Neophyte explores Eocene

By Charles Suslavage USA

 

 

For sale, microscope slide, fossil diatoms, Mors Island, Jutland. Having an interest in microscopic fossils I could not let this opportunity pass. So I purchased the slide and I am very pleased with the acquisition. Mors Island, Denmark, is a source of fossils from the Paleocene (65 55 Ma) to the late Eocene (55 34 Ma). The Paleocene epoch is the geologic time zone that followed the mass extinction event at the end of the Cretaceous, the end of the dinosaurs. The Eocene epoch is the dawn of modern mammalian fauna, the small furry critters that eventually grew larger and evolved into lions, deer, elephants, you, and me. The Paleocene was warm and humid and the temperatures increased in the Eocene to more then 14C above today's global average temperature. The polar ice caps had not yet appeared and would only form after tectonic forces changed the earth and brought about a cooling trend ending the Eocene epoch. This fascinating slide presented the anticipated fossil diatoms along with good examples of silicoflagellates and fragments of radiolarians.

     

As a novice I find that I am easily confused and often frustrated when trying to identify diatoms. Reference material available in general texts is a good starting point but I find it hard to apply when observing at the microscope. Most often diatoms are presented in the literature as two dimensional, valve view and occasionally girdle view. Observe any strew slide of diatoms and the simplicity of two dimensions quickly vanishes. The problem of identification for a beginning novice like myself is exaggerated when working with microfossils. Nonetheless, it is a lot of fun trying and I am continually surprised as to what I learn as I proceed down what seems an endless dark tunnel.

Diatoms join together to form chains, I know this from pictures, text, and pond water. However, while examining this slide I felt as if I had personally discovered this characteristic for fossil diatoms, it is exciting to put the observed pieces together and to feel the moment of discovery. Fig 1 presents three views of Trinacria exculpta, valve view center top, girdle view center bottom, and two flanking oblique views. Putting these views together gave me a clearer picture of this diatoms geometry, something not possible if only a valve view is observed. Fig 2 presents two examples of how the valves would join together to form a chain. Now I ask myself a question, why do diatoms link together to form a chain? T. exculpta also aids in verifying the age of these fossil diatoms, it is characteristic of the late Paleocene and early Eocene neritic environments (shallow waters near a sea coast).

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Fig 1.Trinacria exculpta (Heiberg) Hustedt,
type reference: Heilberg, 1863, as Solium exculptum

Extending from the corners of the valve are four spines, the pole like extensions. Fig 1, lower center, at the approximate center of each spine a dark horizontal line can be observe, a junction between valves? the same junction is easily observed in the spines pictured in Fig 2.

Fig 2.

Fig 2.

A pleasant surprise came when attempting to identify the Silicoflagellates. I learned that Silicoflagellates are excellent stratigraphy markers. The genera Naviculopsis and Corbisema are extinct having died out before the middle of the Miocene epoch ( around 15 Ma).  Naviculopsis aspera, Fig 3a, and Corbisema hastate cunicula, Fig 3c, were swimming the seas around the Paleocene/Eocene boundary and Corbisema triacantha, Fig 3b, around the mid Eocene. Thus, I have additional verification of the relative time period of these microscopic fossils. At first the Silicoflagellates were an interesting curiosity, poor cousins of the diatoms, but now knowing their value as a stratigraphic tool I will forever view these with greater respect.

Fig 3.

Fig 3.

Fig 4, below, is a collection of centric diatoms from this slide in valve view. The valve upper right measures 82 microns in diameter while the diatom lower right measures 51 microns in diameter.

Fig 4

Fig 4.

Centric diatoms come in all shapes. Fig 5 shows two Triceratiums. The diatom on the left I believe is Trinacria regina a marine fossil from the early Eocene. Being the novice that I am, I first thought that all centric diatoms should have a valve face that is circular. I failed to remembering that centric means radiating from a central point.  I wonder why a diatom might choose a triangular or square valve shape over a circular? It is interesting to note that if a diatom dose not have a raphe or a pseudoraphe  it may be grouped with the centrics making the centrics an interesting and varied collection of forms.

Fig 5.

Fig 5.

Fig 6 is a champion among fossil diatoms, measuring 190 microns in diameter! The second image is focused on the outer edge of the valve.

Fig 6.

Fig 6. Coscinodiscus?

Strew slides like this are both a blessing and a curse. Strew meaning the diatoms are strewn about under the cover slip. They are good in that there is a large number of diatoms of different types in various orientations. Bad because many of the fossils are broken fragments, or lying atop one another, or laying at angles that are impossible to image. An example of bad is that all the Radiolaria on this slide were broken, a disappointment but for this novice still of great interest. I would say without hesitation that my journey back some 50 million years with this slide was well worth the insignificant price paid, the price being much less than the price of a single movie ticket.

The black bar in the lower right hand corner measures 10 microns. The images were all acquired using a green filter and then converting to gray scale, the images are not digitally enhanced, but were cutout and pasted onto a gray field using Paint Shop Pro.

The author welcomes comments or suggestions .

References:

F. E. Round, R. M. Crawford, D. G. Mann. the diatoms Biology & Morphology of the Genera. Cambridge University press. 1990

H. M. Bolli, J. B. Saunders, K. Perch-Nielsen. Plankton Stratigraphy Volume 2. Cambridge University Press. 1985

David B. Richman. Classic and Antique Diatom Slides. Micscape. Nov 2006.