Dinosaur extinction

Everybody knows the ‘dinosaurs went extinct 65 million years ago’ theory.

How can scientists be so sure they’ll never find any dinosaur fossils younger than that in the future?

They can’t. But it’s not for lack of trying.

And before anyone does it, I’ll throw in the obligatory birds are dinosaurs bit.

Why couldn’t a ‘real’ dinosaur which existed say 68 million yrs ago have existed more or less in the same form say 62 million yrs ago?
Why is the 65 million year number always thrown at us as some sort of deadline after which they’re not supposed to exist?

As with anything in science, nothing is an absolute certainty. All that we can say is that since we find non-avian dinosaur fossils abundantly up until the end of the Cretaceous period (which ended 65 million years ago), and none after that,* that the dinosaurs went extinct at that point. However, the coelacanth is a type of fish that was also believed to have become extinct at the end of the Cretaceous, but which was found to still exist in 1938.

*There have been some claims that some non-avian dinosaurs survived the end of the Cretaceous, based on fossils found in deposits from the next period, the Paleocene. However, these fossils are generally thought to have been eroded out of Cretaceous deposits and redeposited in ones from the Paleocene.

It could have, but we have no evidence it did.

Because that’s what the evidence seems to indicate.

Surely the portion of the planet that has been tread on by palaentologists is miniscule compared to the gazillion places where such evidence could lie…

Sure, but it’s not like dinosaur fossils have only been found on some out-of-the-way island somewhere. AFAIK, the fossil record is pretty consistent world-wide regarding the end of non-bird-type dinos. Which would indicate some kind of global cause to their demise. And, if you’re digging, newer fossils are generally found higher up (that is, easier to find).

I don’t think scientists are saying, or feel like they can say, that every non-avian dinosaur died exactly 65 million years ago. So few fossils are preserved (as a % of living creatures), that I think it’s quite plausible that some pocket of non-avian dinosaurs hung on somewhere for an extra half million of years or so, and then collapsed without leaving any trace of its existence. Such things could well have happened, but even if they did, it doesn’t really change the fundamental fact that within a very (geologically) brief period, the fossil evidence of dinosaurs went from copious to nil–so something super-dramatic happened, and whether or not it killed every single last dinosaur or virtually every single last dinosaur is not nearly as interesting–nor as potentially answerable–a question as what the hell happened to cause the shift.

I mean, if it absolutely stopped raining in your city for 50 years, it would be really hard to prove that nowhere in that city did a single drop of rain ever fall–but you could be pretty damn sure that the basic state of “rainlessness” had existed, and that state is what you’d really be interested in.

Sure, but there are many places where Mesozoic rocks are exposed, and many places where Tertiary rocks are exposed; large numbers of vertebrate fossils are known from each of these time periods. Many thousands of dinosaur fossils have been found in Mesozoic rocks, and none from later periods (except for the apparently redeposited fossils I mentioned above).

Again, it’s not impossible that a dinosaur fossil might be found from after the end of the Cretaceous; it’s just that the probability is small.

Well, when and if scientists find some younger dinosaur fossils, they will correct the error. But it would be ludicrous to stay agnostic on the subject until every rock is unturned.

I mean, how do you know there aren’t any flying pigs out there somewhere?

Try this: throughout the Tertiary (65 million-2 million years ago) and Pleistocene (2 million-12,000 years ago) there are extensive fossils of proboscideans (elephants, mammoths, mastodons, gomphotheres, deinotheres, and all their kin); rhinocerotids; either ground sloths (four large families in the New World) or chalicotheres (Old World). Together they composed the largest herbivorous forms throughout most of the world, from tundra to tropics. Today only three species of proboscidean survive, in South Asia and a part of sub-Saharan Africa; only five species of rhinocerotids, three of them confined to small nature preserves and the other two to a small area of Africa; chalicotheres are extinct; and only one species of ground sloth survives (as an arboreal ‘tree sloth’). While they’re mostly not totally extinct, the likelihood of fossils from Recent strata in the future is small, and for all practical purposes many of the families would be extinct without human protection.

We’ve now defined a massive die-back. That a few species of dinosaur might have survived as relict forms past the K-T extinction event is quite possible, there is no fossil evidence of it, and it would be the equivalent of saying, the rhinocerotids are not totally extinct. That’s triue, bot they’re not the dominant large herbivore much of anywhere any more.

Similarly, ornithopods, particularly hadrosaurs, and coelurosaurs were everywhere. Ankylosaurs and nodosaurs were sparse but widespread – think of large deer like elk and moose for what I mean here. Ceratopsians and tyrannosaurs were common in western North America and eastern Asia. Titanosaurs (the surviving sauropods) were common throughout the southern continents, especially South America, and including New Zealand, and locally common in Eurasia and North America as well. At the K-T extinction, that ends. If anythng survived, it would be the equivalent of the surviving Asian rhinocerotids or the two-toed sloth – a few relict populations in specific local areas, or one species surviving in a particular econiche not likely to produce many fossils.

