3.2. The degree of preservation of the model SBO in different sediments.
In order to quantitatively assess the degree of preservation of the buried nauplii, we assigned them to five preservational groups which reflect successive stages of decomposition: From the perfect preservation of the body shape, the limbs, and sometimes also the gut (preservational groups 1 and 2) to partial (groups 3, 4) or complete (group 5) decay of the limbs and the gut (see Materials and Methods for details).
The best preserved carcasses (group 1) from the montmorillonite, kaolinite, and silica retained very fine external anatomical structures such as the filter apparatus on antennae 2 and the chaetae (Figure 2 F, G). They also preserved the naturally shaped remains of the gut (Figure 2 A, E, Figure 3 A), which usually rapidly dissolves when buried in liquid media without sediment (Butler et al., 2015). We never observed such fine anatomical details in the representatives of groups 3, 4 and 5.
Group 2 was comprised of well-shaped rigid external integument with finely preserved limbs but completely decayed internal anatomy (Figure 2 D). This group is specific to decay in sediment as it was abundant in all sediments but almost absent in the sediment-free control (Table 3; Butler et al., 2015; Naimark et al., 2018b).
The montmorillonite experiment continued for 5 years, and we expected the majority of the remains to be in groups 4 and 5. Contrary to our expectations, the carcasses from this five-year-old sediment were the best preserved (Figure 2 A, 3 A), with the majority of the remains belonging to groups 1 and 2.
To quantitatively compare the preservation of the nauplii in each sediment, we used the ratio of the number of well-preserved specimens (groups 1 and 2) to the number of poorly preserved ones (groups 3, 4, and 5) (Table 3). This ratio (we called it “degree of preservation”) varied widely across the sediments, being the highest in the montmorillonite and kaolinite and the lowest in the clinochlore. However, the score for the clinochlore was still higher than for the sediment-free control (Table 3).
The results imply that the preservation potential of SBO improves considerably when the carcass is buried in sediment, and that different sediments may provide different opportunities for preservation. In order to elucidate the main factors controlling the preservation, we analyzed changes in pH and mineral composition of the sediments and in the elemental and chemical composition of the carcasses.