Friday, 15 June 2012

From the Tethys to the Niobrara


About 70 million years after the muds and shales of the Oxford Clay Sea were being deposited, the continents began to shift toward their present day configuration. As the Atlantic Ocean widened the once mighty super-continent of Gondwanaland broke up and the continent of North America was subject to volcanism, mountain building and particularly subduction. This, in turn, led to higher sea levels and at least a third of the continent that we are familiar with today was flooded and the great Western Interior Seaway was born.
One of the most famous fossil bearing deposits laid down during this time was the Niobrara Formation that extends from south western Kansas to southern Manitoba. The deposit sweeps across in a broad arc across the continent and is best exposed in the north west of Kansas and consists of a soft yellow chalk that is hundreds of feet thick. At this point it is worth remembering how this chalk was formed.
It is still a matter of fascination to realise that the chalk is made of billions of microscopic plants known as coccoliths. Coccolithophores (to give them their full title) were part of the planktonic foraminifera that flourished and diversified during this part of the Cretaceous. Conditions were perfect for them as the climate was warm and seasonal change negligible. Coccoliths were algae that shed their platelets via cell division which fell to the sea floor and this formed the basis for the Niobrara chalk.
The algae was also consumed in copious amounts by a vast array of marine life which ranged from other planktonic life forms right through to the largest filter feeding fish and those very same platelets were then excreted and also fell to the sea bed. The Niobrara chalk, therefore, is a combination of coccoliths, salt and vertebrate remains. Indeed, chalk deposits worldwide represent 70% of the total carbonate sediment deposited over the last 100 million years (Pollastro and Scholle 1986).
The chief outcrop for vertebrate fossils in the Niobrara is the Smoky Hill Member. This chalk deposit is underlain by the Fort Hays Limestone Member and overlain by the Pierre Shale. The Smoky Hill Member contains a rich profusion of vertebrate marine life such as mosasaurs, plesiosaurs, turtles, pterosaurs, sharks and other predatory fish. Early birds are also represented by well known taxa such as Hesperornis and Ichthyornis.
Because the chalk is soft it is constantly under attack from the elements and erodes readily. Not only are new fossils being constantly exposed but the terrain is always being transformed and over the years the formation of structures such as spires and hoodoos gives the Niobrara a sometime eerie and other world-like aspect.
Naturally, such a rich fossil bearing strata was sure to attract attention and the Niobrara has been worked by some of the biggest names in palaeontology such as Marsh, Cope, Williston and the Sternbergs.  They, and other workers, have amassed a vast collection of specimens over the years and we have a real decent understanding of the life in the Niobrara Sea.
Fish dominated the ancient sea and are represented by multiple genera. Chief amongst these were the sharks and the cretoxyrhinids were apex free swimming carnivores and reached lengths of over twenty feet. Nearly as big, and just as formidable, were the primitive bony fish such as Xiphactinus audax which was a fast tarpon-like pelagic hunter with powerful skulls and a mouthful of razor sharp teeth.  
Xiphactinus audax

Sharing the oceans with the fish were, of course, the famous marine reptiles and these too formed a diverse and spectacular community. The ichthyosaurs had vanished during the Cenomanian-Turonian extinction event and this accounted for over 25% of all marine life that swam in the seas at this point. Soon large land living reptilian squamates returned to the shallow seas to take advantage of predatory niches left open by the extinction event. In a short period of time they had evolved to become the dominant predators in the chalk sea - they were the mosasaurs.
Animals such as Platecarpus, Globidens, Clidastes and Tylosaurus are all well known from the Niobrara, were all large taxa and were only vulnerable to the equally large sharks and probably to larger mosasaurs. They were superbly adapted predators that flourished throughout the Late Cretaceous only to disappear at the K-T boundary.
Mosasaurus hoffmanni
Plesiosaurs are also represented in the chalk but are nowhere near as common as the mosasaurs. The long necked elasmosaurids are particularly impressive but their scarcity in the chalk is likely to be due to environmental considerations and it is thought they may have stayed in very shallow waters to avoid predation or maybe that they were only migrating through these waters.
Turtles are well represented in the chalk and there are many different taxa. These were all fully marine and some displayed conical shaped defensive structures mounted dorsally along the mid-line of the carapace which earnt them the very apt nickname of “saw turtles”.
Pterosaur remains are abundant in the chalk which probably represents the greatest pterosaur graveyard in the world. Naturally, though, many of the fossils are crushed and some elements are very incomplete which is due, in no uncertain manner, to the fragility of the pterosaurian skeletal structure. Having said that, many specimens are truly spectacular with the most famous pterosaur of all, Pteranodon, represented by multiple specimens and some of these had a wing span approaching 9 metres.
Birds are represented by the hesperornithorms and ichthyornids and are well known, even to the palaeontological layman, since the beaks were partly lined with teeth betraying their obvious reptilian ancestry. Birds such as Hesperornis demonstrate a dense skeletal structure which would have helped these flightless creatures overcome their natural buoyancy as they dived under water for food. Ichthyornis, on the other hand, was an extremely agile flyer and appears to have been very tern-like which is probably just as well considering the need to avoid predation, both in the air and on land, which was filled with dangers.
It is also worth noting that dinosaur remains have also been recovered from the chalk but these are exceptionally rare (as they are in the Oxford Clay) and probably only represent the odd bloated carcass that may have drifted out to sea. Hadrosaurian elements have been recovered represented by Claosaurus but the majority of dinosaurian elements are from the armoured nodosaurs. It is apparent now that armoured dinosaurs from all over the world must have preferred coastal environments since their remains are often found in fully marine deposits such as those near Lyme Regis in the UK as well as the chalk in the Niobrara Sea.
I have only skimmed the surface of this fascinating world but for a wealth of data can I point you to Mike Everhart’s superb website, Oceans of Kansas and also to check out Anthony Maltese’s blog over at the RMDRC Paleo Lab where he blogs about both fieldwork in the chalk and highlights their preparation work on some of the truly spectacular specimens they have recovered.
Reference

Pollastro, R. M. and Scholle, P. A. (1986) Diagenetic relationships in a hydrocarbon-productive chalk--The Cretaceous Niobrara Formation: in Studies in Diagenesis, F. A.Mumpton, ed., Geological Survey Bulletin 1578, 219-236.

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