Monday, September 03, 2007

The Late Triassic Mass Extinction


(image credit:Andy Crawford / David Donkin)

I still owe a few posts on the Permian Mass Extinction, I know I especially want to get done the one on the Late Permian Ecology and the other on the WI the Extinction didn't happen, but I thought I'd move on a bit to the Triassic so I could try to get a post up for the second Boneyard paleo blog carnival. I was lucky enough that I had a post selected for the first one, but I didn't submit anything intentionally. This time around, I am going to do just that.

For those of you that have either known me for some time, especially online, know I have a fascination with Deep Time: dinosaurs, synapsids, paleoclimate, the first tetrapods, paleoecology, etc. One topic that keeps looming over anyone that reads about Deep Time is that of the great mass extinctions. You can't escape it: no matter how hard you try. After all where are the dinosaurs? Where are the frighteningly massive dragonflies, spiders, and other invertebrates of the Carboniferous? Where are the terror birds? Where are mammoths?!

Sometimes its a Grindstone. Sometimes its a Piano.

In some cases, animals and plants of the past were merely the victims of normal extinctions: species come and go but remains the same either way. Normally, there's as many or more species evolving into existence each year at the same time: this is the so-called origination rate. It is an interesting fact that not many people realize that species are coming and going without human intervention. It's a great illustration of the passage of "This too shall pass." in the natural world. Extinction rates can go up past the origination rates during times environmental stress. Likewise, during moments of opportunity, such as when a new island emerges from the ocean, origination rates can exceed the extinction rates. These are often when ecologies become simpler or more complex over a prolonged time. However, mass extinctions are a beast apart from these slower, grinding changes to the ecosystems.

People often like to throw around the term "mass extinction" and often use it incorrectly. Let's start with what a mass extinction is not. If all of the members of a single species gets wiped out - like say all the members of the species of the Ivory-billed Woodpecker or Grizzly Bear, then this is not a mass extinction. If all the members of a single genus die out, while no others effected, then this is not a mass extinction. Mass extinctions are for all the critters simultaneously, plants, animals, fungi, algae, etc. For the formal definition to be in play[1], then I'll hold that at least 20% of all known genera, not species, have to get wiped out. There have been something on the order of 20 mass extinctions. However, five of them from the Deep Time have stood out. These are the so-called Big Five[2].

The two most famous of the Big Five Mass Extinctions are the Cretaceous-Tertiary Extinction - the great wipe out of the dinosaurs - and the Permian-Triassic Extinction - the Great Dying. During these events, sixty and 70 to 90 percent of everything died respectively. Everybody's heard and wondered about the dinosaurs dying out. Recently, there has been a great deal of publicity about the Permian Extinction because of its massive trauma that it did to the biosphere. However, there are three more extinctions that compromise the Big Five. However, the other three mass extinctions are the Ordovician, Late Devonian, and, the topic of this post, the Late Triassic (LTE).

The world is changed. I feel it in the water. I feel it in the earth. I smell it in the air.
Much that once was is lost, for none now live who remember it.

The Late Triassic is one of the lesser understood extinction events. There is a fair amount of work that is underway and is just starting to bare fruit. Ten years from now, we'll probably be calling the LTE an written book, not yet closed, but largely understood. Right now, however, there's a nontrivial issue with what exactly happened. Or if it was a major mass extinction at all! Before we get into the extinction itself, we ought to look at the period that it ended.

The apocalyptic vision of the Permian End Times opened the Triassic. The world was a hot, dry, dead place. One that lacked ice caps what-so-ever. Deserts ruled. However, to paint the whole of the world with the same brush would be a terrible mistake as Nick Fraser points out in his work. There might have even been megamonsoons that make anything we've experienced in our day and age rather trivial in comparison. There are also indications that the atmospheric content was rather different than our own: the oxygen level would seem very, very low compared to ours and the world would look rather strange to us if we were so lucky as to glimpse it.

