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Author | Topic: Extinction of Dinosaurs: Consensus Reached . . . mostly | |||||||||||||||||||||||||||||||
Taq Member Posts: 10045 Joined: Member Rating: 5.3 |
Scientist is as Scientist does. Ever the tentative bunch, scientists have reached a "final" consensus on the cause for the extinction of the dinosaurs. The final verdict?
"Combining all available data from different science disciplines led us to conclude that a large asteroid impact 65 million years ago in modern day Mexico was the major cause of the mass extinctions."--Peter Schulte of the University of Erlangen in Germany, in this MSN article. So the debate ending, everlasting conclusion is that the Chicxulub impact was a "major cause". First of all, no shit. I don't think anyone has really debated that it was a factor, or even a major factor. However, this doesn't end the debate on how much other factors played in to it including supervolcanism in modern day India (the Deccan traps) or the already dwindling numbers of dinosaurs species that led up to the Chicxulub impact. So this leaves the debate right where it was before, with the Chicxulub impact being a player but also other players without a defined role. To be fair, I have only read MSN article and not the Science article. Still, I find it funny that scientists can't stop being scientists even when trying to portray certainty in a layman fashion. The big consensus? The Chicxulub impact made the dinosaurs go extinct . . . mostly.
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Admin Director Posts: 13023 From: EvC Forum Joined: Member Rating: 1.9 |
Thread copied here from the Extinction of Dinosaurs: Consensus Reached . . . mostly thread in the Proposed New Topics forum.
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Coragyps Member (Idle past 756 days) Posts: 5553 From: Snyder, Texas, USA Joined: |
The Science article sums up:
The correlation between impact-derived ejecta and paleontologically defined extinctions at multiple locations around the globe leads us to conclude that the Chicxulub impact triggered the mass extinction that marks the boundary between the Mesozoic and Cenozoic eras ~65.5 million years ago. So maybe the reporter was the waffly one here. The article itself also discusses some of the alternate scenarios. Warning: it's now the K-Pg boundary, not K-T. Palaeogene seems to have supplanted Tertiary........
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.4 |
I think the paper does link the K-T (aka K-Pg in the paper) conclusively with the Chicxulub impact and provide good reason to think volcanism and multiple impact scenarios fail to explain the spread of evidence.
But there's still no explaination as to why the Chicxulub impact wiped out the non-avian dinosaurs, specifically, whilst leaving the crocodilians, birds and mammals. Similarly for the various other groups that survived or went extinct. The paper also, bizarely, claims that "[t]he scale of biological turnover between the Cretaceous and Paleogene is nearly unprecedented in Earth history". And supports it with a reference neatly detailing the other mass extinctions... umm? And that's the real problem I have with impact explainations for the extinction of the dinosaurs - there is no evidence of impacts that co-incide with prior mass extinctions. It seems to me that the search for abiotic explainations of mass extinctions is missing the big picture: extinction seems to be a property of evolutionary systems. Chicxulub happened, it explains the geological features of the K-T boundary. But it cannot be considered an answer to dinosaur extinction question until it can explain the distribution of survivors.
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Blue Jay Member (Idle past 2720 days) Posts: 2843 From: You couldn't pronounce it with your mouthparts Joined: |
Hi, Mr Jack.
Mr Jack writes: The paper also, bizarely, claims that "[t]he scale of biological turnover between the Cretaceous and Paleogene is nearly unprecedented in Earth history". And supports it with a reference neatly detailing the other mass extinctions... umm? It’s a poor choice of words, but, since that scale of turnover only happen four other times in half a billion years (and only two of those were at equal or greater intensity), I think it counts as nearly unprecedented. I certainly wouldn’t have phrased it that way, though. -----
Mr Jack writes: Chicxulub happened, it explains the geological features of the K-T boundary. But it cannot be considered an answer to dinosaur extinction question until it can explain the distribution of survivors. I'm going to have to disagree with you on this. First, I don’t think science can really answer questions of the form, Why didn’t _____ happen? Second , I think there are very good hypotheses about how each of those types of organisms survived: omnivory, small body size, low metabolism, and/or ability to find shelter are all considered likely explanations for why various groups survived. -Bluejay (a.k.a. Mantis, Thylacosmilus) Darwin loves you.
