I need a break again from all things HPC and rocketry related. So, I am going to bang out over the next few days a post on the paleoecology of the Carboniferous. This is related to the post on carbon cycling and was intended to be a second half of it. I ran out of time and had to split the effort into two posts. This is the second post.
There is an online friend that I keep mentioning that really deserves the credit for what I am about to write. Carlos made a comment on his old blog, Halfway Down the Danube, when he did a paleo ecology post about how the Paleozoic ecology was pretty segregated and could be interpreted as a caste system for those of us a little more familiar with the historical and cultural studies. This was a while back as I was just getting back into my paleo interests once again and it has really, really stuck with me. It fits so well with what I keep encountering time and again. Carlos has since passed on from the blogosphere, alas, but this post is a hat tip to him and his polymathism.
The world we live in is quite old. I mean that literally and figuratively. The world is billions of years old, somewhere around its 4.5 (+/-) billionth year: after all, who keeps counting after a while? However, I also mean this in terms of what has transpire in the 600, 650 billion years of multicellular life's evolution. There are times when it seems we live in an advanced and fastening world with a lot of highly developed complexity that often gets portrayed as the triumphant, overdetermined end result of all that evolution. We like to look back at the past and proclaim our superiority. A false superiority. Alternately, our modern ecosystems are sometimes portrayed as an impoverished place with ossified lifeforms with little chance for change and are relatively soon doomed by either Man or some devastating environmental catastrophe. The vast gaps of diversity from what has gone extinct can seem pretty depressing at times and not just the recent 6th mass extinction victims either. The truth is neither is really an accurate picture.
If past is truly a different place, Deep Time is an alien world. We touched on that in some of the simulations of the paleoclimate, especially in the Permian Extinction time frame. We talked about it in the previous post on the Carboniferous carbon cycling (or lack there-of). I have pondered the Mesozoic and its rather different and diverse organization of its ecology. As strange as the Mesozoic ecology was the Paleozoic ecology was truly different. The Carboniferous Period epitomizes this.
When Carlos talked about the Paleozoic ecosystem as being a caste system, he meant that different groups of animals and plants lived and did specific things and only their close relatives did anything similar. Bob and his kin are the warriors. Nadya and hers are the priest(esse)s. Hiro and his are the peasants. Tomas and his are the aristocracy. However, instead of social roles being set, in the natural environment, the plants and animals were 'niched out' this way. This was especially true for plants.
Diversification in Phytic Deep Time
Plants originally invaded the terrestrial environment in the Ordovician Period before 470 million years ago. That's when we have evidence of the first land plant spores from the fossil record. While terrestrial plant evolution would cotinue to allow plants to adapt for some time from the Ordovician through the Silurian, it was in the Devonian that the major diversification took place. This is when vascular plants seem to have broken off into their four major groups. We'll get to them. However, the timing of that diversification is interesting.
Wignall and Hallam referred to the Devonian as "one damned thing after another" in their volume, Mass Extinctions and Their Aftermath. There were major mass extinctions that often get lumped together into the Late Devonian Mass Extinction (or here). In the marine environment the extinctions blotted out the placoderms most famously and in the terrestrial environment seem to produce a possibly complete wipe out of the early terrestrial tetrapods. This is often referred to as Romer's Gap. Whether this is a sampling artifact or not, remains to be seen, but for the moment, it does appear that there is an interesting and possibly, even probably if its not a sampling issue, tied to the mass extinctions in the Late Devonian. While this was going on, the plants were busy bushy-ing the family tree.
Speciation often requires isolation. Recently, especially, mass extinctions have been tied to diversification, as contrary as that may be to most people's thought processes. If the mass extinctions of the Devonian sufficiently isolated the different plant populations, it is possible, even probable that the populations would have started diverging. A lot. As the isolated plant populations faced different environments, their favored adaptations would have produced some really different planforms. One of those is what we think of as a tree.
DiMichele, Stein and Bateman in Ecological Sorting of Vascular Plants During the Paleozoic Radiation that there are, based on their cladistic analysis, produced a classic branching tree for the interrelatedness of the plant kingdom. They give quite a long explanation, including a detailed reasoning and weighting of the characters for their analysis. I'll give a quick summary of their phylogeny, but this is only meant as background. It appears that all terrestrial vascular plants are divided in one of two groups, the zosterphylls and the trimerophytes. The majority of vascular plants we know today - apple trees, ferns, and what have you - are all trimerophytes.
