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Weird Science: Oceanic Microfossils

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Microfossils are a group of fossils that are distinct from other fossils because of their small size, usually less than 4 millimeters wide (SF Fig. 7.11). The photos in SF Fig. 7.11 A–C were taken with a scanning electron microscope (SEM). SF Fig. 7.11 D was taken through a compound light microscope. By contrast, macrofossils can usually be seen with the naked eye.

<p><strong>SF Fig. 7.11.</strong> (<strong>A</strong>) Polychaete marine worm jaws, the largest piece measures 1 mm</p><br />
<p><strong>SF Fig. 7.11.</strong> (<strong>B</strong>) Single-celled foraminiferan shell with scale bar indicating 0.2 mm</p><br />


<p><strong>SF Fig. 7.11.</strong>&nbsp;(<strong>C</strong>) Sponge spicule with scale bar indicating 0.4 mm</p><br />
<p><strong>SF Fig. 7.11.</strong> (<strong>D</strong>) Pine tree pollen with scale bar indicating 0.05 mm</p><br />


 

Microfossils represent a wide diversity of plants and animals and are a common feature of the geological record from the Precambrian (prior to approximately 540 million years ago) to the present. Since they do not represent a particular classification of taxa, they are often grouped by their structures (SF Table 7.3). They often require a light microscope or electron microscope to be seen. Microfossils form through various processes and usually involve mineralization in sedimentary rock. In the marine environment, the remains of organisms sink to the ocean floor where sedimentary rock is created.

SF Table 7.4. Microfossils can be grouped by the chemical composition of the remains
Group Description Examples
Calcareous Organisms that contain calcium carbonate structures Coccolithophores, foraminiferans, calcareous dinoflagellates, ostracods
Phosphatic Organisms that contain phosphate mineral structures Oral structures of chordates, shark spines and teeth, fish scales and teeth
Siliceous Organisms that contain silicate material structures Diatoms, radiolarians, sponge spicules
Organic Organisms that have hard carbon based structures Pollen, marine invertebrate egg cases, worm jaws, fungal remains

 

The world ocean and continents are constantly changing. Many areas that were once ancient seafloors are now accessible by land. For example, a large interior sea used to cover much of what is now Montana, South Dakota, and Wyoming during the late Cretaceous period (80–65 million years ago). During that time marine organisms were deposited in sediment and fossilized. The inland sea has since receded but fossils remain. Another example of accessible marine fossils are the Dover Cliffs of England (SF Fig. 7.12 A). Approximately 70 million years ago, this part of England was submerged under a shallow sea. Numerous calcareous phytoplankton, called coccolithophores (SF Fig. 7.12 B), sank to the ocean floor when they died, creating the calcareous sedimentary rock with a chalk like appearance. Since then the crust uplifted above current sea level and the white Cliffs of Dover are a prominent nautical feature.

<p><strong>SF Fig. 7.12.</strong> (<strong>A</strong>) The White Cliffs of Dover in England are composed of sedimentary layers of calcareous microfossils called coccolithophores, giving them their white color.</p><br />
<p><strong>SF Fig. 7.12.</strong>&nbsp;(<strong>B</strong>) Individual coccolithophore cells, seen here under a scanning electron microscope, are covered with calcium carbonate plates.</p><br />


 

Microfossils are very useful to scientists because they can provide information from the past over long time scales. The field of correlating relative ages of layers of sedimentary rock, known as strata, using fossil assemblages is known as biostratigraphy. Paleontologists can use biostratigraphy to identify the time periods that strata from different locations represent by identifying the types of microfossils present. This is very useful because the mineral composition of the strata can change from location to location. Since microfossils are so small, getting samples of them from rock is not as simple as getting larger macrofossils samples. Sediment samples are collected from rock cores or uplifted outcroppings (e.g., the Cliffs of Dover). The microfossils are then extracted using physical and chemical laboratory techniques. Centrifuge techniques can be used to extract materials of different densities. Sieves can be used to extract fossils of different sizes. The microfossils are plated on slides and observed through light microscopes or electron microscopes (SF Fig 7.12 B).

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.