Palaeoart of three Allosaurus kicking up dust as they travel through a clearing in a prehistoric forest.

Allosaurus was a meat-eating dinosaur that lived at the end of the Jurassic Period in what is now North America and Western Europe. It was almost 10 metres long and hunted other large dinosaurs, such as Stegosaurus. © Daniel Eskridge/ Shutterstock

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The Jurassic Period: How did dinosaurs go from basal to bulky?

Stegosaurus, Allosaurus and Diplodocus are among the Jurassic Period’s most famous faces.

But how dinosaurs went from a small and unimportant group of reptiles to ruling our planet for millions of years is a bit of a mystery. 

The Jurassic is a geological period that began 201.4 million years ago and ended 145 million years ago. It’s part of the Mesozoic Era – the part of our planet’s prehistory known as the age of the dinosaurs.

The Jurassic Period is named after the Jura Mountains, a mountain range that stretches along the France-Switzerland border. It’s where naturalist Alexander von Humboldt first noticed that Jurassic rocks were distinct from those of the Triassic Period that came before it.

We can divide the Jurassic Period into three key parts, known as epochs:

  • the Early Jurassic, 201.4 million to 174.7 million years ago
  • the Middle Jurassic, 174.7 million to 161.5 million years ago
  • the Late Jurassic, 161.5 million to 145 million years ago

The world’s continents were once joined together into a massive supercontinent called Pangea. During the Jurassic, it started splitting into two giant landmasses known as Laurasia and Gondwana that were divided by a prehistoric body of water called the Tethys Ocean. The Atlantic Ocean also started to open up during the Jurassic as these landmasses began to split up further into the continents we know today.  

The planet was much warmer than it is now, with no ice caps covering the North Pole or South Pole. Sea levels were also different, they rose throughout the Jurassic – at times reaching more than 140 metres above the present-day average. 

A fossil fish found between paper-thin layers of rock.

A fossil fish found in paper-thin rocks laid down by the Sundance Sea, which covered parts of North America during the Jurassic Period. 

How did the Jurassic Period start?

The end of the Triassic Period, 201.4 million years ago, is marked by one of our planet’s top five major mass extinction events.

What caused it isn’t clear, though massive volcanic activity could be to blame. It’s possible that the release of huge amounts of sulphur dioxide and carbon dioxide significantly disrupted Earth’s climate.

Up to 80% of all species died out as a result. But dinosaurs survived and went on to thrive in the Jurassic Period.

“Whether it’s chance or whether there’s some competitive advantage that they had is a matter of debate, but what happened in the aftermath is that the dinosaurs really radiated and then became the dominant terrestrial organisms until 66 million years ago at the end of the Cretaceous Period,” explains our dinosaur researcher Dr Susie Maidment.

A golden age of dinosaurs

Dinosaurs weren’t a big part of the Triassic ecosystem. In the group’s early years, they were generally small, bipedal omnivores or carnivores. In the Jurassic, they started off much the same way.

Some of the earliest Jurassic dinosaurs are the heterodontosaurids. These were small reptiles that only grew up to about two metres long.

Heterodontosaurus is one of the earliest Jurassic dinosaurs that we know in time, but where it fits in the evolutionary tree of dinosaurs is still a bit of a question,” says Susie. “They’re currently thought to be relatively basal or primitive ornithischians.”

Most dinosaurs have the same type of teeth all the way along their jaws, whereas heterodontosaurids have multiple different types of teeth. They have chewing teeth as well as noticeable tusks or fang-like teeth. 

A Heterodontosaurus skull

The primitive ornithischian Heterodontosaurus had different types of teeth, including canine-like fangs. 

“It’s been suggested that these were possibly omnivorous, but they might have been herbivorous. If they’re omnivorous, it’s probably plesiomorphic – as in it’s because they haven’t evolved herbivory yet,” Susie explains.

By the end of the Jurassic, dinosaurs had transformed. They’d gone from a small and relatively unimportant group of reptiles that were all mostly similar in form, such as Heterodontosaurus and Lesothosaurus, to an exceedingly diverse group. Some Late Jurassic dinosaurs could reach enormous sizes, walk on four legs and some even had elaborate display structures. This is when we see iconic giants like Stegosaurus, Brachiosaurus and Diplodocus.

By then, dinosaurs were dominating the land, accounting for all terrestrial animals more than a metre long.

Middle Jurassic mysteries

There’s a big gap in our fossil record, so the secret to dinosaurs’ success is unclear.

“The Middle Jurassic is one of the worst sampled time periods for dinosaurs,” says Susie.

One problem is high sea levels. During the start of the Jurassic, ocean levels were lower than they are today, but then they rose significantly over the period. This means there aren’t as many places for us to look for the fossils of land-based animals from the Middle Jurassic because more of the world was underwater then. 

A fossil coral specimen

Fossil coral from Middle Jurassic rocks in Germany. Higher sea levels during this epoch mean that there are fewer places for us to find fossils of land-dwelling animals. 

