In this article we will discuss about Fossil:- 1. Role of Fossil 2. Uses of Fossil 3. Types 4. Limitations.
Role of Fossil:
Fossils have a potentially important role to play in taxonomy and phylogenetics. However, their use and interpretation has its own pitfalls and contentious issues. Under proper conditions, animals, plants and even microorganisms can leave remarkably good traces in rocks.
In certain cases the actual substance of the fossilized material is replaced by minerals or become greatly changed chemically. But exceptions do occur, and fossils are sometimes formed under conditions that permit preservation of antigenic materials or even DNA, that can be isolated and put to systematic use.
Some well-preserved taxa provide a historic record of life on earth and thus provide a window into the past.
Uses of Fossil:
Any theory of evolution is incomplete without any palaeobotanic evidence. According to Lam (1959) no evolutionary aoctrine would be perfect without palaeobotanic evidence.
Palaeobotanic studies have proved helpful in:
a. The study of comparative morphology – Fossils not only provide details of the structure and sometimes the biology of extinct organisms, they can also provide estimates of the ages of the groups concerned.
b. Providing phylogenetic evidence:
i. They provide additional taxa that can be scored for character states and included in a phylogenetic analysis.
ii. They give a minimum estimate of the age of origin of particular taxa and their character states.
c. Determining the evolution of the floras of the past.
d. Determining the ecological conditions of the past.
The palaeobotanic studies have proved helpful in the phylogeny of angiosperms. According to Dilcher (1979), paleobotany provides important information about several aspects of the origin and diversification, as well as the early history of angiosperms. On the basis of palaeobotanic studies a number of new concepts have been developed about the primitive flow types by Dilcher (1979).
The evolutionary history of present-day plants, are often furnished by fossil records. Since the fossils of land-plants are scattered and poor, difficulties often arise in the interpretation of these data. The entire basis of fossil plant classification was changed by the appearance of angiosperms.
A large number of fossil angiosperms, particularly leaves, seem to belong to extant families and to be related to recent genera. However, in some cases such affinity is unknown, either due to a lack of knowledge of living plant structure or due to poor state of preservation resulting in the absence of diagnostic features.
Types of Fossils:
Both, macrofossils (e.g. stems, leaves, etc.) and microfossils (e.g. pollen) are used as the soul of systematic data.
Petrified wood is the best-known plant fossil. But due to the absence of proper knowledge about living plant structure, this sort of investigation has been greatly hampered for a long time. Spurred on by Metcalfe and Chalk’s key to living wood amatory, the proper study of fossil angiosperm wood has started and progressed considerably.
Leaves are only preserved under rather special circumstances such as in the anoxic bogs that ultimately produced present day coal measures.
iii. Floral Parts:
Although floral structure constitutes the chief basis for finding out relationships and evolutionary trends, flowers are hardly observed as fossils due to their short life span and delicate nature. Even if fossilized, they are often crushed due to which, important taxonomic details are obscured.
However, if they are at all well preserved, the chances of correlation with parent plants are few and conclusions regarding affinity remains uncertain. For example, a number of well preserved petrified flowers with associated fruits from the Eocene Deccan Intertrappean Series of India was referred to the Lythraceae by Shukla (1944), which had relationship with Decodon.
iv. Fruits and Seeds:
Fossil fruits and seeds have been found, although not as numerous as leaves. Some of these have been found to retain anatomical structure in detail, while others, have, been found, to be preserved merely as frustrating casts or impressions.
For example, as classification of grasses is based largely on seed characters, Elias (1946) classified his well-preserved Miocene prairie grass seeds from the high plains of northern mid-America in a similar way.
Fossil pollen provides important data for reconstructing past vegetation. Pollen have evolved a very tough and resistant outer layer of sporopollenin, a durable organic polymer, which preserves well in the fossil record.
This has enabled the scientists to concentrate on the fossil pollen for study. They are preserved best if the sedimentary environment lacks oxygen or is acidic, conditions unfavourable for the organisms that decompose pollen.
Pollen falling into lakes or peat lands accumulates with other sediments, layer after layer, year after year. Some lake sediments typically lack oxygen necessary for decomposing organisms, and if the sediment remains wet, the pollen will be preserved for thousands or even millions of years.
