Dinosaur Extinction

General Questions
#1

We’ve all been told that the prevailing theory of how the dinosaurs got wiped out is that a meteor hit the Earth and kicked up enough dust to cause a nuclear winter, which wiped out the cold-blooded dinosaurs. Alternately, I’ve heard that another theory states that material ejected from the impact of that same meteor left the atmosphere and re-entered, causing the atmosphere to heat up to 1200 degrees F, causing the extinction.
What’s wrong with both of these theories is that amphibians, reptiles, and crocodiles, which predated dinosaurs and which also survive to this day, were not wiped out.
Does anybody else see this inconsistency and have a better theory?

Baffled by Science

#2

You are missing the crucial point.
The environmental upset didn’t kill the Dinosaurs off directly (although it probably killed a huge number of them), it killed off their food source - plants. The smaller animals could survive on what was still able to grow, but the huge animals starved.

#3

Many amphibians, reptiles, and crocodiles did go extinct. A few did not. Many dinosaurs went extinct. A few did not. Many land plants and animals of all orders went extinct. A few did not.

For the most part, the ones that survived tended to be the smaller ones, as smaller creatures can recover from sudden changes more easily.

#4

Read the “Extinction Event” wikipedia. I’ve found it pretty helpful.

#5

So do dinosaurs.

#6

Second time to day I’ve posted the xkcd.

It really does put a different spin on the K-T event. Instead of “dinosaurs went extinct”, it’s “a whole lot of animals in a lot of clades went extinct, especially the particularly large species”.

#7

Very true, and the ancestry is particularly dramatic if you look at the large flightless birds like the Australian emu, which was found to have a good deal of DNA in common with fossilized dinosaur soft tissue samples that were analyzed some time ago. And check out the cassowary, said to be the world’s most dangerous bird. That first picture is like something out of Jurassic Park.

ETA: The dinosaurs in Jurassic Park were neither feathered nor brightly colored, probably based on what was known at the time and artistic license, and the ones in the later movies were just designed to be similar. But it seems quite likely that many dinosaurs were indeed both feathered and possibly brightly colored like the upper parts of the cassowary.

#8

Maybe the OP was listening to NPR recently. I can’t remember which day specifically, but just in the last few days there was an interview with someone claiming that the dinosaurs were wiped out in a matter of about 2 hours, due to the average air temp around the world rising to ~ 1200 deg. One wonders how any creature not hidden safely away could survive that!

And yeah, my first thought was: Well, the birds weren’t killed and they’re dinosaurs, but I honeys don’t know if the group we can “Aves” were differentiated enough from the non-avian dinosaurs 65MYA that they can be thought of as separate from dinosaurs in the way we sometimes think of them being that way today.

wolpup: Not sure if you were implying this, but I don’t believe any extant bird species can be thought of as genetically closer to “traditional dinosaurs” than any other bird species. And all flightless bird species are descended from “flightful” species.

#9

Most dinosaurs (including the avian dinosaurs) are now believed to have been warm-blooded, at least functionally. As has been pointed out, the extinction event preferentially killed off large animals. Many smaller groups survived.

Most of the land animals that survived the event were small, like most reptiles and amphibians (as well as the mammals of the time) are small. They were able to find shelter, and were also able to subsist on insects or seeds in the absence of plants.

Many freshwater animals like crocodilians and turtles survived the event, because the water shielded them. Crocodilians and turtles are also able to hibernate during adverse conditions.

#10

Actually, most birds became extinct at the end of the Cretaceous as well, notably the most common group the enantornithes. (The name means “opposite birds,” because some anatomical features were the opposite of modern birds). It was a matter of chance which avian and near-avian groups became extinct at the end-Cretaceous.

Right, all modern birds belong to a single lineage and are equally-distantly related from other dinosaurs.

And the word you want is “flying,” not “flightful.”:wink:

#11

65 million years ago, a great big rock slammed into the Yucatan Peninsula in southern Mexico with enough force that debris from the impact rained down as far north as Kentucky. There is also a layer of carbon found everywhere on earth at a depth that indicates this layer of carbon also dates to 65 million years ago. The thickness of this carbon layer varies, but the fact that it is everywhere indicates that the entire surface of the earth burned.

