The Cretaceous Period: What was Earth like before dinosaurs went extinct?

Our dinosaur expert Dr Susie Maidment and fossil plant expert Dr Paul Kenrick explore what the world was like back then and the animals and plants that called our planet home.

When was the Cretaceous Period?

The Cretaceous is a geological period that began 145 million years ago and ended 66 million years ago. It is the last period in the Mesozoic Era. It comes after the Jurassic Period and before the Paleogene - the first period of the Cenozoic Era, our current era.

It lasted a long time, nearly 80 million years, making it the longest geological period of the Phanerozoic Eon, which began some 539 million years ago. 

The Cretaceous is split into two smaller time periods called epochs. The Early Cretaceous Epoch lasted from 145 million years ago to 100.5 million years ago and the Late Cretaceous Epoch lasted from 100.5 million years ago to 66 million years ago.

The Cretaceous climate and continents

Our planet's continents were once joined together into one supercontinent called Pangea. It formed about 335 million years ago, but by the end of the Jurassic, this single landmass had begun to break apart. This continued throughout the Cretaceous and by the end of the period the continents had moved almost into the positions they are in today.

Back then Earth was a lot warmer than it is today and there was little or no ice at the North Pole or South Pole. Sea levels fluctuated but were in the most part high. In fact, at times sea levels were 170 metres higher than today.

Shallow seas formed, dividing some continents. In the Late Cretaceous, for example, the Western Interior Seaway split North America into two landmasses. At its largest this sea was more than 3,000 kilometres long, almost 1,000 kilometres wide and 760 metres deep.

Plants in the Cretaceous Period

The plant life of the Cretaceous was quite different to that of today. For example, temperate rainforest grew close to the poles, which back then were ice free.

'We have evidence from West Antarctica of polar forests that would have been dominated mainly by conifers, things like podocarps, araucarias, and probably gingko trees as well, with understories of ferns and cycads,' explains prehistoric plant expert Dr Paul Kenrick.

Unlike the temperate rainforests that exist today in North America's Pacific Northwest, including in Oregon and Washington, each winter the Cretaceous polar forests would have had to survive four months of the year living in the total darkness of polar night. A very long period without Sun for plants to survive!

Today, about 90% of plants are flowering plants, also known as angiosperms. While the origin of flowering plants may go back as far as the Triassic Period, we don't see much evidence of them in the fossil record at the start of the Cretaceous. However, by the end of the Cretaceous, angiosperms made up a much more prominent part of the planet's plant life.

'In flowering plants today, something like 70% are insect pollinated,' explains Paul.

'Insect pollination happens earlier on in the Jurassic with gymnosperms [a group of seed-producing plants], but it becomes much bigger with the flowering plants.'

'So, there is this big evolutionary story going on between plants and animals.' 

Cretaceous Period animals

'Everything that lived on land that was larger than a metre in size in the Cretaceous was a dinosaur,' says palaeontologist Dr Susie Maidment. 

'There were small, furry mammals running around at the feet of the dinosaurs, but they were a relatively minor component of the ecosystem.'

'Birds had evolved and were in the skies, as well as pterosaurs. In the seas, there were mosasaurs, which are big marine reptiles, and there were groups of plesiosaurs and ichthyosaurs,' explains Susie.

When it comes to Cretaceous dinosaurs, we often think of Triceratops and T. rex, though these dinosaurs only lived at the end of the period, around 68-66 million years ago.  

'The Cretaceous is 80 million years long, so there's a lot of turnover in that time,' says Susie. 

'The Jurassic, which ended 145 million years ago, was the time when we have really big dinosaurs in the northern hemisphere. Things like Diplodocus and Stegosaurus. Those seem to go extinct or at least decline in the Early Cretaceous and they're replaced by iguanodontians and ceratopsids.'

In the Early Cretaceous, iguanodontians were some of the first dinosaurs to evolve complex chewing mechanisms rather than just gulping down food like other reptiles.

In the Late Cretaceous, hadrosaurs - the duck-billed dinosaurs - did similar, using their hundreds of tiny teeth to grind up vast amounts of plant matter. Susie calls them 'the cows of the Cretaceous'.

While the northern hemisphere's biggest dinosaurs lived during the Jurassic, it was in the Cretaceous that the southern hemisphere saw its largest reptiles. In fact, during this period, some of the biggest land animals to have ever existed appeared. The largest of all belong to a group of sauropod dinosaurs called titanosaurs.

Patagotitan, a 37.5-metre-long titanosaur from Argentina in South America, might be the largest found so far. But it's possible that one day we'll find even bigger dinosaurs.

'I think before Patagotitan we would have said dinosaurs were on the edge of what is physically possible, and then you find something bigger,' notes Susie.

It's not clear why some dinosaurs and pterosaurs, such as Quetzalcoatlus, got so large during the Cretaceous Period. Some think it could be related to what gases were in the atmosphere, while others suggest that an evolutionary arms race between prey and predators may have been responsible.

'Big dinosaurs couldn't really run so they had to protect themselves in other ways. So, the prey animals got bigger and then the predators got bigger and so on,' explains Susie. 

How did giant dinosaurs affect their environment?

Sauropods might have lived and travelled in big herds. But how could the Cretaceous environment have supported so many giant herbivores?

'We have such a mammal-centric view of the world, but sauropods were so different to mammals. I suspect they had a very different metabolism and probably didn't need to eat as much food. They could also probably survive on lower-quality plant matter,' says Susie.

'They hatched out of eggs a bit smaller than a football and could grow into these 60-tonne animals. We can see that they deposited bone tissue very rapidly, so they were growing very fast, and they probably had to eat a lot to fuel their growth.'