The biotic consequences of the PT Extinction were staggering. The mammal-like reptiles (if we should call them that; truly better called the therapsids) were well on their way to a complex, nigh on modern ecology on land, albeit sans flying critters. That was ended with the great roasting. The biotic crash from the PT Extinction took millions of years to recover from and did not reach the same levels of complexity into well into the Jurassic. The door was opened for the archosaurs to walk through, something that may not have happened otherwise.

The ever noted Dr. Peter Ward has started a project on the Late Triassic Extinction. Others have as well, but Dr Ward is one of the best self-publicists and as a consequence popular science writers out there. He asserted in Out of Thin Air that the Triassic was as important to terrestrial vertebrate life as the Cambrian was to life in general. There was a fair amount of experimentation going on, evolutionary speaking. Mammals arose from the therapsids then. The archosaurs were the true innovators though. True crocodiles may have underwent their interesting and strange metabolic regression. The dinosaurs evolved. When the extinction ended though, so goes the mainstream theories, the dinosaurs stood tall and the mammals were successfully scurrying between the bushes, but the rest were knee-capped at best. In the end, the party, the revolution was over. The dinosaurs ruled the world. The specifics of the massacre are interesting.

What Actually Died


In the seas of 200 million years ago there were a lot of groups getting wiped out. The formerly very helpful conodonts were completely wiped out: for paleontologists these creatures had been very, very useful for biostratigraphy. They had been very common world wide and their turnover was pretty globally consistent. The idea that the ocean might be filled with large actively swimming things other than fish seems a trifle strange to us now, but up until the End Triassic there was definitely the case. Imagine the ocean teeming with 8 inch long eel like critters that were definitely not fish...very different.

The conodonts weren't the only ones that were smacked, but they were one of the more notable extinctions that got all of their species. On the other hand though, the mollusks also were beaten up pretty bad, but did survive as a group. According to Sepkoski, 48% of the marine invertebrates took it in the shorts and it seems that it is plausible that only a single genus of ammonites survived the extinction. The bivalves, clams and their relatives, lose half the genera, but almost all the different families make it through the extinction event with at least one surviving genus. Supposedly, the gastropods actually fair worse here in the Late Triassic than in the Permian Extinction!

Other invertebrates also get their family trees pruned badly. The sponges and scleractinian corals really were abused badly. 78% of the scleractinians were wiped out. The reefs in general were pretty impoverished by the event. Most all of the Paleozoic survivors - those that survived the Permian Extinction - among the brachiopods also die out here. On the whole, though, supposedly the ocean bottom dwellers were hit the hardest much like in the Permian Extinction, but unlike the KT Extinction where the animals and plankton that are dependent on sunlight are.

In the terrestrial environment, things are rather disputed. In North America there seems to have been a very large turnover of fauna and flora. Yet in Europe, there seems to have been less. Elsewhere in the world is not very well understood. Hallam still holds that there might not have been a terrestrial mass extinction the traditional sense. He holds that there was a regional extinction in North America, but that the case for a mass extinction has not been made. To make matters 'worse' Benton has found that the Carnian fauna in Britain continued into the succeeding stages (re Fraser). Another criticism is that the flora, outside of North America, doesn't seem to have changed all that much. NorAm's plants went through some extinctions, but it seems to have been largely regional in scope. However, seed baring flora seems to survive mass extinctions much better than most animals do though.
The conventional view, on the other hand, is that there definitely was a terrestrial aspect to the mass extinction that happened at the end of the Triassic. The proposed victims were the the majority of the Crurotarsi, the majority of the surviving advanced therapsids, a large number of the other archosaurs such as the phytosaurs, Temnospondyl amphibians and, it seems based on recent evidence out of New Mexico, the dinosauromorphs.

The question remains though: what caused that to happen?

What's the Mechanism, Kenneth?!

