by Janet Burke*1
Introduction and background:
Although the microscopic creatures called planktonic foraminifera are still around today, most people have not heard of them. They don’t come to mind when the words “palaeontologist” or “fossil” are mentioned. They don’t have scales or claws, or big sharp teeth. They don’t even have mouths. If you were to visit the lab I work in, you wouldn’t see the specimens, just a row of compound microscopes and funny metal trays, slides and boxes of glass vials a little bigger than a pinky finger. If you look closer at those vials, each one contains hundreds upon hundreds of fossils, and each of those fossils has a story to tell. Etched into the nooks of its chambers and the very molecules of its calcite are facts about the ocean at a brief moment in time. Each one is a little window to the past.
The open ocean is the largest and most productive habitat on earth, but most of the creatures that support it are invisible to the naked eye. They are plankton, a general term for organisms that cannot swim against a current, including representatives of nearly every animal phylum, in addition to a multitude of protozoa and bacteria. Few of them are preserved in the fossil record. However, the remains of some planktonic microorganisms that grow hard shells or ornaments can be preserved in large numbers because they sink and accumulate on the sea floor. One of these groups, the planktonic foraminifera, has a rich fossil record that has been vital to our understanding of the history of Earth’s oceans and climate.
The name “foraminifera” is derived from the Latin word foramen, which means ‘opening’, referring to the apertures in their shells, or tests. Foraminifera, or forams for short, have amoeba-like bodies within tests that are generally made of calcium carbonate. They have one or more openings through which the foram can extend its body by means of thin, threadlike projections called pseudopods. Pseudopods are used for functions such as food capture and moving around. The tests of foraminifera are grown chamber-by-chamber to accommodate the cell as it matures, sort of like adding rooms onto a house. These chambers often coil or stack in a spiral pattern. Foram tests have many shapes, ranging from sphere, cone or disc to a popcorn-like ‘globose’ profile (Fig. 1). A single, average-sized foram test is generally about half a millimetre long, barely visible to the naked eye. Forams are lumped into two groups: benthic foraminifera that live on the sea floor, and planktonic foraminifera that live suspended in the water column. The planktonic forams, which are the focus of this article, first appeared in the fossil record in the Jurassic period, about 201-208 million years ago.
Planktonic foraminifera use their sticky pseudopods to snare food and draw it in towards the aperture, where they can dissolve and absorb it. They have been observed eating phytoplankton, marine snow (organic materials that fall through the water) and even the small crustaceans called copepods. In the lab, omnivorous species of planktonic foraminifera are fed young brine shrimp (Artemia, Video 1).
Video 1 — Orbulina universa eating a live brine shrimp. Credit: Howard Spero, University of California, Davis/YouTube.
Many species of planktonic foraminifera also contain single-celled organisms that create their own energy through photosynthesis (Fig. 2), similar to the zooxanthellae found inside coral cells, although the exact benefit they get from this relationship is unclear.
The lifespan of a planktonic foram is only a few weeks to a few months. Their life cycle ends when they undergo ‘gametogenesis’, the release of reproductive material. The timing of gametogenesis is associated with lunar cycles in many species. After they die, their tests sink and accumulate on the ocean floor, forming layers of sediment. These sediments build up over time, and scientists can access them by drilling out long cores of the ocean floor from specialized ships (Video 2, Fig. 3).
Video 2 — The seafloor drilling and coring procedure of the CHIKYU research vessel. Credit: Japanese Agency for Marine-Earth Science Technology/ YouTube.
