The Paleozoic, the Rise of Plants, and the Age of Dinosaurs..

Main Points:

  1. Precambrian land life very poorly known; could have been lichens.
  2. Most early Paleozoic biomass had to be in oceans.
  3. Plants (Kingdom Plantae) arose in early Paleozoic from green algae; key innovations include an impervious cuticle, and conductive tissues supported by nearly indigestible cellulose.
  4. By Ordovician some remains of vascular plants.
  5. By Devonian becoming common; by Late Devonian there are forests and coals.
  6. Plant life cycle characterized by "alternation of generation": with a diploid sporophyte and a haploid gametophyte.
  7. Devonian and Early Carboniferous characterized by dominance of lycopsids, ferns, and their relatives, reproducing by spores.
  8. By late Devonian Gymnosperms arise - key innovation is the seed.
  9. By Permian seed plants dominant.
  10. Global biogeochemistry massively changes with rise of vascular plants; most biomass now on land.
  11. Terrestrial arthropods follow; very little herbivory at first.
  12. Fish, which appeared in Cambrian diversify through Paleozoic. Lungs probably primitive for bony fishes, at least.
  13. In Devonian some Sarcopterygians (lung-fish relatives) made the transition to land - origin of the Tetrapoda.
  14. By Late Carboniferous there are amniote tetrapods; key innovation is amniotic egg.
  15. Devonian and Carboniferous terrestrial ecosystems dominated by carnivory among animals: very little herbivory.
  16. Delta 13C curve in sediments gives us signal of organic matter burial.
  17. Curve as well as obvious coals shows massive Carbon burial during Carboniferous and Early Permian.
  18. Could the loss of carbon from terrestrial systems be due to a lack of herbivory?
  19. Burial of this carbon associated with massive Ice House conditions of Late Carboniferous - Early Permian glaciations.
  20. In Permian herbivory becomes common in insects and tetrapods.
  21. Burial of Carbon decreases - Hot House conditions prevail. 

I. The Terrestrial Ecosystem

We have seen the major changes that occurred in the Cambrian Explosion with the spread of coelomate animals and the near elimination of the Precambrian type prokaryotic biomat communities.

A similar kind of change with even more far-reaching implications occurred because of the invasion of land by vascular plants.

Precambrian terrestrial life is very poorly known, there may have been fungi, lichens (fungal-algal sybionts), and there certainly would have been various prokaryotic communities in moist areas. However, most biomass was almost certainly in the oceans, so that the ratio of land to ocean biomass was certainly the reverse of what it is now. However, we do not know if the total biomass was close to what it is today.

The effect of terrestrial plants, as we have already seen is to "fertilize" the chemical weathering process and that results in the burial of CaCO3+ from the reaction CaSiO3 + CO2 = CaCO3 + SiO2, and also to provide organic carbon, produced by photosynthesis, that gets buried, with O2 being left in the atmosphere as a result.

Ecosystem Efficiency

An efficiency, usually expressed as a percent, is relative measure of the efficacy of the conversion of one thing to another. The product is viewed as of value. So for a motor that is 50% efficient we would be saying that for a given amount of electrical energy put in we would get 50% of that energy converted to useful work. The rest would be expended as heat, which in this case would be viewed as not useful. If it were 100% efficient, all of the energy put in would produce useful work and none lost as heat. Thus:

( useful product / input ) x 100 = % efficiency

What might an efficient ecosystem be? I feel that from the point of view of the living parts of an ecosystem, the most efficient system would be one in which all of the energy fixed as organic carbon by photosynthesis would be respired by life. All of the energy from autotrophs would thus contribute to the maintenance and growth of the biota and none would be lost to burial in the sediments. In a 100% efficient ecosystem all of the carbon reduced in photosynthesis would end up respired. Thus:

( respired carbon / carbon fixed by photosynthesis ) x 100 = % ecosystem efficiency

Here we assume that the respired carbon produced useful work for the living members of the ecosystem.

In soil the CO2 from respired ecosystem respiration contributes to the chemical weathering process. The carbon that is not respired can end up being buried for geologically long periods of time and thus could contribute to an increase in O2.

If the terrestrial plant biomass is large, it is possible for ecosystem efficiency to be high, resulting in little carbon being buried, and yet the enhancement of chemical weathering could be very large.

On the other hand, if terrestrial biomass were low, then even if ecosystem efficiency were low, carbon burial could be low and chemical weathering could be low.

