Table 4.

Summary of structural and geochemical features of the five states of preservation identified.

Features		Preservation States
		Well-Preserved			Poorly Preserved
		Tertiary Layer		Secondary Layer	Tertiary Layer	Growth Lines	Dorsal Valve	Secondary Layer
Structural	Microstructure	Prismatic crystals with noticeable microlaminae, preferential orientation	Growth lines: original organic-matrix remains and preferential orientation of the crystals	Lath crystals, original organic-matrix remains and preferential orientation	Recrystallisation (e.g., degrading neomorphism), sparite cements, filled fractures, small patches of crystals without a preferential orientation	Recrystallisation, delamination	Recrystallisation, matrix infilling, micritzation, sparite cements, filled fractures	Recrystallisation, delamination, micritized patches, matrix infilling, filled fractures
	Cathodoluminescence	Non-luminescent	Slightly luminescent, luminescence pattern in growth lines	Slightly luminescent, luminescence banding	Non-luminescent	Increasing luminescence near fractured areas	Luminescent	Slightly luminescent
Geochemical	Crystallographic	Highly arranged crystals: c-axis oriented perpendicular to shell growth	Highly arranged crystals: c-axis oriented subparallel to shell growth	Highly arranged crystals: c-axis oriented parallel to shell growth	Poorly arranged crystals: c-axis randomly oriented
	MTE (ppm)	Mg: 3380, Sr: 1737, Mn: 155, Fe: 184	Mg: 4164, Sr: 2208, Mn: 314, Fe: 371	Mg: 4444 Sr: 2283 Mn: 182 Fe: 370	Mg: 4651, Sr: 1763, Mn: 278, Fe: 955	Mg: 3765, Sr: 1922, Mn: 971, Fe: 858	_	Mg: 4849, Sr: 2323, Mn: 117, Fe: 838
	δ13C–δ18O	Low dispersion of the ratio δ18O: −3.78‰ to −2‰; δ13C: 2.09‰ to 3.4‰; grouped in clusters depending on ontogeny development			High dispersion of the ratio δ18O: −6.28‰ to −1.83‰; δ13C: 1.18‰ to 4.27‰
	87Sr/86Sr	Low dispersion of the ratio: 0.707830–0.707860			High dispersion of the ratio: 0.707807–0.707893