Importance of foraminifera fossils:
Although their lives are relatively short, planktonic foraminifera have had a big impact on our understanding of the climate and the oceans. Below are a few of the reasons that the fossil record of planktonic foraminifera is an exceptional resource for reconstructing Earth’s history:
1) Temporal resolution — Fossils that are discovered near each other are not necessarily from the same time period. Time-averaging is the amount of time represented in a single unit of sediment, and it has major implications for how a group of fossils, or an assemblage, is interpreted. Low time-averaging is ideal for making inferences about ecology, environmental changes, extinction events and evolutionary trajectories, because it means that samples found near each other are closer in age than samples in assemblages that are highly time-averaged. Depending on the rate at which foraminiferal tests fall to the sea floor from the surface, the amount of time represented by a centimetre of sediment in a core can vary widely. Sediments that are well suited for the deposition and preservation of foraminifera can have very low time-averaging compared to that in other types of fossil assemblage.
2) Spatial coverage — Communities of planktonic forams can be found from pole to pole and in all the major oceans. This spatial coverage allows palaeontologists to study the global signature of climate change and extinction events to distinguish between local and global phenomena.
3) Sample sizes — The larger the sample a scientist has to work with, the sounder their conclusions can be. It is hard to correctly categorize the variation in a species if you have only a few specimens. Unlike complete skeletons of many famous dinosaurs, which are rare, specimens of a given planktonic foraminiferal species are abundant in well-preserved sediments.
For decades, geochemists have been developing ways to glean environmental information from the tests of foraminifera by measuring the isotopic composition of their calcite shells. Isotopes are atoms of the same element with slightly different atomic weights. How isotopes are incorporated into materials such as foraminiferal calcite can reflect environmental and physiological conditions at the time of deposition. As a planktonic foram builds its tests, the isotopic make-up of the calcite reflects characteristics of the environment, including temperature, acidity and food type. Isotopic data from planktonic foraminifera has had a pivotal role in our understanding of Earth’s climate fluctuations and has helped lend credibility and nuance to predictions about the effects of modern climate change.
Fossil assemblages of planktonic foraminifera have been used to study extinction events and evolutionary processes. The Cretaceous–Palaeogene extinction event around 65 million years ago, which killed the non-avian dinosaurs, was accompanied by a major extinction of planktonic foraminifera. Before the extinction event, planktonic foraminiferal assemblages came in lots of different sizes and contained a range of species. Assemblages from after the extinction are comprised of a few small species that eventually gave rise to the modern lineages of planktonic forams.
The shape of planktonic foraminiferal tests alone is a useful tool for micropalaeontologists. Without genetic information, it is the main way to identify different species. Changes in morphology can be tracked through time to study responses to climate change, exploitation of new niches and even the formation of new species. Furthermore, because many species of planktonic foraminifera exist for a relatively short time (from a geological perspective), planktonic foraminiferal fossils are used to estimate the age of sediments. This is done by carefully recording the first and last appearances of common, short-lived species, and using the presence of those species to indicate the temporal window. This practice is known as biostratigraphy.
The techniques described above only scratch the surface of the body of research on planktonic foraminifera. Although they are small, the questions planktonic foraminifera have been used to explore are some of the most important facing earth scientists today. Paleontologists are masters of making the most of everything the fossil record offers, from the smallest grains of sediment to the largest skulls. Planktonic foraminifera are a reminder that even the littlest objects can tell a big story.
Schiebel, R. & Hemleben, C. Planktic Foraminifers in the Modern Ocean. (Springer, 2017).
Wetmore, K. Foram Facts — An Introduction to Foraminifera. Learning from the Fossil Record. University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/fosrec/Wetmore.html
Kucera, Michal. Chapter six Planktonic foraminifera as tracers of past oceanic environments. Developments in Marine Geology 1, 213–262 (2007). DOI: 10.1016/S1572-5480(07)01011-1
Video: Smithsonian Science How Webcast. Global Change: Reading Ocean Fossils. Q?rius. (Smithsonian National Museum of Natural History, 2015). https://qrius.si.edu/explore-science/webcast/global-change-reading-ocean-fossils
Video: Shelf Life. Episode Six: The Tiniest Fossils. AMNH.org. (American Museum of Natural History, 2018). https://www.amnh.org/shelf-life/episode-06-the-tiniest-fossils
1Department of Geology and Geophysics, Yale University, New Haven, CT, USA