Might be a theory.

[snuffypuffer]

So I'm sitting here thinking about the asteroid that killed off the dinosaurs, and how from the Jurassic to the Cretaceous there doesn't seem to be as much diversity as far as dinosaur species. The giant sauropods were gone by then, for instance. And I got to thinking that maybe by the time the thing hit, dinosaurs were already on the decline, evolving into birds in some instances, probably going in different directions in others. Basically what I was thinking was dinosaurs as we know them now might have been on the way out anyway, and the asteroid just helped them along. Who knows what would have happened if raptors could have kept evolving. We might have never made it onto the scene at all.

I'm posting this idea of mine to see if any of you cats that know more than me about paleontology think this holds any water. Thank you and good night.

[Startraveler]

I believe a few studies of have been done that found no real decreases in dinosaur diversity through the Cretaceous, though that conclusion is far from solid. Regardless of whether dinosaurs were on the way out, though, the K-T event was a mass exctinction--many, many lineages came to an end at the end of the Cretaceous. So if the Chicxulub asteroid merely helped along a handful of dying lineages, there's still a large amount that died out who weren't just sped along to the inevitable. So the impact is still sizable when you look at the bigger picture.

[snuffypuffer]

Never said I thought it was valid.

Sure, the KT event (I wasn't sure it had an actual name) killed off a lot of species. But there are quite a few that it didn't get. So that was what got me thinking, some animals were better suited to survival than others. Or just lucky. You never know.

[frogfish]

The K-T event also started a chain reaction that closed the books on the dinosaurs

[Jack_of_Blades]

Heard one theory that stated "Around the time of the mass extinction

only around 20 species of dinosaurs remained."

[frogfish]

only around 20 species of dinosaurs remained

That's false...20 species remained in Utah...maybe, but much, much, MUCH more around the world..

[Conspiracy]

wasnt there already a theory that dinos were already on the decline and the chain reaction following the asteroid just finnished em off? :/

[frogfish]

wasnt there already a theory that dinos were already on the decline and the chain reaction following the asteroid just finnished em off? :/

That is a thoery, as sauropods have already died out...but the chain reaction following the asteroid was probably the thing that did it...

[Startraveler]

It's important to realize here that extinctions are not always a very clear-cut business, at least not from our vantage point millions of years after the fact. I'm sure everyone knows the basic rule of thumb that the further beneath the ground a fossil is, the older it is. So paleontologists are able to give a decent estimate of when a species originated just by knowing how far down the very deepest specimen ever found is (they call that one the first appearance datum). Similiarly, they can get a good idea of when a species died out from the top-most specimen (called the last appearance datum). In fact, that span is called the species' stratigraphic range (that is, the amount of rock--and thus, time--through which it exists).

But fossilization is a rare process. Which means that just because you have the last specimen that ever got fossilized (or rather, the last one we've found--incomplete sampling is yet another hinderance to getting it right), that certainly doesn't mean you've found the last specimen that ever lived. The species could've persisted for tens of thousands of years after that "last" one died. Paleontologists use a good deal of mathematical analysis to sort out how long a species probably persisted past its last appearance datum. As you can imagine, the probability that the species was still around drops off the farther forward in time (i.e. upward through the rock) one considers.

So suppose we have a healthy, diverse bunch of dinosaurs living all around the world at the end of the Cretaceous 65 million years ago. Suddenly a massive catastrophe occurs (and it's pretty undeniable at this point that a very large asteroid hit the earth at the end of the Cretaceous) and an abrupt mass extinction of all kinds of organisms--not just dinosaurs--occurs ("abrupt," by the way, as paelontologists use the word can still mean a few thousand years; it's not as if everything just instantaneously slumped over dead). What would that look like in the fossil record? Is it possible it might not necessarily appear abrupt to us?

In short, yes. Not every creature has the same probability of getting fossilized (its so-called "preservation potential"); that depends on a number of factors and some species just have higher preservation potentials than others. So some species might have a last appearance datum a while before the extinction event (in fact many will because, as we've said, species tend to persist for a bit after their last appearance datum). Just due to these realities of how preservation (as well as human sampling) works we might expect the mass exctinction to appear smeared out a bit in time. This phenomenon of abrupt extinctions appearing more gradual than they really were is actually well-known--it's called the Signor-Lipps effect. But there are other realities to contend with as well. Any fossil bed you find will experience a degree of time-averaging, meaning that animals that didn't live at the same time as each other (or, oddly enough, even some that didn't even live in the same place as each other) will appear right next to each other in the same layer of rock (which, converting that rock depth into a time, is one geologic instant). Further, depending on the location where a given paleontologist is digging, the temporal resolution provided by the rock just might not be very good. Some places have more gaps, less overall layers (that is, less slices of time).

So you can see that figuring out just how many dinosaurs there were at the instant the asteroid hit is a pretty tough question. Paleontologists can try to look for trends leading up to the end of the Cretaceous but it actually takes a fair amount of math and analysis to try and figure out what the data is really saying. Sometimes an abrupt end can actually look like a more gradual process that began before any catastrophe occurred; luckily paleontologists are very clever folks and they're always working on better and better methods for figuring out ways to distinguish between this or that. And like I said, I don't know that there's ever been any evidence that the dinosaurs were experiencing a major drop-off in diversity before the K-T event. But I certainly can't imagine it's anything but an open question right now.

[frogfish]

Well said startraveler

[snuffypuffer]

See, many very intelligent responses to a question. It can be done!

[Startraveler]

A lot of times the nitty gritty details of paleontology are lost in favor of some of the flashier aspects. It's questions/ideas like yours that let the two mix a bit and bring some of the really important stuff to the forefront. Kudos.