-mating population, the genotype and allele frequencies

Hardy Weinberg Law

“In a large, random-mating population, the genotype and allele frequencies remain constant in the absence of any evolutionary influences from one to another generation. Influences are inclusive of a choice of mate, natural selection, genetic drift, mutation, sexual selection, gene flow, genetic hitchhiking, founder effect, meiotic drive, population bottleneck, inbreeding and assortative mating.”

Genotype frequencies and allele frequencies are related to each other in a way that it is the square expansion of such allele frequencies. In other words, the law conveys that in a population, it is possible to estimate the expected frequencies of genotypes under a certain limited set of

assumptions, provided the frequency of different alleles in a population is already known.

Take a case of a single locus with only two alleles indicated by A and a with corresponding frequencies f(A) = p and f(a) = q respectively, then the genotype frequencies that can be expected

under limited condition being random mating is

f(AA)= p2 for AA homozygotes

f(aa) = q2 for aa homozygotes

f(Aa) = 2pq for heterozygotes

The Hardy Weinberg Equation can be represented by

p2 + q2 + 2pq = 1

The allele frequencies p and q remain constant in the absence of any kind of influences such as mutation, natural selection, genetic drift, etc from one to another generation. This is how the equilibrium can be reached.

Also Check: MCQs on Hardy Weinberg Law

Who Proposed The Law?

The law is named after G.H. Hardy and Wilhelm Weinberg. They were pioneers in mathematically

illustrating this principle also referred to as Hardy–Weinberg equilibrium, theorem, law or model.

Hardy’s thesis centrally paid attention to debunk the view that prevailed in those times that a dominant allele has the tendency to increase in frequency automatically. In today’s times, the uncertainty on selection and dominance is not very remarkable. In the current times, the Hardy-Weinberg genotype frequencies tests are applied to evaluate population stratification and other sorts of non-random mating.

Inferences From Hardy–Weinberg Law

Listed below are a few deductions from the law:

• Only sexual reproduction can take place

• Process of mating is random

• The size of the population is indefinitely large

• Entities are diploid

• Generations do not overlap

• Equality of allele frequencies in terms of sexes

• No traces of gene flow, selection, mutation, migration or admixture

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In case there is any breach with regard to the above-mentioned assumptions, it can lead to discrepancies from the expected outcome. The consequences are completely dependent on the

deduction that has been digressed.

The law mentions that the population shall have the Hardy Weinberg proportions (given genotypic frequencies) once a single generation of random mating is carried out in a population. In case the assumption of random mating is breached, this population will not possess the Hardy Weinberg proportions. The most common source of a non-random mating is inbreeding. It leads to the rise in the homozygosity of all genes.

Breaching any one of these 4 assumptions can cause the population at each generation to still possess the Hardy–Weinberg proportions, however, with time, there will be a change in the allele

frequencies.

Mutation – it has a mild impact on the allele frequencies. The rate of mutation is in this o order 10 -

4 to 10-8. Mostly, modifications to the allele frequencies are of this order. Even if there persists a

sturdy selection against the alleles in the population, recurrent mutations will conserve it.

Selection – typically this leads to a change in the allele frequencies and is a rapid one. Few types of selections, the selected ones can result in equilibrium with no loss of alleles, namely balancing selection, while some other selections such as directional selection can gradually result in the loss of alleles.

Size of the population being small can lead to a random alteration in the allele frequencies which can be attributed to the sampling effect known as genetic drift. When alleles are found in a fewer copy, sampling effects are significant.

Migration – two or more than two populations can be associated together, genetically with migration. Here, amongst the populations, the allele frequencies have the tendency to become more homozygous. Essentially, a few migration models are the Wahlund effect (non-random mating). Hardy–Weinberg proportions typically are invalid for such models.

Applications of the Hardy-Weinberg Principle

Natural populations persistently depict genetic variation altering from mutation, genetic drift, migration, sexual selection and natural selection. The Hardy-Weinberg law provides a mathematical criterion of that of a population that is non-evolving which can be compared to evolving populations. Over time, if the allele frequencies are noted and estimated for the expected frequencies basis the values of Hardy-Weinberg law, then workings that drive the evolution of the population can be hypothesized.

