Taking in the Carboniferous Atmosphere

3 08 2007

In many a time-travel story, little attention is paid to the local ecology of the destination of the intrepid travelers. If you’re headed back to visit Leonardo da Vinci, this probably isn’t going to be a problem, but if you’re headed back much further, like the Carboniferous or Permian, you might run into some trouble. Indeed, nothing could kill such a “time safari” quicker (like the one in L. Sprauge de Camp’s “Crocamander Quest,” collected in The Ultimate Dinosaur) than stepping out of the time portal/capsule and not being able to breathe. Fanciful notions of time travel aside, we can tell something about the ancient atmosphere without having to actually step foot there, and it has some very interesting implications for evolution and ecology.

One of the defining features of the Carboniferous is that things got big. Even though they may have seemed small compared to titanosaurs or mammoths, a 3-foot-long millipede and giant dragonflies are nothing to belittle. Indeed, terrestrial arthropods in all sorts of groups seemed to achieve sizes unheard of today. Why should the Carboniferous be dominated by giant arthropods?

Well, let’s think about the name of the time period we’re talking about: Carboniferous. Carboniferous deposits, sandwiched between the Devonian and Permian, are well-known for containing coal. The coal was originally laid down by the thick plant growth that marked changes in terrestrial ecology, and although the first plants (likely resembling liverworts) colonized the land as early as the Ordovician, the Carboniferous marked the development of lignified bark (“brown coal” is called lignite), which may have led to the huge amounts of coal deposited as bacteria needed some time to evolve a way to break down this new material. Due to Devonian drops in sea level, plants were also had more space to grow, so there was a greater number of swamps and forests produced, filling up with plant and insect life. The changes in global fauna had a major impact on the atmosphere, not only producing more oxygen due to photosynthesis, but the burial of so much biologically produced carbon also increased the amount of oxygen in the atmosphere, perhaps being as high as 35%! (It’s at about 21% today)

Another important piece of the puzzle is that nitrogren levels did not drop, and so not only was there hyperoxic conditions due to plant evolution, but the air pressure was increased as well, and these two factors probably had a lot to do with the evolution of insects flight and insect gigantism. When there is more oxygen in the air, organisms that diffuse oxygen over their trachea are able to obtain more oxygen with less effort, and it doesn’t take as much energy to become large being oxygen is so “cheap.” Likewise, increased air pressure makes flying less costly in terms of energy, as wing size has to go up with decreasing air pressure in order to keep flying animals aloft (as is that case with hummingbirds that live at high altitudes). Not only was it easier to grow large, but it didn’t take as much energy to fly, resulting in the giant dragonflies like Meganeura. In a 1998 paper, Dudley hypothesized that early tetrapods would have taken advantage of the extra oxygen as well, being that they would have “breathed” through their skin. This hypothesis is largely refuted, however, (see the comment of johannes below) and size of early tetrapods does not seem as dependent on the atmospheric oxygen make up, although it likely could have made oxygen intake for amphibious tetrapods more efficient overall.

There are costs to be adapted to an atmosphere with high oxygen content and high atmospheric pressure, however. Part of the cost is the buildup of metabolic “wastes” as a result of taking in so much oxygen, and unless there’s also an increase in enzymes to take care of these wastes, organisms may have led shorter lives. Experiments in the lab with fruit flies, however, have shown that an increase in enzyme production to combat the harmful metabolic products can be selected, and it’s reasonable to think that Carboniferous organisms underwent such selection naturally. Still, atmospheric oxygen levels began to sharply drop at the beginning of the Permian, and by the beginning of the Triassic atmospheric oxygen levels were as low as 15%. Creatures adapted to higher oxygen content and pressure would need to be adapted quickly again if they were to survive, and size/ability to deal with lower oxygen levels probably made a big difference in the great Permian extinction.

Oxygen levels were not always to remain low, however, and it has been hypothesized that hyperoxic conditions (which means more oxygen in the air plus heightened pressure) might be correlated with the evolution of flight, or at least evolution of larger flying organisms. When it’s less expensive to obtain oxygen and less expensive to fly, it would be much easier to develop flight, and there may be correlations with such atmospheric conditions and the evolution of bats, birds, and pterosaurs. Testing still needs to be done (and the more fossils we can get, the better), but if nothing else, the fluctuations in atmospheric oxygen content show us the importance of considering ecology in terms of evolution and extinction.