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.4 |
First, I don’t think science can really answer questions of the form, Why didn’t _____ happen? Maybe, but it certainly should be able to explain what it's trying to explain - in this case why the dinosaurs died out. The impact isn't an answer to that question because it doesn't explain why other groups didn't die out and thus leaves the question unanswered.
Second , I think there are very good hypotheses about how each of those types of organisms survived: omnivory, small body size, low metabolism, and/or ability to find shelter are all considered likely explanations for why various groups survived. It is simply not possible to find clear dividing lines like that between groups which survived and groups which didn't. While the popular face of dinosaurs might be the big beasts, there was a whole diversity of different dinosaurs out there, from tiny omnivores to massive sauropods. A fact that becomes increasingly pertinent as it becomes increasingly clear how similar the smaller theropods were to birds.
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Taq Member Posts: 10045 Joined: Member Rating: 5.3 |
The paper also, bizarely, claims that "[t]he scale of biological turnover between the Cretaceous and Paleogene is nearly unprecedented in Earth history". And supports it with a reference neatly detailing the other mass extinctions... umm? And that's the real problem I have with impact explainations for the extinction of the dinosaurs - there is no evidence of impacts that co-incide with prior mass extinctions. That has always been the issue, from my limited reading on the subject. I would hazard a guess that most experts would have the Permian extinction as the #1 extinction event and there is no known impact event that correlates with the Permian die off. However, the Siberian traps do correlate. In fact, I watched a Discovery channel show (don't worry, I don't consider the Disc Channel as gospel truth) that directly correlated the Siberian traps with the Permian die off. So what do we have with the K-Pg die off? The Deccan traps. On the other hand, we are lucky to have the Chixulub crater. If it occurred in the Pacific, for example, the crater could very well have subducted and all traces gone. The K-Pg iridium layer would have still been seen world wide where the interface was preserved, but there is no guarantee that every impact will produce such a layer (e.g. icy comet). So it is very understandable why scientists are hedging their bet on this one. That's the way it should be.
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Percy Member Posts: 22480 From: New Hampshire Joined: Member Rating: 4.8 |
Taq writes: In fact, I watched a Discovery channel show (don't worry, I don't consider the Disc Channel as gospel truth) that directly correlated the Siberian traps with the Permian die off. There was a History channel episode of How the Earth was Made that highlighted the same theory. --Percy
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Taq Member Posts: 10045 Joined: Member Rating: 5.3 |
There was a History channel episode of How the Earth was Made that highlighted the same theory.
That is the show I was thinking of. Thanks. Discovery, History, NGC, etc. all blend into one in my memory.
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caffeine Member (Idle past 1046 days) Posts: 1800 From: Prague, Czech Republic Joined: |
First of all, no shit. I don't think anyone has really debated that it was a factor, or even a major factor. However, this doesn't end the debate on how much other factors played in to it including supervolcanism in modern day India (the Deccan traps) or the already dwindling numbers of dinosaurs species that led up to the Chicxulub impact. So this leaves the debate right where it was before, with the Chicxulub impact being a player but also other players without a defined role. I don't know if she's a one-off, but wikipedia cites Gerta Keller as arguing that the Chicxulub impact may have had little to do with the extinction event, as it looks like there's another 300,000 years of material deposited in between the iridiuum layer marking the impact and the end-Cretaceous. To quote her from this article on the Geological Society's webpage: Conventional wisdom holds that any such large impact leaving a 175km-diameter crater would cause major mass extinctions. But this hypothesis is based solely upon the assumption that Chicxulub was the K-T killer. None of the other major mass extinctions in Earth history is associated with major impacts. This hypothesis has no empirical support and must be considered false — at least with respect to Chicxulub.
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Taq Member Posts: 10045 Joined: Member Rating: 5.3 |
I don't know if she's a one-off, but wikipedia cites Gerta Keller as arguing that the Chicxulub impact may have had little to do with the extinction event, as it looks like there's another 300,000 years of material deposited in between the iridiuum layer marking the impact and the end-Cretaceous. To quote her from this article on the Geological Society's webpage: Conventional wisdom holds that any such large impact leaving a 175km-diameter crater would cause major mass extinctions. But this hypothesis is based solely upon the assumption that Chicxulub was the K-T killer. None of the other major mass extinctions in Earth history is associated with major impacts. This hypothesis has no empirical support and must be considered false — at least with respect to Chicxulub.
