I’m still trying to get the hang of this. Here is a question I accidently posted to the wrong message board earlier this morning. Please excuse my clumsiness. The following is a statement I have clipped from an internet site on dinosaurs: "But then suddenly they all mysteriously disappeared at the end of the Cretaceous Period about 65 million years ago. " It is typical of statements you will find in any discussion of the disappearance of the dinosaurs. My question: In geological terms, what does “suddenly” mean? Does it mean they died out over a long weekend? Over a couple of centuries? Or does it mean they died out over a period of, say, one million years?
In geological terms, one million years can be a relatively short time for a group to die out, if it had been diverse and widespread at the beginning of that time.
However, if the asteroid impact theory of the extinction of the non-avian dinosaurs is correct, it could have been much more rapid than that: perhaps a matter of a few days or weeks; maybe a few decades or centuries for the last survivors to die off due to changed ecological conditions.
However, the dinosaurs had been in decline in terms of diversity for some time before the end of the Cretaceous. There were relatively few around compared to earlier periods. The group may have been declining due to long term climatic changes (perhaps triggered by volcanism), and the asteroid just finished off the last few.
When I took paleo 101 about a decade ago, there was some question about this.
Those favoring the Alvarez impact theory said the mass extinction was “nearly instantaneous.” By this they meant that dinosaur fossils essentially stopped appearing in geologic strata everywhere at once.
Others, like Robert Bakker, argued that the dinosaurs died off over the course of about 100,000 years, meaning many other causes of extinction were possible, from climate change to cataclysm.
The Geological record is not complete. There is virtually no environment with continuous deposition of sediments. That’s why we see different rocks piled on top of each other. The scale ranges from the sub-millimetre scale, to the Grand Canyon scale. The boundaries are always sudden - but as Colibri points out, suddenly can be longer than Homo Sapiens have been alive.
With reference to the dinosaurs, no one really knows how long the extinction took, but something weird definitely took place. Dinosaurs disappeared, but so did roughly 75% of all species world wide, both terrestrial and marine. The disappearence of the marine plankton was certainly very rapid. 90% of all plankton species went bye bye in a time span of less than an inch of clay. It doesn’t get much quicker than that where rocks are involved.
There’s a nice, not too technical site here with a lot more information.
Actually, this has been much disputed of late. See, for example, here:
Responding to a poll on another board as to what people thought killed off the dinosaurs, I said:
I voted “other” because there were at least three significant causes influencing the decline and extinction of the dinosaurs, and I don’t believe that any one of them can be targeted as “the” cause – that’s like saying that the assassination of Franz Ferdinand caused World War I. It certainly precipitated it, but the range of causative effects before and after that event had much more to do with it than that single event.
First, the Chicxulub impact was unquestionably the “Franz Ferdinand” event. The environmental effects resulting from that impact unquestionably were what pushed dinosaurs over the line into extinction.
But the question that that brings up is, what were they doing so close to the line that that pushed them over it? And that’s where the other two causes fit in.
First, the Deccan Traps were active – the flat-surface equivalent of around 20 1980 Mount Saint Helens eruptions daily over at minimum thousands of years. [Added: No, I don’t have a cite – I did an order-of-magnitude comparison analogy based on the amount of lava extruded.] To think that this had no worldwide climatic effects is to bury one’s head in the sand. It was, of course, gradual; plants and animals did have time to adapt. But it was definitely a stressor effect.
Then one has the Laramide Revolution. In the Western U.S., an orogeny was going on that would build the Rocky Mountains. There were related orogenic events throughout Eurasia and IIRC in South America – I don’t want to get specific here because I’m not sure which were latest Cretaceous and which before or after that time. (Perhaps someone with a good geology reference could fill in the gap.)
But this event caused significant climate change, for a wide variety of reasons – another stressor event.
These stressors and other minor and localized stressor processes had led to a decline in number of species present in the latest Cretaceous in most dinosaur families – the hadrosaurs being a notable exception. Such things had happened before, and as regards other orders would happen again. In general, the process is that during non-stress times families radiate to maximize the number of related econiches they will fit into, and stressor times select from this the best-adapted species. The other species die back, and then after stress times are over, the surviving species radiate again. This is a fairly normal ongoing process in evolutionary biology, documented in family after family, from corals to primates. But it happened that Belly River and Lance times, at the very end of the Cretaceous, were times of species reduction preparatory to a new radiation – largely due to the stressor effects of the Deccan Traps and the Laramide Revolution. In consequence, extinction of an entire family meant that only one species needed to fail to survive. And the Chicxulub impact’s consequences pushed those species over the line.
Thanks, I’m glad to see that. I had thought that it would have been rather coincidental for the asteroid to have hit just as the dinosaurs were declining for other reasons.
Some have proposed that the Deccan Traps eruptions were somehow triggered by the Chicxulub impact, which was directly opposite on the globe at the time. (Similarly, some have proposed the end-Permian Siberian Traps were triggered by an impact in Australia, which coincided with the greatest mass extinction of all.) However, I am not aware that any convincing mechanism has been proposed as to how this could have occurred.
Here’s more on the The Deccan Traps Volcanism-Greenhouse Dinosaur Extinction Theory
Being a bit lazy tonight, rather than trying to Google out the answer, I’ll just ask: my distinct impression from past reading was that Deccan Traps vulcanism preceded the end of the Cretaceous by a significant period of time, and was clearly datable to before the Chicxulub impact. Any of you who can confirm or refute this, or clarify that the dating is not all that precise?
After a bit of Googling, I found articles that indicate that the Deccan Traps date from about 200,000 years before the K-T junction.
On the other hand, there now seems to be evidence that Chicxulub itself dates from some 300,000 years before the K-T junction.
I’m not going to try to sort out the various competing theories and controversies right now, since it’s dinner time, but some info can be found here:
I was always under the impression that the major cause of a post-meteorite die-off would be the impact on plantlife. The meteor strike would cause cloud cover and tempature variations which would kill off plants; herbivores would starve and then carnivores would follow. If this is true the extinction period would probably be measured in months.
There are at least three proximate causes hypothesized in asscoaition with the impact scenario: sudden cooling, acid rain, and global wildfires. In truth, none of those three adequately explain the sum of the patterns of extinction at the end of the Cretaceous. Even taken together, there are groups which ought to have been affected which were not, and groups which outght not have been affected which were. Sudden cooling, for example, ought to have hit amphibians pretty hard, but they remained relatively unscathed.
Further, recent evidence suggests the dark period would have lasted 1 - 10 years, with a subsequent 2,000 year cooling trend. That length of extreme darkness should have hit a number of organisms much harder (or at least, perhaps, differently) than what we actually see. Again, amphibians, for example - no significant extinction was experienced on their part. Turtles weathered the event just fine, but lizards (speaking here of regular ones, not the terrible ones) took it in the proverbial teeth, with up to a 70% loss. Crocodiles, meanwhile, did about as well as turtles.
What is probably closer to the truth is that no single proximate cause was responsible for the whole of the extinction event. As Polycarp noted, there were multiple events occurring at the end of the Cretaceous, the sum of effects of which probably added together to create the observed mass extinction, and with different proximate causes serving as the killer for various groups.
In general, vertebrates associated with freshwater (turtles, amphibians, crocodilians) did relatively well compared to terrestrial forms. They might have been relatively protected from wildfires and possibly cooling (since water temperatures would be more stable than those on land), but one would think acid rain might have had a big effect on them.