What causes the apparent location data for Xenacanthus to be skewed compared to marine sharks?

Its preservation is linked to geological processes preserving fluvial sediments

There is an interesting taphonomic bias when comparing the fossil records of Xenacanthus to marine sharks. Since Xenacanthus lived exclusively in freshwater, its fossil record is inherently tied to the geological processes responsible for preserving inland sedimentary structures, such as ancient riverbeds, swamps, and lake deposits. This means the *apparent* geographical distribution recorded by fossils might be skewed toward areas that possessed excellent preservation conditions for these fluvial sediments, rather than accurately reflecting the absolute highest density of living sharks. Marine sharks, conversely, are preserved in different sedimentary contexts, meaning the comparison of their fossil counts is not directly equivalent due to differing preservation regimes.