AERIAL LOCOMOTOR PERFORMANCE.” The Journal of Experimental Biology 201, 1043–1050 (1998)



14 responses

3 08 2007

You are probably right about the connection between high oxygen levels and the rise of giant arthropods.
As far as the tetrapods are concerned, it seems to be another story, however.
Stem tetrapods were scaly or armoured, unlike modern lissamphibians. I think that rules out skin-breathing for them. Being scaly, like their sarcopterygian ancestors, is probably the primitive condition for tetrapods.
If skin breathing was important for stem tetrapods, one would expect them to increase the surface area of their bodies. In fact, the opposite is the case: most terrestrial stem tetrapods, like Pantylus or Cacops, have a shorter tail and a more compact build than contemporary amniotes. They actually tend to decrease body surface area – perhaps because the contemporary climate was rather cold? What strikes most observers when they first see a picture of Cacops is how compact this animal is: It’s a sort of temnospondyl working terrier, compressing the highest amount of firepower into the smallest possible frame.
I can’t see a decrease in the size of stem tetrapods in post-carboniferous times either; the last temnospondyls from the Cretaceous of Australia are among the largest of their kind. What drove the giant temnospondyls into extinction was probably not falling oxygen levels, but competition by crocs; that’s why they survived longest in the cool, subantarctic and croc-free forests of cretaceous Australia – crocs like it warm.
BTW, the easiest way for a temnospondyl to cope with falling atmospheric oxygene levels would have been to become neotenic and keep its gills.

3 08 2007

Thanks johannes; I was actually hoping for this kind of commentary since my knowledge of the subject isn’t quite as broad. The early tetrapods seem to be mentioned because there seemed to be a correlation, but the causation (as you noted), seems dubious. I’m going to amend the original post to focus on the arthopods, and although I’m sure the oxygen levels did have some effect on early tetrapods, it probably wasn’t as big a factor as Dudley hypothesizes. Thanks!

3 08 2007
Will Baird

Fascinating stuff. Not that different than I knew already, but well written.

What I find interesting about the O2 level studies from the past is that sometimes you have to go ‘hmmm’ about them. It’s not to say they’re wrong, but sometimes I have wonder about them. If O2 levels really got that low in the Triassic and Jurassic, how do you explain the largish insect fossil that Fraser notes in his book or the sauropod size? Yes, the saurischians were supposed to have the bird-like respiratory system, but…what of the metabolic requirements of the mongo-sized sauropods? That’s one helluvah wind pipe to pump such low amounts of oxygen through for such a huuuuge body. Has anyone studied that and published? If O2 levels are so important for flight, then what about the Triassic evolution of the pterosaurs? They took to the skies under low and dropping O2 levels.

Query: mudskippers have developed the skin-breathing technique independently of the lissamphibians. How scaly are they? Sme quick glances at photos online says not, but…

3 08 2007
Will Baird

PS: Giant Bugs a Thing of the Past, Study Suggests:


3 08 2007

Thanks Will; I know it’s probably old-hat for most of the people who come to red, but I didn’t know too much about it (and I didn’t know about the air density issue or the metabolic waste products of hyperoxic environments). I have to go “hmmm” about O2 levels as well, and I’m sure more work was been done since 1998, but the Dudley paper suggests other periods of hyperoxic conditions that may have existed at certain points, although not as high as in the Carboniferous. He even suggests that there might have been an air pressure/oxygen increase during the development of pterosaur flight, although this is just hypothesis and nothing is given to back it up (like you noted, studies so far show that they didn’t have the same advantage as the insects).

Still, as you and johannes have pointed out, we’re comparing creatures across higher levels of taxonomy, and while there might be correlations we can’t generalize what is advantageous or disadvantageous too strictly because there are so many other factors in play. The field is still wide open (and I like the query about mudskippers), so I definitely hope more work is done on how ancient atmosphere changed evolution & ecology.

3 08 2007

Thanks for the extra link, Will! I’ll have to check that one out.

