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The Mississippi River has (in one form or another since the shrinking of the Western Interior Seaway) been flowing for 70 Million Years. https://www.smithsonianmag.com/science-nature/geological-history-mississippi-river-180975509/ This may be more of a current biological rather than paleontological question, but I'm curious about the origin of the current chondricthyan diversity in the Mississippi River? Now a fair amount of you might be confused when I say "Chondricthyan diversity in the Mississippi River", but this is truly a cool case of truth stranger than fiction. The most famous species in this case is the Bull Shark (Carcharhinus leucas), a requiem shark able to tolerate both fresh and saltwater that had been confirmed to be recorded as far up the Mississippi River as Alton Illinois in 1935. Image credit: https://www.thetelegraph.com/insider/article/Researchers-affirm-two-bull-shark-sightings-16308838.php Shell, R., & Gardner, N. (2021, July 1). Movement of the Bull Shark (carcharhinus leucas) in the upper Mississippi River Basin, North America. Marine and Fishery Sciences (MAFIS). Retrieved February 12, 2023, from https://ojs.inidep.edu.ar/index.php/mafis/article/view/181 Shell, Ryan & Gardner, Nicholas & Hrabik, Robert. (2022). Updates on putative bull shark (Carcharhinus leucas) occurrences in the upper Mississippi River Basin of North America. 36. 10.47193/mafis.3612023010101. https://www.researchgate.net/publication/362847015_Updates_on_putative_bull_shark_Carcharhinus_leucas_occurrences_in_the_upper_Mississippi_River_Basin_of_North_America But these are not the only cases of Chondricthyans found in the Mississippi River as shown by the reports and articles here: http://chnep.wateratlas.usf.edu/upload/documents/Essential_habitat_of.pdf (Specifically pg. 10) Rafinesque, C.S. (1820) Ichthyologia ohiensis, or Natural history of the fishes inhabiting the river Ohio and its tributary streams, preceded by a physical description of the Ohio and its branches. W.G. Hunt, Lexington, Kentucky, 90 pp. https://doi.org/10.5962/bhl.title.6892 Jordan, D.S. and Evermann, B.W. (1896). The fishes of North and Middle America. Bulletin of the US National Museum 47:1-1240. https://www.actionnews5.com/story/12943330/memphis-woman-photographs-stingray-in-mississippi-river/ Today, the Mississippi River is one of the most industrialized rivers in the world, with many locks and dams in places that previously allowed Chondricthyans to enter as upstream as Alton Illinois. This is a phenomena that unfortunately is still occurring both in the US and across the world today from places as far away and China and Brazil, Pakistan to Russia, and Cambodia and Australia, disrupting natural river flows that make regions more prone to sever flooding effects and causing a terrifying decline in large bodied freshwater fish that migrant frequently across a whole rivers lengths (and also contributing to the Human induced climate crisis as all the dead animals that pile up at the bottom of these dams produce high amounts of methane). This phenomenon is also something I must add we as a species needs to address urgently and decisively by switching more to more renewable power sources like solar, install fish ladders and workable passages for fish to go around the dams, or legally breach the dams safely whenever possible. But not to get off track here, I'm still curious about the paleontological date of chondricthyans from the Mississippi River. How many fossils of brackish water sharks and rays have been found in the areas of the Mississippi River, was there a historically larger or smaller amount of chondricthyans in the Mississippi during the Late Pleistocene-Early Holocene, and why isn't (at least that I know of) the chondricthyans diversity in the Mississippi River even historically been as high as places as the Amazon river in Brazil? What do you guys think?