Dear Miroslav,

It seems yes, it is fossils, may be Bivalves.

Dear Mr. Ivanov,

the rock has obviously been taken from the Late Triassic Megalodonta Limestone ("Kuhtritt" = cow step limestone). It is a very widespread facies rife with fossils mainly the bivalves which were used as namesake for the denomination of the calcareous rocks.

Best regards

H.G.Dill

the rock in the two photos resembles a lot the Dachstein Limestone of the Northern Calcareous Alps, often containing abundant Megalodon sp.

E.g.: https://commons.wikimedia.org/wiki/File:Kuhtrittmuscheln_Dachstein.JPG

The fossils in the given picture appear to be of  Bivalvia, a class of marine and freshwater molluscs ( early Cambrian–recent).

you can further refer....

R.C. Moore, in Moore, Lalicker, and Fischer, 1952, Invertebrate Fossils,

which gives a practical and useful classification of pelecypods (Bivalvia), based on shell structure, gill type, and hinge teeth configuration.

Bioclasts of different mollusk shells.

I think also different mollusk shells, bivalves and gastropods, widespread

All the best,

Hans

Gasteropods are also represented (the fossil with the two locules)

Brachiopods are also possible

Yes, many of these are sections of bivalves.  Several appear to have sectioned a very thick hinge area indicating one of the large Mesozoic oysters or perhaps Isognomonidae.

Pelecypods  is clearly seen by sections of some intact bivalves

.

it is oyster shell fragments.. (bivalves

)

some of which are bivalves and others are gastropods

packstone (left) to grainstone (right) with oyster shell sections and others bivalves, and some gastropods

Madani's observation suggests me a possible difference between the rock of the two photos and the Dachstein Limestone of the Northern Calcareous Alps.

While the Dachstein Limestone shows the Megalodon bivalves mostly in-situ, fossilised in the position they lived and grew, this rock could be generated by resedimenting a bivalve-rich calcareous sediment, resulting in a graded bed, with a base (right) rich in bivalve shell, and a top (left) with much less bivalves. This would also explain the relative scarcity of connected and articulated shells.

Dear Miroslav,

The hand specimen is likely limestone and is full of shell fragments (bivalves), but most probably it will contain other microfossils too. It is better to prepare good thin sections for microfossil analysis.

Good luck,

Habib. 

Mr. Miroslav,

The thickness and size and irregular sets is typical of oysters, but in my experience this type of oyster/bivalve is more typical of Cretaceous sediments.  It appears the rock is not in place but floating in the rock slope.

Goog Luck 

Paul Grover

I suggest it will be much better if you prepare thin sections of this rock unit in order to check its fossil content and to reconstruct of precise age. Best regards.

Amr Zaky

It certainly looks like molluscs, but just as Paul Grover said above, the age of the rock may be wrong, and it is definitely not an in-place block.

Definitely mollusks - oysters, and some clams and gastropods as stated in the other comments.  The larger, very thick shells and triangular hinge structures are most likely something in the Ostreacea, probably Gryphaeidae.  Some of the genera existed in the uppermost Triassic, but were much more common by the Cretaceous.