During the Carboniferous, three out of four of the dominant groups were trimerophytes. Only one was not.During the Carboniferous, three out of four of the dominant groups were trimerophytes: ferns, sphenopsids, and seed plants. Only one was not, lycopsids. Each of these plants castes lived primarily in different environments. That is to say that they did not live in each other's environments, but rather that they became the primary, sometimes nearly sole, plant group in that environment. Sometimes, btw, it seems that a particular group actually originated somewhere else, remained a bit player there, and then spread, preadapted, to their environment where they ended up dominating.
The Stage and Cast of Phytocastes
During the Carboniferous, there were three major environmental dominions: marshes and swamps, aggragational environments other than marshes (ie flood plains), and upland. Each one of these had a different caste that ran the place. Then there was a fourth caste that came in and set up shop whenever there was an opening. This one evolved to fill the role of opportunist that would move in whenever the environment was disturbed.
That caste was the ferns. They developed with the sole role of rapidly filling in wherever was so disturbed that the other castes were wiped out by an event. During the Carboniferous, as we all know, the oxygen content of the atmosphere was far, far in excess of modern times. Fires were quite, quite common. Other localized catastrophes were also not uncommon as well. When you put that altogether, it presents a unique selective pressure that some plant would be able to take advantage of. Ferns did exactly that. That was their original niche. That isn't what they developed into later on - tree ferns developed after mass extinction opportunities - though some varieties did continue to do that. In fact, the very famous Fern Spike is a good example, albeit on a massive scale, of that exact niche strategy on a massive scale. So what did the regular, normally populated phytocaste domains that existed at the time?
(a variety of lycopsids, credit Devonian Times)
When most people think of the Carboniferous, they think of the grand swamps and marshes and great, great sodden forests. Water is ever present. It's sluggish and frequently standing. It would remind you of a mangrove swamp in some ways and in others just a plain old nasty swamp or marsh. This was only one of the kinds of environments that were present at the time. It was a very, very important one to be sure though and its dominant caste was that of the lycopsids. What's a lycopsid?
A lycopsid is an interesting type of plant. It's a radically different from most modern seed plants in a few particular ways. The structural element, that what keeps the plant upright and vertical, is actually the bark. This is really different than our modern trees which use the wood as the structural element. It also is one of the reasons that you often see in different renditions of the Carboniferous have these odd, strange hollow logs. Those are lycopsid logs that the center, softer 'wood' has rotted away leaving the structural skeleton. The other interesting fact about them is that they did not - do not! - have leaves. They grew to be tree sized and for the marshes, swamps and mangrove forest niches that's precisely what they were. The reason that they were restricted to only the swampish environment was two fold. One was because they still required water to reproduce. The second was that their root system is not that deep or advanced enough to tackle dry ground terribly well. While in the Carboniferous they were the swamp forest trees, today they are represented by the extent club moss. Note: the lycopsids of the Carboniferous were not merely amped up, oversized club mosses: club mosses have evolved for 300+ million years since then and they are very, very distant relatives as a result.
The next phytocaste domain happens to be aggradational environments. Wuh? This is an environment that deposition of sediment is faster than being washed away or settling. Swamps are a classic case of this, but those are explicitly separate from this in the Carboniferous. This dominion was anything but a swamp where deposition was faster. A good, good example of this is a flood plain. In this case, if you were to walk out from the swamps, marshes and sluggish water forests onto the flood plains where there was far more drainage, but still had relatively ready access to water you would find that the lycopsids had given way to another caste. This caste was the sphenopsids. In this case of Carboniferous, the sphenopsids produced many different forms. They had stream side forms like modern ones. They also even produced shrub sized ones. They even produced the flood plain forest "trees." The sphenopsids dominated the nonmarsh, non-upland environments. They were ubiquitous: they were the forest largely from bottom to top. The question is "What's a sphenopsid?"
Sphenopsids have an extent representative: the horsetail. However, like the club moss, remember that the modern horsetail is not the same as the Carboniferous sphenopsids for the same reasons. However, like the modern horsetail, the Deep Time sphenopsids grew in segments, even the tree sized ones. They also had a rhizome that the roots branch off from that. Like the modern horsetail it also had a structural exterior and weaker pith, similar to a lycopsid. As far as we can tell, the ancient sphenopsids had a similar reproductive cycle similar to their modern relatives. Again, there are no leaves. There are several differences though.