Producers vs. Consumers

We can thus partition the processes that effect the carbon cycle into above and below ground components and in each case recognize the role of producers (autotrophs, in this case plants) and consumers (animals, fungi, bacteria and other heterotrophs).

The more above ground respiration there is, the less organic carbon there is that could go below ground.

Organic carbon that goes below ground could contribute to carbon burial if it is not respired and to chemical weathering if it is.

Thus, the relative contribution of plants to the carbon cycle is contingent on the relationship with producers. As during the Cambrian explosion, however, animals and fungi that live off of plants are predators which the plants evolve defenses against. A pattern of trophic escalation thus exists in the relationship between plants and heterotrophs. Sometimes that relationship can become comensal or even symbiotic, however.

Key Evolutionary Innovations

In the evolution of vascular plants, there were several key innovations that represent breakthroughs to physical challenges allowing new ways of life. Many of these key innovations were also challenges to heterotrophs, that apparently took tens to hundreds of millions of years to be matched by countering key innovations.

The early key innovations seen in plants are responses to the challenges of:

The first two are largely structural and the last involves the mode of reproduction. Phylogenetic analysis of living land plants makes it quite obvious that the earliest land plants had alternation of generation.

Alternation of generation involves, what seems like a very complicated reproductive method. There are two generational types, the sporophyte and the gametophyte generations that alternate.

The sporophyte is diploid (paired chromosomes) and produces a minute structure called a spore, which is what the plant uses to spread itself around or disperse (i. e. a propagule). The spore is haploid (only one complement of chromosomes). The spore lands somewhere and grows into a haploid multicellular plant that is the gametophyte generation. The gametophyte produces sperm or eggs (also haploid) that unite to form a zygote (diploid) that grows into the sporophyte plant. Most of the obvious plants you see are the sporophyte generation.

Ferns and their allies have a prominent sporophyte generation and an inconspicuous but still fairly large gametophyte. They are relatively tied to water because the sperm must move through water to fertilize the egg. The sporophyte produces spores (haploid), which are microscopic, cuticle covered entities. These land on a damp surface and grow into the small gametophyte generation that produce sperm and eggs (haploid). These unite to produce a diploid sporophyte that can repeat the process.

Seed plants on the other hand have reduced the gametophyte generation to a microscopic cuticle covered object (the pollen grain for the male). In the female, the gametophyte is reduced to the ovule that remains within the reproductive structure of the plant and produces eggs. The pollen grain is released to the air or carried b insects to the female reproductive structure where the male gametophyte sends down a pollen tube into the female reproductive system. The pollen grain sends sperm down the pollen tube to fertilize the egg produced by the ovum (female gametophyte). The result is a zygote that grows into an embryo which is supplied with a packet of food and encased in a more or less impervious shell.

The advantage of the fern method of reproduction is that they can disperse extremely easily because the spore is a very low Reynolds number propagule, can hence blow about in the atmosphere for years, and is all that is needed to grow a plant (albeit a gametophyte). The disadvantage is that the sperm must travel in water and this greatly restricts the distribution of ferns and their allies.

The adavantage of the seed plant method of reproduction is that the reproduction is no longer directly tied to water and the propagule has a source of food to get a head start. The disadvantage is that seeds are relatively large and thus seed plants cannot disperse as readily as ferns and their relatives.

II. Herbivory and Detritivory

If there were only plants and microscopic heterotrophs (bacteria, protists, and non coelomates) and fungi on land, the rate of degradation of plant material would be very slow.

But, that is not the case because of the action of macroherbivores and macrodetritivors. Both act to vastly increase the surface area of the plant material they ingest, and thus vastly increase the rate at which microscopic heterotrophs can respire the fixed carbon.

Mostly overlooked in discussion of the carbon cycle and its evolution are the effects of macroherbivores:






In this diagram, the gray arrow is respired CO2 and the black arrow is the movement of organic carbon fixed by photosynthesis. The size of the arrows show (very approximately) their relative importance. The lowest downward arrow represents organic carbon burial.

Thus, macroherbivores tend to increase the ecosystem efficiency of the terrestrial system. This is despite that fact that their assimilative efficiency is only about 10%.

III. Phanerozoic Evolution of the Carbon Cycle

The key evolutionary innovations for plants show up in the middle Paleozoic. By the Late Devonian seed plants and surviving ferns, lycopsids, and equisetalians (all spore producers) had clothed much of the world with forests.

