Ancient Atmosphere Giant Insects

Interesting point about the problem regarding insect spiracle respiration. They don't work very well on a large insect because the surface area to volume ratio shrinks very rapidly as size increases without proportional changes.

If the atmosphere was indeed far more dense than it is presently, and/or had far more oxygen, this would have rather far-reaching effects. This does not mean that it is not true, but it does mean that other fossils might show differences associated with atmospheric change.

In particular, the atmospheric changes would effect the gas uptake of other organims, including plants.

The stomata of plants are sometimes preserved in good compression fossils. Since gas uptake (of carbon dioxide for photosynthesis, and oxygen uptake for respiration) would occur much more rapidly in the atmospheric conditions you described, one would also expect plants to need fewer stomata for gas exchange. Plants lose water through their stomata (which is why they are located primarily on the bottom of the leaves and closed when gas exchange is not needed) so it is reasonable to assume that selection would favor minimizing the stomata present on the leaves.

Therefore, if the atmosphere of the time had the properties described, then the stomata seen in compression fossils of plants from that time should be fewer in number (on average) than the stomata on modern

Aren't you basing this on the assumption that modern dragonflies are 'maxed out' in their ability to have increased size in the present atmosphere and oxygen percentage? Are you sure that is a safe assumption? Isn't it also possible that there is no problem in the flight dynamics of larger size, but the modern dragonflies remain smaller because their prey is small and adequete as it is so there is no incentive for modification?

The dynamics of flapping flight like insects do is not at all easy to analyse.

I believe you are wrong about this. It's not the ratio of oxygen to nitrogen that matters, but the absolute amount of oxygen available. Nitrogen is inert, so it cannot dampen fires. In deep sea diving, when people must breathe pressurized air, the composition of the air is carefully adjusted so that the percentage oxygen is very low, but the partial pressure of oxygen is the same as at sea level. If you breathed pressurized air at normal ratios there would be way too much oxygen in each lungfull, and you would get oxygen poisoning. You would also catch on fire rather easily!

However your hypothesis doesn't require a constant oxygen/nitrogen ratio. If you increased the amount of nitrogen in the air without increasing the amount of oxygen we would be able to breath normally, fires would be no more of a problem than they are now, but large insects would have an easier time flying in the denser air.

But because nitrogen is inert I don't thing there is any way to move it in and out of the atmosphere fast enough to make this hypothesis plausible. Since oxygen is more reactive it is easier to imagine that some process might have removed large amounts since the Paleozoic.

That's not much of a problem, actually. I don't have a source for you, but anecdotally, I've heard reports of extensive charcoal deposits (or their petrified equivalents) found from about the right time period. That would, in fact, be consistent with heightened oxygen levels.

You have to remember what's going on at the time (Silurian? Someone correct me if I'm wrong) that these giant insects are found in. Land plants have become established. While there's insects and other arthropods on land, there's not a lot of other things that respire to convert oxygen to carbon dioxide. On top of that, there don't seem to be large oceanic anoxic events or major erosional patterns that would act to sink carbon dioxide. It's very possible that there was a hyperoxygenation of the atmosphere at this time.

***** Daniel Snyder, Graduate Student Department of Geosciencies, University of Florida

WILL HUNT DEAD STUFF WITH BIG TEETH FOR FOOD *****

Not necessarily. 'Charcoal' is manufactured by burning wood chips under almost anaerobic conditions. Only enough oxygen is allowed to just barely keep the wood pile burning. This is why we are left with carbon, instead of only ashes.

Natural charcoal could easily be formed from any biomass which became largely isolated from the atmosphere - like if a flood tore out a lot of vegetation, piled it up and then covered it up with silt. Spontaneous combustion of the compost is a real possibility and if only a little oxygen diffuses into the pile - we get charcoal.

So - the charcoal would suggest far LESS oxygen rather than more ... but since there are several ways to produce this condition without having to change the composition of the whole atmosphere, I think we can safely pick from that list.

-j

: >>the same nitrogen/oxygen ratio, the fire-damping : >>effects of the nitrogen increase also. Perhaps not in : >>perfect proportion to the fire-enhancing effects of : >>pressurized oxygen, but close. This means that you : >>can double the amount of oxygen per cubic centimeter : >>of atmosphere without greatly enhancing the fire : >>danger.