The law offers a prototype which is typically used as a point of origination to study the population genetics of diploid entities, which fulfil the fundamental assumption of random mating, large population, no mutation, migration or selection.

However, the Hardy-Weinberg model is not applicable to haploid pathogens. In the event of a population not being found in Hardy-Weinberg equilibrium, one of the assumptions in this law then gets violated. This conveys that selection, non-random mating or migration has influenced the population, in which case experiments are carried out and hypotheses are advanced in order to understand the reasons behind the non-equilibrium of the population.

I. Complete Dominance

Allele frequencies can be detected in the presence of complete dominance when Hardy-Weinberg equilibrium prevails wherein it is not possible to differentiate between two genotypes. Two genotypes AA and Aa having the same phenotype as a result of complete dominance of A over a, can help determine the allele frequencies from frequencies of the individuals indicating recessive phenotype aa. Here, the frequency of aa individual should be equivalent to the square of the

frequency of the recessive allele.

II. Multiple Alleles


Calculation of genotypic frequencies at a locus with more than two alleles is allowed in the Hardy-Weinberg principle, for instance in the ABO blood groups. Three alleles are present in IA, IB, IC with p,q and r frequencies respectively where p + q + r = 1. With random mating, the genotype of a population will be given by (p + q + r)2

III. Linkage Disequilibrium

Take, for instance, two or more alleles on the same chromosome, at two different loci with 2 or more alleles. As a result of genetic exchange by recombination taking place at regular time

intervals, at two syntenic loci, the frequency of allelic combinations attains equilibrium.

In the event of not being able to attain an equilibrium, alleles are known to be in a linkage disequilibrium, which is as a result of two or more linked alleles to be inherited jointly, more frequently than expected. Such gene groups are also known as supergenes.

IV. Frequencies If Harmful Recessive Alleles

The law can also be applied to estimate the frequency of heterozygous carriers of recessive genes that are harmful. In a population, two alleles, A and a are at an autosomal locus with p and q frequencies respectively, and p + q = 1, then AA, Aa and aa genotypes will have the following frequency, p2 + q2 + 2pq. In case, the aa genotype tends to express a phenotype that is harmful, such as cystic fibrosis, then in the population, the proportion of the affected individuals shall be q2, the recessive allele frequency of the heterozygous carrier shall be 2pq.

Summary

• In a given population, the Hardy-Weinberg principle assumes that the population is indefinite and not influenced by sexual, natural selection, mutation and migration.

• Frequency of alleles can be calculated by the frequency of recessive genotypes. Then estimate the square root of this frequency to find the frequency of the recessive allele

• In a population, the frequency of alleles can be indicated by p + q = 1, with p = frequency of the dominant allele and q = frequency of the recessive allele.

• In a population, the frequency of alleles can be indicated by p2 + q2 + 2pq = 1, where p2 is the frequency of homozygous dominant genotype, q2 is the frequency of recessive genotype and 2pq is the frequency of heterozygous genotype.


SPECIATION

Evolution is the successive modification in inherited traits over a huge span of time, usually over generations. The theory of evolution was first proposed by an English biologist named Charles Darwin. In 1859, he mentioned about evolution in his book ‘The Origin of Species’.

Charles Darwin noted that living organisms change their physical and anatomical structure over a long period of time for better adaptations to the changing environment. The change is by natural process and those organisms which do not adjust to it, find it difficult to survive. This put forward the concept of natural selection and Darwin called it ‘Survival of the fittest’. Speciation is an evolutionary process which resulted in natural selection.

Also Read: Anagenesis

Let’s learn more about the concept of speciation and factors affecting speciation.

Speciation Definition

“Speciation is the process of formation of new species from existing populations.”

What is Speciation?

