Not easy to explain

The majority of the tracks and traces are not well preserved and thus harder to interpret, if the interpretation is possible at all.
I have photographed something that looks like a sequence of rhytmical skratching of the shallow sea floor My first guess was: the small pterosaur swim traces (highlighted in the circles); the second guess is the water bird swim traces; the third is the trace fossils of fish fins swishing along bottom of shallow water;... or the artifact. Which one would you choose?
The age is Mid-Cretaceous (late Albian) of Pula.