The relative locus of biomass had switched from ocean dominance to land dominance.

In plants, the key evolutionary innovations during the Paleozoic were as follows (in order of appearance):

Structure Description and responce to challange First appearance
1. Cuticles Waxy polymer coating preventing desiccation. Ordovician
2. Spore with cuticle Minute propagule produced by sporophyte resistant to desiccation allowing wind (as opposed to water) transport . Ordovician to Silurian
3. Vascular tissue Conducting tissue for transport of fluids and nutrients in sporophyte. Middle ? Silurian
4. Stomata Holes in the cuticle that can be opened or closed allowing gas exchange sporophyte. Late Silurian
5. Enations Lifelike extensions of the stem to increase surface area sporophyte. Early Devonian
6. Wood Cellulose (long chain sugar) and lignin (polymer) structural tissue sporophyte. Middle Devonian
7. True leaves Specialized photosynthetic structures with webbed structures for structural and vascular support in sporophyte. Middle Devonian
8. True seeds and pollen Relatively large propagule consisting of plant embryo and nutritive material, encased in resistant coat: pollen is minute male gametophyte that can be transported by wind or insects. Late Devonian

Heterotrophic Evolution

PROTEROZOIC: Low Ecosystem Efficiency, Low Chemical Weathering

In the diagram at right, and in similar ones in this lecture, the arrows represent the flux of carbon, with their size corresponding in a very diagrammatic way to the magnitude of the fluxes with the colors representing different carbon-containing compounds as follows: gray arrows , flux of CO2; black arrows, flux of organic (fixed) carbon, such as CH2O; and white arrows, flux of HCO32- (along with Ca2+ and Mg2+).

The organisms shown are as follows: a, lichens; b, fungi; c, procaryotic mat in moist areas; d, nematodes (the latter not known from fossils, but may have existed); e, soil bacteria.

LATE DEVONIAN - CARBONIFEROUS: Low Ecosystem Efficiency, High Chemical Weathering

Insects and tetrapods (four legged vertebrates) evolved in the Devonian. However, a striking aspect of Devonian through Carboniferous terrestrial animal communities is the rarity of forms with herbivorous adaptations.

Devonian terrestrial arthropods are numerically dominated by predaceous and detritivorous forms. Insects with sucking mouth parts appear in the Carboniferous, but despite the abundant vegetation folivorous (leaf eating) insects remain uncommon until the Permian.

Devonian and Carboniferous terrestrial verterbate communities were virtually all predatory. The only forms that might have been herbivorous were small forms such as the stocky Diadectes of the Late Carboniferous.

The terrestrial ecosytem might have looked something like the diagram at right in the Late Devonian through Carboniferous.

Symbols for fluxes are: gray arrows , flux of CO2; black arrows, flux of organic (fixed) carbon, such as CH2O; and white arrows, flux of HCO32- (along with Ca2+ and Mg2+).

The organisms shown are as follows: a, a large vascular plant (progymnosperm); b, macro-fungi; c, the giant terrestrial millepede-like arthropod, Arthropleura (a detritivore); d, soil mites and earthworms (the latter not known from fossils, but presumed to exist); e, soil nematodes; f, soil bacteria.

LATE TRIASSIC-EARLY CRETACEOUS: High Ecosystem Efficiency, Low Chemical Weathering

In the diagram at right, symbols for fluxes are: gray arrows , flux of CO2; black arrows, flux of organic (fixed) carbon, such as CH2O; and white arrows, flux of HCO32- (along with Ca2+ and Mg2+).

The organisms shown are as follows: a, a large vascular plant (conifer); b, super-herbivore (Jurassic Brachiosaurus); c, folivorous insects (grasshoppers); d, macro-fungi; e, social insects (e.g., termites); f, soil mites and earthworms; g, soil nematodes; h, soil bacteria.

NEOGENE: High Ecosystem Efficiency, High Chemical Weathering

In the diagram at right, symbols for fluxes are: gray arrows , flux of CO2; black arrows, flux of organic (fixed) carbon, such as CH2O; and white arrows, flux of HCO32- (along with Ca2+ and Mg2+).

The organisms shown are as follows: a, a large vascular plant (angiosperm); b, superplant (grass); c, herbivore (American Bison, a grazer); d, folivorous insects (grasshoppers); e, macro-fungi; f, social insects (e.g., termites); g, soil mites and earthworms; h, soil nematodes; i, soil bacteria.

Updated March 17, 2005
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