They could be surface imprints of wave depositions or they could be a cross bedding of windblown or water current sediments. A third alternative is that they could be some form of irregular weathering. It's hard to tell without seeing a particular specimen.
They are mostly found in Sedimentary Rocks
Fossils are principally found in sedimentary rocks and also in some metamorphic rocks of sedimentary origin.
Fossils are nearly always found in some type of sedimentary rock. Limestone is a sedimentary rock which very commonly contains fossils, as is coal.
They are not found everywhere, in some places metamorphic rocks are to be found and in other places igneous rocks are to be found.
Some fossils are found in the sedimentary rocks - along with shells and sediment.
Sedimentary rocks can be found anywhere rock particles, clay, and organic matter has deposited itself and lithified from overburden pressure or chemical precipitation from a saturated solution. Sedimentary rock covers 75% of the surface of the Earth's continents.
Canada's largest deposits of sedimentary rocks can be found in Alberta and Saskatchewan (the Great Plains area). Some of the minerals found inside of sedimentary rock include coal, oil, natural gas, and potash.
Calcite is a mineral commonly found in sedimentary rocks. It can also be found in some metamorphic rocks, as it can recrystallize under heat and pressure. It is unlikely to be found in igneous rocks except in trace amounts.
Copper is primarily isolated from copper ores such as chalcopyrite, bornite, and chalcocite. These ores are typically found in igneous and sedimentary rocks, as well as in hydrothermal veins. Copper can also be found in some metamorphic rocks.
Nickel is predominantly found in igneous rocks, such as peridotite and basalt. It can also be found in some sedimentary rocks like certain iron ores.
Sedimentary rocks: Metamorphic rocks
It could most certainly be found in sedimentary rock in areas of past volcanism, and is, in fact, found in some limestones.