I have a strong suspicion that conclusions such as this is exactly what they were addressing with this "final consensus". From the abstract of the Nature paper: "The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction." The important bit is "the temporal match". Reading between the lines they are arguing that there is not a gap between the impact and the ecological extinction signal. So they don't seem to be arguing against other causes so much as they are against a gap between the impact and the extinction event. Reading between the lines again, the phrasing "lead us to conclude that the Chicxulub impact triggered the mass extinction" in the abstract allows a lot of leg room for other causes (e.g. supervolcanism) being a factor. Edited by Taq, : No reason given.
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Larni Member Posts: 4000 From: Liverpool Joined: |
whilst leaving the crocodilians, birds and mammals. Similarly for the various other groups that survived or went extinct. I've always wondered that. I assumed that crocs could go donkey's years with out eating and live on carrion. Mammals could burrow and eat the eggs of anything that did survive. Birds could....I dunno, I don't suppose they could do much but they must have done something right.
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Percy Member Posts: 22480 From: New Hampshire Joined: Member Rating: 4.8 |
Hi Caffeine,
If above the K-T layer there are really 300,000 years of layers containing dinosaur fossils then she's right and there could be little argument, but are there? She's basing her conclusions on a stratigraphic sequence from a single region around a thousand miles from the Chicxulub Crater. The layers she examined above the K-T layer were marine, and she claims no species went extinct, i.e., that no species below the K-T layers were absent above, but dinosaurs were not marine animals. In other words, you have to read between the lines in that Geological Society article, but she isn't talking about dinosaur fossils, nor even the giant marine reptiles of the period. The dinosaur extinction was not like the Permian where 90% of marine species were wiped out (including the trilobyte). If Keller is claiming that marine species were not much affected for 300,000 years after the impact in the region she studied then I believe her, but it doesn't say much about whether the Chicxulub impact was primarily responsible for the dinosaur's demise. Like any scientific hypothesis the current consensus is tentative, but it can only be overturned in the same way it become accepted. It's the best explanation fitting the available evidence, and it will remain so until some other hypothesis fits the evidence better. --Percy
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kbertsche Member (Idle past 2153 days) Posts: 1427 From: San Jose, CA, USA Joined: |
quote:??? What about the iridium anomaly near the Eocene-Oligocene boundary? Or the one at the Triassic-Jurassic boundary? Or shocked quartz at the same boundary? Or chondritic meteorite fragments and shocked quartz at the Permian-Triassic boundary? All of these are evidence of impacts.
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Matt P Member (Idle past 4797 days) Posts: 106 From: Tampa FL Joined: |
Hi kbertsche,
I've been getting into the impact field a fair bit as of late and of the P-T, T-J, and Eocene-Oligocene extinction events, only the Eocene-Oligocene event seems to have any really good evidence. The Eocene-Oligocene event has a crater (2 actually, Chesapeake Bay, Popigai), with distributed impact glasses (the bediasites). That said, the Eocene-Oligocene extinction was much smaller in scale compared to the K-T/Pg extinction, and may be due to other things instead of an impact. The T-J impact has some evidence of impact, but it's not found throughout the T-J boundary. There are about 3 craters that correspond roughly to the T-J boundary in terms of age, but none of them really match the T-J boundary date. The shocked quartz associated with this layer has also been dismissed as tectonic in origin. The iridium enrichment is about 1/10th-1/20th the enrichment of the K-T/Pg layer, which can be attributed to a slow-down in sedimentation rates. So the good evidence, like that found at the K-T/Pg layer, isn't there for the T-J layer. The P-T impact evidence is really poor. Bedout "crater", which is claimed to be impact-derived, is more likely volcanic in origin. The various impact evidences linked to that crater are also very dubious. While the presence of chondritic impactors in Antarctic rocks associated with the P-T boundary have been claimed (Basu et al., Science 2003), the claim is based on a small amount of metal in these rocks. Metal is rare in geologic samples, but is known to form during lightning strikes (see for instance: Lightning-induced reduction of phosphorus oxidation state | Nature Geoscience ), some of these can have a fair bit of nickel in them.
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