Sunday, June 24, 2018

Regarding forams

Life started out microscopic (at least to humans) and most of it has stayed that way. Of course, many microscopic organisms have poor fossil records, due to factors like lack of hard parts and the whole "microscopic" thing (finding and studying microfossils takes special equipment and expertise that aren't used for collecting, say, brachiopods). However, a subset of microscopic organisms have very significant fossil records. We saw the ostracodes a few years ago, but there are also a number of groups of single-celled organisms that produce hard parts suitable for fossilization. Among the most important are: coccolithophores, phytoplankton which form skeletons of scale-like objects known as coccoliths, micron-scale structures that make up chalk (and which are sometimes called nannofossils because they're so darn small); diatoms, phytoplankton with cell walls made of silica; dinoflagellates, which form organic-walled cysts; radiolarians, protozoans that form body structures of silica; and the subjects of today's entry, the foraminifera, which can be described glibly as "amoebas with shells".

A living foram, the brackish-water benthic calcareous species Ammonia tepida, showing strands of pseudopodia surrounding the coiled test. What do all these terms mean? Read on! (Photo from Wikimedia Commons; unfortunately, no scale, but you'll get an idea of the size of what we're dealing with in the photos to come.)

Foraminifera are more familiarly known as "forams", which saves a few letters and the confusion of the whole singular/plural thing and knowing when to capitalize. They are also sometimes known affectionately (or otherwise) as "bugs", usually in the context of being manipulated for study, which is done by such means as wetting a single paintbrush hair and picking up the individual microfossils while looking through a microscope ("picking bugs"). (As you might imagine, this takes steady hands and a lot of patience.) Your basic foram is an amoeba-like single-celled eukaryotic critter that interacts with its surroundings via pseudopodia, which are long thin strands of the foram's cellular material. The pseudopodia can be used to catch food, creep along surfaces, and so forth. Forams catch food and sometimes acquire photosynthetic symbionts. They are largely marine, but can show up anywhere with enough moisture, including, apparently, soils. Most forams are benthic (or benthonic), meaning they live at or near the seafloor, but a small number of species are planktonic (or planktic), drifting with the currents in the upper ocean. Most forams are smaller than a millimeter in diameter, meaning you might spot them with the naked eye if you are looking for them, but probably won't pick out many details. However, some have attained truly titanic proportions for single-celled organisms. In the fossil record, a couple of groups that left fossils as much as a couple of inches across include the late Paleozoic fusulinids, which resemble grains of rice in shape, and the middle Cenozoic nummulitids, which look like discs or coins. (The trivia that always gets repeated about nummulitids is that the pyramids of Egypt are basically made of them, due to the type of rock used in their construction.)

As a high schooler and undergrad I did some work with forams from the Oligocene of coastal South Carolina. This and the next images are from photos I took back in 2001 (a lot of the pictures are kind of blah, due to inexperience). The three forams here are examples of the benthic genus Uvigerina, which I saw in great abundance. The tick marks are 1 mm apart.

A foram protects itself with a "shell", technically known as a "test". A few forams have organic tests, but most have either agglutinated tests (made up of bits of stuff from the surroundings) or calcareous tests (made of calcium carbonate). There is quite a bit of fine structural variations among agglutinated and calcareous tests that will be omitted here. Forams have come up with a number of different ways of building tests, from simple (cylinders, spheres, flasks, blobs) to slightly more complex (spiral tubes, chambers stacked in a single row), to intermediate (miniature coiled nautilus shapes), to things like chambers in three alternating columns or spirals turning into columns as the foram got older, to the complex layers of chambers formed by the giant fusulinid forams. Foram tests can be abundant enough to more or less make up entire marine formations. The foram fossil record goes back at least to the Cambrian, and they are quite abundant by the late Paleozoic, when the fusulinids were on the scene. Forams were knocked down by the end-Permian extinctions, which killed off the fusulinids, but bounced back in the Mesozoic. One significant Mesozoic innovation was the beginning of modern planktonic groups, which can easily be distributed worldwide. Benthic forams have all kinds of shapes, but planktonic forams tend to look "inflated", like tiny coiled nautiloids with balloons for chambers.

These I thought might be Lenticulina.

Forams are among the most useful types of fossils. It's a lot easier to get large samples of well-preserved specimens of forams than it is for most other groups, and they can be studied in many ways. Some common uses of forams include:
  • Biostratigraphy: foram species can be good index fossils for relative dating;
  • Paleoecology: some foram species are sensitive to such factors as temperature (usually expressed as water depth) and salinity, although of course ancient forams may have differed from their descendants in ways we can't know;
  • Paleoclimatology and "handedness": it turns out that some modern planktonic forams coil differently in different water temperatures, as shown by modern polar and tropical populations of the same species. The exact genetic mechanisms are not understood, and it is not clear how far back this information can be applied (p. 241–242 at link);
  • Paleoclimatology via stable isotopes: isotope geochemistry is very complex stuff, so let us just say that isotopes are varieties of the same element with different numbers of neutrons and thus different masses, stable isotopes do not decay unlike radioactive isotopes, and different stable isotopes of the same element will fractionate differently due to the different masses (e.g., water molecules with heavier isotopes of oxygen do not evaporate as easily as water molecules with lighter oxygen isotopes, so water vapor is composed of a slightly lighter fraction of water than liquid water). Foram tests can be used to look at a number of different stable isotope systems, which relate to various climatological properties like temperature and salinity. As you might imagine, planktonic forams record near-surface climates, while benthic forams record what's going on deeper in the ocean.
  • Studies of evolution in foram lineages over time.

I attributed specimens of this form to Florilus.

Forams have also long been used in the oil industry. Their status as biostratigraphic and paleoenvironmental indicators are useful for determining if someone is drilling in the right rocks. In addition, the coloration of agglutinated foram tests can be used to determine how "cooked" a rock unit is, something you want to know if you're looking for petroleum (the rocks need to have been heated, but not too much). Although people often think of paleontologists as digging dinosaurs for a museum or university, in practice the petroleum industry has been a major employer of paleontologists due to the use of forams and other biostratigraphically important fossils.

These are a couple of planktonic forams, with inflated chambers.

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