Figure 5.

Representative FT-IR spectra of the studied fossil, Polysphaeroides filiformis, and modern specimens of Calothrix sp. BCCM/ULC003 (sheath), Stigonema ocellatum SAG48.90 and Bangiopsis franklynottii CCMP3416

The bands represented by the green lines are more similar between P. filiformis and the two cyanobacteria in contrast to B. franklynottii, which also shows these bands but with a smaller intensity. The red line represents the major band of the polysaccharidic region of B. franklynottii that is more intense compared to bands with green lines. This red line, although also visible on P. filiformis spectrum, is less intense than in B. franklynottii CCMP3416. There is an inversion between the major bands of the polysaccharidic region of P. filiformis and modern cyanobacteria versus B. franklynottii CCMP3416. These observations suggest a closer similarity between P. filiformis and the studied cyanobacteria and a similar polysaccharidic composition for the fossil sheath to sheaths of the two modern cyanobacteria (Calothrix sp. and S. ocellatum SAG48.90). The blue lines represent bands related to aliphatic CH₂ and CH₃. The green and red lines represent the bands related to C-O-C, C-O, C-C functional groups.