paleontolgi microfossil

7/27/2019 paleontolgi microfossil

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PALEONTOLOGY

Microfossils

Microfossils are by far the most abundant of all fossils in the paleontological record. Microfossils,

moreover, generally survive the action of the drill bit, and quantities of them are retrieved from wellcuttings, whereas macrofossils become fragmented and are rendered generally unidentifiable by drilling

operations. Microfossils are most often found in the cuttings of fine-grained sediments like shales and

limestones, but are relatively rare in sandstone. Because various types of microorganisms flourished in

different depositional settings, their fossil remains provide extremely valuable indicators of sedimentary

environments.

The other maor application of microfossils is in biostratigraphy, as chronometers of geologic time. !aunal

evolution throughout the "hanero#oic $%ambrian to present& has allowed the ages of rock samples to be

determined and strata to be correlated by the recognition of the distinct species and assemblages of

microfossils they contain.

The two most important varieties of microfossils employed in environmental analysis are foraminifera and

ostracods. The main reason for this importance is that some suborders and genera of foraminifera andostracods are benthic $bottom dwellers& and live only in specific environments according to depth and

salinity. Thus, when found as an assemblage of fossils in the same place they lived, termed a biocoenose,

they can provide a direct indicator of depositional setting.

ForaminiferaForaminifera $'forams'& are the most widely used microfossils. These single-celled organisms

have existed since the (rdovician, with )*++ genera and +,+++ species $*,++ still in existence&

having been classified.

The skeleton, or test, of foraminifera averages +. mm in si#e and is distinguished by chambersinterconnected by an opening, or foramina

$!igure ) , Chamber construction in Dicorbis&.
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Figure 1

lthough they have a simple structure, foraminifera tests come in a wide variety of shapes $ !igure / ,Variety of shapes of foraminifera&.
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Figure 2

!oraminifera, both planktonic $floating& and benthic $bottom-dwelling& forms, inhabit a wide variety ofaquatic environments, from very shallow water to depths of +++ in, and in waters ranging from brackish to

hypersaline. They are notfound, however, in fresh-water lakes.

0t has been determined that the habitat of recent foraminifera depends largely on the composition of their

tests. Three basic groups of test composition are recogni#ed1

agglutinated 2 tests formed from material borrowed from the habitat1 grains of quart#

$arenaceous tests&, flakes of mica, clay material, and various skeletal debris. The material

0s bonded together by a secreted cement, either chitinoid $celluloselike& or calcitic. Therelative proportions of cement and agglutinate vary with species.

porcelaneous 2 opaque, calcareous tests that appear white and brilliant in reflected

light.

hyaline 2 calcareous tests characteri#ed by their glasslike transparency.

3alinity. Most benthic foraminifera are marine. 4owever, certain groups having porcelaneous tests live

equally well in hypersaline lagoonal environments. (ther types, such as certain agglutinates and hyalines

prefer brackish lagoons and estuaries. 3ome foraminifera may be found in all environments.

5ater 6epth.!igure $Depth distribution of recent benthic froaminifera& shows the depth distribution ofrecent benthic foraminifera.


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Figure 3

7enerally, benthic foraminifera with porcelaneous tests live in shallow waters, whereas hyaline tests occureverywhere but in deepest waters. gglutinated types similarly occur everywhere, with noncalcareous

benthic types surviving at great depths $up to +++ in&.

fourth group of foraminifera, characteri#ed by microgranular tests, flourished in the upper "aleo#oic but

are now extinct. !igure *$Stratigraphic range of some foraminiferan groups& ) illustrates the stratigraphic

range of the families within the four main groups.


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Figure 4

The standard reference on foraminifera is 8oeblich and Tappan $)9*:&. More recent works include 4aynes

$)9;)& and Bu#as and 3en 7upta $)9;/&.

Ostracods

Ostracodsare the most advanced forms of microfossils used. They are crustaceans and can beregarded as shrimps within a bean-shaped calcitic shell $!igure ) ,Lateral view of a podocopid ostracod,

without the left valve: A ! " # appendages$ a # anus$ c # carapace$ dg # digestive system$ e # eye$ f #

furca$ go # genital organs$ m # mouth %&''(&.


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Figure 1

5hen the ostracod dies, the body disappears, leaving the shell $carapace& to be fossili#ed $ !igure / ,)hotomicrographs of ostracod shells&. (stracods have existed since the %ambrian< their fossils abound in

sediments of all kinds but are particularly abundant in clays and marls.

