What environment did Xenacanthus prefer over the open ocean like most relatives?

Freshwater systems

The ancient shark Xenacanthus is noted for presenting a compelling puzzle regarding its historical distribution because, unlike the majority of its close relatives which inhabited the open ocean, its preferred and obligatory habitat was freshwater systems. This crucial distinction means that tracing its historical locations requires examining sedimentary basins and geological formations that represented large, stable, non-saline bodies of water, such as extensive river networks or ancient lakes, rather than mapping modern marine coastlines or ancient seas. This fundamental ecological separation determined where its remains could be successfully preserved as fossils millions of years later, setting it apart from typical marine shark discoveries from the Paleozoic Era.