It is one arguably one of the most pivotal stages in our history. 

But what really happened around the time when an asteroid smashed into the Earth 66 million years ago? 

Scientists may finally have the answer, after discovering the remains of the last surviving dinosaurs in New Mexico.

Their fossil evidence suggests that there was wide variety of dinosaurs in New Mexico at the time of the impact – including Alamosaurus, a huge creature the size of a blue whale.

According to experts from the University of Edinburgh, this indicates that, contrary to popular belief, the dinosaurs were in their prime when the fateful day arrived.

'An inconvenient truth is that until now paleontologists have had few fossils of dinosaurs unequivocally dated to the last few hundred thousand years of the Cretaceous, before the asteroid hit, so much of our understanding of the extinction was extrapolated from older fossils and statistical analyses,' said Professor Steve Brusatte, co–author of the study. 

'Now in New Mexico we have fossils of dinosaurs that were there right at the end and when we compare them with the only other fossils accurately dated from this time, from further north, we can see they are much different. 

'There clearly were many types of dinosaurs thriving up until that moment the asteroid ended it all.'

For the study, Professor Brusatte and his colleagues from around the world analysed fossils representing around a dozen different dinosaur species, found over the course of 10 years at the San Juan Basin of northwestern New Mexico. 

This region is about 1,500 miles further north from the Yucatán Peninsula of Mexico, which received a direct hit from the space rock. 

Before the impact, the San Juan Basin was dominated by Alamosaurus – a large, long–necked dinosaur about 30–50ft tall, weighing 30–80 tons. 

Alamosaurus wouldn't have been much smaller than a blue whale – the largest creature on Earth today – although it was half as heavy.

'Alamosaurus was one of the biggest dinosaurs to ever live, heavier than a Boeing 737,' Professor Brusatte told the Daily Mail. 

'It had a long noodle-shaped neck, tiny head, bulging belly, Greek columns for arms and legs, but it was a plant-eater.

'If we'd been alive at the time, it would probably take no interest in us, be no threat to us, unless we got too close and then we'd have to worry about being squashed with a single footfall.'

Other dinosaurs that left fossils at San Juan Basin for the team to analyse were the famous T. Rex, the horned dinosaur Ojoceratops, the omnivorous bird-like dinosaur Ojoraptorosaurus, and the raptor Dineobellator. 

Results of the analysis showed that the dinosaurs at San Juan Basin were of the same age – dating back 66 million years, challenging assumptions that they were from several million years before the asteroid arrived. 

This finding also counters a long-held idea that there was a long-term decline in dinosaur diversity leading up to the impact event that made them more prone to extinction. 

'There is no sign this ecosystem is in any sort of trouble, that dinosaurs were in any way weak or in decline, but then, the asteroid hits and it's all over,' Professor Brusatte told the Daily Mail. 

According to the academic, there's a small chance the actual fossils studied could have been from individual dinosaurs alive the day the asteroid hit. 

If not, they would have been alive at some point no more than 300,000 before that day. 

'There's just no way to accurately tell if a single particular fossil was there on a single particular day 66 million years ago,' he told the Daily Mail. 

'But the Alamosaurus fossils from New Mexico can be definitively accurately dated to within the last 300,000 years or so of the age of dinosaurs. 

'That's the precision of the methods we have for dating rocks, and in geological terms, that is a very short amount of time. We can therefore be very confident that Alamosaurus and these other dinosaurs were there when the asteroid hit.' 

The team say overall dinosaur communities at New Mexico were different to the ones at the famous Hell Creek region much further north, covering Montana, North Dakota, South Dakota, and Wyoming. 

At Hell Creek, the dominant dinosaurs 66 million years ago were Triceratops, known for its three horns, and Edmontosaurus, the large lizard–looking herbivore. 

Again, this suggests there was high dinosaur diversity across North America generally.

'Diversity was high and fairly constant until the end of the Cretaceous, and that there were distinct dinosaurs living in the north and south of North America when the asteroid hit – another sign of their strength, as they were adapting differently to different environments,' Professor Brusatte added.  

When the space rock hit it would have been 'carnage', even at San Juan Basin further away from the immediate impact zone. 

'At San Juan, you would not have been immediately vaporized as you would have been if you were closer to ground zero where the asteroid hit on the Yucatan,' Professor Brusatte told the Daily Mail.

'You would have seen a blinding flash of light, and heard a few ear–shattering booms, and then a bit later the ground would have started to rumble, the Earth turning into a trampoline as some of the biggest earthquakes of all time shook the planet. 

'Then the air would have turned hotter than an oven, as bullets of glass rained down, the stuff that was liquefied by the asteroid fell back to earth and solidified. 

'It was so hot it would have spontaneously combusted the forests, wildfires raged, the air was choked with smoke, and that was just in the first few hours. 

'Over the next few days the whole world went dark and cold, and stayed that way for many years, as all of the smoke and gunk clogged the atmosphere, and blocked out the sun.' 

The new findings are published today in the journal Science.