Paleontology 137
Dr. J Bret Bennington
Micropaleontology and Palynology
Stromatolites
Although stromatolites are megascopic organo-sedimentary structures more within the realm of sedimentology than micropaleontology, they are produced by bacteria (prokaryotic organisms) so we will consider them along with other fossil microbes.
The term Stromatolite is defined as an accretionary organosedimentary structure, commonly laminated, megascopic, and calcareous, produced as a result of the growth and metabolic activities of mat-forming, filimentous prokaryotes - primarily cyanobacteria ("blue green algae", which are not algae at all - algae are eukaryotes). Sedimentary particles carried by currents and tides settle out onto the mat surface. Bacterial cells grow upward through the sedimentary material, trapping and binding it, and forming a new mat surface. This process is continuously repeated to produce laminations of sediment bound by older mat material with the living growing microbial mat remaining on the outer surface of the mound.
Stromatolites can be stratiform (flat), columnar (mound-like), or spheroidal (round). Thrombolites are unlaminated stromatolites with a characteristic "clotted" fabric. Because stromatolites can build up considerable relief from the seafloor, they are, in essence, the microbial equivalent of organic reefs produced by algae and invertebrates.
Suspected stromatolites are known from some of the oldest rocks on earth. They are abundant in marine sedimentary rocks of the Late Archean and Proterozoic, becoming scarce by the end of the Cambrian. Apparently, the evolution of mat-grazing organisms such as gastropods made it nearly impossible for cyanobacterial mats to persist in most marine environments after the Cambrian. Modern stromatolites are only found in abundance in hypersaline environments such as Shark Bay, Australia, that exclude grazing invertebrates.
Micropaleontology
Having finished discussing biostratigraphy, this is a good point in the course to consider micropaleontology – the paleontological study of microscopic fossils. Microscopic fossils are often of great utility in biostratigraphic studies for reasons mentioned previously:
Small size = abundant in small samples such as drill core cuttings
Often planktonic or windborn = good dispersal, facies independent
Because of their small size, microfossils require specialized techniques to extract them from rocks, describe, and identify them. Also, the microfossils themselves represent groups of organisms that are not dealt with by invertebrate paleontologists and most paleobotanists.
"Animal" and "plant" microfossil groups
Technically, any microscopic fossil is a microfossil. However, because the techniques for dealing with different microfossil groups are different, paleontologists tend to specialize and work either with animal microfossils (invertebrates and protistans) or plant microfossils (marine algae and plant propagules such as spores and pollen).
Three animal microfossil groups of great utility and interest in invertebrate paleontology are foraminifera, conodonts, and ostracods.
In addition to these groups, there are a variety of other microfossil groups that are usually placed under the aegis of Palynology.
Palynology was originally limited to the study of spores and pollen, but the techniques for extracting spores and pollen from rock are also needed to extract a variety of other types of fossils, many representing types of marine 'algae'. Therefore, palynology has grown to encompass a variety of organisms, including:
Dinoflagellates and Acritarchs
Diatoms
Coccolithophores
Radiolarians
Spores and Pollen
Let's now have a quick look at these different groups and say a little bit about their usefulness in paleontology.
Protistan Microfossils
Foraminifera
Foraminifera (forams, foraminifers) are single-celled organisms closely related to amoebas that usually secrete a chambered shell or test . The test is either composed of calcium carbonate, or it is made of a variety of sedimentary particles cemented together by the foram. Some forams are particularly selective, choosing only certain sediments, for example, sponge spicules, to construct the test.
Overhead: Foram test morphologies
Forams first appear in Cambrian rocks and are still abundant today. Forams are ubiquitous in all marine environments.
There are two main types of forams:
Benthic foraminifera: These forams live in the sediments on the seafloor. Benthic forams range from the Cambrian to the recent. They are useful as biostratigraphic fossils from the late Paleozoic to the Recent. In the Pennsylvanian and Permian a group of large, grain-shaped forams called fusulinids became abundant in shallow waters and form the basis for biostratigraphic zonations of the Upper Paleozoic.
The depth and geographic distribution of benthic foram species is better known than that for any other type of marine organism. Because of this, benthic forams are used extensively to interpret the depth of formation of ancient sediments (particularly in the Tertiary).
Planktic foraminifera: This group of forms only lives floating in the water column in the open ocean. They first evolve in the Mesozoic, but are most abundant and widely distributed in the Cenozoic. Cenozoic marine strata are extensively zoned using planktic forams, and the resulting zonation is unparalleled by any other group of organisms for its stratigraphic precision and widespread application.
