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Diversity of Seed Plants and their Systematics
Gymnosperms I
Geological Time Scale
Dr. NUPUR BHOWMIK
Department of Botany
University of Allahabad
Senate Hall
Allahabad – 211002
[email protected]
Date of submission: 27/06/2006
Keywords Evolution, sedimentary, radiometric, isotopes, compression, half-life, Cryptozoic, Phanerozoic, Archaeopterid,
Rhyniophytes.
The crust of the earth is defined as those rocks that overlie the mantle. After the formation of the earth's core and
driving off much of earth's volatile elements particularly gases, partial melting of the entire mantle resulted. The
solid crust of the earth was formed only when sufficient cooling by radiation at the surface had taken place. And
geological evolution began after a solid skin, the earth's crust was formed. The earth we live has changed
through geologic times and prehistoric life inhabiting the earth in the geological past have been preserved in
the rock records as fossils. Study of these plant fossils reveals the nature of the plants that inhabited the earth
throughout the geological time. A vast majority of fossil plants are preserved in sedimentary rocks.
The sedimentary rock was built by constant deposition of sand and silt carried by moving water. Conversion of
the sediments into rock involved removal of water followed by varying degrees of compaction and cementation.
Presumably deposition of sediments had been going on in this way since the earth's crust became solid. In such a
case there would have been a continuous sequence of strata from the beginning to the present with the oldest
strata at the bottom and youngest strata on the top. Often plant parts like leaves, twigs, seeds etc. are also
carried by the stream and if not decayed were incorporated into the sediments and were finally included in the
rock. These plant parts are preserved as compressions in sedimentary rock if there is abundance of particles of
sedimentary material as silt, clay or fine sand available in the environment. As the plant parts accumulate in the
body of water, they become covered with the sediment and entombed in the subsequently formed rocks.
The strata of rocks deposited thus, form a geological column (see chart 1 and 4). Fossil plants from one part of
the geological column differ in size, shape, level of complexity and abundance from those from another part of
the column, because there are changes in the types of plants through geologic time. A study of the geological
record of fossil plants reveals the possible time at which various major groups originated, the time each group
reached its maximum development and in case of certain groups also the time they became extinct (see Chart
2).
Dating of fossil plants is important in order to know when the various groups of fossil plants inhabited the earth
in the geologic past. A number of methods were used to date the various rocks but the most common method to
"age dating" is that involving radioactive decay. However, there are other processes like using tree-rings of
dating archeological wood (Dendrochronology) etc.
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Radiometric Dating :
Decay at atomic level is the basis for dating technique. Dating of the rocks is determined by the rate at which the
radioactive minerals found in some rocks lose particles. Certain mineral elements have radioactive isotopes that
are sometimes referred to as geologic clocks. These isotopes lose particles (decay) at a known uniform rate,
breaking down ultimately to a stable (non-radioactive) state. For example, 1 gram of Uranium238 (radioactive)
yields 0.5g U238 + Pb206 in 4.5 billion years. In other words, the time required for half of 1 gram of uranium238 to
break down to lead206 is 4.5 billion years. Half of the original U238 will still be present. Four and a half billion
years is called as the Half life of U238. If a given sample of rock contains both U238 and Pb206, the ratio between
them can be used to determine how long ago the rock was formed. Some minerals used in radiometric dating are
:
Original
Element
Element formed
by radioactive
decay
Time required for decay of half of
original element
Uranium238
Lead206
4.5 billion years
Uranium235
Lead207
710 million years
Thorium232
Lead208
14 billion years
Rubidium87
Strontium87
47 billion years
Potassium40
Argon40
1.2 billion years
Carbon14
Nitrogen14
3.6 thousand years
At present the oldest rock of earth's crust to be measured by this method gives an age of about 4.5-4.8 billion
years.
The earth's geological history encompassing 4.5 billion years (b.y.) is divided into relevant time intervals which
are accepted globally. A number of methods have been used to achieve geologic time units and from time to
time a large number of terms have been used to signify a particular geologic time. Some of the methods were
based on rock succession or sratigraphy using, systems, series, stages etc. and others based on occurrence of
important events like Eons, Eras, Periods, Epochs, Ages etc. Besides this prefixes like Early-, Mid- or Lateor Upper or Lower were also used to specify geological intervals. Even then the division indicating the
different stages of the Earth's history seem to have remained unsatisfactory. To cite an example, the vast
Precambrian which stretches for nearly 4 billion years has been divided into only two major divisions - the
Archaean and the Proterozoic while the comparatively smaller Phanerozoic time period spanning the last 570
million years has been divided into 3 major divisions which have been further divided into 15 sub-divisions
based on the abundant fossil data easily available.
