Platy limestones are of primary interest to many palaeontologists as they are considered to be Fossillagerstätten – sedimentary deposits exhibiting extraordinary fossils in exceptional states of preservation, oftentimes even with soft tissue. As defined by Seilacher (1970), these are accumulations of fossils that in terms of their numbers and/or quality of preservation, give an unusually large amount of palaeological data (in addition to fish and reptiles, here the occasional fossil starfish is also found!). Already at the end of the 19th century, Viennese geologist Stache introduced the term ichthyfern shales into the scientific literature, referring to the platy limestones of Komen in Slovenia and the coastal areas of Croatia (Hvar), and since then this has been a subject of numerous papers and debates. Recognised as a separate facies within the thick carbonate sequences of the younger Mesozoic, the fish-bearing layers in the stratigraphic study of the Dinarides have been interpreted in various ways.
This facies with special palaeographic and sedimentary properties settled in the shallow seas of the one large tropical Tethys Sea, which extended between the Eurasian and African continental blocks from the Palaeozoic to the Tertiary. The platy limestones appear only in some areas within the great shallow sea carbonate basin, dominated by the biolithic complexes of the rudist limestones. Due to the constant vertical oscillations of spaces between these bodies, they occasionally took on the property of being closed lagoons and protected areas with somewhat reductive properties, where the fish died, dropped to the bottom and were quickly covered with sediments.
On the other hand, fish fossilisation is an exceptional process (as dead fish are more prone to rapid decomposition than other vertebrates), directly connected with the complex morphology of these vertebrates, and depends on other biological and physical parameters controlled by the special conditions on the sea bed, as a prerogative for fossilisation. It must be emphasised that not all platy limestones contain fish fossils, nor will fish fossils only be found in platy limestones. Evidently, Fossillagerstätten only appears under those conditions in which all the palaeoecological parameters are in perfect interaction.
This facies with special palaeographic and sedimentary properties settled in the shallow seas of the one large tropical Tethys Sea, which extended between the Eurasian and African continental blocks from the Palaeozoic to the Tertiary. The platy limestones appear only in some areas within the great shallow sea carbonate basin, dominated by the biolithic complexes of the rudist limestones. Due to the constant vertical oscillations of spaces between these bodies, they occasionally took on the property of being closed lagoons and protected areas with somewhat reductive properties, where the fish died, dropped to the bottom and were quickly covered with sediments.
On the other hand, fish fossilisation is an exceptional process (as dead fish are more prone to rapid decomposition than other vertebrates), directly connected with the complex morphology of these vertebrates, and depends on other biological and physical parameters controlled by the special conditions on the sea bed, as a prerogative for fossilisation. It must be emphasised that not all platy limestones contain fish fossils, nor will fish fossils only be found in platy limestones. Evidently, Fossillagerstätten only appears under those conditions in which all the palaeoecological parameters are in perfect interaction.
Thin layers of the facies of platy limestones, island of Hvar
In the middle of the Cretaceous period, the sedimentary space of the carbonate platform featured substantial diversity. In the favourable climate conditions, rudist biolithic bodies were created, protecting certain palaeoareas like a barrier against the influences of the open sea, creating a closed lagoon as an area of protected sedimentation. Due to oscillations of the seabed in the basin, some areas were deepened and, in these reductive areas, micrites containing the remains of fish began to form, and the sediment took on the properties of platy limestone that often laterally and vertically became stromatolite limestones. In the shallower segments of the lagoons, in the littoral areas and even on the slopes towards these local depressions, algal formations were created, stromatolites, by the accumulation of micrite materials on algal filaments. This is how the stromatolite limestones with laterally linked hemispheroids (LLH type) were formed.
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