Planktic forams drift throughout the world's oceans and dead tests continually rain down on the seafloor. In deep ocean regions where there is little input of other sediments, vast numbers of foram tests accumulate to form a calcareous mud called an ooze. Many such modern deposits are composed principally of the genus Globigerina and are called globigerinid ooze.
As we will discuss in detail later, the oxygen isotope composition of planktic foram tests is sensitive to water temperature so that fossil forams are often employed as paleothermometers to estimate changes in ocean temperature over time.
Because planktic forams drift with the currents, their pattern of distribution in ancient sediments can provide clues to the movements of ancient ocean currents.
Radiolarians
Radiolarians are heterotrophic protists related to amoebas and forams. They secrete a very delicate, opaline silica test that is the glory of the protistan world! There are two main groups of radiolarians:
Spumellarians - evolved in the Cambrian and are generally spherical, often with long, radiating spines.
Nacellarians - evolved in the Mesozoic and are generally helmet shaped, often with a single apical spine and several adapical spines.
Radiolarians are planktic and occur in the uppermost few hundred meters of the open ocean. In deep ocean basins (such as the North Pacific) radiolarians rain down and accumulate in concentrations at depths below the CCD (carbonate compensation depth) where they form siliceous oozes. Many Mesozoic and Tertiary chert deposits appear to be formed from radiolarian oozes.
Radiolarians are used as biostratigraphic index fossils to correlated sediments that are lacking calcareous material due to dissolution.
Animal Microfossils
Ostracods
Ostracods are small crustaceans that secrete a calcareous bivalved shell that superficially ressembles a tiny clam. They live in both marine and freshwater environments and are known from the Cambrian to the Recent.
Ostracods are mostly benthic organisms, although a few planktonic species are known. They are very useful biostratigraphic fossils, particularly in fresh water lain strata where marine biostratigraphic fossils such as forams are absent. Ostracods often have highly ornamented shells that allow different species to be confidently distinguished.
Conodonts
Conodonts are an extinct group of marine organims know almost exclusively from what are assumed to be their teeth – tiny calcium phosphate skeletal parts called elements. Conodont elements are common in marine rocks from the Cambrian to the Triassic.
The conodont elements can be easily liberated from limestones by dissolving the rock slowly in dilute acids, and then the elements can be concentrated by separating them from other residue using heavy liquids.
Although the conodont animal itself was completely unknown until 1982, the elements have been used for many years as very good biostratigraphic index fossils. Conodonts are widespread, and some species are relatively facies independent, leading paleontologists to long suspect that the conodont animal was either planktic or nectonic in its life habit.
Originally, a separate species designation was assigned to each type of element. By the 1940's however, it was known that some types of conodont elements must have belonged to the same animal. It was noticed that certain "species" of conodonts always occured together and in constant ratios. The discovery in some rock samples of different conodont elements preserved together in a symmetrical arrangement confirmed that the individual elements were just part of a larger apparatus.
In 1982 a conodont animal was discovered on a slab of shale in a museum drawer in Edinburough, Scotland. The impression preserved is that of a worm-like animal, about 4 cm long, with a posterior fin-like structure and an anterior bilobed mouth containing a complete conodont apparatus. The conodont animal bears some interesting similarities to living arrow worms (chaetognaths). Chaetognaths are planktic predators that employ a battery of spines and teeth at the head end to capture small prey. Although differences in the composition and form of chaetognath teeth and conodont elements suggests that they were not closely related, their overall convergence of form argues for a similar life habit. Conodonts have recently been placed within the vertebrata on the basis of similarities in the histology of conodont elements and vertebrate tooth enamel.
In addition to their common use in biostratigraphy, conodonts have an important use in determining depth and duration of sediment burial and the geothermal history of the rocks that contain them. It had long been noted that conodont elements occured in a range of colors, from pale yellow to amber to black and even clear. Anita Harris of the USGS determined that this color range was dure to different amounts of heating. By placing conodonts in her oven at home for different times at different temperatures, Dr. Harris demonstrated that as conodonts are heated to higher and higher temperatures they change color along the path described above. This work produced a color alteration index for conodonts (running from 1 to 8) that can be applied in the field to produce maps that show the thermal maturation of rocks over a large area. Because petroleum formation is temperature dependent (too cool - no mature hydrocarbon, just right - oil, too hot - natural gas) these maps assist in locating conditions favorable to hydrocarbon production.
