65-Million Year Old Mystery Resolved?

[Correa Neto]

Just like the K/T boundary case (better say the K/T boundary case before the impact evidence became clear), there are a number of possibilities for each of the other mass extinction events. To date, AFAIK there are no actual "winners". Personaly, my wild educated guess is that ech one may have had a diferent cause (or group of causes).

I would guess, for example, that those related to runaway greenhouse effects, massive methane release, anoxic events and similar things have an edge when it comes down to events which happened when there were supercontinents around.

Note also that one must be very lucky to find evidence for oceanic impacts older than the K/T boundary. Finding the crater itself would be very unlikely, given the oceanic crust recycling generated by plate tectonics. Tektite/spherulite layers would be a better guess, since even examples from the Archean are known. But you would still be left with the problems of finding and correlating them.

Now, as for amonites, no idea. I can only make some guesses. Maybe the competition with other species was too fierce after the blast, when the race "re-started". Maybe their larval stages was too vulnerable to the climatic changes (if they ate phytoplankton, for example). Maybe they were already on the declining curve. Who knows? All we know is that trilobites followed the same path.

 

[Correa Neto]

I'm still confused:

Why would the impact take out the marine dinos?
And why didn't it take out alligators and turtles?

nitpick- There were no marine dinos, but large marine reptiles.

If phytoplankton dies, the whole net crumbles. Since it seems most of them were big active predators (and probably quite specialized), they would be very vulnerable in such a situation.

Small to medium-sized oportunistic and scavenging critters, especially if not very active and/or able to store and save energy somehow (say by hibernate within holes, accumulating fat, etc.) woul have a great edge in such after-impact situation. Perfect for turtles, crocs and rat-like critters (not to mention cockroaches).

 

[Landrew]

I'm also fairly "sold" on the asteroid impact theory, but never am I hermetically sealed against any new evidence which might suggest otherwise.

I believe some investigators have attempted to "debunk" the asteroid theory by suggesting that some dinosaur species may had survived the event, because their fossils appear to exist above the k-t layer.

I don't doubt that some survived the event, but in the radically-changed environment which favored a whole different set of organisms, they probably failed to adapt to it and may have hung on for a time, but probably became extinct a bit later than most of their kin.

 

[MG1962]

I'm still confused:

Why would the impact take out the marine dinos?
And why didn't it take out alligators and turtles?

I can not speak for turtles but with alligators and crocs they have the ability to go insane amounts of time without food. I would imagine in the first days after the event the amount of carrion available would have been enormous, and simply they would have stuffed themselves and waited it out

 

[Correa Neto]

Same thing. Low metabolism and can accumulate fat.

Possibly did the same crocs and gators did. Stufed their bellies with carrion and lived from the stored fat. Not sure about what the veggie ones did. Maybe just got dormant inside holes waiting for better days.

 

[rjh01]

It would take a few months for the plants to die and decompose. Once this has happened then you need to be able to wait for the plants to come back. When it does most of the animals would be dead. The rest would be starving. If you do survive then it would be a matter of who can breed fastest. The climate would be bad.

 

[Landrew]

Basically, nearly every animal over a certain size perished. Perhaps it was easier for smaller animals to live on carrion and stay buried underground during a period of intense cold.

 

[SpuriusMaximus]

Those knew when to dodge

 

[Damien Evans]

Nope we've lost stuff over the years. Sometimes due to poor record keeping (do you really know what is in all those boxes in the cellars?) theft (fossiles are valuable), physical damage (from straightforward handleing to more serious issues) and other record keeping issues (how sure are you as to where stuff came from?).

And, of course, War. Lotsa Museum collections damaged in wars.

 

[DecayingAngel]

Small Survivors

Although the size/weight theory is not 100% accurate, I read many years ago that basically anything over about 50 lbs. in weight (with exceptions) did not survive. This was due largely to how much food an animal would need to live, so those requiring a smaller amount would subsist on a lot less nourishment.

I was watching a documentary on Sarcosuchus Imperator and it would make sense why it went extinct since it needed a huge amount of food. With fewer large dinos around, I could see how quickly this species would disappear.

Its smaller relatives, very similar to the crocodilians we know today, not only require a lot less food but as has been mentioned once having eaten they can go for very long times until the next meal.

I can remember that before the asteroid theory was proposed, one of the biggest questions involved the size/survival puzzle. Now that seems to have at least partially been resolved.

But I do think that the other major extinctions in earth's history are worth a closer look. There is a chance they may point to some kind of cyclical event, who knows?

 

[quarky]

I'm not against the asteroid theory, but weren't there lots of small dinosaurs that bit the dust along with the large ones?

 

[Simon39759]

I'm still confused:

Why would the impact take out the marine dinos?
And why didn't it take out alligators and turtles?

Being nitpicky here but there were no "marine dino".

As for the core of your question, the mass extinction was not as much species being blasted out, it was more the impact causing a nuclear winter and a environmental crisis that lead to a crash of the ecosystem.
The dinos, being warm blooded and yet having limited thermic isolation, were very poorly adapted to a drop in temperature or a restriction in available food...


ETA: Doh; it seems I hadn't see the next page with all the answers, sorry all!

 

[Delvo]

Although the size/weight theory is not 100% accurate, I read many years ago that basically anything over about 50 lbs. in weight (with exceptions) did not survive. This was due largely to how much food an animal would need to live, so those requiring a smaller amount would subsist on a lot less nourishment.