A species is a group of organisms with similar characteristics and can interbreed to give fertile offspring. Speciation is an evolutionary process of the formation of new and distinct species. The species evolve by genetic modification. The new species are reproductively isolated from the previous species, i.e. the new species cannot mate with the old species.

Also Read: Concept of Species

Speciation Types

There are two major types of speciation:

Allopatric Speciation

Allopatric speciation is the type of speciation caused by geographical isolation. In this, the population is separated by a physical barrier.

Parapatric Speciation

This is a type of allopatric speciation in which the species are not formed by any physical barrier. Instead, they are beside each other. This occurs by an extreme change in the habitat. Though the individuals in these areas can interbreed, they develop different characteristics and lifestyles.

Peripatric Speciation

This is a type of allopatric speciation in which new species are formed from an isolated peripheral population. In this, the populations are prevented from exchanging genes Therefore, it is difficult to distinguish between them.

Sympatric Speciation

It refers to the evolution of new species from the surviving ancestral species in which both the species continue to live in the same geographical region.

Factors Affecting Speciation

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There are several factors which lead to speciation. Two of them are:

Geographical Isolation

Due to some geographical changes, few members of a species get isolated from other members. Later, this isolated group grows in a different land and eventually evolves as a new species with new adaptations according to its environment. Natural selection and genetic drift have a major role to play in this.

Also Read: Natural Selection

Let’s understand this concept with an example.

Suppose earlier there was a species of flies living on land A. Some of them fed on dead animals. Evidently, there was a flood which washed off the dead animals and the flies feeding on them. Consequently, a few groups of flies get isolated from the other members to another land B. The species on land A and land B are too far to unite. Moreover, the environmental conditions in land B are different from those on the mainland A. The groups of flies which now live inland B start to adapt themselves according to their environmental conditions. Gradually, the individual’s structure and functions alter to give rise to a new species. This is speciation.

The new species are different from the flies in mainland A as well as from the flies who got introduced to land B by the flood. Even if this new species was reintroduced to the mainland A, they would not mate with those flies. New species start to mate amongst themselves. Thus a population of new species arrives.

Hybridization

Hybridization is an artificial method of developing a new species. In animal husbandry, two parents from different species are mated to form a third species. Hybridization has numerous and various impacts on the process of speciation.

There are many hybrid animals, which have been crossed between the same species and the

genus. Below is the list of a few successfully crossed hybrid animals:

• Zebroid- It is a hybrid cross between a male zebra (Equus quagga) and a female donkey (Equus asinus) or with any other female members of the horse family.

• Liger – It is a hybrid cross between a male lion (Panthera leo) and a tigress (Panthera tigris)

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ISOLATION

Isolation, in evolutionary term, means segregation of different populations into smaller

units by certain mechanism so as to prevent interbreeding among individuals.

1. Geographical isolation

When the populations are separated by a geographical barrier, such as river, sea,

mountain, deserts and for aquatic animals land, they are physically prevented from

interbreeding. Such populations are termed as allopatric and are forced to evolve

independently and accumulate genetic differences. Geographical isolation may be

different for different species. For example, a small stream may be an effective barrier

for land insects and small mammals while for birds even mountain and oceans may not

be barriers.

2. Reproductive isolation

It is the property of individuals that prevents interbreeding in populations that are

actually sympatric (living in the same area).

Classification of reproductive isolating mechanisms

A. Premating mechanisms

They prevent interspecific crosses in sympatric populations.

1. Seasonal isolation: Also called temporal isolation, in which potential mates do not

come in contact with each other because of differences in breeding seasons of two

species, e.g. different flowering seasons in plants. Bufo americanus breeds in early

rainy season (May), while Bufo fowleri breeds in late rainy season (July) in USA.

2. Habitat isolation: Also called Ecological isolation, in which also potential mates do

not meet each other due to differences in habitats, requirements of food, space, climate

etc. Potential mates live in different areas and therefore do not come in contact with one

another. For example spawning grounds of riverine fishes are in different tributaries,

which prevents interbreeding.


