Where the Age of Dinosaurs met its end.

The site of impact, not far from where I sit. Image credit: David Fuchs
The site of impact, not too far from where I sit. Image credit: David Fuchs

Here is the location of Playa del Carmen, for comparison. At least I’m on the same peninsula.

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Image credit: Vacations to Go

Chixulub is the Mayan name for a town, with “ch’ic” meaning a flea or tick, and “xulub” meaning a devil or demon. It’s also named for a massive crater discovered nearby, which measures 180 kilometers in diameter (110 miles) and has been dated to approximately 66 million years ago. That also happens to be the same time that levels of iridium, an element common in asteroids, spikes in layers of the Earth’s crust around the world.

Of course, it’s also the time when all non-bird (or non-avian) dinosaurs disappeared.

What's that in the sky? Image credit: Corbis
What’s that in the sky? Image credit: Corbis

The conclusion made by Luis and Walter Alvarez, called the Alvarez hypothesis, is that all of these events are linked. The Chixulub crater came from the impact of a massive asteroid turned meteor, which then vaporized and settled as iridium-rich dust around the world. The dust kicked up from the explosion blocked much of the sun’s light for many years, making it very difficult for large animals (which need lots of food) to survive. Indeed, pretty much everything went extinct that was larger than a cat.

If they were both alive at the time, only one of them would have survived. Image credit: lovemeow.com
If they were both alive at the time, only one of them would have survived. Image credit: lovemeow.com

Of course, there weren’t any actual cats around back then, or horses. Or elephants or whales or kangaroos. Mammals were among the tiniest of creatures at that time, their evolution having been long suppressed under the reign of the dinosaurs.

cretaceous_mammals_multituberculate_ metatherian_ eutherian
Our ancestors really weren’t very impressive back then.

When I was growing up, our concept of dinosaurs underwent a radical shift. When I was little, I read books on how dinosaurs were big, stupid, cold-blooded reptiles that were so big that some could barely even walk on land.

The old image--a big dummy in the water.
The old image–a big dummy in the water.

They were supposed to be inferior to mammals, and they went extinct because the smart, warm-blooded animals were better able to compete. Surely this was mostly a product of our own biased beliefs, since we happened to be mammals, and so naturally mammals must be the best.

But that’s not how science works. When given a chance, scientific inquiry can overturn beliefs, provided that enough evidence is gathered.

Who's inferior now? Image credit: Daniel Eskridge
Who’s inferior now? Image credit: Daniel Eskridge

Here’s what the evidence shows. First, mammal relatives, known as paramammals (or mammal-like reptiles) dominated the age before dinosaurs, called the Permian.

Dimetrodon was related to mammals; it was definitely not a dinosaur! Image credit: DiBgd
Dimetrodon was related to mammals; it was definitely not a dinosaur! Image credit: DiBgd

As time went on, the paramammals became more and more mammal like.

Inostrancevia was practically a Permian lion, feasting on a turtle relative (Scutosaurus). Image credit: Dmitri Bogdanov
Inostrancevia was practically a Permian lion, feasting on a turtle relative (Scutosaurus). Image credit: Dmitri Bogdanov

Judging from the way things were going in the Permian, it almost looked like mammal-like reptiles were poised to rule the world for millions more years…

…then came the Permian extinction.

dinogorgon_923_600x450
The time when nearly everything died. Image credit: Jonathan Blair

Though the extinction event of 65 million years ago was major, and the extinction event that is currently happening could be just as bad, the Permian  extinction even was by far the worst. 96% of all animal species in the oceans, along with 70% of the backboned animals on land, died within a short period of time, triggering a huge increase in fungus that fed on the remains.

But what made it happen? In this case, there are many different hypotheses. Some say it was an asteroid impact, others say it was volcanoes and coal fires, while others say it was due to releases of methane, either from underwater traps or bacteria. At the present time, there doesn’t seem to be enough evidence one way or another in order to completely sway scientists’ opinions.

What is clear is what happened after the extinction event, when the playing field was leveled. Who would dominate the Earth? Mammal-like reptiles had pulled through, becoming even more mammal-like as time went on, but so had the dinosaur’s ancestors. The stage was set for major competition.

Thrinaxodon_BW
In this corner, we have Thrinaxodon, a clear mammal relative. Image credit: Nobu Tamura
And in this corner, we have Proterosuchus, an early archosaur (the group that includes dinosaurs, crocodiles, and birds).
And in this corner, we have Proterosuchus, an early archosaur (the group that includes dinosaurs, crocodiles, and birds).

As for who won, it isn’t really a secret. Dinosaur relatives became larger and evolved into a wide variety of sizes and shapes, while mammal relatives became smaller, furrier, and mostly had to retreat into burrows during the daytime, only to come out at night.

By the end of the Triassic Period, which followed the Permian, it was becoming clear who'd come out on top. Image source: Palaeos Earth
By the end of the Triassic Period, which followed the Permian, it was becoming clear who’d come out on top. Image source: Palaeos Earth
And by the Ju
And by the Jurassic Period, it was obvious who’d won. Image credit: Udo Schöter

 

The largest mammal in the Cretaceous Period, Repenomamus, was about the size of a badger. Image credit: Nobu Tamura
The largest mammal in the Cretaceous Period, Repenomamus, was about the size of a badger. At best, it could feed on baby dinosaurs. Image credit: Nobu Tamura
The largest dinosaurs, on the other hand, were easily bigger than a bus! Image credit: Matt Martyniuk
The largest dinosaurs, on the other hand, were easily bigger than a bus! Image credit: Matt Martyniuk

 

Here's a picture with more detail. Note that the largest sauropod (long-necked dinosaur) is dfferent--this is the subject of some controversy! Image credit: Zachi Evenor
Here’s a picture with more detail. Note that the largest sauropod (long-necked dinosaur) is dfferent–this is the subject of some controversy! Image credit: Zachi Evenor

I mentioned in an earlier post that some dinosaurs even sprouted feathers and started to fly (over a couple dozen million years).

A bird "family tree." Image source: Introduction to Birds
A bird “family tree.” Image source: Introduction to Birds
Haplocheirus_poster
Image source: Introduction to Birds

Now while it is true that the diversity of dinosaurs declined from time to time during their reign, they always bounced back with a new diversity of forms.

One of those times where dinosaur species declined was around 10 million years before they finally went extinct, and you can see on of them (a dinosaur mummy, in fact) described here by one of my favorite paleontologists, Robert Bakker (who was born and raised in New Jersey):

The key here is that while the number of species of dinosaur declined from time to time, they always bounced back. That is, of course, until an asteroid hit at exactly the wrong time (which I will remind you happened right off the Yucatán Peninsula).

All that could survive the catastrophe, aside from turtles and lizards, snakes, small crocodiles, and a variety of insects and amphibians (as well as smaller creatures in the rivers, lakes and oceans), were mammals and birds. Of course, birds were better adapted for flying, having lost the fingers on their hands, although certain bird species did dominate the land for a time.

Still, they were not as effective at dominating the land as their dinosaur ancestors, which finally gave mammals the chance to thrive. After 150 million years hiding from the dinosaurs in burrows and in trees, species began to diversify, including upright-walking apes that have dominated more and more environments between about four million years ago and the present.

Like it or not, that's your prehistoric cousin. Image credit: John Gurche
Like it or not, that’s your prehistoric cousin. Image credit: John Gurche

And don’t forget that it all started with a crater on the Yucatán! 

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