Right now there seems to be three different leading theories. They are divided into the usual suspects, truth be told, such that it seems that sometimes paleontologists are a little unoriginal. However, in reality it is simply that the world has a limited set of possibilities for these things to have been played out. The first one is the theory that gets a lot of press. It's dramatic with a blink tag. It is, of course, the bollide impact theory. The second one is that volcanoes did the deed like they did in the Permian. The last is that there was a near extended ecosystems and environmental failure that I had best explain in detail.



The first theory is one that has been presented as one of the contenders for The Grand Unified Mass Extinction Theory. This is a tendency that the two big groups with their two pet theories to try to shoe horn all the extinctions into their theory. Personally, I find this annoying. However, here I think they have a better case for universality than most. The theory itself is that the inner workings of the Earth are more dangerous to life's great web than anything else. When those inner workings relieve themselves strongly upon the surface in a big way, things die. A lot of things die. The idea is that major volcanoes are the main reason for the mass extinctions. There is a nontrivial amount of evidence in favour for this theory. Vincent Courtillot has compiled a large database to compare when there is a major extinction and when there was major volcanic activity. He was even able to get Walter Alvarez to note that there was an interesting correlation.

However, there are a few constraints on the theory. The first is that not just any sort of volcanic activity can cause a mass extinction. It seems that only those that are sustained flood basalts will cause mass extinctions. The ones like what Toba, Pinatoba, and others likewise are not seemingly the causes of mass extinctions, but definitely are for local extinction. The suspicion is that the Toba et al extinctions have too short of a period of influence to affect a mass extinction. The Year Without a Summer didn't wipe out most animals at all. On the other hand, it’s been pretty definitely tied that the Siberian Traps were the cause the Permian Extinction. This was a massive flood basalt eruption that lasted for millions of years. Sustainable negative impact on the ecosystem seems to be key wrt volcanoes and their possibility for causing a mass extinction. There's a big hit here against it: the Deccan Traps did not cause the KT Boundary Extinctions. In fact, keeping in mind that India was an island during the end of the Cretaceous, Indian paleontologists have found dinosaur fossils between the layers of basalt flows meaning that the Deccan Traps didn't even wipe out the dinosaurs on the Indian Subcontinent. Please keep in mind that it was an island continent at that point and pretty remote from the other continents as far as megafauna would be concerned. hrm.

Even so, traditionally, the Late Triassic Extinction has been attributed to the vulcanism that helped split the great supercontinent of Pangaea. The strongest evidence of this is the massive flood basalts that make up the CAMP (Central Atlantic Magnetic Province) and helped produce the Newark supergroup. The duration was long enough. The vulcanism was pretty energetic. It wasn't as strong as the Siberian Traps. The extinction was actually lesser so than the Permian too, so the model seems to be consistent. On the other hand there might be a problem with the geochronology of the eruptions that might make the eruptions and the extinction to be chronologically disjointed: they might not have happened at the same time. There is evidence from the Milankovitch cycles that they happened 60,000 years too late!




The second theory that keeps competing for the Grand Unified Mass Extinction Theory is one that has already, like the vulcanist theory, successfully claimed a mass extinction as having been caused by this theories Murder Weapon: bollide impact, off the Yucatan, at the KT boundary, killing dinos+. There has been a lot of push since the Alvarez team published their paper in 1980 about the bollide explanation to find if all the mass extinctions are caused by the impact of asteroids, comets, whatever kind of bollide crashed into the Earth and killed off the huge percentages of life. As noted, the KT Extinction seems to have been an asteroid impact. There is strong evidence for the FF Boundary Extinction during the Late Devonian as being impact caused. There is also some evidence for an impact cause for the Late Triassic Extinction as well. Like the vulcanism theory though, it does have some nontrivial issues, namely with geochronology.