'It's been suggested that their metabolism changed as they grew. So early on they had a really fast metabolism and as they got big their growth slowed and they actually had a much slower metabolism.'

Giant sauropods would have stripped cellulose-rich leaves off conifers and these may have taken a long time to process in their digestive systems. They might have also eaten the cones from these trees.

'The cones contain the seeds. They're different to the leaves because they contain starches rather than just cellulose, so you get more for your bucks by eating them,' says Paul.

Herbivorous - plant-eating - dinosaurs might also have been ecosystem engineers, meaning they changed the places where they lived through their behaviour. When these dinosaurs ate plant seeds, they may have passed through their guts and out in their poo, which helped to spread the seeds across the animal's habitat as they moved around.

'If you think about the analogies in the modern world, such as elephants, then seed dispersal may have been an important function of big dinosaurs,' notes Paul.

But was there any way Cretaceous plants could prevent themselves from becoming a dinosaur's dinner? Paul points out a few groups of plants that might have done just that!

Of the around 370 species of cycads alive today, almost all of them are toxic to most mammals, with only a few primates able to stomach them. If cycads were this toxic during the Jurassic and Cretaceous, it's possible this could have been enough to put dinosaurs off eating them. 

Bennettitales are an extinct group of plants that looked a lot like cycads.

'Cycads have cones that are produced at the top of the plant, whereas in many Bennettitales, cone-like structures were embedded in the trunk,' Paul explains.

'It might be that these plants were protecting their reproductive parts by encasing them in an armour-like trunk rather than allowing them to be exposed and being easy to predate by dinosaurs.'

Some plants might have survived by being robust. The monkey puzzle tree, Araucaria araucana, for example, looks almost armour plated, covered in thick, scale-like leaves with sharp edges. The group this plant belongs to was at its most diverse during the Jurassic and Cretaceous.

Cretaceous Britain

The name Cretaceous comes from the Latin 'creta' which means chalk. It's named for the large quantities of chalk rock laid down at this time in Western Europe. The UK's famous White Cliffs of Dover are just one of many Late Cretaceous chalk deposits.

Chalk is laid down in marine environments, and the fossils within it can tell us about the creatures that lived in the ocean. But it also means scientists have less of an understanding of the plants and land animals that lived in Britain at the end of the Cretaceous Period.

We have a much better idea of what lived on land here in the Early Cretaceous, particularly thanks to rocks near Bournemouth in southern England known as the Wealden Group. This area is well known for the discovery of Iguanodon. In fact, the UK is particularly well known for iguanodontians, with 12 genera having been found here from the Early Cretaceous. 

We know from the fossil record that rather than dense deciduous woodlands, back in the Cretaceous Britain's landscapes would have featured conifers with an understory of ferns and cycad-like plants.

These landscapes would have been relatively open. Today, big areas dominated by grasses and with sparse tree cover are known as savannahs and prairies. In the Cretaceous, open landscapes like these would have been dominated by ferns rather than grass.

'We think of ferns as being these little delicate things that like wet places, but back then you had ferns that were quite robust and lived in semi-arid environments,' says Paul.

How did the Cretaceous Period end?

The Cretaceous-Palaeogene extinction 66 million years ago is possibly the most famous mass extinction event. It was caused by a large asteroid crash-landing off the coast of Mexico, which changed the climate of the planet dramatically.

'It vapourised carbonate and sulphate rocks, which caused acid rain, and threw lots of ash, dust and dirt into the atmosphere, blocking out the Sun. This caused a global collapse of the food chain,' explains Susie.

'There also would have been a thermal heat pulse that caused wildfires - that's evidenced by charcoal in the rock record - and huge tsunamis washed across the ocean basins.'

Adding to the chaos was the formation of the Deccan Traps, one of the planet's largest volcanic features. The vast quantities of sulphur released would have cooled the atmosphere.

The Cretaceous extinction wiped out about 65% of all species. 

Plants were affected, though not in quite the same way as animals.

'With animals, whole groups disappeared. Whereas with plants, you see a lot of extinction, but you don't tend to see whole groups of plants disappear,' explains Paul.

'In many respects, plants are more robust than animals, more resilient to physical disturbance.'

Paul likens this to cutting the grass. Even after cutting a plant in half with a lawnmower, it can regenerate, something that is impossible for animals to do. In catastrophic circumstances, plants can regrow from rooting systems, buds and other parts.

Plants also have the advantage of creating vast quantities of seeds that, in some types of plants, can survive in the soil for decades until conditions are right for them to begin to grow.

Ecological variation within groups also helps in the face of catastrophe.

'You see plants that produce big trees and small shrub-like things. You might have families that have annuals and long-lived perennials. You might have groups that are adapted to hot and cold climates in the same family.'

'The family level of plants may be more diverse than animals, so that's why you don't get the massive losses of these big groups, because some element of them finds a way to survive extinction.'

What came after the Cretaceous Period?

When the Cretaceous Period and Mesozoic Era ended 66 million years ago, the Palaeogene Period and the Cenozoic Era began.

In the Palaeogene, the continents drifted even closer to their present-day positions, and during the following Neogene Period the world was cooler and the effects of seasonality were more widely felt.

Many modern plants evolved in this period and grasses began to spread.

In place of pterosaurs, birds became the dominant animals in the skies and the extinction of the non-avian dinosaurs allowed mammals to rapidly diversify and grow.

While it would still be many millions of years before ancient human relatives would appear, evidence of some of the earliest primate-like mammals comes from the Palaeogene.