m long, that's about five people lying head to toe! The icthyosaurs were very highly adapted for spending their entire lives in water. Unable to crawl onto land, they gave birth to live young, as mammals do. This is called viviparity. The evidence for this comes from several icthyosaur fossils which have been found with their young still inside the womb.

This icthyosaur is part of the Bristol University Geology collection. It was collected from a quarry in Somerset. The specimen is from the Triassic/Jurassic boundary, which makes it about 200 million years old. Thanks to Dr Liz Loeffler, who helped me to take the photo. Please don't lift this image, as it would cause copyright problems. Thanks!

PLESIOSAURS ('ribbon reptiles')

Plesiosaurs fall into two groups. The 'plesiosauroid' plesiosaurs had elongated necks with quite small heads. Their mouths were filled with dagger-like teeth to stab into fish. They would have twisted their neck around to help them to catch their prey and probably attacked their prey by ambush. The 'pliosauroid' forms had much shorter necks and were heavily built and had huge heads. They probably chased their prey actively, unlike the 'pleiosauriods'.With teeth of up to 23cm they would have been ferocious, even able to kill other plesiosaurs! They would have also preyed upon turtles and ammonites. There is some speculation that if 'Nessie' is real then the animal in Loch Ness is some sort of plesiosur! Both groups had short tails and would have used their large, paddlelike flippers to swim. They would have probably moved in a similar fashion to seals,'flying' underwater.

MOSASAURS ('reptiles from the Meuse')

The Late Cretaceous saw the appearence of these giant marine lizards.These predatory animals had long, flexible bodies and short necks. Like icthyosaurs they had teeth for eating fish, although some had teeth that suggest that they were able to lever shellfish off the rocks and eat them. Although they were large, (4.5-9m in length) they would not have been able to evade the 'pliosauroid' hunters or the giant sharks that cruised the Cretaceous seas. Despite arising so late in the Mesozoic they are known in great numbers from the chalky deposits of North America and Northern Europe. The Meuse is a river in northern France, in case you were wondering.

WHO ATE WHO IN THE MESOZOIC SEAS?

Just as in modern marine ecosystems, all life needed energy to survive and reproduce. Ultimately, nearly all forms of life depend on solar energy. Humans get their energy by eating vegetables and crops that grow due to sunlight, or by eating animals that have eaten these plants. This type of structure is known as a food chain. You can look at a rough Mesozoic food chain below. Such chains still exist today, but with different organisms in the slots. At each step up the food chain there are fewer numbers of animals at that level, because a given food chain can only support so many individuals and bigger species need more energy. Remember that the waste products of the living members of a food chain and the bodies of the dead are decomposed by scavengers and bacteria, which help to return nutrients to the system.