A large asteroid struck our planet and killed dinosaurs about 66 million years ago, according to archaeological studies. 
This theory is widely accepted, but new research shows that the dinosaur population and the various species were in a state of steep decline many million years before the asteroid hit Mexico.

The Fate Of The Dinosaurs

<ul><li>Crocodiles are lizards are ectothermic(cold-blooded) animals.</li><li>Humans and birds are endothermic(warm-blooded).</li><li>Dinosaurs are somewhere in between these two and are classified as mesothermal.</li></ul>Dinosaurs needed a warm climate to survive, and any sharp temperature decrease could have been fatal for them. Changes in vegetation and climatic deterioration could also be factors in the eventual decline of the dinosaur, paving the way for primates and humans.

Dinosaurs: Mammals Or Reptiles

The other three were herbivores: the Ceratopsidae (represented in particular by the Triceratops), the Hadrosauridae (the richest of all the families in terms of diversity) and the Ankylosauridae (represented in particular by the Ankylosaur, a dinosaur covered in bony armour with a club-like tail).We knew that all these families had survived until the end of the Cretaceous, marked by the fall of the asteroid. Our goal was to determine at what rate these families diversified – formed new species – or became extinct.For five years, we compiled all the known information on these families in order to try to work out how many of them there were on Earth at a particular time, and which species were in each group. In palaeontology, each fossil is given a unique number for the sake of traceability, which allows us to follow it through the scientific literature over time.<picture><source media="(min-width:1200px)" srcset="https://ychef.files.bbci.co.uk/1600x900/p09p7md0.webp" type="image/webp"><source media="(min-width:1200px)" srcset="https://ychef.files.bbci.co.uk/1600x900/p09p7md0.jpg" type="image/jpeg"><source media="(min-width:880px)" srcset="https://ychef.files.bbci.co.uk/1280x720/p09p7md0.webp" type="image/webp"><source media="(min-width:880px)" srcset="https://ychef.files.bbci.co.uk/1280x720/p09p7md0.jpg" type="image/jpeg"><source media="(min-width:576px)" srcset="https://ychef.files.bbci.co.uk/976x549/p09p7md0.webp" type="image/webp"><source media="(min-width:576px)" srcset="https://ychef.files.bbci.co.uk/976x549/p09p7md0.jpg" type="image/jpeg"><source media="(min-width:224px)" srcset="https://ychef.files.bbci.co.uk/624x351/p09p7md0.webp" type="image/webp"><source media="(min-width:224px)" srcset="https://ychef.files.bbci.co.uk/624x351/p09p7md0.jpg" type="image/jpeg"><img title="The research suggested a number of dinosaur species saw a long, gradual decline toward extinction (Credit: Tara Moore/Getty Images)" src="https://ychef.files.bbci.co.uk/976x549/p09p7md0.jpg" alt="The research suggested a number of dinosaur species saw a long, gradual decline toward extinction (Credit: Tara Moore/Getty Images)" width="687" height="386"></picture>The research suggested a number of dinosaur species saw a long, gradual decline toward extinction (Credit: Tara Moore/Getty Images)The work was tedious – we made an inventory of most of the known fossils for these six families, which represented more than 1,600 individuals from around 250 species. It’s not easy to properly categorise each of the species and date them correctly: one researcher might have given a record a certain date and species, and then another might re-examine it and make a different analysis. In these cases, we had to make our own calls – if we had too many doubts, we eliminated the fossil from the study.Once each fossil was properly categorised, we used a statistical model to estimate the number of species that evolved over time for each family. We were thus able to trace the species that appeared and disappeared between 160 and 66&nbsp;million years ago and estimate, again for each family, the rates of speciation – the evolution of new species – and extinction over time.