Algal Microfossils
Coccolithophores
Coccolithophores (coccoliths) are extremely small, single celled, photosynthetic algae (Division Haptophyta) that secrete tiny calcarous plates called coccospheres. Coccoliths require sunlight to photosynthesize and therefore live in the upper layers of the open ocean where they are the main constutuent of the nannoplankton. Coccoliths have been abundant in the world's oceans since the Mesozoic (possible late Paleozoic coccoliths have also been reported). At times they have been extremely prolific. For example, the vast world-wideUpper Cretaceous chalk deposits are composed predominantly of coccospheres.
Coccospheres range in size from about 1 micron to 15 microns. It has been estimated the a cubic centimeter of oceanic coccolith ooze contains at least a trillion coccospheres!
Coccoliths are extremely useful for biostratigraphy for the same reasons that planktic forams are.
Diatoms
Diatoms are a type of Golden-Brown Algae (Division Chrysophyta) that live in both the benthos and plankton in both marine and freshwater environments. They evolved in the Cretaceous and have been abundant ever since. Diatoms secrete a two-part test composed of opaline silica. The two parts (frustules) fit one inside the other much like a pill-box or petri dish. There are two main types of diatom:
pennate diatoms: these are elongate diatoms the live in the benthos or in / on other organisms. They are among the first organisms to colonize any surface where there is light (in the photic zone). They grow on the bottom surface of sea ice, and as an infestation in the skin of whales.
centric diatoms: these are round diatoms that are mostly planktic. Centric diatoms are very abundant in nearshore waters and are the most important primary producer at the base of the marine food chain.
Where conditions are favorable, centric diatoms can be extremely abundant, forming deposits on the seafloor called diatomites or diatomaceous earths. Such deposits are found in the Tertiary of California where they are mined and used in the filtration and purification industries and in household abrasives.
In the north and equitorial Pacific, diatoms have been used extensively for biostratigraphic correlations of Miocene to Recent sediments.
Sporopollenin Microfossils
Sporopollenin is a high molecular weight polymer of C-H-O that is extremely resistant to all forms of chemical degredation except oxidation. Sporopollenin microfossils are very durable under most conditions of burial and can be extracted from virtually any type of rock through a complicated technique called maceration, which involves using a combination of very strong hydrofluoric and hydrochloric acids to dissolve both silica and carbonate from a rock sample, leaving only the organic material remaining.
Spores and Pollen
The gametes of terrestrial plants are encapsulated in resistant coatings of sporopollenin and released as spores and pollen. Spores are known from the Silurian onward, while pollen appears in the Early Cretaceous with the evolution of flowering plants.
Spores are generally larger than pollen, having a flattened and oval, circular, or triangular shape. Pollen grains are very small and often spherical in shape. Conifer (pine) pollen has a distinctive shape consisting of a central spherical body with two smaller spherical bladders attached at the ends.
Both spores and pollen are distinctive at the genus and sometimes the species level. Both are useful for correlating terrestrial strata with nearshore marine strata. Because tree species are very sensitive to climate conditions the distribution of pollen in Pleistocene lake sediments has been used extensively to reconstruct the climate changes that have occurred throughout the current ice age.
Dinoflagellates
Dinoflagellates are protists assigned to the algal Division Pyrrhophyta (flame algae). Living dinoflagellates are present in both marine and fresh waters and are mainly planktic. In the ocean dinoflagellates are responsible for bursts of bioluminescence that one sees when the surface water is disturbed. Periodic blooms in the nearshore populations of dinoflagellates result in 'red tides' that occur in late spring and summer. Red tides are often dangerous to humans because some dinoflagellates secrete potent toxins that can be concentrated in fish and shellfish that feed on these species.
Dinoflagellates have a life cycle that includes a period of time during which the cell is dormant. To prepare for dormancy, the dino secretes an outer cyst of sporopollenin. The cyst settles to the seafloor. When dormancy ends, the cell departs the cyst and returns to the water column, leaving the cyst to be buried in the sediments.
Dinoflagellates appear in the Triassic, but do not become abundant until the Cretaceous. From the Cretaceous to the Recent they are important biostratigraphic index fossils.
Acritarchs
This is a 'junk basket' group that includes all marine sporopollenin cysts that cannot be identified as dinoflagellates. Undoubtably they represent a variety of cyst-forming marine algal groups that have not yet been identified. Acritarchs are present in rocks from the Late Precambrian to the Recent. They are sometimes used as biostratigraphic fossils in pre-Mesozoic rocks where dinoflagellates are not available.