3. Ethological isolation: It is a behavioral isolation, in which potential mates meet but

cannot mate, due to differences in courtship displays or other specific signals that are

necessary rituals before mating. The signals may be of the following three types, which

stimulate the opposite sex for mating:

(a) Visual stimuli: Feather displays and dancing in male birds is necessary to attract

the female, e.g. peacocks, pheasants and birds of paradise. The colour and shape of

the feathers as well as display pattern is so unique for each species that mating

between two different species is not possible. Collection of the nest material and

construction of the nest as by the weaverbird male is also a very specific display that

cannot be imitated by the other species.

(b) Auditory stimuli: Songbirds like cuckoos, mynas, nightingales, parakeets etc. use

auditory signals to attract the opposite sex. Sometimes the singing goes on for several

days before the pair can actually come together for mating. Auditory communication is

used by a large number of animals, viz. frogs, toads, cicadas, gibbons, monkeys,

jackals etc.

(c) Chemical stimuli: This includes odors of the animals that attract the opposite sex

for mating. For example scent of musk deer and musthing in elephants attract the

females. In insects, particularly Lepidoptera, females produce highly specific

pheromones that can be detected by the highly specialized antennae of males from a

distance of about 2 kilometers.

4. Mechanical isolation: In this case the above isolating mechanisms are not present

and therefore mating is attempted but is not successful due to mechanical problems

such as differences in the structure of genitalia. Dufour (1844) described “Lock and key

mechanism” in the genitalia of insects. In the species of Drosophila the genitalia are so

different that copulation is mechanically not possible.

B. Postmating mechanisms

These reduce the success of interspecific crosses. In case premating mechanisms fail

to prevent mating then several postmating mechanisms prevent the success of mating

and hybridization. There are 4 such mechanisms, which are given below:


1. Gamete mortality: Mating and sperm transfer takes place but egg is not fertilized.

In Drosophila vaginal wall swells killing spermatozoa should interspecific crosses take

place. If mating takes place between Bufo fowleri and Bufo valliceps, sperms cannot

penetrate the egg membrane of each other, leading to mortality of gametes.

2. Zygote mortality: Egg is fertilized but the zygote dies. Eggs of many species of

fishes may be present in the spawning grounds and some may be fertilized by the

sperms of different species forming zygote but such zygotes fail to develop due to

differences in chromosomes.

3. Zygote inviability: Zygote develops and hybrid is produced but is physically weak

and inviable due to physiological disturbances in the body. It fails to survive for long and

prematurely dies. Such cases have been recorded in different species of ducks.

4. Hybrid sterility: Hybrid is viable, physically strong and physiologically sound but is

sterile due to differences in chromosomes and different gene arrangements. Mule is a

cross between male donkey and female horse and Hinny between female donkey and

male horse and both are sterile, albeit physically strong.

Sometimes all isolating mechanisms break, leading to fertile hybrids, which are

generally not reproductively isolated from the parents and can produce fertile offsprings

by Introgression (hybrids backcrossing with parents to produce fertile offsprings). This

will be instant speciation.

Significance of isolating mechanism

• Wasteful courtship is avoided. If isolating mechanisms are distinct and specific only

individuals of the same species indulge in courtship.

• Isolating mechanism protects gene pool of a species and prevents hybridization.

• It prevents wastage of gametes and energy.

• A weak isolating mechanism leads to production of new species through

hybridization.

• Absence of isolating mechanism leads to production of new species by instant

speciation.


• Geographical isolation followed by reproductive isolation ultimately leads to

production of new species.

• Isolating mechanisms protect the identity of a species, which all species fiercely

guard.