<BLOCKQUOTE><h2>Our results show that the number of species was in steep decline from 10 million years before the asteroid strike until the dinosaurs were wiped out</h2></BLOCKQUOTE>To estimate these rates, we had to take several confounding factors into account. The fossil record is biased: it is uneven in time and space, and some types of dinosaur simply do not fossilise as well as others. This is a well-known problem in palaeontology when estimating the dynamics of past diversity.Sophisticated models can account for uneven preservation over time and between species. In doing so, the fossil record becomes more reliable for estimating the number of species at any given time. But it is important to be cautious, because we are talking about estimates, and these estimates may change if we find more fossils, for example, or new analytical models.A steep declineOur results show that the number of species was in steep decline from 10&nbsp;million years before the asteroid strike until the dinosaurs were wiped out. This decline is particularly interesting because it is worldwide, and affects both carnivorous groups such as Tyrannosaurs, and herbivorous groups such as Triceratops.<picture><source media="(min-width:1200px)" srcset="https://ychef.files.bbci.co.uk/1600x900/p09p7mrd.webp" type="image/webp"><source media="(min-width:1200px)" srcset="https://ychef.files.bbci.co.uk/1600x900/p09p7mrd.jpg" type="image/jpeg"><source media="(min-width:880px)" srcset="https://ychef.files.bbci.co.uk/1280x720/p09p7mrd.webp" type="image/webp"><source media="(min-width:880px)" srcset="https://ychef.files.bbci.co.uk/1280x720/p09p7mrd.jpg" type="image/jpeg"><source media="(min-width:576px)" srcset="https://ychef.files.bbci.co.uk/976x549/p09p7mrd.webp" type="image/webp"><source media="(min-width:576px)" srcset="https://ychef.files.bbci.co.uk/976x549/p09p7mrd.jpg" type="image/jpeg"><source media="(min-width:224px)" srcset="https://ychef.files.bbci.co.uk/624x351/p09p7mrd.webp" type="image/webp"><source media="(min-width:224px)" srcset="https://ychef.files.bbci.co.uk/624x351/p09p7mrd.jpg" type="image/jpeg"><img title="The ecosystems at the end of the Cretaceous system had changed markedly due to climate change and new vegetation (Credit: BJDLZX/Getty Images)" src="https://ychef.files.bbci.co.uk/976x549/p09p7mrd.jpg" alt="The ecosystems at the end of the Cretaceous system had changed markedly due to climate change and new vegetation (Credit: BJDLZX/Getty Images)" width="687" height="386"></picture>The ecosystems at the end of the Cretaceous system had changed markedly due to climate change and new vegetation (Credit: BJDLZX/Getty Images)Some species declined sharply, like the Ankylosaurs and ceratopsians, and only one family out of the six – the troodontids – shows a very small decline, which took place in the last five million years of dinosaurs' existence.What could have caused this strong decline? One theory is climate change: at that time, the Earth underwent a period of global cooling of 7-8C.We know that dinosaurs need a warm climate for their metabolism to function properly. As we often hear, they were not ectothermic (cold-blooded) animals like crocodiles or lizards, nor endothermic (warm-blooded), like mammals or birds. They were mesotherms, a metabolic system between reptiles and mammals, and needed a warm climate to maintain their temperature and thus perform basic biological functions. This temperature decrease must have had a very strong impact on them.