From fish to…us!
The rocks along the Jurassic Coast span the period of time known as ‘the Age of the Reptiles’, which includes the era of the dinosaurs. Reptiles evolved from fish that crawled out of the sea up to 400 million years ago to become amphibians. Some of these amphibians evolved into reptiles. And some reptiles branched off to become mammals, including us!
Evolution of reptiles
There are three broad groups of reptile, classified by holes in the skull known as ‘temporal fenestrae’ or ‘windows in the skull’. These holes are where muscles were attached to work the jaws, and they also helped make the skull lighter.
Anapsids – primitive tortoises and turtle-like reptiles
The anapsids are the most primitive form of reptile, and had no holes in the skull. Anapsids look very like tortoises and turtles. Only three skulls have been found in the Triassic rocks of East Devon that we know of.
Synapsids – our ancestors and us
This group had one opening behind the eye and evolved into mammals, including humans
Diapsids – crocodiles, dinosaurs and birds
The diapsids had two openings behind the eye. They included marine reptiles, crocodiles, flying reptiles and dinosaurs (dinosaurs evolved into birds).
Today’s snakes and lizards are also diapsids, but lizards have lost one of the holes in the skull, while snakes have lost both. This tells us that although crocodiles look similar to lizards, they are in fact more closely related to dinosaurs! Recently scientists have begun to think that modern-day turtles are diapsids that evolved back towards the more ‘primitive’ anapsid form.
These facts show just how complex evolution is, and fossils don’t provide all the answers. Most animals that have ever lived have been eaten, smashed up, broken or dissolved away and therefore not fossilised. Scientists are left to work with what’s left – like a jigsaw puzzle with many of the pieces missing.
Triassic reptiles and amphibians
Strange creatures of the desert
The Triassic rocks of East Devon formed in a desert which had huge branching rivers running through it. Strange reptiles grubbed about on the river banks, such as the rhynchosaur (below left) and a tortoise-like animal known as a procolophonid. There were also massive amphibians with heavily-armoured skulls.
We also know that there was a fearsome predator about in those times, but not enough remains have been found yet to indicate how it looked. Amongst the clues are razor-sharp teeth that could have belonged to a monstrous four metre-long carnivore.
When these animals died, their bodies tended to be broken up and carried down the river beds, so most finds are of isolated bones and teeth. But these fragments of Triassic reptiles and amphibians are extremely rare worldwide, which makes the East Devon coast a very important site.
Monsters of the deep!
At the start of the Jurassic period, the sea flooded the Triassic desert – bringing with it the marine reptiles. One of these creatures was to evolve into the scariest sea animal of all time, the pliosaur.
We have fossils of three amazing types of marine reptile in our collections. The plesiosaur was nothing like anything that lives today. It had a long neck and tail, a barrel-like body and four big fins. It later evolved into the pliosaur, a massive creature, about 18m long, with huge toothed jaws. The ichthyosaur was a giant marine reptile that resembled a dolphin.
Copyright Bob Nichols 2014 palaeocreations.com
Sinking onto a stagnant seabed
Occasionally, almost-complete skeletons of these animals are found. This is because they lived in the open water above a stagnant and poisonous sea floor. When the animal died, it sank to the seabed where other animals couldn’t reach it. As a result it became buried without being scavenged and broken up.
But more often, when these creatures died, the gasses created by decomposition bloated the bodies which then floated back up to the surface. There they were pulled apart by other animals. That’s why most finds are single or scattered bones which sank back to the seabed to become fossilised. Some of these bones even contain bite marks made by other marine reptiles.
Fossil poo (coprolites) can tell us a lot about what these animals ate. They are packed with undigested food remains such as fish scales and sometimes the bones of other marine reptiles.
Battered bones and footprints
Dinosaur fossils on the Jurassic Coast are rare. Why? Because when dinosaurs were around (in the late Triassic, Jurassic and Cretaceous periods), most of the rocks of the Jurassic Coast were under the sea. So finds usually consist of fragments of dinosaurs that had died, been washed into the sea, then scavenged before becoming fossilised.