ARIYALUR FOSSIL SYSTEM

STUDIES ON FOSSILS OF ARIYALUR, TAMILNADU

ABSTRACT

INTRODUCTION

A fossil is the impression of parts of a dead organism in a sample of rocks. A wide

range of things can be fossilised such as plants or animals and in some cases their footprints

or tracks. Fossils are the preserved remains or traces of animals, plants, and other organisms

from the remote past. The totality of fossils both discovered and undiscovered and their

placement in fossiliferous rock formations and sedimentary layers is known as the fossil

record. They help us to understand about animals and plants that lived in the past. They also help

to date the different parts of earth and rocks lie in layers. Animals found in the same rock layer

obviously lived and died at about the same time. Some animals alive today are not found in

rocks 400 millions ago, such as birds. Some have been found as ancient fossils, and yet are still

alive today, such as brachiopods.

India, once a member of the lost supercontinent Gondwana, broke away from it and made

a solitary northward excursion and finally collided with Asia. During its long voyage, India

remained isolated for 100 million years and is expected to be characterized by stunning

endemic biodiversity. But this is not recognized by the terrestrial faunal and floral content, and

their distribution patterns. For example, the Inter-trappean vertebrate faunas of India, which

lived during “India-in-exile”, do not show any made-in-India assemblages, but rather betray

a mixed biota having both Gondwanan and Laurasian affinities. These differential distribution

patterns of fauna and flora, and their affinities with those of other areas, prompted many workers

to envisage an array of suggestions regarding the time of India's final separation from Gondwana,

the time of northward drifting and different palaeopositions during its long journey. But closer

examination of the nature of the vertebrate fossil records reveals that the so-called exclusive

endemicity of Indian fauna during its sojourn is in fact a product of taxonomic artefact.

The majority of the faunas have been described on the basis of poor fossil data, and

comparisons for biogeographic correlations are made at higher taxonomic levels, which

perhaps masked India's faunal distinctiveness. Yet, the biological processes that constrain


biogeographical distribution operate at the species level. In the present study, fossils of Ariyalur,

Trichy was investigated.

OBJECTIVES OF THE PRESENT STUDY

1. To survey the fossils available in the Ariyalur area.

2. To understand the evolutionary significance of the animals.

3. To relate fossil forms with that of evolutionary period.

4. To understand the period in which the fossils are flourished.

MATERIALS AND METHODS

Overview of Ariyalur fossil (Study area)

Ariyalur is a small town in Tamil Nadu, southern India where many cement

manufacturing units are situated that depend on the limestone raw material. It is about 60 km in

the direction of Northeast from the temple town of Tiruchirappalli (Fig. 1).

The Cretaceous Formation of the Ariyalur area (Ariyalur District, Tamil Nadu) is one

of the best- developed sedimentary sequences in South India. Blanford (1862) was the first to

work on the stratigraphy of this formation and he divided the litho-units into three groups:

Uttatur, Trichinopoly and Ariyalur. The geology and the stratigraphy of this area are

accounted by many workers (Rama Rao, 1956; Ramanathan, 1968; Banerji, 1972, Sastri et

al, 1972; ONGC, 1977; Sundaram and Rao, 1979; Ramasamy and Banerji, 1991; Banerji et

al, 1996; Gonvindan et al, 1996).

Ramasamy and Banerji (1991) have revised the stratigraphic framework of the exposed

Pre-Ariyalur sequence based on detailed lithological and petrographical variations. Banerji et al.,

(1996) have redefined the Uttatur Group and identified within it four distinct formations

comprising reefoidal bodies, sandy clay, coarse sand bar and gypsiferous silty- clay units.

Madavaraju (1996) has presented a detailed geochemical and petrographical account of

Ariyalur Group of sediments and Kallamedu Formation is the youngest unit of this group. The

sedimentary rocks of Cretaceous – Palaeocene age are well developed in the Ariyalur area, which

consists both clastic and carbonate facies.

According to Chandrasekar, (2000) the limestone sediments at Ariyalur and Perambalur

were of a marine origin, and were deposited there by a rare geological event called marine


transgression. “The only other example of this phenomenon is found at Wales, United Kingdom.

The sea seems to have invaded the land between Puducherry in the north to Karaikal in the

south millions of years ago, stayed put for around 81 million years, and then for reasons

unknown, seems to have regressed to its present location. Having covered over one lakh

hectares, the sea has left behind fossilised traces of marine life making the region a geologist‟s

treasure trove.

