Fairy tales from the sea near the mouth of the river: How climate change has affected oysters

In terms of lithostratigraphy, Profile Hôrka belongs to the Orlové sandstone and is located at the foot of the hill of the same name by the village Považské Podhradie. Orlové sandstones were formed during the Cenomanian and Turonian (Upper Cretaceous) periods. Many geologists and palaeontologists have studied them in the past. However, to date, there has been no systematic study of the numerous colonies of the oyster species Rhynchostreon suborbiculatum.

The sedimentary conditions prevailing in the high-energy environment near the estuary facilitate a wide range of methods applicable to their study. Sediment grain size, population analysis of the species, taphonomic analysis of the shells, and the potential of geochemical methods (especially the main oxides) have become the basis for the high-quality output of this work.

One of the most intriguing methods applied is the orientation of oyster shells to examine the reconstruction of flow, energy characteristics, and variability of palaeoconditions over time. In general, articulated oyster shells indicate low-energy conditions that are characteristic of the ocean’s rest regime. However, the increase in dynamics due to the occurrence of storms results, among other things, shows in an irregular arrangement of shells in the sediment. The increased environmental energy is also evoked by the presence of exclusively disarticulated oyster shells in these positions (Fig. 1).

Fig. 1: Scheme of behavior of a living oyster colony during one episode of resting and less resting cycle in a high-energy environment; A-C individual phases of the cycle, author: Jakub Rantuch

The decrease in the intensity of freshwater inflow into the peripheral-marine system (and thus the expected increase in the salinity of water bodies) is mainly due to appreciable climate change, most likely aridisation (a process in arid areas where evaporation is greater than precipitation). The increase in the ratios of selected main oxides (Na₂O/TiO, Na₂O/K₂O, Na₂O/Al₂O₃ ratios) also corresponds to such an interpretation (Fig. 2).

Fig. 2: Selected ratios of major oxides showing wet and dry cycles, author: Jakub Rantuch

These changes represent a significant palaeobiological selection pressure, which has led to morphological changes in the construction of the left oyster valve. Significant correlations in changes in size and other parameters can be observed (Fig. 3). An increase in the size of the shells accompanied by a decrease in the size of sedimentary particles may indicate regional eustatic changes in sea levels, which is also reflected in an increase in the bioturbation index (a scheme for quantifying the degree of sediment bioturbation).

Fig. 3: Changes in selected ratios (average values) of biometric parameters in correlation with the average size of the left shell. Abbreviations: H / L (height to length ratio), W / L (width to length ratio), W / H (width to height ratio). The size of the left dish is represented by the parameter L (length can be considered the most suitable indicator of ontogenetic age). There is a significant correlation in the decrease of the measured ratios together with the increase of the parameter L (upwards), author: Jakub Rantuch

This intensive long-term work challenges generally accepted knowledge of the studied oysters as a strictly stenohaline element (an organism that can only live under certain salinities of the aquatic environment). The researched profile provides us with valuable information on the alternating influence of freshwater inflow and the alternation of drier and wetter cycles within the Upper Cretaceous period.

I would like to thank the author of the study for his valuable notes to the first draft!

Jan Geist

Jakub Rantuch (2021) Behaviour of the oyster Rhynchostreon suborbiculatum assemblages in a Cenomanian-Turonian river dominated marginal-marine environment: multidisciplinary study from the Pieniny Klippen Belt (Western Carpathians, Slovakia), Historical Biology, 33:12, 3340-3360, DOI: 10.1080/08912963.2020.1867124