A notable exception is Scelidosaurus, the Charmouth dinosaur (shown below). Two almost-complete skeletons of this plant-eating creature have been found so far. There is another tantalising fact about Scelidosaurus. Its neck, back and tail were covered in armour. This must have been protection from a predator – but remains of this mystery carnivorous dinosaur have yet to be found!
In the early part of the Cretaceous period, Dorset was covered by swamps and lagoons. Dinosaurs walked along the shore of these lagoons, leaving footprints that still exist today.
But what actually makes a dinosaur? Why is a crocodile or a lizard not a dinosaur? They have scales and claws, sharp teeth and tear things apart with them… The answer is simple – dinosaurs have upright legs while crocodiles and lizards have splayed legs.
Pterosaurs (flying reptiles)
Bones too flimsy for fossils
The reptiles also managed to reach for the sky in the form of pterosaurs.However, their fossils are very rare along the Jurassic Coast, partly because the bones were too fragile for fossilisation.
In order to be able to fly, the pterosaurs had very light, hollow bones. These bones were unlikely to sink to the sea floor and become fossilised. As a result, nearly all pterosaur finds consist of isolated or scatterings of bone – those in our collections come from the Kimmeridge Clay and Purbeck Beds.
Anning’s amazing pterosaur
A near-complete pterosaur, called Dimorphodon was found by the famous Lyme Regis fossil collector, Mary Anning, in 1824.
Crocodiles and turtles
Nature’s true survivors
It’s staggering to think that crocodiles and turtles once walked and swam alongside the dinosaurs. They have survived – little changed – for millions of years. Fossils from the Dorset part of the Jurassic Coast record an important stage in the evolution of crocodiles. This was the transition from simpler forms to the creatures we would recognise today.
In the Fossil Finder database we have used the common name ‘crocodile’ but all of our specimens are actually ancestors to modern crocodiles and should really be called ‘crocodilians’. Modern crocodiles, alligators, caimans and gharials (fish-eating crocodiles) appeared around 85 million years ago.
Swanage crocodile and Portland turtle
Goniopholis (below left) is the most ancient ancestor of our salt water crocodile – only found today in places like India, Asia and Australia. And yet, 135 million years ago it lived in Swanage!
Similarly, the ‘Portland turtle’ (below right) is one of the earliest records of its type in Europe.
Extinct creatures Q&A
What’s the difference between dinosaurs, marine reptiles and crocodiles?
Dinosaurs have upright legs whereas crocodiles, turtles and lizards have splayed legs. Marine reptiles have paddles.
How are birds related to dinosaurs?
Like dinosaurs, birds have upright, scaly legs and a reptilian eye – when the eye moves it darts around like a lizard’s. Take a closer look the next time a seagull tries to grab your sandwich! Recent research suggests that birds are even more closely related to dinosaurs than originally thought. Some people say that they are dinosaurs that survived the great extinction event, 65 million years ago.
What caused the death of these prehistoric creatures?
Unfortunately, the very end of the Cretaceous period is not recorded in the rock layers along the Jurassic coast. This was the time of the ‘great extinction’, when the massive dinosaurs, plesiosaurs and flying reptiles disappeared.
Most scientists agree that this catastrophic event had two main causes. Massive volcanic eruptions in what is now southern India (the Deccan Plateau) spewed huge amounts of dust and gasses into the atmosphere which started to change the climate. Then, 65 million years ago, a giant meteorite collided with the earth in what is now Mexico’s Yucatan Peninsula. This was the end of two-thirds of all life on the planet, including the dinosaurs.
What colour were they?
As no one has ever seen a living dinosaur or marine reptile, how do we know what colour they were? The answer is that we don’t know for definite, but we can take an educated guess. The marine reptiles were clearly predators, being armed with rows of sharp teeth. There would be no point in being brightly coloured because their prey would see them coming. So they were most likely to be camouflaged like modern whales, dolphins and fish. Recent research, using the latest technology, has started to find high levels of pigment in rare fossilised skin, suggesting some were dark or even black in colour.
The colour of dinosaurs is trickier to define. Technology is now enabling scientists to examine the molecular structure of rare fossilised skin and even feathers – which might contain clues about colour. In the past, illustrators pictured them in drab colours like the large mammals of Africa today. But modern reptiles can be very colourful, so why not the dinosaurs…?