3 08 2007
Zach Miller

Love those huge bugs. What always makes me wonder, when considering gigantism in insects, are the structural supports necessary for an Eagle-sized dragonfly. One wonders how thick and long the wings on a thing like that would have to be. Would the muscles anchoring the wings have needed to be significantly larger? Since insects don’t have internal skeletons, I wonder how thick a gigantic milipede’s carpace would have to be? At a certain point, doesn’t an enormous external skeleton hinder movement? Wouldn’t a giant dragonfly become so heavy that it couldn’t take off anymore?

Despite atmospheric conditions during the Carboniforous, I’m going to assume that gravity remained a constant. And I’d think that gravity “weighed” heavily (get off the stage!) on ancient insects. I mean, it’s clear that prehistoric arthropods overcame those challenges, but I’d just like to know HOW. Great post, though. Interestingly, Meganeura inspired “Godzilla vs. Megaguiras,” in which the villain was an enormous dragonfly, mutated by radition from a rift in space time (or something) as well as Yanmega, a new evolution of Yanma (Pokemon games here) who looks positively prehistoric.

4 08 2007

This is not my field but quite interesting. You describe how the atmospheric oxygen could have risen to high levels with the proliferation and evolution of plants from the Devonian to the Carboniferous. However what change occurred that caused the O2 levels to drop so low by the Triassic?

4 08 2007

There was a movie that came out a few years ago called Mimic that featured giant, people-eating insects, and the explanation given for why they had become so big is that the bugs had evolved lungs and therefore could process oxygen more efficiently (if memory serves me correctly). But I believe the bugs in the Carboniferous were about as big as they can get, since their exoskeletons couldn’t support body structures as large as people — although there was the giant centipede/millipede creature, Arthropleura.

I know Peter Ward has written a book titled Out of Thin Air in which he argues that lower-than-modern-day oxygen levels were a major force of evolution and were the reason why the dinosaurs dominated for so long. I haven’t read it yet, and I’m not sure what type of reception it has received among other paleontologists.

5 08 2007

Thanks for all the comments everyone. I know I didn’t address why the oxygen levels dropped so low by the Triassic, mostly because I have to admit that I don’t know (it’ll make for another post). I’m aware of Ward’s book on the subject (it’s on my amazon.com wish list), although I haven’t had time to read it yet. I’m probably going to read the book Oxygen on just this topic in the near future, however, so expect more on this subject.

As for gravity, I think it would be a limiting factor as far as size/construction goes for flying arthropods, but being that it is constant and the air pressure was fluctuating, it seems from the data mentioned above that wing size/overall obtainable size for flying animals is correlated with higher air pressures, so less energy is needed to keep them aloft. Obviously you couldn’t get too big without just being too heavy to fly, but heightened air pressure certainly seemed to allow for larger animals to get into the air. I do wonder about the thickness of the large arthropod shells, however; even though they could get bigger, shells would have to change to accommodate the larger size as well as more materials used in shell-building would have to be obtained from food. Indeed, it seems I’ve got more questions out of this post than answers (but that’s a good thing 🙂 )

I remember Mimic too (“Mr. Funny Shoes”), although I never did brave the cheese-fest that is sure to be Mimic 2.

6 08 2007
Zach Miller

Mimic 2 was so bad that I turned it off ten minutes in and immediately burned (read: sold) the DVD. And Brian brings up a good point–since arthropods build their exoskeletons out of (mainly) chiton, one wonders where they were getting all the minerals necessary to build such structures.

The Carboniforous was a very different place back then. Let’s just leave it at that! 🙂

16 11 2011

I must share this site with my friends on Facebook… Fab stuff!

24 12 2012
Hank Roberts

‘vouchers’ was a spambot.

Air pressure — any info on how much it changed? I see the climate-denial folks still citing the large insects as proof that air pressure must have been vastly greater, and then saying that proves CO2 was much higher, e.g. this one: http://pubs.acs.org/subscribe/archive/ci/30/i12/html/12learn.html

But they ignore the oxygen numbers, when they assert that stuff.

Another question — isn’t that high level of oxygen enough to support combustion in many materials that wouldn’t burn in current conditions, so forest fires would have been far more aggressive at that time?

7 03 2017

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