“Across the places on Earth where we’ve looked for dinosaurs and other terrestrial vertebrates – North America and Europe, primarily – we actually have marine rocks deposited over this time,” Susie explains.

That’s a problem for palaeontologists. Without an ample fossil record, our understanding of how several major groups of dinosaurs came to be remains relatively poor – stegosaurs, armoured dinosaurs and iguanodontians, for example.

Modern birds are dinosaurs, but their initial evolution may also have its roots in the Middle Jurassic. Archaeopteryx, sometimes known as the first bird, is a Late Jurassic dinosaur. But Susie notes that there’s a growing body of evidence that suggests birds may actually have been around much earlier.

“There’s certainly very bird-like footprints from a number of places – we described some from Morocco [in 2023]. They’re really suggestive that birds or something very closely related to birds had already evolved.”

A fossilised bird-like footprint in rock.

Susie and her colleagues described some very bird-like footprints from Middle Jurassic rocks in the Atlas Mountains, Morocco. Their findings were published in 2023. Image courtesy of Dr Susie Maidment. 

Dinosaur footprints aren’t the only exciting things Susie and her colleagues have found in Morocco’s Atlas Mountains.

“One of the really cool things is that it’s one of the places in the Midde Jurassic where we do have terrestrial rocks. There’s some there, some in China and a little bit in Kyrgyzstan, but generally around the world it’s quite rare.”

“They’re not very well explored, the north African and central Asia rocks. So, in the Moroccan record, everything we’re finding is basically new. It’s really interesting because they are some of the earliest representatives we have for the kinds of things that we know better from the Late Jurassic.”

Susie and the team are shedding light on a time when dinosaurs were really getting going, with discoveries including the oldest ever ankylosaur, Spicomellus afer, and one of the earliest stegosaurs, Adratiklit boulahfa.

Sites like this show that the information we’re seeking about dinosaur evolution is likely out there, but that we just haven’t looked in the right places enough yet

The rib bone of Spicomellus afer showing spikes emerging from the dorsal surface.

Ankylosaurs are known for their tough armour, but the bizarre Moroccan ankylosaur Spicomellus afer has its armour fused directly into its bones. No other animal has anything similar. 

Jurassic reptiles in the UK

During much of the Early and Middle Jurassic, southern England was underwater. We can tell this from the many fossils of ocean-dwelling animals that turn up on the 150-kilometre-long Jurassic Coast. This area includes Lyme Regis, where famous fossil hunter Mary Anning gained her reputation for finding and identifying prehistoric marine reptiles such as ichthyosaurs and plesiosaurs in the 1800s. 

Palaeontologists have had more luck finding dinosaurs where islands sat in shallower water, such as in modern-day Oxfordshire and Gloucestershire. Our scientific associate Dr Simon Wills spent his PhD looking at tiny teeth and in doing so helped to reveal just how diverse Britain’s dinosaurs were. His finds include some of the world’s earliest dromaeosaur teeth.

A Megalosaurus jaw bone fossil

This lower jaw belonged to the Middle Jurassic theropod dinosaur Megalosaurus. It was found in Oxfordshire. Megalosaurus was the first dinosaur ever described in scientific literature. Image by Josh Davis. 

Further north, there’s evidence of dinosaurs and other reptiles on the Isle of Skye in Scotland, and in Yorkshire

“There are lots of dinosaur footprints from Skye in the Middle Jurassic. They were probably underwater but shallow – beaches and things like that,” notes Susie.

We also find pockets of Triassic and Jurassic rock in the Vale of Glamorgan, South Wales, which were once islands in a shallow sea. Jurassic ammonites, bivalves, mammaliaform teeth and lizards are just some of the fossils that have turned up here.

Jurassic Period plants

As for the plant kingdom, the Jurassic was the age of the gymnosperms – a group of seed-producing plants that includes conifers, cycads and ginkgos. A lot of the gymnosperm diversity has died out and what we see today are only the remnants of the group. 

Half of a seed cone from the coniferous tree Araucaria mirabilis.

This lemon-sized, fossilised seed cone is from the coniferous tree Araucaria mirabilis. These Late Jurassic trees are found in what’s now South America. In life they were wind pollinated and had two types of cones – seed cones and pollen cones.

The equator and low latitude tropics are now the most biodiverse regions on Earth. But in the Jurassic, these were much drier places and most plants thrived in the mid-latitudes instead.

There’s some speculation that flowering plants, also known as angiosperms, appeared in the Jurassic, but currently there’s no clear evidence to support this. If they were around in this period, then it’s likely flowers weren’t a big part of the planet’s plant life.

Compared to dinosaurs, plants from the Middle Jurassic are a little easier to find. In fact, the UK is home to one of the best Middle Jurassic, fossil-plant sites in the world.

