So God created great sea creatures and every living thing that moves, with which the waters abounded, according to their kind, and every winged bird according to its kind. And God saw that it was good. And God blessed them, saying, “Be fruitful and multiply, and fill the waters in the seas, and let birds multiply on the earth.”(Genesis 1:21-22) NKJV
Sedimentary deposits containing sharks and bony fish can be assigned to the Mississippian because of how the evolution narrative, of development from primitive to advanced, overlays with the geological timescale.
Assigning fossils to periods of time has nothing to do with radiometric dating. Within the evolution narrative bony fish (Teleosts) are considered to be an evolutionary advancement compared to cartilaginous fish (Elasmobranchs) which means that sediment with teleost fish in it can be placed later in the narrative. Ironically sharks are Elasmobranch fish, but, and here’s the fudge in the narrative, supposedly more advanced than the primitive Elasmobranchs. Primitive and advanced are a microcosm of circular reasoning which is only relevant in phylogenetic systematics. The creation is spherogenetic, not phylogenetic. As such the geological timescale is whack-a-mole induction.
The narrative is establishing the premise that older Devonian sediment contains fossils of primitive Elasmobranch fish, and more recent Carboniferous sediment contains fossils of advanced Teleost fish. Except for the inconvenient fact that the Mississippian sediment contains both.
Not only that but there’s no direct way to correlate sediment containing sharks and bony fish with the buried remains of a Mississippi swamp. There are no common indicator fossils, the sediments are in different locations. All there is, is the narrative.
Popular science (SciPop) doublespeak is used to make it seem as if the Mississippian is a period of time which included all kinds of ecosystems, even though they weren’t buried together. We may deduce that Mississippian sediment was deposited during the inundation phase of Noah’s flood (Middle Noachian).