There are such things as “Lazarus taxa” – the coelacanth is a perfect exmple: an animal surviving many millions of years after its relatives were presumed extinct. There are several Lazarus taxa showing up in Tertiary sediments that were otherwise extinct at the K-T. None of them, to date, are (non-avian) dinosaurs, big or small, meat- or plant-eating, bipedal or quadrupedal – a dozen groups of large animals went from dominating the fauna to zilch at the K-T event. I won’t rule out survivors into the Tertiary – but I’ll be very surprised to see a find of any of them announced.

Ever seen a crocodile? Not everything died 65 million years ago. Dinosaurs weren’t just one thing. There were lots of different types of them, and some do survive to this day.

65 million years ago, this great big rock slammed down into Mexico so hard that debris from the impact rained down on Tennessee. Remnants of fire from the event have been found all over the world, so aside from the damage of the impact, you can imagine the entire world burning. The shock waves triggered a lot of volcanic activity, so between the stuff thrown up into the atmosphere by volcanos and the stuff thrown up into the atmosphere from the impact itself, you didn’t get much sunlight there for a while. Plants that weren’t burnt from the fire raining down from the sky died due to lack of sunlight. Animals that depended on the plants died. Animals that ate those animals then ran out of food and also died.

A few types of crocodilians survived, because they have the ability to go without food for fairly long periods of time. Many species of avian dinosaurs (birds) survived. Frogs and a lot of amphibians survived. So it’s not like nothing survived this mess. A lot of stuff died though, and it wasn’t just the dinosaurs.

Your typical non-avian dinosaurs needed to eat plants or needed to eat animals that ate those plants, so those pretty much all died out from starvation. With that kind of extinction event, it’s not realistic to think that viable populations of these animals lived for very long after the event. This is why scientists would be very surprised to find any of these animals after the extinction event.

Another thing is the K-T boundary which (clearly?) indicates something major happened, and has hugely different ‘fauna’ above -v- below it. Comet, volcanoes, who knows for sure? But something happened that might’ve been relatively instantaneous or taken much longer in duration, but still quick geologically. I hope I say this right…

Dinosaurs (as we define the term) are not found above this level/time for the most part. Above it, things are a lot different and as the levels get ‘younger’ (closer to surface), different things are found/discovered that are radically different than typical dinosaurs found in older (~65 mil+ yrs) - things that would not be copacetic to dinosaurs needs as we understand. Kind of like color going from white to black quickly with possible greys in between and the duration of grey is debated still (I think). Black would be the point above K-T layer, fwiw. Probably a bad analogy.

Since nothing of dinosaur-like life is really found above the time of 65mil yrs (or so) and with the Earth being so very different then, it is a strong assumption that during that period, all dinos died off. How long did it take? Shrug. Until dino stuff is found above K-T (reliably dated at ~65mil), science is strong on that time-frame.

I likely missed something of detail, but that K-T boundary tells a LOT about time(s) and what went on ‘back then’. I think.

ETA: I see others give much better detail and relate K-T into it as well. Thanks!

Since others have danced around this point, I’ll just come out and make it clear: Birds are dinosaurs. I can hear dinosaurs singing outside my window right now, I ate a dinosaur-meat sandwich for lunch on Thursday, and a dinosaur pooped on my bike seat a couple weeks ago.

We’re not dancing around it; somebody mentioned it in post #2, for pete’s sake. The point is that ‘dinosaur’ in popular parlance includes only the creatures that died off at the K-T event or before, and not the paleognath and neognath dinosaurs of the Tertiary and Recent. (A friend from Surrey uses an avatar of an angry emu’s head that looks for all the world like a photograph of an attacking dromaeosaur.)

You have the same problem as the old use of “reptile” – it’s often convenient to have a generic term for amniote tetrapods that are not birds or mammals, despite the fact that that’s a cladistic nightmare. Likewise using “dinosaurs” to mean “ornithischians and non-avian saurischians” is also often convenient, however cladistically incorrect.

** walks off humming “There’ll be Blue Dinos Over the White Cliffs of Dover” and “When the Dinosaurs Come Back to Capistrano” **

Colibri in post #2 referred to “non-avian dinosaurs”, but didn’t pause to explain why he put in the qualifier.

Which is really the answer to the OP, right there: The reason that people say the dinosaurs all went extinct 65 million years ago at the latest is that anything that didn’t go extinct then, we don’t call dinosaurs. Why don’t we call them dinosaurs? Because they didn’t go extinct.

I think the term is self-explanatory, since birds were already identified as dinosaurs in the post before that.

No, the OP was clearly using the term dinosaur to mean non-avian dinosaurs. If we discovered a non-avian dinosaur that lived 40M years ago, we’d call it a dinosaur.

[Extreme sarcasm! Do not take seriously!!]
After Darwin founded the Church of Evolution, his followers spent decades going around the globe destroying all evidence that dinosaurs had coexisted with Adam and Eve’s descendants up until the Flood.

Apologies, John Mace. I somehow managed to completely overlook that post. Even, apparently, after it was pointed out to me.

Yes, I just said “non-avian dinosaurs” because John Mace had already pointed out that birds are dinosaurs.

Actually, since birds survived the K-T event it’s a little surprising that other small dinosaurs didn’t. All large terrestrial animals died off in the event; however, many other dinosaurs were as small as some birds, and you would think that a few of these might have made it through as well. But it seems that only the small avian dinosaurs, the ones that could fly, got through.