One of the formations in North America targeted by Othniel Charles Marsh during the 1870’s and later George and Charles Sternberg, this is the most important Cretaceous marine formation of all. The best-known unit of this formation is the Smoky Hill Chalk Member, a series of chalk and shale formations situated in much of Kansas.
It shows a record of life in the whole of the Western Interior Seaway that covered the center of North America during much of the Cretaceous. This sea, also called the Niobrara Sea after the rock formation, stretched from the Arctic Ocean to the Gulf of Mexico which was part of the vast Tethys Ocean.
These shallow seas are known as epicontinental seas. North America itself was cut into two islands, Laramidia in the west and Appalachia in the east.
Quite ironically, it was the smaller island of Laramidia that bore the greatest diversity of stem-birds, also with countless hadrosaurs and ceratopsians as well as numerous tyrannosaurs. It was very geologically active, with the Rocky Mountains under construction toward the end of the Cretaceous. This was the Laramide orogeny.
It was an age of continual mountain-building and the sea shrank and grew with the forces exerted by these geological processes. At this time the North American plate came into contact with some others and this caused subduction, thus raising new mountains.
As the period drew to an absolute close, more land was lifted up from the north. This is now known as the Laramie Formation of Colorado. At this time the sea began to regress more than it had before.
It had shrunk away into the smaller Pierre Sea. This arm of the Western Interior Seaway persisted as late as 66 million years, to the end of the Mesozoic Era.
The Niobrara Formation records life at the heyday of this seaway, from 87 to 82 million years ago between the Coniacian and the Campanian Stages of the Late Cretaceous. Prior to the Niobrara was the rather poorer Dakota Formation and after that comes the Pierre Shale. Contemporaries to Niobrara include the Mooreville Chalk of Alabama.
As in any sea there was a massive variety of fish, ranging from typical herring and sardine-like plankton feeders to much more massive groups. The biggest bony fish here was a filter feeder as well, the big-mouthed Bonnerichthys.
It was 6 meters long, hardly the largest fish here. The macropredatory shark Cretoxyrhina or ‘ginsu shark’, named after its slicing teeth, was the biggest carnivorous fish here.
The fanged tarpon-like speedster Xiphactinus, or ‘bulldog fish’, is just as huge but much more streamlined. It was a member of a group called ichthyodectids. These were large, agile pelagic animals which both hunted smaller fish and filtered their food. However, not even they are as big as the largest Ptychodus.
This shark was a bottom-dweller which lived in the rudist and clam reefs of the time. The clams it ate were as big as a tub and it too grew to more than 10 meters long, the size of modern whale sharks.
It probably resembled a nurse shark, but on a far more awesome scale. The largest species was Ptychodus mortoni. Ptychodus went extinct in the Santonian Stage, 85 million years ago while Cretoxyrhina’s reign of terror ended about 82 million years ago.
The smaller ‘crow shark’ Squalicorax with its worldwide distribution and generalized eating habits however, managed to sail by with no problems. The biggest animals here were marine reptiles, specifically mosasaurs and plesiosaurs.
Mosasaurs were no more than highly adapted lizards but they looked like a cross between a reptile and a shark. They had very robust, fish-like bodies and a tail fin for speedy swimming and perfect maneuverability. Although they evolved from small lizards ten million years earlier, they ended up as absolute leviathans by the end of their reign.
The very first wave of advanced mosasaurs arrived 87 million years ago and kept growing. The biggest was Tylosaurus. This huge reptile is known from remains of its skin, stomach contents and bones, and it grew to 15 meters in length.
However it was still very much lighter and leaner than some later mosasaurs. Its range has been extended to Alaska, where evidence of scavenging has been found. The mosasur's dinner was the carcass of a duckbill dinosaur and the scavenger was the most well-known species, T. proriger.
Some of the most spectacular mounts of this genus include the ‘Bunker’ specimen from Western Kansas and ‘Bruce’ from Manitoba’s Bearpaw Shale. The latter was a separate species known as T. pembinensis.
Also, we know that mosasaurs had a similar coloration to modern whales like the Bryde’s whale. They had darker backs and light undersides.
The plesiosaurs are famous for having been cast as sea monsters in popular media. Though they were unable to curve their necks like popular depictions of the Loch Ness Monster, they could raise their necks straight up out of the water if necessary.
The largest of the long-necked plesiosaurs was probably Elasmosaurus, at 14 meters in length. These massive animals were able to tackle both small and moderately large prey by using the massive gape of their jaws.
Polycotylus was a short-necked, medium-sized plesioasur as big as an open-water shark, with lengthy jaws for grabbing fish. It was probably very much faster than its huge cousins.
One particular fossil shows a mother with a single, large baby still in her belly. This suggests that plesiosaurs had fewer young than most reptiles. They also invested more care in their offspring, similar to modern whales. Mosasaurs also gave live birth, being far too specialized to leave the water. Plesiosaurs used all four flippers to swim due to having a highly reduced tail.
The birds here include the large diving bird Hesperornis and the gull-like Ichthyornis. Hesperornis was like a penguin in looks, but had lobed feet like a grebe.
As for the pterosaurs, the most iconic of all was Pteranodon with its bony crest. This pterosaur can be reconstructed with a great deal of confidence as a fish-eater similar to a gannet or an albatross. It would have soared over the waves, plucking out prey from just below the surface.
Or if it felt a bit more adventurous, it would have dived. Alongside it was the small, short-lived Nyctosaurus. This animal had a huge, thin antler-like crest projecting from its head. It also lacked the other fingers on its hands, sporting only the wing finger.