: >I believe you are wrong about this. It's not the ratio of oxygen to : >nitrogen that matters, but the absolute amount of oxygen available. : >Nitrogen is inert, so it cannot dampen fires. In deep sea diving, when : >people must breathe pressurized air, the composition of the air is : >carefully adjusted so that the percentage oxygen is very low, but the : >partial pressure of oxygen is the same as at sea level.

: The absolute ratio IS relevant - to burning anyway. : As a flammable substance burns, either an oxygen or : nitrogen molecule will randomly strike the combustable : element (carbon, hydrogen, maybe sulfur). If it's an

I think John Brock has it right. When you're in ideal gas territory, the partial pressure of O2 should determine the reaction rate (by the law of mass action). Granted, the higher density of the medium will affect the heat transfer rate too, but I doubt it would really offset the 2X change in the reaction rate to the point of 'damping' fires. Higher pressure would certainly increase liquid flash points, but I think ignition temperature is a more relevant measure of the likelihood of natural fires breaking out, and I don't think that will change significantly with 2X pressure in most cases.

: >But because nitrogen is inert I don't thing there is any way to move it : >in and out of the atmosphere fast enough to make this hypothesis : >plausible. Since oxygen is more reactive it is easier to imagine that : >some process might have removed large amounts since the Paleozoic.

: But was nitrogen the only 'inert' gas in the ancient : atmosphere ? The higher level of volcanic and tectonic : activity way back then suggests that levels of CO2 : and SO2 were likely higher - and possibly methane as : well from both geochemical and biological sources. : All of these are reactive too - and heavy. I would : have to do some calculations ... say a mix of CO2 : and methane with a little SO2 ... how great a : percentage increases atmospheric weight by how : much ?

CO2 is probably a better thing to look at. If you've got enough SO2 to double your pressure, you've got a really strange, toxic atmosphere (and one which would probably be geologically obvious). Really, any of those three gases in such quantities would imply a strikingly different atmospheric chemistry. That much methane would probably ignite the whole planet the first time lightning struck somewhere!

: And nitrogen isn't ENTIRELY inert either. Many plants : and bacteria can fix nitrogen into compounds that : plants can use to grow. Could the 'missing' nitrogen : be tied-up in biomass ? Solar radiation and lightning : can also combine nitrogen and oxygen into nitrous : and nitric acid anhydrides ... which dissolve in : water and form nitrates and nitrites which can be : used by biologicals or form minerals. Give it a : couple hundred million years - it could add up.

Not very likely. That's an awful lot of nitrogen to sequester. Even if the mechanisms existed to do it, we would certainly have found geological evidence of the vast lakes of underground ammonia, or whatever the hypothesized reservoir is.

Ben Buckner

Try Steven Vogel. If anyone's put a dragonfly in a wind tunnel, he has, or knows who has.

One thing that there wouldn't have been is many airborne predators. These large & presumably clumsy insects would pretty much have had the skies to themselves in the absence of pterosaurs, flying birds, mammals, & reptiles (although some kids in New Jersey found a very well preserved fossil of a unique Triassic (I think) gliding lizard a number of years ago).

There wouldn't have been much of a downside to being large, other than increased energy requirements, presumably met by having lots of plants to themselves. Then as other members of the animal kingdom conquered the air the larger and more clumsier insects would have been easy prey. Evolutionary pressure could have then favored smaller, and more agile flying insects that we have today.

Bob Mozark

You have to remember what's going on at the time (Silurian? Someone correct me if I'm wrong) that these giant insects are found in. Land plants have become established. While there's insects and other arthropods on land, there's not a lot of other things that respire to convert oxygen

I've posted to sci.chem about this combustion question. Hopefully, someone there has experience with the partial/absolute pressure thing. Stay tuned.

-j (hey, it's easy to type !)

Hello,

There is significant correction needed in terms of some of the comments on the age of giant insects (and the Paleozoic oxygen high). The giant insects (as well as several other 'giant' arthropod groups including diplopods, arthropleurids, and scorpion) were found in the Carboniferous, particularly the Late Carboniferous, i.e. Pennsylvanian. The oxygen pulse is thought to have started in about the mid-Devonian and ended with decreaseing oxygen levels during the Permian.

The Graham and others (1995) reference suggested by Del Cotter is a good place to start.

Rick Toomey Illinois State Museum

It's OK to cross post in cases like that. It draws people from other backgrounds in to help with the discussion and we can all see the posts without chasing over to the other newsgroup to find out.