<BLOCKQUOTE><h2>One big question remains: what would have happened if the asteroid had not crashed?</h2></BLOCKQUOTE>It should be noted that we found a staggered decline between herbivores and carnivores: the grass-eaters declined slightly before the meat-eaters. It's probable that the decline of herbivores caused the decline of carnivores. This is what we call cascade extinction.The knockout blowOne big question remains: what would have happened if the asteroid had not crashed? Would dinosaurs have gone extinct anyway, due to the decline that had already begun, or could they have rebounded?It's very difficult to say. Many palaeontologists believe that if the dinosaurs had survived, primates and therefore humans would never have appeared on Earth.<picture><source media="(min-width:1200px)" srcset="https://ychef.files.bbci.co.uk/1600x900/p09p7nn1.webp" type="image/webp"><source media="(min-width:1200px)" srcset="https://ychef.files.bbci.co.uk/1600x900/p09p7nn1.jpg" type="image/jpeg"><source media="(min-width:880px)" srcset="https://ychef.files.bbci.co.uk/1280x720/p09p7nn1.webp" type="image/webp"><source media="(min-width:880px)" srcset="https://ychef.files.bbci.co.uk/1280x720/p09p7nn1.jpg" type="image/jpeg"><source media="(min-width:576px)" srcset="https://ychef.files.bbci.co.uk/976x549/p09p7nn1.webp" type="image/webp"><source media="(min-width:576px)" srcset="https://ychef.files.bbci.co.uk/976x549/p09p7nn1.jpg" type="image/jpeg"><source media="(min-width:224px)" srcset="https://ychef.files.bbci.co.uk/624x351/p09p7nn1.webp" type="image/webp"><source media="(min-width:224px)" srcset="https://ychef.files.bbci.co.uk/624x351/p09p7nn1.jpg" type="image/jpeg"><img title="Would the dinosaurs have died out regardless if the asteroid had not hit? (Credit: Jacobs Stock Photography/Getty Images)" src="https://ychef.files.bbci.co.uk/976x549/p09p7nn1.jpg" alt="Would the dinosaurs have died out regardless if the asteroid had not hit? (Credit: Jacobs Stock Photography/Getty Images)" width="687" height="386"></picture>Would the dinosaurs have died out regardless if the asteroid had not hit? (Credit: Jacobs Stock Photography/Getty Images)An important fact is that a possible rebound in diversity can be very heterogeneous and group-dependent, so that some groups would have survived and others not. Hadrosaurs, or "duck-billed" dinosaurs, for example, showed some form of resilience to the decline and might have bounced back afterwards.What we can say is that the ecosystems at the end of the Cretaceous period were under significant pressure due to climatic deterioration and major changes in vegetation, and that the asteroid dealt the final blow.This is often the case in the disappearance of species: first they are in decline and under pressure, then another event intervenes and finishes off a group that may have been on the verge of extinction anyway.This article&nbsp;<a href="https://theconversation.com/dinosaurs-were-already-in-decline-before-the-asteroid-wiped-them-out-new-research-163624" dir="ltr">originally appeared&nbsp;</a>on The Conversation, and is republished under a Creative Commons licence.&nbsp;--Join one million Future fans by liking us on&nbsp;<a href="https://www.facebook.com/BBCFuture/" dir="ltr">Facebook</a>, or follow us on&nbsp;<a href="https://twitter.com/BBC_Future" dir="ltr">Twitter</a>&nbsp;or&nbsp;<a href="https://www.instagram.com/bbcfuture_official/" dir="ltr">Instagram</a>.If you liked this story,&nbsp;<a href="http://pages.emails.bbc.com/subscribe/?ocid=fut.bbc.email.we.email-signup" dir="ltr">sign up for the weekly bbc.com features newsletter</a>, called “The Essential List”. A handpicked selection of stories from BBC Future, Culture, Worklife, and Travel, delivered to your inbox every Friday.

