Microfossils in 3D – stereophotography of ancient micro-organisms

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).

Figure1a
Fig. 1. Simple methods for the generation of stereoscopic photographs of diverse microfossils: (a) Method 1: the production of stereo-images with different perspectives by slight rotation of the glass slide on which the sample is placed; and (b) Method 2: production of the stereo-images by changing the focus point of the sample.

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 (570mya), the occurrence of conodont animals seems to be restricted to the time period ranging from the Middle Cambrian (about 550mya) to the end of the Triassic (about 160mya). Diatoms (Silicoflagellata) are increasingly in evidence since the Middle Cretaceous (about 100mya).

Methodology

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.

Method 1: 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.

Figure2
Fig. 2. Stereophotograph of star sand from the Okinawa archipelago, showing shells of Baculogypsina sphaerulata.

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