There is a very promising crater in Quebec, Canada - the Manicouagan Crater - that is almost the right age. The problem is that it precedes the extinction by around twelve million years. This causes issues with causation: the one confirmed impact caused mass extinction (the KT) was a swift killer, not delayed by millions of years. Now there might be a case where things need to be given a better bit of study, but it seems that this isn't the case for now. Something that hurts the claims for the impact causing the Late Triassic Extinction is that the there are few indications that n impact took place on the order - or even remotely close to - the Chixculub Crater. There is no evidence of the shocked quartz on the scale of the KT Boundary. There are no isotope anomalies such as the KT Boundary's global iridium layer. There are no tektites or spherules. This is rather unsupportive of this theory, but this might be just that there isn't as much work being done on the LTE as the other extinction events.

The last theory is rather complicated. The idea is that the ecology simply broke down and the LTE was a cascade of those breakdowns or a general systems failure on the part of the ecology. Let me see if I can articulate this theory, but I am unsure if I will do so very well. The idea is that mass extinctions are a product of a failing ecology. The idea is that ecologies are intimately linked entities. When the certainly parts of it die it, other parts begin to die out. This makes sense in a way: if you kill off the plants that certain animals need, you are going to kill off those animals and that in turn will kill off the predators for those herbivores. etc. If this chain reaction happens to reach above a certain level, the links between ecologies cause localized, single biome centric extinction to progress to larger mass extinction. This theory has been applied to the Late Triassic Extinction.

The idea is that the combination of two facts made this possible. The ecologies of the Triassic were simplified by the harrowings of the Permian and the Carnian. Furthermore, during the Triassic, the continents were all congealed into one great supercontinent. This would, in theory, make the number of species less as there were actually less isolated environments. Less diversity means that there are less critters that are variant enough to try to survive a mass extinction. So, the idea is that when the big reduction in force took place at the Late Triassic, the interconnectedness of the largely similar ecologies broke down and did so such that more dependent species died out and would continue to die out for an extended period. It was a systems failure of a supercontinent wide magnitude.

The biggest problem is with this last theory is that the last stage of the Triassic, the Rhaetian, is only four million years old. This would argue the mass extinction was a very short term affair. This would favor something that was relatively quick and the ecological unhinging theory is anything but a quick kill mechanism. Secondly, the nasty aspect is that humanity in what has been termed the 6th mass extinction, has already done this whole sale all over the earth, yet we don’t see things dying off left and right from the mere absence of numerous links in the chain. This brings about some questions as to whether or not this theory is even applicable at all to any mass extinction, nevermind the LTE. Considering then, the bollide impact and vulcanism are the leading candidates.

If we were to leave it there, without the newest research of the past two years, we would find that the Late Triassic Mass Extinction was an enigma still. Until 2005, no one had found any conclusive evidence of anything strange in the isotope ratios. The carbon ratios seemed to stay consistent across the boundary. The oxygen ratios were nothing to talk about either. There was no iridium signature like the KT Boundary either. No one – even now – seems to have discovered much about the strontium or other worthwhile trackable isotopes as far as having any noticeable changes. It’s rather perplexing. It could be that there are simply not enough sites with sediment left over from the Rhaetian to draw conclusions. In fact, this is an argument that some use to state that there was not a mass extinction at the Late Triassic at all!


The total number of sites with Rhaetian sediments is exceedingly few. There are so few of them that, if my sources are not too badly out of date, you can count the number of known, good locales on one hand. That's not too promising to do a statistical analysis with. Additionally, some authors have attacked the locales as examples of interrupted sedimentation which would call into question their usefulness because they would give false impressions about how fast the faunal turnover was between the Late Triassic and Early Jurassic. This is a true danger of having too few samples: it can give very bad and wrong impressions as to what happened. With recent discoveries of the dinosauromorphs surviving through the Carnian, it seems very, very possible that the Late Triassic Mass Extinction may be the extinction that never was.

However, because the folks that chase fame if not fortune through exploring and explaining the mass extinctions of the past. Walter Alvarez, Jaan Smit, and others have had their names carved into the annals of science by explaining the KT Extinction. Erwin, Wignall, and others have carved their name into the same annals for the PT Extinction. There are wars still over the Late Devonian Extinctions and all sides have staked a claim on it (vulcanist, impactists, and the rest). This argument is ongoing. The Ordivician, at least for now, seems to have a low key argument going on (most seem to think it was glacially induced, but others are disagreeing). The Late Triassic was the least studied until recently. Those that participated in the other two ‘settled’ extinctions would like to get their name in the lead for their ‘own’ extinction.

