An early late Campanian transgressive lag, a 'shark tooth bed', in the local base of the Demopolis Formation overlying the Coffee Formation at the Frankstown site in Prentiss County, northeastern Mississippi, reveals a diverse vertebrate fauna of fragmentary remains pertaining to a wide variety of habitats of the northern Mississippi Embayment. By comparing the Frankstown taxa to those of less mixed faunas elsewhere in America, it is possible to separate the large primary shallow-water marine fauna from the smaller secondary deep-water marine, estuarine, fluviatile and terrestrial faunas mixed with it. The site includes 53 taxa---the most diverse Cretaceous vertebrate fauna known from Mississippi, including a chimaeroid, 13 sharks, 5 rays, 15 bony fish, 6 turtles, a lizard, 5 mosasaurs, a plesiosaur, 2 crocodilians, and 4 dinosaurs. Many of these (26) are new records for the state. A review of the 42 known Mississippi Late Cretaceous vertebrate taxa not present at the site is also included, 26 of which are also new records for the state. The deep-water (pelagic) marine fauna was likely washed inshore by storms, but the other secondary faunas were probably reworked from underlying beds. By separating the various faunas of a diverse transgressive lag assemblage, a broad overview of the paleoecology of the area can be obtained from a single site. Vertebrate faunas from the base of a stratigraphic sequence are fundamentally different in nature from those of the deep-water marine (maximum transgression) middle part of a sequence, and not, as has been thought, just a poorly preserved high-energy version of the well-preserved low-energy middle sequence fauna Based on X-ray diffraction analyses done for this study, it is argued that, in an early stage of lag formation, unstable phosphate was leached from the vertebrate material of the lag, and redeposited as a more stable form in wood, rip-up clasts, and steinkerns Contrary to previous work, it is shown that the shallow-water marine vertebrate fauna of the American Cretaceous changed little through that time---there was no major faunal change after the Turonian
