Chitin-protein complex in theOrdovician organic fossil Byronia (Scyphozoa).

CHITIN-PROTEIN COMPLEX IN THE ORDOVICIAN ORGANIC MICROFOSSIL?

Grażyna Mierzejewska and Piotr Mierzejewski

Acta Medica Polona 1979, 20, 1, 33-34

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Graptolite Net | Graptolites and Graptoliters | Evolution of Graptolites | Scolecodonts | Organic Microfossils

Organic skeletons of Palaeozoic marine invertebrates have been studied for several years with the methods of electron microscopy. In the past the structural material of Chitinozoa, Graptolithina, jaws of Polychaeta and other organic microfossils was regarded to be chitin. Ultrastructural studies have not shown any traces of chitin-protein complexes in all studied microfossils, although chitin is the most abundant organic skeletal component of invertebrates. In the present note the first though slightly doubtful finding of chitin-protein complex in the periderm of an organic microfossil is described.

Material and methods. - The material under study consists of fragments of Ascosyrinx robustus Kozłowski, 1967 [now classified as the scyphozoan Byronia robusta (Kozlowski)], an enigmatic microfossil known from the erratic Ordovician boulders of Poland. This fossil was etched from the limestone with 10% acetic acid and embedded in Epon 812. Sections were cut with the diamond knife on a Tesla microtome and examined under a Tesla transmission electron microscope. Supplementary studies have been performed using scanning electron microscope Stereoscan 180.

Results and discussion. - SEM studies on the fine structure show that periderm of A. robustus is constructed of numerous layers. TEM sudies confirm these observations and show that each layer is composed of arcuate fibrils (Fig. 1, x 23300). The fibrils are made of electron dense substance. The matrix surrounding fiblis is made of electron lucent material and is essentially homogenous. The presence of layers with arcuate fibrils forms specific ultrastructure usually defined as parabolic patterns. The fine structure of this type is typical for periderm of scyphopolips and the cuticle of Arthropoda, i.e. skeletons composed of chitin-protein complexes. According to Bouligand's model, the parabolic patterns derive from oblique sectioning of helicoidal supestructure. It is quite likely that the observed fine structure of A. robustus periderm represents also a chitin-protein complex. The best technique for revealing the parabolic patterns of chitin-protein complexes in thin sections is to stain the surrounding matrix with potassium permanganate and lead citrate (negative contrast of Neville and Luke, 1969). The parabolic patterns are well visible in thin sections of A. robustus without staining. The staining with the mentioned above method has been completely ineefective in this case. These facts suggest a deep degradation of chemical composition of A. robustus periderm as well as "self-staining" and "selffixation" of fine structure during the process of fossilization. The "self-staining" is positive in character in contradictinction to the negative contrast of Neville and Luke.
It must be remembered that ultrastructure if form paraboli patterns is quite common in animals. Similar parabolic patterns can be formed by chitin-protein complexes, proteins- cholesteric liquid crystals, tunicin fibrils as well as chromosomal DNA. For this reason the problem of chemical composition of A. robustus periderm requires palaeobiochemical studies. Nonetheless, we suppose that the presence of a chitin-protein complex in the studied microfossil is convincing, The application of a highly specific, accurate and quantative enzymatic method of Jeuniaux (1963) might resolve this problem finally.

References:
Jeuniaux C.: Chitine et chitinolyse, Masson, Paris, 1963.
Neville A.C., Luke B.M.: Molecular architecture of adult locust cuticle at the electron microscope level. Tissue and Cell 1, 355-356, 1969.