Ward et al and others have started research into the LTE. In 2005, they published a discovery from their expeditions to the Triassic/Jurassic Boundary in the Queen Charlotte Islands. They discovered there was a carbon signature there that matched a previous ‘settled’ extinction: the Permian. There was also work done with the oxygen isotope ratios that indicate a similar behavior as well. Ward then argued in his work, Under A Green Sky, that if you follow the fossils as well – some of the bivalves and ammonites that are collected throughout the Rhaetian are adapted to warm, very low oxygen waters – you get a picture that the murderer was volcanism, in the brand spanking new North Atlantic with an overload of carbon dioxide, anoxia, hydrogen sulfide producing bacteria coming to near the surface of the seas, a UV blast, a methane hydride release, and global warming on a massive scale, albeit a lesser one than the Permian End Times.

In wrap up, it seems that there is a very strong contender for the LTE kill mechanism: volcanism stimulated methane-carbon dioxide cooking with a seasoning of hydrogen sulfide poisoning and ultraviolet burning. However, this ought not be thought as a closed book or even a written book. We have an issue here that this science – in terms of refutation or confirmation – is very, very new. It will take time to confirm – or deny! – this work conclusively. More research is underway. Scientists are still digesting the work and considering the implications. It is worrisome that the carbon and oxygen signatures have taken so long to uncover and from so few sites. There ought to be more just to verify that these are not merely local phenomena that are being sampled. Until this is done, the LTE should be considered an extinction that is as yet unsolved. Ward himself makes the case for caution with his own discussion of the Permian bollide impact that Becker et al reported for the Permian Extinction…and its rather poor science that seems to have been documented therein: wait for the work to be done before deciding that the mystery has been solved. Even so, to be fair, Ward does seem to have a very strong case for this paleontological version of Clue.

1. Hallam, in Catastrophes and Calamities, holds that a mass extinction merely needs to be broadly based enough that it is statistically significant and catastrophic. Others have it be more specific and I'm hold up something of a synthesis of those to make it more specific.

2. Wonderful Life expounded on the history and oddities found exquisitely preserved in the Burgess Shale. Some of the hypotheses that were put forth accreted around the idea that there was a massive experiment by life to see what worked during the Cambrian Explosion and then there was a mass extinction that wiped out a number of the forms that were rather different than today. However, at least a few and possibly many of the critters that were actually tied surviving lineages. The iconic example is Hallucigenia being a Velvet Worm. There have been several paleontologists, including Tony Hallam, that have questioned whether or not there was a mass extinction at the time of transition from the Cambrian to the Ordovician. The general consensus is that there was, but that it seems to have been less impressive than first thought[3].

3. That would mean the much trumpeted 6th Mass Extinction that is being aided and abated by humanity is really the 7th.


Bibliography:

Catastrophes and Lesser Calamities, Hallam, 2004, ISBN: 0-19-280668-8

Dawn of the Dinosaurs, Fraser, 2006, ISBN: 0-253-34652-5

The End of the Dinosaurs, Frankel, 1999, ISBN: 0-521-47447-7

Evolutionary Catastrophes, Courtillot, 1999, ISBN: 0-521-58392-6

Extinction, Raup, 1991, ISBN: 0-393-30927-4

Mass Extinctions and Their Aftermath, Hallam & Wignall, 1997, ISBN: 0-19-854916-4

Modeling Extinction, Newman & Palmer, 2003, ISBN: 0-19-515946-2

Out of Thin Air, Ward, 2006, ISBN: 0-309-10061-5

Terrestrial Ecosystems Through Time, Behrensmeyer et al, 1992, ISBN:0-226-04155-7

Under a Green Sky, Ward, 2007, ISBN: 978-0-06113791-4

Wonderful Life, Gould, 1990, ISBN: 978-0393307009

13 comments:

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Will Baird said...

