Dinosaurs Nonavian dinosaurs have been found in the Niobrara Chalk despite it being located hundreds of miles out to sea at the time. The most reasonable theory is that the carcasses drifted out to sea. It is unlikely that the bodies were carried out by outgoing tides along the shorelines where they died, but rather it is more probable that the dinosaurs were carried offshore by floodwaters during a storm. In the shallow waters the bodies would have begun to
decompose and bacteria within the carcass would have produced gasses that would have accumulated in the gut, thereby making the body
buoyant. Next, the prevailing winds and currents would have carried it out to sea, where it would eventually settle to the bottom and be buried in sediment. A few
caudal vertebrae from a
hadrosaur have been found with bite marks and have been eroded, suggesting at one point they were digested. A single tooth belonging to
Squalicorax was found
in situ under the vertebrae. This suggested the shark consumed the
posterior end of the tail of a floating hadrosaur carcass and had partially digested it before fossilization. Most dinosaurs in the chalk were
nodosaurs. The dinosaurs found here were endemic to
Appalachia.
Non-avian Birds Three genre of birds are present in the formation, although rare. They were unrelated to modern birds, as they still retained teeth.
Baptornis and
Hesperornis were large flightless aquatic birds suited for diving.
Ichthyornis was a seabird that resembled the
gulls and
petrels of today. Both probably preyed on small fish and were preyed upon by sharks, large bony fish such as
Xiphactinus, and mosasaurs.
Mosasaurs '' specimen which was found with a plesiosaur in its stomach Mosasaurs are the most common marine reptiles in the Niobrara Chalk and the most successful ones in the sea at the time. Several different genera representing the four different subfamilies of
Mosasauridae: the
Tylosaurinae,
Plioplatecarpinae,
Mosasaurinae, and
Halisaurinae, were present in Niobrara. They were the dominant carnivorous marine reptiles and ate cephalopods, fish, turtles, pterosaurs, birds, and even plesiosaurs. There is evidence of them consuming other smaller mosasaurs. Despite this, mosasaurs often fell prey to some of the large sharks at the time, such as
Cretoxyrhina. The presence of young mosasaurs in the formation suggests that mosasaurs were
viviparous and gave birth hundreds of miles out to sea, as Niobrara was in the middle of the Western Interior Seaway at the time. Juveniles would likely have been vulnerable to predation by the many large mid-ocean predators present in the ecosystem.
Plesiosaurs Plesiosaurs are present from two different families within
Plesiosauroidea in the Niobrara Chalk: the
Polycotylidae, or short-necked plesiosaurs, and the
Elasmosauridae, or long-necked plesiosaurs. Polycotylids superficially resemble
pliosaurs, which are not present within the formation, but are unrelated. They were fast swimmers, unlike the Elasmosaurs that used their long necks to catch fish. Plesiosaurs are rare in the formation and were therefore likely uncommon in the Western Interior Seaway at the time. Specimens become much more numerous in the
Pierre Shale situated above the chalk.
Pterosaurs Two genera of pterosaurs are present in the formation, both within
Pterodactyloidea: the
pteranodontid Pteranodon (sometimes broken into several genera like
Geosternbergia and
Dawndraco, though this is dubious) and the
nyctosaurid Nyctosaurus. They are large pterosaurs with elongated cranial crests. The pterosaurs of Niobrara probably spent most of their time at sea and rarely went on land, with
Nyctosaurus being a probably fully pelagic animal.
Pteranodon probably foraged on the ocean surface, while
Nyctosaurus was a
frigatebird-like aerial predator.
Turtles Sea turtles have been found from the Niobrara Chalk that reached large sizes. The biggest,
Archelon, was considerably larger than its distant relative, the
leatherback sea turtle, which is the largest of the sea turtles alive today. The sea turtles most likely fed on ammonites, squid, and other
cephalopods. ==Invertebrates==