On the Arctic Cretaceous

By now, those of you who follow paleontology news have probably heard of Nanuqsaurus hoglundi, the newly described genus and species of small Arctic tyrannosaur. The obligatory and nearly instantaneously produced Wikipedia article can be found here, and the scientific description can be found here. Nanuqsaurus comes from rocks of the North Slope of Alaska, and lived about 70 million years ago. It's just the "tip of the iceberg", so to speak, of the North Slope dinosaurs, which also include the hornless horned dinosaur Pachyrhinosaurus perotorum, a bonehead named Alaskacephale, the ubiquitous duckbill Edmontosaurus, and various small theropods. This is pretty typical for the latest Cretaceous of North America, give or take an armored dinosaur.

The Arctic Late Cretaceous has become a sort of romanticized setting. It's the perfect excuse to draw dinosaurs in twilight, or getting snowed on, or breathing out little clouds in the cold and frosty air. Let's face it: when we think wintery conditions and geologic time, we think of the recent glacial periods, or Snowball Earth, not "The Age Of Reptiles", so dinosaurs in snowdrifts is historically a novelty. The Arctic has received increasing attention from geologists over the past few decades, not the least because there is growing interest in how the area works in the absence of ice. Unlike some places, like India, Arctic areas of North America have been near their present latitudes since the Late Cretaceous, so we can look at the rocks and see what was going on at various warmer times. If you prefer mammals, Jaelyn Eberle of the University of Colorado–Boulder has done a lot of work on the Eocene rocks of the Canadian High Arctic (think about 50–40 million years ago). Conveniently for those of us looking at the Cretaceous, there is also a marine record that is slightly older than the North Slope fossils.

When I was a graduate student at the University of Colorado–Boulder, my supervisor was Karen Chin, known for her work on coprolites (fossil feces). I came on board as she was working on a project describing coprolites from Cretaceous sediments of Devon Island (the marine Kanguk Formation and the base of the overlying terrestrial Expedition Fiord Formation). I was her graduate assistant for this project: for three years, I supported myself by quite literally polishing turds: one of my major duties was making thin sections from the many coprolites she'd excavated from the island. I also discovered that my tiny handwriting made me a natural at labeling specimens. Each specimen was labeled, measured, and often illustrated and photographed in loving detail at macroscopic and microscopic levels. So, what did we find?

The material Karen brought back included a diverse assemblage of shallow marine and coastal terrestrial fossils. Wood was washed in from land and bored by shipworms, and pollen and spores from land plants drifted into the sea from the air or were washed in. The water was full of microscopic plankton, including radiolarians, diatoms, silicoflagellates, and dinoflagellates. The seafloor was populated by glass sponges, brachiopods, inoceramid bivalves, and crustaceans. Above them swam cephalopods, sharks, ratfish, bony fish, mosasaurs, plesiosaurs, and flightless hesperornithiform birds (check out fellow CU-Boulder alum Laura Wilson's description of the hesperornithiform here). I have a soft spot for the sponge, Nunavutospongia irregularia, because it got me scientific immortality. The picture at the link to the name does not quite do it justice: specimens look a bit like pan flutes crafted by octopuses, and are composed of tiny caltrop-like spicules of silica. The lack of calcitic fossils is probably due to the pH of the water, while the abundant siliceous microfossils show that the shallow shelf was extremely productive. The food chains appear to have been rather short, as seen in the modern Arctic, except with plesiosaurs and large filter-feeding fish instead of marine mammals. Overall, the setting was probably a place of seasonal plankton blooms, tied to the yearly extremes of sunlight, with substantial input of nutrients from the coastal forest. This is all written up in Chin et al. (2008), which can be downloaded here.

The coprolites come in two major varieties, both reaching several inches long. One type is composed of greensand, the greensand being a mineral known as glauconite that often forms on continental shelves with slow sedimentation (often converting tiny fecal pellets and the "shells" [tests] of marine plankton). The greensand coprolites are frequently dotted with small white pellets that were probably left there by small mobile invertebrates, and often contain large inclusions, such as crustacean pieces and bone fragments. These coprolites were probably left by vertebrates (plesiosaurs?) that were grabbing or scavenging things from the seafloor, and getting a lot of sediment at the same time. The other type of coprolites are black or off-white, composed of phosphate, and laced with small invertebrate burrows. These coprolites are chock-full of siliceous microfossils (diatoms and radiolarians), indicating filter feeders (some kind of fish?).

Although it's an evocative image, it probably wouldn't be appropriate to draw plesiosaurs or mosasaurs swimming in the vicinity of a tyrannosaur shuffling through twilight snow on a beach, because the marine reptiles would probably have been mostly interested in the seasonal boom times with 'round-the-clock sunlight, not the nutrient-poor days of darkness.

References:

Chin, K., J. Bloch, A. Sweet, J.Tweet, J. Eberle, S. Cumbaa, J. Witkowski, and D. Harwood. 2008. Life in a temperate Polar sea: a unique taphonomic window on the structure of a Late Cretaceous Arctic marine ecosystem. Proceedings of the Royal Society B 275(1652):2675–2685.