The terms used in the geological time scale like Cambrian, Ordovician, Silurian etc. describe the entire
stratigraphic record from the point at which abundant fossils appeared. These terms have be an applied to rocks
and fossils when they are labels or geological systems, or to time when they designate the periods. Additional
terms sub-divided the systems into Series and Stages and the periods into Epochs and Ages. The periods are
each assigned to large units, the Paleozoic, Mesozoic and Cenozoic Eras and these in turn composed the
Phanerozoic Eon. The science of sub-dividing and labeling geological time is known as chronostratigraphy.
The past history of the earth was divided into two major Eons : The Cryptozoic (hidden life), now called as the
Precambrian and the Phanerozoic (visible life). The Precambrian includes the span of time from the
beginning of earth history to about 590 million years ago, some 80% of the geologic time. Precambrian time is
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divided into two sub-eons: the Archean which extends from the beginning of earth history upto 2.5 billion years
ago and the Proterozoic which spans the time from Archean to the Cambrian, around 590 million years ago.
The term "Hadean" (introduced by Preston Cloud) is used to designate geological time from the origin of the
earth, some 4550 Ma ago to the oldest terrestrial rocks now known at 3800 Ma.
It is believed that in the oldest rocks dated at 3800 Ma, many geological, biochemical and perhaps biological
events had remained unrecorded in the rocks.
The possible clues of earliest life are the Isua supracrustals. Though highly metamorphosed, nature of
metasediments indicates deposition under aqueous conditions and apparently under anaerobic conditions.
Carbonates (implying presence of atmospheric CO2), sedimentary banded iron formations (BIFs) and reduced
carbon in the form of graphite are also found. The Isua carbon is enriched in C12 which suggests presence of
autotrophs.
However, the oldest unequivocal remains of life come from slightly metamorphosed rocks about 3500 Ma old.
The cherts show presence of stromatolites and filamentous microfossils. Stromatolites are deposits of
limestone, dolomite, or chert that have been laid down in concentric or eccentric layers. Individual stromatolites
are fused laterally to one another to form a solid mass of rock (see Figure 3). Certain blue-green algae can
produce pebble-sized rocks of limestone commonly called water biscuits. The concentric layers of limestone
are seen when water biscuits are broken open and they have been formed by organisms that grew on the surface.
More than 28 species of algae mostly blue-green algae have been obtained from limestone matrix as well as
from the surface of water biscuits. Many modern blue-green algae are able to form laminated limestone
deposits. Thus, in the Precambrian, about 3.5 billion years appeared the first evidence of cellular organization,
where both one-celled and filaments of cells having shape, size and organization of prokaryotic bacteria and
blue-green algae were present. Some of them were accompanied with photosynthetic mechanisms whereas
others were heterotrophs.
Further upwards in Precambrian about 2.1 billion years (the Proterozoic) coccoid and bacillus type cells
resembling modern bacteria and blue-green algae forming filamentous colonies had existed (see Figure 2).
In the younger Precambrian, about 900 Ma ago, diversification of blue-green algae and appearance of
unicellular organisms resembling extant green algae had occurred. Many organisms had nucleus like the
eukaryotes. Besides algae, fungi of Phycomycetous type had also evolved.
Above the Precambrian is the Cambrian and Ordovician between 570 and 435 million years. By this period,
great diversification of algal types had occurred and together with the blue-green, the green, brown and red alga
had also evolved. The organisms had become multicellular, showing evolution of eukaryotic conditions. Rocks
also showed presence of spores with triradiate marks indicating occurrence of meiosis and sexual
reproduction in the plant life cycles.
In the Mid-Silurian (Wenlockian) about 420 Ma ago appeared the first vascular land plants. They were small,
had naked, dichotomizing axes with terminal eusporangia. The parenchymatous axis contained primary xylem in
the centre. The sporangia were multicellular and contained microspores (see Figure 1).
In Lower Devonian, between 395 and 374 million years, appeared the Rhyniophytes and Zosterophylls with
diverse morphologies (see Figure 4, 5 and 6). Evidence of land characters like cuticle and simple stomata on
axis were seen for the first time. Some axes bore microphylls with single vein. Evolution of stele from circular
protosteles to centrarch and exarch condition had occurred. Sporangia were borne at tips of main axis
(Rhyniophytes) as well as on tips of short lateral branches (Zosterophylls). Phycomycetes were also present.