The arrival of a cataclysmic asteroid strike helped quicken the end of the dinosaurs. But might they have been on the decline anyway?

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The other three were herbivores: the Ceratopsidae (represented in particular by the Triceratops), the Hadrosauridae (the richest of all the families in terms of diversity) and the Ankylosauridae (represented in particular by the Ankylosaur, a dinosaur covered in bony armour with a club-like tail).

We knew that all these families had survived until the end of the Cretaceous, marked by the fall of the asteroid. Our goal was to determine at what rate these families diversified – formed new species – or became extinct.

For five years, we compiled all the known information on these families in order to try to work out how many of them there were on Earth at a particular time, and which species were in each group. In palaeontology, each fossil is given a unique number for the sake of traceability, which allows us to follow it through the scientific literature over time.

The research suggested a number of dinosaur species saw a long, gradual decline toward extinction (Credit: Tara Moore/Getty Images)

The work was tedious – we made an inventory of most of the known fossils for these six families, which represented more than 1,600 individuals from around 250 species. It’s not easy to properly categorise each of the species and date them correctly: one researcher might have given a record a certain date and species, and then another might re-examine it and make a different analysis. In these cases, we had to make our own calls – if we had too many doubts, we eliminated the fossil from the study.

Once each fossil was properly categorised, we used a statistical model to estimate the number of species that evolved over time for each family. We were thus able to trace the species that appeared and disappeared between 160 and 66 million years ago and estimate, again for each family, the rates of speciation – the evolution of new species – and extinction over time.

Our results show that the number of species was in steep decline from 10 million years before the asteroid strike until the dinosaurs were wiped out

To estimate these rates, we had to take several confounding factors into account. The fossil record is biased: it is uneven in time and space, and some types of dinosaur simply do not fossilise as well as others. This is a well-known problem in palaeontology when estimating the dynamics of past diversity.

Sophisticated models can account for uneven preservation over time and between species. In doing so, the fossil record becomes more reliable for estimating the number of species at any given time. But it is important to be cautious, because we are talking about estimates, and these estimates may change if we find more fossils, for example, or new analytical models.

A steep decline

Our results show that the number of species was in steep decline from 10 million years before the asteroid strike until the dinosaurs were wiped out. This decline is particularly interesting because it is worldwide, and affects both carnivorous groups such as Tyrannosaurs, and herbivorous groups such as Triceratops.

The ecosystems at the end of the Cretaceous system had changed markedly due to climate change and new vegetation (Credit: BJDLZX/Getty Images)

Some species declined sharply, like the Ankylosaurs and ceratopsians, and only one family out of the six – the troodontids – shows a very small decline, which took place in the last five million years of dinosaurs' existence.

What could have caused this strong decline? One theory is climate change: at that time, the Earth underwent a period of global cooling of 7-8C.

We know that dinosaurs need a warm climate for their metabolism to function properly. As we often hear, they were not ectothermic (cold-blooded) animals like crocodiles or lizards, nor endothermic (warm-blooded), like mammals or birds. They were mesotherms, a metabolic system between reptiles and mammals, and needed a warm climate to maintain their temperature and thus perform basic biological functions. This temperature decrease must have had a very strong impact on them.

One big question remains: what would have happened if the asteroid had not crashed?

It should be noted that we found a staggered decline between herbivores and carnivores: the grass-eaters declined slightly before the meat-eaters. It's probable that the decline of herbivores caused the decline of carnivores. This is what we call cascade extinction.

The knockout blow

One big question remains: what would have happened if the asteroid had not crashed? Would dinosaurs have gone extinct anyway, due to the decline that had already begun, or could they have rebounded?

It's very difficult to say. Many palaeontologists believe that if the dinosaurs had survived, primates and therefore humans would never have appeared on Earth.

Would the dinosaurs have died out regardless if the asteroid had not hit? (Credit: Jacobs Stock Photography/Getty Images)

An important fact is that a possible rebound in diversity can be very heterogeneous and group-dependent, so that some groups would have survived and others not. Hadrosaurs, or "duck-billed" dinosaurs, for example, showed some form of resilience to the decline and might have bounced back afterwards.

What we can say is that the ecosystems at the end of the Cretaceous period were under significant pressure due to climatic deterioration and major changes in vegetation, and that the asteroid dealt the final blow.

This is often the case in the disappearance of species: first they are in decline and under pressure, then another event intervenes and finishes off a group that may have been on the verge of extinction anyway.

This article  <a dir="ltr" href="https://theconversation.com/dinosaurs-were-already-in-decline-before-the-asteroid-wiped-them-out-new-research-163624">originally appeared  </a> on The Conversation, and is republished under a Creative Commons licence.

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Were dinosaurs doomed before the asteroid hit?

The Fate Of The Dinosaurs

Dinosaurs: Mammals Or Reptiles

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