It looks to me like a fossil that was hollow, top layer eroded. In the bottom pic,whilst it is covered in seashells it is still obvious that the rock type where the arrows are pointing is dffernt to the surrounding rock.
i dont think this is a fossil, but an unusual sedimentary structure. layers of these rocks have been put down in layers and in this case ether before it became rock or afterwards it has become deformed into these ripples. the sea has then eroded away the top serface leaving this pattern. where was the picture taken?