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TIMESTAMPS

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I. BARREN LANDSCAPES

Our solar system is 4.6 billion years old. Within our vast universe there are millions of older solar systems, many of them probably with planets and life. It is entirely possible that more intelligent life forms may exist elsewhere. If so, what may have they seen as they passed Mars and approached Earth?

If our visitors had passed by Mars early enough, they may have seen a planet with a rocky landscape with some active volcanoes and a considerable quantity of surface water. Over the land surface, barren rocky soil totally devoid of life.

Let's say our visitors came by recently, oh.. ...some 480 million years ago (90.6% of the time from Earth formation to today). Our visitors would first be struck by the vast watery expanse of our oceans, similar in area and volume as today. Different would have been the shape of the oceans and distribution of continents.

As our visitors moved closer, for a better view of the continents, they would have observed the same variety of topography as today, vast flat plains to high snow covered mountain ranges. Yet to these explorers, Earth's land surfaces would have been more like Mars, where they had just visited. WHY? Because there was no visible life, no green, no plants, except for perhaps some very limited cyanobacteria.

So when did plant life first inhabit the continents? That is the subject we now explore. First, plants had to solve the problems of desiccation (drying), support, and the effects of gravity. Land plants probably evolved from marine plants, moved into freshwater and finally onto land. This probable transition is from marine green algae to simple bryophyte type plants to vascular plants:

II. BACKGROUND

• No fossil evidence of transition from marine algae to non-vascular plants

A. Non-vascular Land Plant Characteristics:

• Lack specialized cells for the movement of nutrients and water.
• They are small and limited mostly to low, moist areas.

• Phylum: Bryophyta: "amphibious" plants lack:

1. fluid transport system - small

2. means of preventing gamete desiccation - need moist environment

B. Vascular Land Plant Characteristics

• (not all evolved at the same time)
• roots: to gather water and nutrients
• leaves: photosynthesis
• vascular system: for fluid transport
• cuticle (cutin in outer wall layers): prevent drying
• support: lignin and cellulose

III. A BRIEF SUMMARY OF MAJOR STEPS IN PLANT EVOLUTION

• The oldest fossils reveal evolution of non-vascular plants by the middle to late Ordovician Period (~450-440 m.y.a.) on the basis of fossil spores.
• These first non-vascular, "amphibious" plants, were limited primarily to lowland, wet areas, of temperate to tropical latitudes. Still most land areas were barren.
• By some 420 m.y.a., seedless vascular plants evolved which were bigger and more diverse than their ancestors. These seedless vascular plants required water for sperm to reach the eggs and were still limited in their distribution to wet climates. These seedless plants grew to tree size, reaching 10 or more meters by the late Devonian (~370 m.y.a.). Lowland swampy environments had abundant seedless plants.
• After the 1st land plants evolved (approximately 450 m.y.a.) they underwent rapid expansion and radiation with great forests by Middle Devonian Period. (385 m.y.a.). Although these forests were limited in their extent, vast quantities of organic material were deposited and preserved in swampy environments. Through time and burial, this organic material was converted to coal, forming the first significant coal deposits of Earth history.
• At a similar time (late Devonian, ~375-360 m.y.a.), seed bearing gymnosperm plants (non-flowering) evolved. The development of the seed allowed plants to proliferate and spread to drier areas of continents. Gymnosperms became the dominant plant type between ~290 to 145 m.y.a. and are still common today.
• During the final days of the dinosaurs (Cretaceous), the final chapter of plant evolution began. This was the evolution of flowering plants (angiosperms). During the Cretaceous Period, angiosperms began overcoming non-flowering plants as the dominant land plants. Since this time they have become the dominant land plants.

For most of the history of planet Earth, landmasses have been barren of life. Even with the advent of vegetation, plants first clung to lowland wet regions. Much later, with the arrival of gymnosperms, they spread to drier regions. Only within the last 100 m.y. did the planet's plant world begin to resemble the Earth we know.