“It’s on the Yorkshire coast – known as the Yorkshire Jurassic,” says our fossil plant researcher Dr Paul Kenrick. “It was a river delta and an estuary, and it’s famous for its plants.”

We also have the Portland Stone and lower Purbeck sequences from the Late Jurassic, which feature a fossil forest that stretches across southern England.

Portland Stone is a limestone rock that’s used in many famous British landmarks in London, including St Paul’s Cathedral. During the last parts of the Jurassic, a very salty, shallow lagoon with a forest of conifers growing around it sat just on top of these rocks. 

A fossil tree on the UK's south coast.

This doughnut-shaped rock in Lulworth Cove on the UK's Jurassic Coast is a stromatolite that developed around the base of a tree trunk in a flooded forest.  © Vaide Seskauskiene/ Shutterstock

“The trees were quite short in stature, around 12 to 15 metres in height, and we can tell from how they’re spaced that this wasn’t a dense forest,” Paul explains. “They were quite small, scrubby conifers.”

Paul was part of a team that worked on documenting one of these trees in 2023. The specimen was over 11 metres long, weighed more than 2 tonnes and was excavated in over 100 pieces. Some of these pieces were very large and heavy, so the team used laser scanning to reconstruct the tree, making it the largest 3D reconstruction of a plant fossil to date.  

Sitting around the trees in this forest were short, dumpy plants called Bennettitales. These looked a lot like cycads but were a distinct group that grew their cones inside their trunks, rather than at the top of the plant.

“Then you would have had lots of plants related to modern Ephedra – small, shrubby gymnosperms – and ferns.”

A trunk of the Bennettitalian Cycadeoidea microphylla.

This is the trunk of a Bennettitalian called Cycadeoidea microphylla that was found on the Isle of Portland, a tied island off England’s south coast. Trunks like these were once mistaken for gigantic fossilised bird’s nests or bee hives. 

Back then, Britain sat closer to the equator than it does today and was quite arid, with conditions similar to the Mediterranean. Summers were hot and dry, and winters were cool and wet.

“If you look at the growth-ring structure of the trees you can see that there’s quite a lot of variability in the climate. They were probably living right on the edge of the environment that they could grow in.”

“They had foliage that’s like a leylandii-type hedge, with tiny, little leaves, rather than needle-like leaves. These are typical of conifers that grow in arid conditions today.”

The fossilisation of this Jurassic forest may be linked to the rifting of Pangea. Scientists think that as the Atlantic Ocean formed, earthquakes might have shifted the ground downwards, causing the lagoon to flood the forest, killing and preserving the trees. 

A fossilised Pagiophyllum peregrinum twig with scale-like leaves.

This Jurassic Pagiophyllum peregrinum twig looks scaly, but these scales are actually the plant’s leaves. This is an adaptation that tells us the plant was likely living in a dry environment. 

How did the Jurassic Period end?

The Jurassic’s beginnings are relatively clear, but how the period came to an end 145 million years ago is murkier.

In the northern hemisphere, the fossil record tells us that there was a significant shift in the kinds of animals roaming the planet. But it was more of a transition than a sudden change. Stegosaurs, for example, went extinct in North America but continued on in Europe for a while.

This suggests that the Jurassic didn’t end with a dramatic global mass extinction event – not on land at least.

“There’s this really big faunal turnover and what caused that isn’t totally clear,” Susie explains. “Early analyses of biodiversity change through time did notice that there was an extinction event at the end of the Jurassic in the marine realm. It’s not as large in magnitude as one of the top five mass extinctions, but whether whatever caused it also occurred on land is unclear.”

Our knowledge of this time tends to be biased towards the North American rock record. This is where we have the Morrison Formation, which represents the world’s best known Late Jurassic terrestrial ecosystem.

But there’s a gap in this record. It spans five million years at minimum and stretches to 10–15 million years in some places. A lot seems to have happened in this time because when we finally tune back in, iguanodontians and ankylosaurs are dominating the ecosystem. 

Sophie the Stegosaurus on display in Earth Hall.

Sophie the Stegosaurus, which is on display in our Earth Hall, comes from a section of the Morrison Formation in Wyoming, USA. 

“In North America, there was a big mountain-building event going on at that time. So, it’s possible that the habitats of the dinosaurs were no longer there, and they went extinct regionally,” suggests Susie.

“In Europe the picture is less clear. Partly it’s that the rocks aren’t very well dated, so exactly where the Jurassic ends is a little bit more debatable. But it does seem that the Jurassic faunas continued for a little bit longer there.”

“If there was an extinction event in North America, it’s not clear whether that’s the case elsewhere or if the faunal turnover was as rapid, but that’s because the fossil record isn’t as well explored elsewhere.”

As the Jurassic Period ended, the Cretaceous Period began. Over the next 79 million years, our planet’s continents inched closer to their present-day positions, 60-tonne titanosaurs ruled the southern hemisphere and the non-avian dinosaurs met their end in a colossal asteroid strike