Cement city

The 25 kilometre stretch with a depth of 20 metres between Kadur and Yelaakurichi

villages in Ariyalur is said to be the deepest part of the sea. The limestone, gypsum and

phosphatic nodule repositories here form the primary source of raw material for a thriving

cement industry. The eight major cement factories in Ariyalur alone, account for nearly 80

per cent of Tamil Nadu‟s cement production. Consuming tones of limestone every day, the

industries sometimes destroy rare pieces of fossils as well. The nearly 320 types of fossils found

here are also high grade limestone, and the sedimentary deposits costs much less to grind as

against the metamorphic limestone deposits found elsewhere in the state. There are three

traverses which include: I. Karai – Kulattur Traverse; II. Chittali – Kunnam Traverse; III.

Kunnam – Periyanagalur Traverse.

Fig. 1. Arialur District map

Method of collection of fossils


The materials needed for collecting fossils depends on the location and the rock type.

Some fossils are found in hard rocks and other in loose soil. When collecting fossils in

rocks geological hammer and chisels were used. For very soft rocks, a coarse saw and shovel

were used to extract the fossils. A shovel was used. After collected the fossil, it was packed

well for transportation. T he details of location and the specific geologic layer were noted and

fossils were identified. These fossils were collected from Ariyalur surroundings (Kunnam,

Odiyam and Kaattupringiam) during the period of January 2014 to August 2014 (Fig. 2).

Fig. 2. Sampling stations: Arialur fossil mine

RESULTS AND DISCUSSION

The fossils were collected from Ariyalur sourrounding (kunnam, odiyam and

kattuperingium area of Ariyalur) during the period of January 2014 to August 2014. The

identified fossils their classification and their evolutionary significance has been presented. It may

be stated that the cretaceous period was ranged 145 million years ago until 66 million years ago.

The dinosaurs became extinct at the end of the Cretaceous period almost certainly because

of a large meteorite impact. Ammonites were flourishing during the Cretaceous, but they also

became extinct. Mammals and flowering plants began developing.

1. Nautilus


Fig. 3 Nautilus

The order of Nautilida belongs to the class of ink fish like animals (Cephalapoda) and the

phylum Mollusca. The "sipho" (tube which connect the chambers) is centered in the middle of

the chambers. The animal can go up and down in the water column by regulating the amount of

water in the chambers. The "suture lines" are less developed than in ammonites. They occur in

sediments from the end of the Cambrian period until recent.

2. Rastellum gregareum

Fig. 4 Rastellum gregareum

Rastellum gregareum from the Upper Jurassic period fossil (150 Million years ago)

and this specimen has the ridges coming from a more central line as opposed to along the

edges. This one must have anchored to another clam shell as it seems to have grown upwards

from the base and reminds of a flower pot. (Sowerby, 1815) identified in Vaches Noires (France).

Some species get long and curved and are nicknamed "Denture Clams".

3. AMMONITES


Fig. 5 Ammonite

Ammonites are the most widely known fossil; they are cephalopods and first appeared

in the seas 415 million years ago, in the form of a straight shelled creature known as Bacrites.

During their evolution three catastrophic events occurred. The first during the Permian period

(250million years ago), only 10% survived. They went on to flourish throughout the Triassic

period, but at the end of this period (206 million years ago) all but one species died. Then they

began to thrive from the Jurassic period until the end of the Cretaceous period when all species

of ammonites became extinct. Ammonite fossils are found on every continent in the world.

Because of their rapid evolution and wide spread distribution they are an excellent tool for

indexing and dating rocks.

4. Rastellum carinatum

Fig. 6 Rastellum carinatum

Rastellum carinatum has wide, angled ribs that have led to it being called the 'denture

clam'. The zig-zag join between the two shells stopped coarse dirt and debris entering the

shell and damaging its soft body. Like modern oysters it lived in shallow coastal waters including

the intertidal zone during Cretaceous (late), cenomaninan (early) period (144 - 65 million years).