1. Prior to plant colonization of land, how would geologic processes (such as erosion) have differed from today?

2. Why are land plants important as eventual providers of a fossil fuel source?

3. In what type of environment are plants preserved as fossils?

4. When did upland and higher latitude areas become forested? By what type of plants?

IV. PLANT EVOLUTION

A. FERNS & RELATIVES - dominated through most of the Paleozoic
1. no seeds
2. no effective way to prevent gamete desiccation - require moist environment

B. GYMNOSPERMS - conifers and other groups, first appear in Carboniferous, dominant land flora by Triassic.
1. seeds and pollen

C. ANGIOSPERMS - flowering plants - first appear in early Cretaceous
1. flower and enclosed seed

• Reproduction of seedless plants: primitive reproduction, gametes not produced on large leafy plant but on separate small leafless plant - called gametophyte.
• See text page 370 (Box 13.3) for an illustration of the reproductive history of a seedless vascular plant. Compare with the illustration of the reproductive history of a primitive seed plant (gymnosperm).
• From the illustration note: - alternation of sporophyte and gametophyte plants. - need moist soil for gamete protection.

A. Psilopsids -

• most primitive,
• most lack roots and leaves,
• simple vascular system,
• abundant only in Devonian.
• Seedless Trees - leaves and strong root systems; evolved from small herbs

• huge trees of coal forests (up to 30m).
• Arose in the Early Devonian (small), large by Late Devonian
• Lepidodendron - scale trees
• Sigillaria - scale trees
• During the Carboniferous, coal swamps with abundant trees of this type accumulated, forming the greatest coal deposits of all geologic time.
• Elongate leaves emerged directly from trunks and branches, leaving distinctive leaf scars (shown in class).
• Living club moss a representative
• In many, sporangia in clusters called cones, male and female cones on separate plants (advantage for variability)

• joint stemmed plants (scouring rushes)
• Calaminites - common in Carboniferous
• Equisetum (horsetail) - only living representative
• Seedless trees overshadowed in Late Carboniferous and Permian by gymnosperms.
• see fossil Sphenophyta

• true ferns
• Huge trees in Carboniferous
• survivors small
• over 10,000 living species

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Reproduction of gymnosperms

• seed not enclosed (exposed), e.g., woody scales of pine cone
• great advantage over seedless ancestors: reproduce without external moisture
• retain gametophyte stage but no sporophyte plant
• See text page 370 (Box 13.3) for an illustration of the reproductive history of a primitive seed plant (gymnosperm). Compare with the illustration of the reproductive history of a seedless vascular plant.
• From the illustration note: - Seed bearing plants bear male and female cones. - The pollen fertilized seed is protected and may survive dried conditions for a period of time before growing. This adaptation allowed plants much wider distribution that possible by seedless plants.

• First plants to bear seeds
• First in lower Carboniferous to Late Devonian
• Common through Permian extinct in Early Cretaceous
• only major division to go extinct declined rapidly with rise of other gymnosperms

• Fossil Reord of the Cycads 
• Introduction to the Cycads
• Fossil Record of the Ginkgoales
• Introduction to the Ginkgoales
• dominant trees of Triassic and Jurassic
• declined rapidly in Cretaceous
• Cycads - palm-like, survive today in warm climate
• Ginkgos - fan-shape leaf, 1 species left

 

• First in Carboniferous
• Pines, firs, cedars arose in Early Mesozoic
• Dominant vegetation where water sparse, poor soil, cold

RISE & DOMINANCE OF FLOWERING PLANTS ANGIOSPERMS

Very dominant group today - 96% of all vascular plant species

Success the result of development of flower and enclosed seed

• specialized flower reproductive structure: - both seed and pollen producing structures
• colored leaves to attract pollinators
• Double fertilization: one pollen grain fertilizes ovary, and a second triggers vigorous growth of the nutritious fruit, nut, or kernel.
• seed covering: soft, fleshy, edible, aid in seed dispersal
• other dispersal mechanisms
• exhibit great diversity
• geologic range: few in Triassic and Jurassic; dominant since Cretaceous
• fossil record of Angiosperms



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