Sorry, Roger, I am not taht interested in the pet stories. Better luck elsewhere.

Zach Miller said...

No pet stories?! Aw, Will, you're such a killjoy. :-)

Wonderful post, Will. I have a question, though. Now supposedly the higher therapsids (at least) were fully endothermic. Hair, scent glands, parasaggital posture, the whole nine yards. And they basically ruled the northern hemisphere.

Now those dinosaurmorph upstarts were tiny, lived in a fairly isolated area (South America), may NOT have been endothermic, probably didn't have much more than scales, and...well, a parasaggital posture. But why would the therapsids, which were clearly amazingly successful and physiologically advanced, be so...oh, what's the word...RIPE for extinction in the face of the ornithodirs?

I've often heard the excuse that because the world got so much HOTTER, then ectotherms would have won out over endotherms, because if it's too hot out, endotherms overheat and DIE, whereas ectotherms do not. Well, that's not entirely true. In the heat of the day, most ectotherms run for the shade, too. They sun themselves when the day is beginning, but are hiding by the time the sun is directly overhead.

So that theory doesn't fly with me. Do you have some idea why dinosaurmorphs "won out" over therapsids?

Will Baird said...

Oy. You're not going to like the answer I am going to give you on this, Zach. Not at all.

First off, of those that are researching the mass extinctions there seems to be a consensus of some things that I find amazing, if not a little weird. The main one, which I think is going to piss ya off, Zach, my friend, is that the basal archosaurs were endothermic! The evidence that is often being cited now is the crocodilian (or is it crocodylian?) nearly four chambered heart. Something that those that are 'in the know' say is an artifact of having been endothermic and then lost (!!) the trait. I find it mind boggling, but I'm not a true paleo type. There's a reason I put a link into the 'strange metabolic transformation of the crocodiles' there. The endothermy was supposed to arise during the PT Event as a way to cope for both the therapsids and the archosaurs.

Secondly, I believe that Ward's camp states that the at least the saurischians and possibly more had far better respiratory systems than the therapsids at this time. The O2 content of the earth's atmosphere was suppsoed to bottom out during the Jurassic, but had already hit very low during the end Triassic. Again, I'm skeptical, but it's always possible.

Better lungs for low O2 + endothermy beats just endothermy so goes the theory.

Zach Miller said...

Wait a second. Basal archosaurs were endothermic, and then somehow went back to being ectothermic? I prefer to think of the modern crocodilian heart as convergence due to some pressure that I can't figure out. I don't think it's physiologically POSSIBLE to go from endothermic to ectothermic, but I'm no biologist.

I am perfectly willing to accept the repiration hypothesis, though. Because the synapsid line, which includes us, seems to have a piston lung system (whereupon the diaphram does most of the work in "suctioning" air into the lungs), ornithodirs developed a more avian-type repiration system fairly early on. It's more efficient and sucks more O2 out of the air.

Good answer, though. I don't think the fact that MODERN crocodilians have a pseudo-four-chambered heart is evidence that endothermy is primitive for the Archosauria. That's my only beef, as you correctly predicted. :-)

Will Baird said...

Wait a second. Basal archosaurs were endothermic, and then somehow went back to being ectothermic? I prefer to think of the modern crocodilian heart as convergence due to some pressure that I can't figure out. I don't think it's physiologically POSSIBLE to go from endothermic to ectothermic, but I'm no biologist.

Heh. Both Ward and Fraser buy it. They from different camps as far as I can tell.

I am trying to grasp how endothermy would have been any sort of asset during the PT Event though. I cannot for the life of me figure out why. I mean, you have resources getting wiped out left and right so what do you do? You evolve a need for MORE of them.

