they could have pursued active prey, though they probably could not have sustained high speeds. Overall, it may have been a moderately-good swimmer, capable of open-ocean travel. Archelon, like other marine turtles, probably would have had to have come onshore to nest; like other turtles, it probably dug out a hollow in the sand, laid several dozens of eggs, and took no part in child rearing. The right lower flipper of the holotype is missing, and stunted growth of remaining flipper bone indicates this occurred early in life. It may have been the result of attempted predation by a bird while a hatchling and trying to escape to the sea, bitten off by some large predator such as a mosasaur or a Xiphactinus, or was crushed off by larger adults while herding on the shore. However, the latter is unlikely as juveniles probably did not frequent the coasts even during breeding season. Brigitta is estimated to have lived to 100 years, and may have died while partially covered in mud brumating–a state of dormancy–on the ocean floor. However, the longstanding belief that marine turtles brumate underwater like freshwater turtles may be incorrect given the high surfacing-frequency needed to prevent drowning. Paleoecology Archelon inhabited the shallow Western Interior Seaway; the muddy, oxygen-depleted seafloor was probably, on average, no more than below the surface, and average water temperature may have been in the Campanian. The Late Cretaceous Dakotas were submerged in the Northern Inland Subprovince, an area characterized by moderate to cool temperatures, with an abundance of plesiosaurs, hesperornithiform seabirds, and mosasaurs, particularly Platecarpus. There is no fossil evidence for vertebrate migration between the northern and southern provinces. Though sharks were generally more common in the southern province, several sharks are known from the Pierre Shale, including Squalus, Squalicorax, Pseudocorax, and Cretolamna. Other large predatory fish include ichthyodectids such as Xiphactinus. The Pierre Shale's invertebrate assemblage includes a variety of mollusks, namely ammonites–from the Pierre Shale Placenticeras placenta, Scaphites nodosus, Didymoceras, and Baculites ovatus–bivalves–such as the giant Inoceramus– the squid-like belemnites, and nautilus. As the seaway progressively migrated southward, it is possible Archelon was unable to migrate with it. The increasing threat of egg or hatchling predation by new marine or mammalian species may have led to the extinction of Archelon, and the disappearance of gigantic protostegids seems to have coincided with the increasing size of dermochelyids. Protostegidae is more-or-less absent in