The final environmental dominion that we haven't discussed as yet is the upland environment. This is where the chance for water is greatly reduced and the soil is frequently and mostly dried out. The climate is frequently harsher and unfriendly. As you would expect, the seed plants dominated here. It should be noted, though, that seed plants did not originate in the uplands. They apparently, according to DiMichele et al, had a long evolutionary history in the lycopsid domain as a bit player before they developed for whatever reason the seeds that allowed them to strike out to colonize what had been a largely untapped region. They would very quickly diversify in that environment. As a consequence, btw, they had a far, far greater diversity in forms as a consequence as a side effect of the relative marginal nature of their domain.
Seed plants wouldn't make almost any headway until the climatic changes of the Permian towards making inroads on the other plants' dominions. They probably would not have even come to dominate the plant dominions like they do now without the multiple intervening mass extinctions. If they did not have the intervening mass extinctions hadn't taken place, there's no indication that the seed plants would have come to dominate the lycopsid and sphenopsid dominions.
As an aside, DiMichele et al, make an interesting assertion. I am not really qualified to comment, but apparently there was some consternation on their part about the angiosperm - flowering plant - classification. They argue very, very strongly that it is an accident of history that the angiosperms were classified as a phylum, but in reality they feel that it belongs as a class, a very species rich one to be sure, but not nearly as high up in the Linnean hierarchy as they get. They are merely a very successful seed plant clade as far as the authors are concerned.
A Walk through the Carboniferous Countryside
Slogging through the swampland forest, you really wish you hadn't been talked into ditching the boat. You're about halfway out now and you've seen a few critters that really, really made you wish you'd stayed in the boat. Scaly salamander-like tetrapods that were the size of crocodiles. Eels that came up to gulp air. Giant sized bugs. oy. What's really fastinating and creepy at the same time is the trees. No leaves. Zilch. They make you think of thorn bushes grown into trees with no leaves and photosynthetic spikes. There are logs floating in the water that are, well, hollow and look more like organic pipes. The only reassuring element is the common and familiar looking ferns that seem to be skulking waiting for some break in the local defenses.
Ah here we are. Finally! You're out of the swamp and moving inland. There seems to be something of a flat plain, a with better drainage. It's beautiful in its own strange way but you're watching your step. There are cat sized spiders running around that make the tarantulas back home seem tame dachshunds in comparison. There are also plenty of what look like slightly misshapen lizards and more of those scaly salamanders as well. There are giant millipedes...sorta...too. The forest seems more like something from your own time...but only if you were ant sized. The trees look like wildly contorted horsetails. It almost feels as though you ate the wrong side of the mushroom and that you ought to expect to see a white rabbit with a pocket watch at any time. Even here, the ferns seem to lurk in corners awaiting some violence upon the order of things.
At last, some hours later, you find the ground moving upwards. You're moving into the hills. The vegetation changes again. Now though, its looking more like home. The plants actually have what appears to be leaves and needles. There are actually seeds on the ground. It's not home, or even close, but it feels a little less alien. Besides, those freaky salamanders are gone. Yep, those criminal ferns seem to be here, too. You spend a lot of time climbing, but you do reach a vista and look out.
What you see is different. It's odd. You see a world where the forests do transition but each area looks really different and they sit almost atop one another. In some ways it looks like a bulls eye. Its rings of vegetation with very different if blurred textures. In the lowest, swampiest areas you see the lycopsids with their spiky looking branches. Then the strange fractal, brobdingnagian horsetails. Then in the mountains around you, the seed plants that don't seem quite right. Then you see off in the distance, there's a hillside that looks a like it has some blackened trees and there in the wind you see the greenery dance almost grass like, but you know its not. Its those ferns. They're ariot with growth in an area recently savaged by fire.
You can't help but note. This world is segregated. Far, far more than your own with its blend of this and that. This world, and its plants, all have their place and from that place, unless you are of the fern-ish underworld, you do not move. Its a caste ecology. Almost set in stone. It would probably ossify if not for a major shake up. One you know with dread-relish is coming.
You had best be moving on, there's not a lot to eat unless you like gamy, wild reptile meat or roasted bugs. Your digestive system just didn't evolve in this world. It's not home and it won't be home, not for 300 million years.
One Last Word
I would like to make a note that the interconnectedness of these different phytocastes was pretty low. Each had a very different ecology, even if they were side by side. They just. didn't. mix. The diversity of the lycopsids was actually pretty low, btw. It also contributed to their demise as megaflora when the extinctions came.
At any rate, I am out of time and I have Team Phoenicia work to get done tonight and tomorrow (visitors!), so we'll have to end it there. I hope you enjoyed the little tour and I hope I didn't embarass Carlos too much with this.
Fire away with any screwups, thoughts and comments, as always!
Actually, I may append this later.