Figure 2.

(a–e) Microstructural features of Gigantoproductus sp. Shells under petrographic microscopy, cross-polarized micrographs; (a,b,d–f), and cross-polarized with lambda plate micrographs. (c) Scheme of the shell with underscored shell regions. On the right, a line which divides the secondary and tertiary layer into the thick region (micrographs (b,d,f)). (a) The laminar microstructure of the dorsal valve with micritized patches (upper part), micrite infilling between the dorsal and the ventral valves, and inner region of the ventral valve (lower part). Scale bar 2 mm. (b) Innermost part of the thick region, showing ventral valve with columnar microstructure and several growth lines. Scale bar 1 mm. (b1) Inset showing a laminar growth line. Scale bar 100 µm. (b2) Inset showing a diffuse growth line. Scale bar 100 µm. (b3) Inset showing pseudopunctae in a growth line. Scale bar 100 µm. (d) Umbonal region of the ventral valve with several growth lines and a chaotic crystal arrangement. Scale bar 1 mm. (e) The thick region of the ventral valve with the largest columnar crystals and several growth lines. Scale bar 1 mm. (f) Thin region of the ventral valve with columnar microstructure and several growth lines. Scale bar 1 mm.