Dr Robert Sturm (Austria)
In the past 60 years, microfossils have increasingly attracted the attention of earth scientists for several reasons. Firstly, they are highly useful in biostratigraphic respects; secondly, they can be easily determined by light- or electron-microscopic studies in most cases; and thirdly, sampling, preparation and storage of microfossils is carried out according to well-defined and mostly simple procedures.
By definition, microfossils are the petrified relics of micro-organisms, which have mainly colonised aquatic habitats of the Tertiary or older epochs of earth history. Most of these ancient micro-organisms measured less than 1mm in size, so any scientific documentation of their remains requires a magnifying glass or, better still, a microscope. In certain cases, the size of microfossils is between 10µm and 100µm, which necessitates the use of an electron-microscope to elucidate their structure and to determine the species. Fossils measuring less than 10µm in size chiefly belong to another category of fossils, that is, nannofossils (for example, coccoliths).
Typical representatives of microfossils include radiolaria, foraminifera, ostracods and diatoms, which may be used as index fossils within certain local or regional strata. A special role is taken by conodonts, which are small dental structures belonging to the so-called ‘conodont animal’. This is supposed to be distantly related to the lancet fish (Brachiostoma). While foraminifera, radiolaria and ostracods have colonised the earth with varying abundances since the Early Cambrian (570Ma), the occurrence of conodont animals seems to be restricted to the time period ranging from the Middle Cambrian (about 550Ma) to the end of the Triassic (about 160Ma). Diatoms (Silicoflagellata) are increasingly in evidence since the Middle Cretaceous (about 100Ma).
The production of 3D-images of microfossils is quite easy, but requires sophisticated equipment. For microfossils measuring less than 100µm in size, low amounts of crushed rock or sediment samples containing the objects of study are transferred onto a glass slide and then embedded in resin with a high light refraction (for example, Canada Balsam, n = 1.53). The preparation is finally covered with a thin coverslip.
As illustrated in Fig. 1, there are two principal methods of microfossil stereophotography, in which two perspectively different images of an object are recorded and may be distinguished.
The first technique is based on a modification of the viewing direction on the object. This is simply achieved by firstly producing a stereo-image of the microfossil assuming a ‘normal’ position on the glass slide relative to the light rays of the microscope and then producing the second image after application of a slightly rotated position (rotation angle α). The angle of rotation should ideally range from 5° to 10° to avoid so-called trapezial errors, which may cause a remarkable diminution of 3D-image quality.
However, the technique allows for the possibility of sample rotation, which is normally possible in modern light-microscopes. If the sample is not capable of being rotated, the hobby microscopist can manage the problem by placing some coverslips below the left edge of the glass slide and producing the necessary inclination of the microfossil-containing sample in this way.
The second technique is even simpler, but requires a high degree of experience with microscopes to obtain adequate results. The production of stereo-images is achieved by using two different focus points on the sample, which is expressed by different distances between sample and objective (difference d). In other words, for the first image, a frontal point of the object is focussed on and, for the second image, a more distal point is used. The distance between the focus points should not exceed 5µm to avoid significant lack of sharpnesses.
For microfossils measuring more than 1mm in size, embedding single study objects in highly refractive resin is less necessary. In this case, sediments or crushed rocks with microfossil content are simply spread on a glass slide or a comparable sample holder and are subsequently studied using a reflected-light microscope. However, taking stereo-images can be done in the same way as described above for transmitted-light microscopy. Both method 1 and method 2 are easy to use and the second technique does not present any great problems, because microscopic work generally takes place with much lower magnifications. As a result, a greater depth of focus can be achieved.
The main problem arising for the hobby microscopist in connection with the production of stereophotographs is the availability of the correct camera equipment. Most light-microscopes are affordable to non-scientific people, but do not automatically include a high-quality photographic device. Therefore, alternative strategies have to be contemplated. One of these includes the recording of photographic pictures with the help of a mobile phone camera. This photographic device has the remarkable advantage of having a rather small objective lense, with the help of which imaging can take place through one of the two eyepieces. For the production of good stereophotographs, recording of the left and right stereoimage has to be conducted using the same ocular and, equally important, identical conditions of exposure, magnification and depth of focus.
Among the Foraminifera, one of the most impressive application examples for 3D-imaging is the so-called ‘star sand’, which can be found (among other places) on Okinawa, on the Ryukyu Islands and on Hatoma Island, 200km east of Taiwan. On Hatoma Island,there are numerous coral sands composed to a large extent of foram tests. However, most of these shell structures are highly worn, which makes them barely recognisable.