If it had been a matter of food requirements, warm-blooded animals would have been gone. A small mammal eats something like 10 times as much as a lizard of the same size, and it's even higher for birds than for mammals. At the very least, if size & food requirements were the issue, then endotherms and ectotherms should have had very different weight limits, not the same one.

I do notice that the only group of dinosaurs to survive was small, but there were no large mammals, so saying that things over a certain size limit went extinct is essentially just re-phrasing the statement that non-avian dinosaurs did, plus maybe some large reptiles if there were any at the time. That's not adding much information; it doesn't really separate the question of size from the question of different taxons and their various other advantages or disadvantages.

weren't there lots of small dinosaurs that bit the dust along with the large ones?

The small size ranges were pretty much filled by reptiles and mammals.

 

[The Fallen Serpent]

If it had been a matter of food requirements, warm-blooded animals would have been gone. A small mammal eats something like 10 times as much as a lizard of the same size, and it's even higher for birds than for mammals. At the very least, if size & food requirements were the issue, then endotherms and ectotherms should have had very different weight limits, not the same one.

I do notice that the only group of dinosaurs to survive was small, but there were no large mammals, so saying that things over a certain size limit went extinct is essentially just re-phrasing the statement that non-avian dinosaurs did, plus maybe some large reptiles if there were any at the time. That's not adding much information; it doesn't really separate the question of size from the question of different taxons and their various other advantages or disadvantages.

The small size ranges were pretty much filled by reptiles and mammals.

If there were large lands mammals present I suspect they would have gone extinct as well. Prior to archosaurs such as the dinosaurs becoming the dominant large animals the synapsida were the dominant large land vertebrates. For whichever reason is actually true (there are competing theories) archosaurs had an advantage in competition between large animals. So after the Permian–Triassic extinction event^WP archosaurs flourished, diversified and filled the niches of large land vertebrates. After the Cretaceous–Tertiary extinction event^WP the environment was different. Non-avian dinosaurs are believed to have been warm blooded and that made a big difference between dinosaurs and crocodillians survival rates. Hence more and larger crocodillians survived than dinosaurs.

The evidence suggests that mammals did not largely begin to diversify immediately after the K-T boundary. The environment was different than before the K-T boundary that gave mammals an advantage and allowed them to dominate the niches of large land vertebrates, but it doesn't appear they got the chance to fill such niches for a very long time.

It appears that fish largely passed the K-T boundary with small rates of extinction. Does someone know if the evidence suggest that plesiosaurs were warm blooded?

Basically I am agreeing that the weight limits for warm-blooded and cold-blooded creatures would be different. The majority of mammals were the size of rats at the time of the K-T extinction event. A 50 lbs limit would be an absolute limit for all creatures and a smaller limit would be in effect for warm-blooded creatures. Mammals and avians were the predominant warm-blooded creatures to survive and they did not begin to grow in size again until after the K-T event.

 

[DecayingAngel]

Came From Very Outdated Book

I only mention the size/weight theory because it appeared in a book I had read as a child, and that book was already quite old, coming as it did before the asteroid theory was even proposed. My uncle used to keep a big library upstairs in our house and I was like a kid in a candy store with his books. Some of them, as the one I am referring to, were quite old and thus very outdated as far as modern theories are concerned.

I just mentioned it as a matter of interest. The thought did spook me as a kid, though, and it seemed to be such an unfathomable, scary mystery!

 

[DecayingAngel]

Early Paleontologists Used to Blast Away

And, of course, War. Lotsa Museum collections damaged in wars.

It is sad when one thinks of all the wonderful collections (not just scientific but of art and of course, many beautiful buildings) that were destroyed by war.

Another practice which probably destroyed almost as much as it helped to find fossils was blasting. Early paleontologists used to dynomite sections to get at possible fossils and in so doing one can only imagine the havoc this created and the damage and destruction of the very objects they were seeking.

 

[Correa Neto]

I think the issue must not be approached only through the size angle, but also from the especialization aspect. Small specialized critters, evolved to use very specific niches would most certainly be at the top of the extinction list.

Imagine for example small, chicken-sized species of dinos, adapted to live among trees, some species maybe even gliding between trees. Impact, KABOOM, sun blocked by ash and cinder, trees die, dinos die too.

 

[Landrew]

It is sad when one thinks of all the wonderful collections (not just scientific but of art and of course, many beautiful buildings) that were destroyed by war.

Another practice which probably destroyed almost as much as it helped to find fossils was blasting. Early paleontologists used to dynomite sections to get at possible fossils and in so doing one can only imagine the havoc this created and the damage and destruction of the very objects they were seeking.

Dyn-o-mite!
As Jimmy Walker used to say.

 

[Landrew]

That particular extinction event has the hallmarks of a nuclear winter-type effect. Plants that would survive intense freezing, and animals that could hide in caves, like rats, bats,lizards and birds seem to have survived. Crocodiles and champsosaurs probably stuck it out in lakes and rivers which didn't freeze up completely during that time, or perhaps there was enough geothermal heat in some areas to keep them alive. The champsosaurs died out a bit later, probably losing out to competitors in a drastically changed ecosystem.

 

[quarky]

Forgive my ignorance on the subject:

The large marine reptiles...Did they crap out during this same time frame? If so, why?