5. Sea urchin

Fig. 7 Sea urchin


Sea urchin belongs to the phylum of Echinodermata. They have a radial symmetry of

five. Sea urchins live on the seafloor. The shell of sea urchins often in well preserved .The

shell is made of calcite. The spines usually fall of and are found separately. Sea urchins first

occurred in the Ordovician period. There are two types of sea urchins. The regular sea urchins

are totally five radial symmetrical. Their anus is on top, and their mouth is on the bottom.

Irregular sea urchins have their mouth and anus both on the bottom. These sea urchins are less

symmetrical.

6. Astarte legans

Fig. 8 Astarte legans

Astarte legans are bilaterally symmetrical and show no torsion. Head is greatly reduced;

no tentacles but have a foot that can often be seen sticking out of the valves. The two valves are

joined by hinges and other structures that help the valves to open and close. They are completely

aquatic. Bivalve has been around for a long time, over 540 million years ago.

7. Pseudopectin equivalvis

Fig. 9 Pseudopectin equivalvis

Pseudopectin equivalvis was a lower Jurassic period fossil which is similar one

identified by Sowerby, 1815. The shell of a marine bivalve, a type of mollusc and


ecologically it is facultatively mobile low-level epifaunal suspension feeder. The anterior and

posterior areas are smooth. According to Hallam (1987), this distinct species has a long

stratigraphic range from the early Sinemurian 190-197 million years to the end of the Pliensbachian

period (183-190 million years of lower Jurasic period).

8. Astarte gueuxi

Fig. 10 Astarte gueuxi

It is small brownish bivalves, usually sculptured with concentric furrows. The ligament is

external, lunule distinct. Soft parts commonly brightly colored. There are many species,

distributed chiefly in cool areas. An elongate shell, sturdy and strong. Anterior end rounded

and short, posterior long and broadly rounded. Surface with strong, flattened radiating ribs,

scaly on posterior.

9. Gryphaea

Fig. 11 Gryphaea

Grypheaea is characterized by its distinctively convoluted shape. These fossils ranged

from Jurassic to Cretaceous periods (between 199.6 million and 33.9 million years ago). The left


valve, or shell, was much larger and more convoluted than the flattish right valve. Fine markings

extended across the shell at right angles to the direction of growth.

10. DINOSAUR’S EGG

Fig. 12 Dinosaur’s egg

It is interesting to note that Geologists have found cluster of fossilized dinosaur eggs, said

to be 65 million years old, in a village near sendurai taluk of Ariyalur District in TamilNadu. The

cluster of eggs, of what is believed to be the most aggressive Carnosaur, buried in a river bed in

the village, were discovered by researchers from Periyar University, Salem; Bharathiyar

University, Coimbatore, and Bharathidasan University, Trichy. Layer upon layer of spherical

eggs and body parts of dinosaur and each cluster contain eight eggs were reported. The eggs,

about 13-20cm (5-8 inches) in diameter and lying in sandy nests about 1.2m wide (4feet), were

discovered during a study funded by Indian and German scientific institutions also.

The Kallamedu formation has been known to contain the remains of dinosaur since the

report of the collection, by Blanford (1862), of an isolated tooth and ill-preserved bones from

the area north of Kallamed village. Yadagiri and Ayyasami (1987), excavated the fossil remains

of the carnosaur Bruhatkayosaurus matleyi from the sediments of the Kallamedu Formation.

In the present study numerous dinosaurs eggs were collected in the same area.

Conclusion

The present study revealed that number of dinosaur’s egg, ammonites, bivalves and


cephalopods of mollusks were excavated in the Ariyalur mines which indicated that the area might

have been submerged under the sea. These organisms might have been deposited during the rare

geological event called marine transgression. It is suggested to preserve the rare fossils of

Ariyalur, which may through light on evolution.

BIOJOURNAL JUNE – 2015 Rajan et

al.

-mating population, the genotype and allele frequencies

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