The Mandible Ultrastructure

EDS analysis on one specimen demonstrates that the mandibles have the following chemical composition in percents of the total weight: C (58-53%); O (30%); S (2%); Si (1-5%); Fe (1-2%), and Ca (less than 1%). The lack of significant amounts of iron and calcium indicates that neither pyrite nor calcite is an important preservational element in these fossils. Even more significant is the lack of phosphorus. Many soft tissues in fossil coleoids and the chitinous mandibles of all fossil cephalopods are commonly replaced by phosphorus in the form of apatite (for examples, see Dagys and Dagys, 1975; Mapes, 1987). However, the EDS data indicate that replacement by apatite or any other phosphorus mineral has not taken place in Austrotrachyceras. Thus, the preservation of the mandibles in these ammonoids is by concentrated carbon. For the ultrastructural comparison, the mandibles of modern squid (Loligo) were studied with SEM (Fig. 11. 8E, F).

Most mandible studies have utilized standard light microscopy for the descriptive analysis. The only study we are aware of utilizing SEM analysis on

Fig. 11.8A-F (continued) after one year drying. E. Broken surface of a longitudinal fracture showing the steplike conchoidal fracture surface; faint laminae are present. Scale bar = 60.0 pm. F. Enlargement of the fracture surface of the mandible showing a faint granular texture. Blob with fractures near the center is a SEM "burn" showing that the organic material is very sensitive to even short term exposure of the electron beam. Scale bar = 6.0pm.

Fig. 11.8A-F A-D. Austrotrachyceras sp. NHMW2005z/0006/0014, mandibles. A. Mosaic fracture pattern of the mandible. Scale bar = 0.3mm. B. Enlarged detail of A showing the conchoidal fracture surface. Scale bar = 30.0/jm. C. Enlargement of the fracture seen in B showing the stepped fracture surface. Scale bar = 3.0 fim. D. Fracture surface showing the ultrastructure of microporosity of small microslits. Scale bar = 1.5 fim. E, F. Modern squid (Loligo) mandibles

Fig. 11.8A-F A-D. Austrotrachyceras sp. NHMW2005z/0006/0014, mandibles. A. Mosaic fracture pattern of the mandible. Scale bar = 0.3mm. B. Enlarged detail of A showing the conchoidal fracture surface. Scale bar = 30.0/jm. C. Enlargement of the fracture seen in B showing the stepped fracture surface. Scale bar = 3.0 fim. D. Fracture surface showing the ultrastructure of microporosity of small microslits. Scale bar = 1.5 fim. E, F. Modern squid (Loligo) mandibles ammonoid mandibles is the present report. It could be concluded that the original material was organic and that the carbonization had taken place to preserve the mandibles in situ.

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