FOSSILS:OUR BRIDGE TO THE PAST

Biodiversity changes not only spatially, but also temporally [7]. This change can be measured, on numerous levels, by the number and variability of: genes, species and ecosystems [7]. Paleontological data is therefore essential for us to comprehend how biodiversity, through evolutionary processes, was generated and changed temporally [7]. Fossil records remain the integral key to reconstructing past biodiversity and are essential to gaining a historical viewpoint of the current “biodiversity crisis” i.e. sixth extinction [8] conservationists are becoming increasingly aware of.
Evolution is the driving force behind biodiversity, but this process occurs over millions of years and many taxa have become extinct over time. Fossil records are essential in reconstruction of biodiversity because not only do they allow paleontologists to study extinct taxa but also these taxa’s morphological and anatomical adaptations that developed through evolution but is now lost in extant forms of those taxa [10].

One of the most detailed fossil records is that of the vertebrate fossil group: Ichthyosaurs [6]. Fossil records indicate that ichthyosaurs originated during the upper part of the Lower Triassic but by the end of the Triassic had become highly diverse and enjoyed a global distribution [6] indicating the success of this group of vertebrates. The fossil record of ichthyosaurs emphasize the importance of this paleontological data as changes in morphological features were observed in specimens (during the early Jurassic) indicating the evolution of these animals to a more efficient aquatic body shape i.e. rear limbs became shorter, physically more powerful shoulder girdle and the caudal section of the vertebral column bent downwards [6]. Some amazing fossil specimens have the soft-tissue outline preserved (Figure 1); more detail and therefore more information could be derived from these records. A dorsal fin, a lunate caudal fin (tail) and forelimbs extending from their skeleton were observed in these unique specimens. Paleontologists could therefore derive that ichthyosaurs from this era used these adaptations to move fast within their aquatic environments, for example the lunate tail would be used for thrusting the animal foreward and the elongated forelimbs would be used for, “underwater flight”[6]. Even more of an exciting discovery was that some, very rare, ichthyosaurs’ fossil specimens contained preserved integument and could actually be examined to provide further evidence of ichthyosaur’s efficiency i.e. less drag effect in locomotion in their aquatic environments [5]. Fossil records are not only important in understanding how extinct taxa lived but also how they perished, by the middle and late Jurassic a major decrease in ichthyosaur’s diversity was observed and by studying other faunal and floral fossil records deductions can be made as to why this species eventually became extinct.


Figure 1: An Ichthyosaur fossil specimen [5].

New discoveries of fossils are increasingly important as each new discovery leads to a better understanding of biodiversity and a more accurate timeline of how diversity evolved [11]. Fossils are also used to reconstruct past environments and therefore past ecosystems. Comparing fossils of pollen, seeds and fruit with linked fauna, all of which characterize various energy levels within an ecosystem, allow the ecosystems where mammoths once lived during different stages of the Pleistocene to be reconstructed [3]. By studying the fossils of fish, microbes, pollen, plants, molluscs and invertebrates a reconstruction of the Connecticut River Valley, in its state between 135 and 225 million years ago, was possible [1]. Ichnofossils (from invertebrates) also play an important role in reconstructing past ecosystems, as they are the result of organism-substrate interactions eg. burrowing and therefore provide information on both morphological and behavioral characteristics [4] Paleosols (soil) result from interactions of different organisms with different types of substrates, paleosols together with ichnofossils proved traces of numerous extinct fauna and flora. These traces are indicators of many physical characteristics in the past environment i.e. temperature, precipitation, water chemistry, levels of oxygen even the water table level at the time all of which add to a more defined reconstructed ecosystem [4].

There are many techniques used to reconstruct biodiversity using fossil specimens. One technique uses peels of coal balls (Figure 2), coal balls are solid masses made up of majority of calcium carbonate that precipitated in ancient peat beds [2]. A large proportion of the anatomical structure of plants, which lived in these ancient coal swamps consisting of peat, was therefore preserved [2]. Coal balls are cut with a saw, producing longitudinal and transverse sections; uncovered surfaces are then etched with hydrochloric acid [10]. A sheet of cellulose acetate, with acetone, is then applied to the exposed surface, which implants fossil cell walls to the sheet, once dried the peel is removed and studied under a microscope [2]. Another technique that uses acid is the analysis of preserved sores and pollen of different plant species. The shale in which some pollen and spore fossils are preserved can be dissolved in acid, thereby exposing the small structures for further study [1].


Figure 2: Above are examples of prepared coal ball peels used in the reconstruction of a 305 million old Carboniferous coal swamp [2]

Another interesting technique used on fossils is called slice data acquisition, used to produce three-dimensional images of fossils [9]. Three-dimensional images are highly informative, as they capture morphological information not possible with the conventional two-dimensional images. There are two approaches to this technique, namely the non-destructive and destructive approach. Non-destructive approaches include: Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) whereas destructive approaches include: serial slicing and serial grinding [9].

Without fossils we would not be able to put together the many puzzle pieces biodiversity has left for us over the past millions of years, so much information on how life operates and its driving force would be lost. Biodiversity, as mentioned above, at present is in a state of crisis and if we ever have a hope of comprehending this situation, and the consequences it will have to life on earth, we have to look back to the past. We have to look to fossils.

References:
1. Abrams J, Riley e (2002) A Reconstruction of the Biodiversity of the Connecticut River Valley Using Fossil and Geological Evidence. The Traprock 1:18-22
2. Anon. Reconstructing an ancient environment. [Online]. [Cited 2007 Mar 13] Available from: www.nmnh.si.edu/paleo/PaleoArt/Techniques/pages/reconstuct9.htm
3. Borodin AV, Strukova TV, Trofimova SS, Zinoviev EV. Reconstruction of mammoth environments at different stages of the Pleistocene in the West-Siberian Plain. [Intenet]. [Cited 2007 Mar 13] Available from: www.cq.rm.cnr.it/elephants2001/pdf/267_271.pdf
4. Hasiotis ST, Dubiel RF, Demko TM. A Holistic Approach to Reconstructing Triassic Paleoecosystems:Using Ichnofossils and Paleosols as a Basic Framework. [Internet]. [Cited 2007 Mar 13] Availablr from: www.nature.nps.gov/geology/paleontology/pub/grd3_3/pefo2.htm
5. Lingham-Soliar T (2001) The ichthyosaur integument: skin fibers, a means for a strong flexible and smooth skin. Lethaia 34:287-302
6. Martill DM (1996) Fossil explained 17: Ichtyosaurs. Geology Today
7. Sepkoksi JJ (1997) Biodiversity:Past, Present and Future. Journal of Paleontology 71 (4):533-539
8. Smith AB (2001) Large-scale heterogeneity of the fossil record:implication for Phanerozoic biodiversity studies. The Royal Society 356:351-367
9. Sutton MD, Briggs EG, Siveter DJ, Siveter DJ (2001) Methodologies for the Visualization and Reconstruction of Three-Dimensional Fossils from their Silurian Herefordshire Lagerstätte. Palaeontologia Electronica 4:1-17
10. Wang SJ, Hilton J, Galtier J, Tian B (2006) A large anatomically preserved calamitean stem from the Upper Permian of southwest China and its implications for calamitean development and functional anatomy. Plant Systematics and Evolution 261:229-244
11. Zhou Z (2004) The origin and early evolution of birds:discoveries, disputes, and perspectives from fossil evidence. Naturwissenschaften 91:455-471