Figure 2


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3hell si#e ranges from +.) to /+ mm for living species and up to ;+ mm for specimens found in "aleo#oic

rocks. (stracods inhabit every aquatic environment, both marine and fresh water. Many varieties are

benthic and prefer quiet, still waters where finegrained sediments are rich in organic material. Basicallythree broad assemblages are recogni#ed based on sensitivity to salinity1

fresh water*generally with thin, smooth shells.

marine* robust shells, often ornamented.

brac+ish water* various shell markings, depending on salinity.

ssemblages of marine benthic varieties can also be used to give marine water depth< shelf $less than /++

in&, bathyl $/++-++ m&, and abyssal $greater than ++ in&.

!or additional information on ostracods, the reader is referred to =an Morkhoven $)9:/-& and Bate,

>obinson, and 3heppard $)9;/& .

(ther common Micro !ossilss summari#ed by Bignot $)9;&, other common microfossils, some of which have value as environmental

indicators, include calcareous nanofossils, radiolarians, diatoms, conodonts, spores and pollens, and

dinoflagellates.

Calcareous nanofossils2 minute $) to micron& rosette-, star-, or button-shaped plates called coccoliths

2 are the most common variety $!igure ) ,Discoasterid calcareous nannofossils and other presumed

coccoliths&.


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Figure 1

Because of their small si#e, they are best studied with the electron microscope. !ossil coccoliths, common

since the ?urassic, are found mostly in marine sediments deposited far from coasts. @o coccoliths have been

found in lacustrine sediments.

adiolarians2 marineplanktonic microorganisms, whose preserved siliceous skeletons consist ofneedlelikespicules2 are found in rocks ranging from the (rdovician to >ecent< the presence of

radiolarian microfossils indicates marine deposition in waters of normal salinity $ !igure / ,Different types

of siliceous radiolarians&.


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Figure 2

Diatoms are single-celled algae whose siliceous microfossils date from the 8ate %retaceous $ !igure ,

Different types of diatoms&.


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Figure 3

They are found in all types of aquatic environment regardless of salinity< many varieties are benthic, andmost species are subect to strict ecological controls. !ossil diatoms should thus be expected to "rovide

information on depositional environments.

Conodonts are small structures, averaging ) mm in length and consisting of calcium phosphate, that appear

to be the teeth or hooks of larger organisms

$ !igure * 6iagram of a composite conodont&. They occur only in "aleo#oic and Triassic marine

sedimentary rocks, both in near-shore and glauconitic sands and in deeper, fine-grained facies.


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Figure 4

Spores and pollens originate from higher land plants, and generally indicate continental paleoenvironments.

4owever, because they are subect to wide dispersal by both wind and water currents, they may also be

found in a wide variety of marine deposits. %ommon since the 6evonian, spores and pollens provide

excellent biostratigraphic markers.

Dinoflagellates are single-celled algae whose fibrous cellular walls are preserved as fossils, termed cysts $

!igure ,Different types of dinoflagellates&.


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Figure 5

6inoflagellate cysts range in si#e from :+ to )+ microns, and in age from "ermian to >ecent.6inoflagellates occur as plankton in the surface waters of all environments ranging from marine to lagoonal

to lacustrine. Because of their wide dispersal, they are not reliable indicators of depositional environment.

By way of summary, !igure : $Simplified classification of a-uatic environments and salinity ranges for

some groups of living microorganisms& illustrates the salinity ranges for some common groups of living

organisms that provide fossils in the geologic record.


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Figure 6

Trace Fossils.race fossils constitute another valuable type of biological information that can be used in environmentalanalysis. 3ubsurface information on trace fossils, however, may not always be available. Anlike microfossil

data easily obtainable from well cuttings, subsurface information on trace fossils can only be derived fromexpensive conventional cores. Trace fossils, produced by bioturbation, are simply fossili#ed tracks, trails,

burrows, etc. made by animals within sediment or on the sediment surface. They provide a reliable record

of benthonic organic communities because, unlike other fossils, they always occur in situ and cannot be

reconcentrated by reworking. These structures also include plant trace fossils, like root molds and casts.

3eilacher $)9:& devised a series of ichnofacies based on the observation that certain types of animal trace

fossils characteri#e particular environments $!igure ), characteristic ichnofacies for variousenvironments&.


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Figure 1

Cach ichnofacies consists of a suite of trace fossils which can be used as an indicator of bathymetry.

S+olithos ichnofacies consist of vertical burrows made in sandy or firm mud bottoms of the littoral$intertidal& #one by suspension feeders, i.e., by organisms that feed on food particles suspended in the

agitated #one of shallow water.

Cru/iana and/oophycos ichnofacies consist of more hori#ontal and increasingly patterned burrows made in

soft muds of the neritic #one by sediment feeders.

0ereites ichnofacies consist of crawling traces made in the soft muds of the bathyl #one by sediment-

feeding organisms.

lthough the animals responsible for the trace fossils have undergone evolution, these ichnofacies have

remained remarkably constant through "hanero#oic time.

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