Fossil pollen can be used for the following:
a. Fossil pollen is a very important kind of proxy data for reconstructing past climates, as vegetation is sensitive to climate. They can help reconstructing past environments for a number of reasons:
b. Pollen, which is released into the air, is transported some distance. The assemblage of pollen in the sediment of a particular region is representative of the vegetation of that region.
c. Abundant fossil pollen will be present in the sediment of virtually any permanent, natural lake. A cubic centimetre of lake sediment may contain tens or hundreds of thousands of pollen grains.
d. Scientists can reconstruct vegetation change over thousands of years, as pollen accumulates continuously, year after year. They can determine when and how rapid changes occurred. Like the annual ring of trees, some lakes even have annual layers that make it possible to reconstruct short-term climate variability over hundreds or thousands of years.
e. Paleopalynology (study of fossil pollen) helps in relative age dating (biostratigraphy).
f. The colouration and type of palynomorphs also provide information on the thermal maturity and type of organic material in the rock – important features for assessing hydrocarbon potential.
vi. Fossil DNA:
DNA sequence information from fossils can also help to resolve many phylogenetic questions relating to both recently and not so recently demised species. The potential of the recently developed technique of polymerase chain reaction (PCR) not only helps in the analysis of DNA samples from living organisms, but also has the potential to make gene sequences available from at least some recently fossilized organisms.
This is possible because of the surprisingly robust nature of DNA. DNA, like all other biological macromolecules, is very unstable, and spontaneously breaks down. In living cells, DNA is maintained by repair mechanisms, but after death DNA self- destructs at a rather rapid rate.
Thus, although fossilized material usually contains very little of its original DNA and whatever is present may be largely denatured, PCR allows to amplify many million-fold whatever small amounts of DNA is present, which can be further sequenced as if it were new.
Further, most fossil DNA is difficult to sequence because of oxidation of the bases, especially the pyrimidine’s and the sequence able fragments tend to be rather short, only a hundred or so base pairs long. Sykes (1991) has commented that in vitro estimates of the rate of spontaneous hydrolysis imply that no DNA would remain intact much beyond 10,000 years.
Hence, it seems feasible that useful DNA sequences tens of thousands of years old could be recovered, particularly if the fossil has been retained at low temperature.
Clearly therefore, the success of obtaining usable DNA sequences from fossils largely depends on the circumstances of preservation. Nevertheless, even with these and with carefully selected probes, it may be possible to obtain phylogenetically usable data.
Recently, for example, PCR has made it possible to obtain gene sequences from 17-20 million years old Magnolia leaves from lake bottom sediments of Miocene age deposits in Idaho, which had become buried in the lake under volcanic clay deposited rapidly creating a virtually anoxic environment.
Most of the DNA, which has been recovered is from plants preserved in amber, as for example from a Hymenaea leaf thought to be 25-40 million years old.
Limitations of Fossilization:
a. Fossilization almost invariably involves distortion, chemical degradation and replacement in addition to the normal processes of death and decay. Thus, fossils are seldom if ever perfect. Unfortunately the palaeontologist has to work on with whatever material is available and this often means interpreting less than ideally preserved fragmentary or distorted material.
b. As fossils are by their very nature incomplete representations of past organisms and often poorly preserved, interpreting what is a species from fossils is not easy. Further, it is highly unlikely that the fossils will display adequately the sorts of small differences that are so often used today to separate extant taxa.
Thus, fossil ‘species’ will only correspond to biological species notions if the latter happen to be well correlated with fairly obvious morphological differences. Hence, if fossils are to be classified up to species level, they may require a modified species concept.
c. Another problem, which a palaeontologist face is the fact that, the full details of how species arise over time, is not well understood from fossil data:
i. According to Hennigian philosophy and modern cladistic techniques, all speciation events involve dichotomous (or polychotomous) splits. This type of speciation, which, results from a lineage splitting into two (or more) separate species is referred to as cladogenesis (Fig. 8.30).
In contrast to this, many fossil records suggest that speciation can occur over time without any necessity for splits to occur. This type of speciation, which, results from accumulation of change over time in a single population is termed anagenesis.
ii. Secondly, according to Darwinian concept, the evolutionary change is likely to be a gradual process, with new species often forming over geologically active periods of time, which is referred to as phyletic gradualism.
If it is so, then any near complete fossil sequence would be expected to show a gradually evolving sequence of forms, in which case, it would be very difficult for any taxonomist to decide where to draw the line between species. Unfortunately, very few fossil series are complete enough to permit assessment of phyletic gradualism.
d. The incompleteness of the fossil record often poses as a drawback in including fossil data in cladistics analyses. Many data sets are incomplete and most extant species are still unknown to science.