On the beaches of Okinawa and the Ryukyuin the Far East, lots of fresh examples of foram tests can be found and subjected to detailed photography. In Fig. 2, star sand from the Okinawa archipelago mainly contains the species Baclogypsina sphaerulata, which is shown in Fig. 2 as an anaglyphic (3D) image, for which red-green glasses are required to perceive the three-dimensional form. Single shells range in size from 1mm to 3mm and are therefore barely determinable with the naked eye. Under the microscope, the beauty of the bright shells, with their darker pores, is apparent; and no two objects are exactly alike.
The shells are characterised by five to seven extensions of different lengths, whose spatial extent can easily be investigated by stereophotography. Another foram species, which is very suitable for 3D-imaging, is Ammonia beccarii. This differs from Baclogypsina sphaerulata insofar as its single shell only measures 0.2mm to 0.4mm in diameter, so their photographic potential using a transmission-light microscope is high (Fig. 3). In this context, additional 3D information may be used to estimate the dimensions of single shell chambers.
Within the Radiolaria phylum, a great variety of shell geometries can be recognised, which are always worthwhile imaging using stereophotography. Since most fossil shells are smaller than 100µm, they are best studied by transmission-light microscopy or, if available, by electron-microscopy. Fig. 4 shows a spherical example of this monocellular group of organisms, in which the single extensions that can be seen are especially emphasized by 3D imaging, and are thought to provide buoyancy for the siliceous tests and defence from predators. Another example of a more bizarre shell structure, which is reminiscent of a wooden crown, can also be seen in Fig. 4.
The radiolarian species with this exceptional structure is Lithocircus sp. From a more scientific point of view, the shell is constituted by a ring that produces external irregular teeth and branches. In certain species, there can also be several smaller rings or siliceous arches, which can be combined to form a larger, more complex structure. The size of a single shell varies between 50µm and 200µm.
The last example in this small review of 3D photography of microfossils concerns the class of Ostracoda, which are small crustaceans with a carapace that are sometimes known as ‘seed shrimps’. They typically vary in size between 0.2mm and 2mm, but can also reach unusual dimensions of up to 30mm in the case of Gigantocypris. Their laterally flattened bodies are protected by a bivalve-like shell, which may be formed from chitinous or calcareous material. Ostracods colonise marine as well as fresh water habitats, in which they either follow a planktonic or benthic way of life. The photographic example in Fig. 5 is Juxilyocypris schwarzbachi, measuring about 0.8mm in length and about 0.35mm in width. The shells of male representatives of this species are characterised by a very irregular and highly porous surface, which makes them easily recognisable under a microscope.
As I think is confirmed by several spectacular 3D images in this article, stereophotography of microfossils represents a highly worthwhile hobby, but which requires good microscopes and photographic equipment to achieve results of scientific value. However, the importance of stereophotography has been understood by palaeontologists and biologists for a long time, so the number of images and related publications has multiplied during past decades. In fact, it should be expected that the photographic techniques discussed above will become more important over the coming years.
“Marvelous Microfossils: Creators, Timekeepers, Architects“, by Patrick De Wever, foreword by Hubert Reeves, translated by Alison Duncan, The John Hopkins University Press, Baltimore, Maryland, USA (2020), illustrated edition, 255 pages (hardback), ISBN-13: 978-1421436739
“Microfossils” (2nd edition), by Howard A Armstrong and Martin D Brasier, Wiley-Blackwell (2005), 306 pages (softback), ISBN: 978-06320527-9-0
Sturm, R.: Fascination 3D – stereo microphotography of fossil and recent mollusc shells. Mikrokosmos, 97, 75-80 (2008).
Sturm, R.: 3D photography of fossils: Ammonites from the Northern Limestone Alps of Austria. Deposits Magazine, 18, 10-13 (2009).
Sturm, R.: Coccoliths: tiny fossils with immense palaeontological importance. Deposits Magazine, 20, 42-45 (2009).
Sturm, R.: Microphotography of the shells of gastropod fossils from the Paratethys. Mikrokosmos, 98(6), 331-336 (2009).
Sturm, R.: 3D photographs of fossil gastropods from the ancient Paratethys Ocean. Deposits Magazine 26, 12-15 (2011).
Sturm, R.: Aquatic protozoans examined more closely. Part 1: Foraminifera. Mikrokosmos, 102(1), 20-25 (2013).
Sturm, R.: Aquatic protozoans examined more closely. Part 2: Radiolaria. Mikrokosmos, 102(2), 91-96 (2013).
Sturm, R.: Aquatic protozoans examined more closely. Part 3: Diatoms. Mikrokosmos, 102(3), 148-154 (2013).
Sturm, R.: Use of stereophotography for the clarification of scientific questions. Mikroskopie, 3(2), 86-100 (2016).