Around the same time as the impact 65 million years ago, there was a huge increase in volcano activity in an area called the Deccan Traps. While not exactly on the opposite side of the world from where the impact hit, it’s kinda close, which means that this were likely an area where volcanoes were already prone to happen, but were also spurred on and made a lot worse by the shock wave from the impact, which would travel around the world and would concentrate again on the opposite side of the globe from the impact.

So not only did the impact burn everything to a crisp and throw up a huge amount of hot stuff into the atmosphere (you can basically imagine it raining molten lava for a bit), but then it was followed by huge volcanic eruptions that threw even more stuff into the atmosphere and kept throwing stuff into the atmosphere for decades.

The earth wasn’t a very happy place for a very long time after that.

The impact itself killed a lot of the life on earth. You’ll note that most of the survivors were small creatures that could burrow underground or could dive under the water to get away from all of the burning stuff. The creatures that did manage to survive the impact and the rain of fire then faced a new problem. There wasn’t any food. A lot of the plants that were burned didn’t die off completely, and would have sprouted new leaves and grown new fruit, but the lush forests were gone. Only a tiny fraction of the plant life remained. Creatures that lived on those plants starved to death. Creatures that ate the plant eaters then also starved to death.

Crocodilians did fairly well because they can go a long time in between meals and not starve to death. This is why they survived while other similar sized animals perished. IIRC, the crocodilians were the largest land animals to survive, partly because they could go underwater to survive the burning, and partly because they could go a long time without eating.

In the ocean, things weren’t quite as bad because the ocean didn’t burn (obviously). The lack of sunlight really wreaked a lot of havoc there though. Ocean plant life died off in large numbers, causing the plant eating animals to starve, which then caused the ocean predators to starve.

75 percent of the species on earth were completely wiped out. The 25 percent that remained had their numbers drastically reduced, but managed to squeak by and survive.

There’s nothing inconsistent or strange in the species that survived vs. those that were completely wiped out.

#12

Iridium, not Carbon. And Carbon Dating wouldn’t work for something that old.

#13

The layer contains high levels of carbon, which is the indication that stuff burned, and also contains abnormal levels of iridium, indicating an impact event.

Cite:

“Accretion of Extraterrestrial Matter Throughout Earth’s History”
edited by Bernhard Peucker-Ehrenbrink, Birger Schmitz

#14

It’s actually the peculiar selectivity of extinctions - heavily weighted to large land animals - that helped point to an impact or other catastrophic event.

#15

High levels of carbon is generally indicative of organic matter, not necessarily “stuff burned”. Finding Carbon is not the same as finding charcoal or ash. But the defining characteristic of the impact event, as you note, is the presence of iridium-- rare on earth, not so rare in asteroids and comets. I just found it strange to mention the K-T boundary layer without noting the presence of iridium, since that was the key clue that pointed to an impact event in the first place.

And even your cite notes that the charcoal and ash is found in “many” of the boundary layer sites, not “everywhere on earth”. This paper, published after you cite, notes the absence of charcoal at certain of the K-T boundary layer sites.

I honestly don’t know if there is a consensus on the idea of a global burning event due to the impact. One hears different scientists speak confidently about different hypotheses all the time.

#16

Nitpick: Iridium is rare in the crust of the Earth. It’s expected to be relatively abundant in the core, though (well, at least as abundant as it is in asteroids).

#17

Yes. Point taken.

#18

I heard part of the show yesterday - it was Radio Lab.

#19

Nitpick – tektites (solidified drops of molten glass) have been found in the ocean on the opposite side of the earth that are believed to be from the impact event. In layman’s terms, it threw melted rock completely around the earth.

Much of the evidence we see about surviving species is consistent with the idea that small, burrowing animals were preferentially selected. That’s not incontrovertible proof, but it does tend to support the idea the world burned.

#20

Fight my ignorance, we have Dino DNA?