PALEONTOLOGYMicrofossils

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 well cuttings, 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 major application of microfossils is in biostratigraphy, as chronometers of geologic time. Faunal evolution throughout the Phanerozoic (Cambrian 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 and ostracods 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 Ordovician, with 1400 genera and 30,000 species (4,500 still in existence) having been classified.

The skeleton, or test, of foraminifera averages 0.5 mm in size and is distinguished by chambers interconnected by an opening, or foramina ( Figure 1 , Chamber construction in Dicorbis).

Figure 1

Although they have a simple structure, foraminifera tests come in a wide variety of shapes ( Figure 2 , Variety of shapes of foraminifera).

Figure 2

Foraminifera, both planktonic (floating) and benthic (bottom-dwelling) forms, inhabit a wide variety of aquatic environments, from very shallow water to depths of 5000 in, and in waters ranging from brackish to hypersaline. They are not found, however, in fresh-water lakes.

It has been determined that the habitat of recent foraminifera depends largely on the composition of their tests. Three basic groups of test composition are recognized:

agglutinated — tests formed from material borrowed from the habitat: grains of quartz (arenaceous tests), flakes of mica, clay material, and various skeletal debris. The material Is bonded together by a secreted cement, either chitinoid (celluloselike) or calcitic. The relative proportions of cement and agglutinate vary with species.

porcelaneous — opaque, calcareous tests that appear white and brilliant in reflected light.

hyaline — calcareous tests characterized by their glasslike transparency.

Salinity. Most benthic foraminifera are marine. However, certain groups having porcelaneous tests live equally well in hypersaline lagoonal environments. Other types, such as certain agglutinates and hyalines prefer brackish lagoons and estuaries. Some foraminifera may be found in all environments.

Water Depth. Figure 3 (Depth distribution of recent benthic froaminifera) shows the depth distribution of recent benthic foraminifera.

Figure 3

Generally, benthic foraminifera with porcelaneous tests live in shallow waters, whereas hyaline tests occur everywhere but in deepest waters. Agglutinated types similarly occur everywhere, with noncalcareous benthic types surviving at great depths (up to 5000 in).

A fourth group of foraminifera, characterized by microgranular tests, flourished in the upper Paleozoic but are now extinct. Figure 4 (Stratigraphic range of some foraminiferan groups) 1 illustrates the stratigraphic range of the families within the four main groups.

Figure 4

The standard reference on foraminifera is Loeblich and Tappan (1946). More recent works include Haynes (1981) and Buzas and Sen Gupta (1982).

OstracodsOstracods are the most advanced forms of microfossils used. They are crustaceans and can be regarded as shrimps within a bean-shaped calcitic shell ( Figure 1 , Lateral view of a podocopid ostracod, without the left valve: A1 - 7 = appendages; a = anus; c = carapace; dg = digestive system; e = eye; f = furca; go = genital organs; m = mouth (x100)).

Figure 1

When the ostracod dies, the body disappears, leaving the shell (carapace) to be fossilized ( Figure 2 , Photo micrographs of ostracod shells). Ostracods have existed since the Cambrian; their fossils abound in sediments of all kinds but are particularly abundant in clays and marls.

Figure 2

Shell size ranges from 0.15 to 20 mm for living species and up to 80 mm for specimens found in Paleozoic rocks. Ostracods 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. Basically three broad assemblages are recognized based on sensitivity to salinity:

fresh water — generally with thin, smooth shells.

marine — robust shells, often ornamented.

brackish water — various shell markings, depending on salinity.

Assemblages of marine benthic varieties can also be used to give marine water depth; shelf (less than 200 in), bathyl (200-500 m), and abyssal (greater than 500 in).

For additional information on ostracods, the reader is referred to Van Morkhoven (1962-3) and Bate, Robinson, and Sheppard (1982) .

Other common Micro Fossils As summarized by Bignot (1985), other common microfossils, some of which have value as environmental indicators, include calcareous nanofossils, radiolarians, diatoms, conodonts, spores and pollens, and dinoflagellates.

Calcareous nanofossils — minute (1 to 35 micron) rosette-, star-, or button-shaped plates called coccoliths — are the most common variety ( Figure   1 , Discoasterid calcareous nannofossils and other presumed coccoliths).

Figure 1

Because of their small size, they are best studied with the electron microscope. Fossil coccoliths, common since the Jurassic, are found mostly in marine sediments deposited far from coasts. No coccoliths have been found in lacustrine sediments.

Radiolarians — marine planktonic microorganisms, whose preserved siliceous skeletons consist of needlelike spicules — are found in rocks ranging from the Ordovician to Recent; the presence of radiolarian microfossils indicates marine deposition in waters of normal salinity ( Figure   2 , Different types of siliceous radiolarians).

Figure 2

Diatoms are single-celled algae whose siliceous microfossils date from the Late Cretaceous ( Figure   3 , Different types of diatoms).

Figure 3

They are found in all types of aquatic environment regardless of salinity; many varieties are benthic, and most species are subject to strict ecological controls. Fossil diatoms should thus be expected to Provide information on depositional environments.

Conodonts are small structures, averaging 1 mm in length and consisting of calcium phosphate, that appear to be the teeth or hooks of larger organisms

( Figure   4 Diagram of a composite conodont). They occur only in Paleozoic and Triassic marine sedimentary rocks, both in near-shore and glauconitic sands and in deeper, fine-grained facies.

Figure 4

Spores and pollens originate from higher land plants, and generally indicate continental paleoenvironments. However, because they are subject to wide dispersal by both wind and water currents, they may also be found in a wide variety of marine deposits. Common since the Devonian, spores and pollens provide excellent biostratigraphic markers.

Dinoflagellates are single-celled algae whose fibrous cellular walls are preserved as fossils, termed cysts ( Figure   5 , Different types of dinoflagellates).

Figure 5

Dinoflagellate cysts range in size from 60 to 150 microns, and in age from Permian to Recent. Dinoflagellates 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, Figure   6 (Simplified classification of aquatic 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.

Figure 6

Trace Fossils Trace fossils constitute another valuable type of biological information that can be used in environmental analysis. Subsurface information on trace fossils, however, may not always be available. Unlike microfossil data easily obtainable from well cuttings, subsurface information on trace fossils can only be derived from expensive conventional cores. Trace fossils, produced by bioturbation, are simply fossilized 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.

Seilacher (1967) devised a series of ichnofacies based on the observation that certain types of animal trace fossils characterize particular environments  ( Figure 1 , characteristic ichnofacies for various

environments).

Figure 1

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

Skolithos ichnofacies consist of vertical burrows made in sandy or firm mud bottoms of the littoral (intertidal) zone by suspension feeders, i.e., by organisms that feed on food particles suspended in the agitated zone of shallow water.

Cruziana and zoophycos ichnofacies consist of more horizontal and increasingly patterned burrows made in soft muds of the neritic zone by sediment feeders.

Nereites ichnofacies consist of crawling traces made in the soft muds of the bathyl zone by sediment-feeding organisms.

Although the animals responsible for the trace fossils have undergone evolution, these ichnofacies have remained remarkably constant through Phanerozoic time.