Anyways, apparently it's plausible and possible, just very unlikely to go ectothermic->endothermic->exothermic. The idea is that the crocodilian niche - lying in wait for critters at the water's edge - would have selected for slower and slower metabolisms until finally you cross that point where you are no longer endothermic. I wonder if there's 'genetic scars' so to speak if you were to flip that bit (bad analogy, but...) in the DNA you'd get a screwed up endothermy in a croc. That's actually a rather frightening thought, really. Endothermic crocs. *shudders*

Think of it this way, Zach, you have two groups of surviving archosaurs: birds and crocs. The birds are endothermic. The crocs have traits that are consistent with having had an ancestor that was endothermic. What's the simpler explanation? It evolved twice in the same lineage? Or...

Yeah. It's a bit...odd. Interesting. The only way I could buy it after all this time is that truth is stranger than fiction.

Will Baird said...

A note on the respiration hypothesis: I am not sure that I buy it. Not because it wouldn't explain the situation, but rather what's driving it: dropping O2 levels to very low levels < 15% through whatever reasons. (possibly aridification and lower productivity in plant forms, but that's a guess). The problem with the O2<15% theory is that there are some side predictions that ought to be happening. There's one that comes to mind that bugs me. Nick Fraser rants a bit about it in his work: insects ought to be nontrivially smaller than now. They really ought to be itsy bitsy.

They're not. There's at least one that was over 8 inches, iirc, that Fraser brought to the reader's attention. That's odd. That's bigger than NOW and we have some 20% percent O2. hrm.

I might be wrong. That doesn't sound right to me. Anyone have any sort of data about insect size and altitude?

Zach Miller said...

Well, okay. If you consider modern crocodilians to be a slowed-down version of endothermy, perhaps "lost" from an ancestor...I guess I can buy that. You're right--a resting metabolism that slows down in times where energy is not being expended...that could work.

Still wierd, though. As for respiration, if O2 levels were dropping, then an avian-type lung system would really be an advantage for the ornithodirs. However, low O2 levels would not be great for endotherms...hmmm...

Will Baird said...

Well, okay. If you consider modern crocodilians to be a slowed-down version of endothermy, perhaps "lost" from an ancestor...I guess I can buy that. You're right--a resting metabolism that slows down in times where energy is not being expended...that could work.

Yeah. There's a strong possibility that there is a bit of genetic code to confirm this.,..or not. There's a fish in the Antarctic waters that has no red blood cells (!!) that apparently evolved this way back 55 million years ago (huh?! Why? It was frackin warm then! See Eocene Ocean post) Also the croc's "high walk" is supposed to be the basal archosaur's normal walking position. IE modern crocs are highly derived despite earlier assumptions.

Still wierd, though.

Amen! Hallujah! Pass the ammo, please!


As for respiration, if O2 levels were dropping, then an avian-type lung system would really be an advantage for the ornithodirs. However, low O2 levels would not be great for endotherms...hmmm...

AND the calorie requirements go way up too. Just when calories are undergoing a scarcity crunch too.

uh. yeah. Something doesn't sound right. to me.

Zach Miller said...

I just keep posting comments!

It's been long known that a modern croc's unique posture is derived from a more parasagittal one. During the move to the aquatic zone (dee dee dee dee), the shoulder and femur articulations moved laterally to aid in...swimming, I guess.

Will Baird said...

It's been long known that a modern croc's unique posture is derived from a more parasagittal one. During the move to the aquatic zone (dee dee dee dee), the shoulder and femur articulations moved laterally to aid in...swimming, I guess.

And to save energy, iirc, on land. It requires less, as I think I read, to use the croc belly crawl than the parasagital one.

Begs the question though why no other diapsids have gone with the moreupright stance since the dinos bit off some flaming asteroid though...

GrAveTzT said...

That is a very interesting story.

It is impossible to determine why almost everything became extinct, though. There is a handful of many "guess" that scientists have made.

Will Baird said...

I would not call it a guess. It's a theory based on evidence. There's a lot more weight and work to mass extinction conclusions than merely 'guesses.'

Welcome to the blog. Check out the other mass extinction posts as well.