Tech UPTechnologyFossils as small as a few millimeters help us...

Fossils as small as a few millimeters help us decipher the past.

Often when we think of Paleontology , large fossilized bones , or complete skeletons of huge animals from the past, mounted in the center of a large museum hall, come to mind.

But these large, fascinating remains are often not the most abundant, nor are they the most informative.

Although they are often far from occupying privileged spaces in exhibitions, the smallest fossils are pieces of enormous value , due to the large amount of information they provide for the knowledge of the fauna of the past. But they are also the ones that allow us to know details of the environments inhabited by the different extinct species.

Two great examples that size is not proportional to the information provided by fossils are: foraminifera and rodents .

Foraminifera are a group of unicellular organisms, inhabitants of marine environments with an extensive fossil record. And despite their size, they are tremendously informative.

The foraminifera secrete a shell with a great variability of shapes and sizes. During some intervals they came to have “gigantic” forms, with shells of several centimeters! When these organisms die, their shells accumulate on the seabed.

What makes these fossils so interesting?

These organisms have short-lived species but as a group they cover a very wide temporal range (from the Cambrian to the present). This allows specialists in this group to be able to determine the ages of the deposits based on the different groups of foraminifera that appear in the deposits. Thanks to this condition, it has been possible to define different biozones, zones of different ages, with high resolution to date strata where species of this group appear.

In addition, foraminifera are very useful as indicators of the environments in which they lived. Data on the temperature and depth conditions of the water in which they lived can be inferred from their study. One example is the study of the stable isotopes present in the shells of these animals, which have made it possible to establish graphs of global climate change . Very detailed climatic curves can be obtained from oxygen isotope curves of deep-sea benthic foraminifera.

Another characteristic of this group is that the foraminifera have an important interest in the oil industry and are called “guide fossils” to identify hydrocarbon deposits.

The tools to get the small fossils of foraminifera are not proportional to these animals. It is necessary to use large special scientific vessels , equipped with a drilling tower very similar to the one used in oil prospecting, to obtain soundings of the seabed where foraminifera have been accumulating. The greater the accumulation, the better the climatic record that can be obtained.

In the case of rodents, the fossil remains that we find of these small mammals are usually millimeter-sized loose teeth. The excavation and study of these fossil remains requires particular processes . Contrary to what we might imagine, its extraction is particularly hard and requires special techniques.

First of all, several tons of soil have to be collected from the levels potentially rich in these fossils that are put into sacks. The extracted material is then processed by putting the collected sediment into buckets. There it will be soaked and then put to dry. This apparently contradictory operation may have to be repeated several times. Through this process of moistening and drying the sample, the mineral material that surrounds the fossil remains will expand and contract: the clays. Afterwards, they go through a pressurized water washing process through what we call “washing table” and which are large sieves (giant strainers) with different mesh sizes. Thus, the largest remains will be deposited in the first sieves with larger holes, letting the rest pass that will be collected in the following ones to finish with sieves with a mesh size of 0.5 mm . Only clay passes through these tiny holes, which is the smallest mineral fraction. The smallest fossils will be retained in this last sieve.

The material retained in the different sieves is collected and labeled and goes on to the next phase: separation and identification of fossil remains. To do this, it is necessary to examine the sample of material collected under a binocular loupe and with the help of a small brush, the fossil remains found are separated from the granites of minerals that have remained after washing. Fossils are identified by an expert and labeled for later study.

All this great detail work is necessary and will be rewarded with the great amount of information that these small fossils provide us.

But… Why so much information contained in those tiny fossils?

Rodents have very high reproductive rates, so their populations can accumulate changes very quickly. In addition, being communities with such high reproduction rates, they have large litters, that is, many individuals that can potentially leave many fossils.

One of the advantages of studying these fossil remains is: the large amount of material, which allows statistical studies to be carried out. In this way we can, for example, obtain information with a high degree of detail on variability.

What kind of information do they reveal to us?

In addition to the systematic and taxonomic information, that is, from their study we can describe to which species these teeth belong and what the animals to which these fossils belonged were like. We can also obtain ecological information that will allow us to draw the landscape of the site where they lived. By comparing these communities of small rodents and the ecological data they provide in space and time, we will be able to detect environmental changes at a continental level .

On the other hand, the fossils of these small mammals have been described as very good “biochronological” indicators. So, as was the case with foraminifera, rodents also make it possible to establish the age of the site where they are found and can be correlated between different areas. This characteristic has made it possible to establish a very detailed system of ages between deposits from different regions.

In particular, in areas such as the Iberian Peninsula or Siwalik (India), there is such a good known fossil record that they have made it possible to establish time series based on the study of rodent fossils.

So we can see how tiny pieces of the fossil record give us huge clues to describe the past . It is very important to take into account the amount of information that these fossils of only a few millimeters can contain.


Duleba, W., et al. (2018). Environmental impact of the largest petroleum terminal in SE Brazil: A multiproxy analysis based on sediment geochemistry and living benthic foraminifera. PLoS One, 13(2), e0191446 DOI: 10.1371/journal.pone.0191446

Kimura, Y., et al. (2021). Tempo and mode: evidence on a protracted split from a dense fossil record. Frontiers in Ecology and Evolution, 9, 642814 DOI: 10.3389/fevo.2021.642814

Kranner, M., et al. (2022). Calculating dissolved marine oxygen values based on an enhanced Benthic Foraminifera Oxygen Index. Scientific Reports 12, 1376. DOI: 10.1038/s41598-022-05295-8

 Oliver, A., and López-Guerrero, P. (2017). The Hill of the Saber-toothed Tigers: the Miocene sites of the Cerro de los Batallones (Torrejón de Velasco, Community of Madrid). The Micromammals: Techniques of Extraction and study (pp. 530-534). La Caixa Social Work.

Zachos, J., et al. (2008). An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics. Nature 451, 279–283 DOI: 10.1038/nature06588

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