There was also the appearance and diversification of Trimerophytes.
In the Middle Devonian, between 374 and 395 Ma other trimerophyte derivatives and aneurophytes made their
appearance. There was increase in plant size leading to arborescence in some progymnosperms, lycopods and
cladoxylales. There was also the occurrence of cambium producing secondary xylem and secondary phloem.
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In Upper Devonian, between 359 and 353 Ma, there existed Archaeopterids, aneurophytes, lycopsids,
sphenopsids and ferns (see Figure 12). There was the evolution of eustele from protostele, and well defined
free-sporing heterospory. There was also the first evidence of liverworts.
In Upper Devonian, between 353 and 345 Ma, were present the Pteridosperms and arborescent sphenopsids.
There was also the appearance of preovules in cupule-like structures.
In the Mississippian and Pennsylvanian (Carboniferous) between 345 and 280 Ma, the Carboniferous floras
consisted of cordaites, seed ferns, herbaceous and arborescent lycopsids, sphenopsids, ferns, both
leptosporangiate and eusporangiate type (see Figure 7 and 8). There were plants with well developed ovules and
cupules and embryos were associated with megagametophytes. There was also the appearance of Ascomycetes
and Basidiomycetes and also mosses.
In Permian and Triassic between 280 and 195 Ma, there was extinction of carboniferous pteridosperms,
arborescent lycopods and sphenopsids. Ferns and Voltziales (transition conifers) showed diversification. First
evidence of cycads and cycadeoids. Glossopterids became conspicuous in Southern Hemisphere,
Ginkgophytes also appeared and conifers of Northern Hemisphere showed evolution of compound seed cones.
In the Jurassic between 195 and 141 Ma, there was representation of a variety of cupulate pteridosperms (see
Figure 9), maximum abundance of diverse ferns, cycads, cycadeoids, conifers and ginkgophytes (see Figure
10). Glossopterids became almost extinct. Sphenopsids and lycopsids became less conspicuous.
In Lower Cretaceous between 141 and 100 Ma, undoubted evidence of angiospermous leaves, pollen and
flowers. Both monocots and dicots were present. Flowers were wind as well as insect pollinated. Groups like
cycads, cycadeoids, conifers and ginkgophytes were on the decline. Mesozoic pteridosperms had become
extinct.
In Upper Cretaceous between 100 and 65 Ma, many angiosperms were present some of them having
characteristics of extant families of angiosperms. Ferns and conifers continued to be represented. Variety of
ginkgophytes and cycads had declined. Cycadeoids became extinct.
In Tertiary between 65 Ma and present there was evolution of new angiosperm types (see Figure 11). Diversity
of conifers on decline but ferns continued to expand.
The successional changes of organisms through geological time also provide fundamental basis for the study of
evolution of plants. While the angiosperms showed rapid evolution in short time, the blue-green algae were an
example of a group that evolved slowly during the Precambrian and have shown little morphological change
since that time (see Table 1).
While concluding it may be that mentioned, efforts for making the geological time scale (GTS) more precise
are going on and the latest GTS (2004) embodies significant changes. While the term Precambrian is used for
rocks older thank Cambrian, it does not form a geo-chronologic unit of GTS (2004). The Proterozoic Eon has
been sub-divided into Paleoproterozoic, Mesoproterozoic and Neoproterozoic Eras. The Archean Eon has
also been sub-divided into - Eoarchean, Paleoarchean, Mesoarchean and Neoarchean Eras. The new GTS
does not favour usage of terms like Tertiary and Quaternary System / Periods but recommends the term
Neogene which includes Miocene, Oligocene and Pliocene (which were previously a part of the Tertiary
Period) and Pleistocene and Holocene (previously Quaternary) (see Chart 3).
Suggested readings 1.
Paleobotany and the evolution of Plants by Wilson N Stewart, Cambridge University Press, Cambridge
New York (1987).
2.
Recent Revisions to the Geologic Time Scale by A.V. Sankaran. Current Science, Vol. 89, No.7,
(2005).
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3.
Nature Library - Fossils by Richard L Moody.
4.
The Cambridge Encyclopedia of Earth Sciences, ed. by David G. Smith. Cambridge University Press,
Cambridge London (1981).
5.
Evolution and Plants of the Past by H.P. Banks, Belmont , Calif, Wadsworth (1970).
6.
The Biology and Evolution of Fossil Plants - by T.N. Taylor and E.L. Taylor. Prentic Hall, Englewood
Cliffs, New Jersey (1993).
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