Saturday, December 13, 2014

The Last American Dinosaurs at the Smithsonian ~ Paleoecology of the Hell Creek Formation

In the last two million years of the Cretaceous Period, the area of Montana and the Dakotas presently marked by the Hell Creek Formation was decidedly green, a humid and semi-tropical landscape featuring rivers and forests.  The formation itself, according to paleontologists John R. Nudds and Paul A. Selden, “is a fluvial deposit, laid down by meandering rivers, which frequently flooded onto a broad alluvial coastal plain on the eastern side of the Rocky Mountains.  The rivers flowed east across this plain into a large epeiric [shallow, inland] sea, the Western Interior Seaway, which during Cretaceous times was retreating southwards and eastwards, exposing the coastal plain.”  (Fossil Ecosystems of North America, 2008, p. 182.)

The Last American Dinosaurs:  Discovering A Lost World, the new exhibit at the Smithsonian’s National Museum of Natural History, features the Hell Creek Formation, then and now.  Perhaps the exhibit's most immediately appreciated gifts are the dinosaurs which end the barren months that followed the closing of the museum’s fossil hall earlier in the year for renovation.  Two mounted skeleton casts of a Tyrannosaurus rex and a Triceratops horridus dominate the entrance to the exhibit (the casts affectionately known as Stan and Hatcher, respectively), and several other Triceratops skulls and the skull of an Edmontosaurus appear as well.

But, the heart of the new exhibit is captured by its subtitle.  If visitors make the effort to go beyond the dinosaur hook and consider the story being told by the exhibit, they will be rewarded, learning a bit about that 66-million-year-old “Lost World” – what it was like in its ecological complexity, and also how we have come to know it.  So much more than dinosaurs.

The best of the exhibit is its middle where the opposing walls offer complementary narratives.  On one wall is an array of recent pictures of the contemporary, arid landscape of the Hell Creek Formation showing museum scientists scouring the rocks for fossils, such as tiny teeth and impressions of plants.  Here is the fieldwork that underpins the exhibit.

  Among these images is a particularly lovely one focused on that critical piece of fieldwork equipment – toilet paper.

More importantly, fieldwork is placed in its proper context.  As visitors step back from this montage of pictures, they encounter a display, not only of field equipment, but also of what happens after the fieldwork, from how specimens are safeguarded in the field to the careful and painstaking prep work in the museum’s labs.  In fact, someone standing before this montage need only turn to the right to look through the windows of the FossiLab and see ongoing work on fossils.

On the opposite wall from the pictures of collecting at Hell Creek is a wonderful mural by Smithsonian scientific illustrator Mary Parrish.  It’s primary element is her rendering of a stream scene in the Hell Creek area some 66 million years ago.  She acknowledges that she’s filled it with more species than were likely to be in such a location at one time, but the scene captures the essence of what that environment was like – wet, lush, and green.  The key to this scene is that dinosaurs do not dominate it; they are a part of an environment graced by plants and other animals.  It’s an ecosystem, not a blockbuster movie scene.

I hope visitors take the couple of minutes needed to watch the video associated with the mural.  In it, Parrish explains the process she follows in creating her artwork.  It’s excellent; indeed, all of the videos in this exhibit are first rate.  (Several of the videos, including Parrish's, can be found here.)  Still, I do have a complaint about how her mural is treated.  The mural itself includes more than this pre-extinction scene and some of the material that went into making it.  It encompasses the destruction of the extinction event and the recovery of the landscape in the Paleocene Epoch, yet visitors, I suspect, will not quite get that.  Perhaps because of the unfortunate position of a pillar, it was decided to interrupt the mural’s flow with a tall, metal frame holding displays about the asteroid hit.  It connects to the pillar and stands at right angles to the mural, dividing it in two.

The fossil material around the Parrish’s stream scene reinforces its message.  Visitors will see a beautiful cast skeleton of Didelphodon vorax, the largest mammal living 66 million years ago.  I am quite taken by this cat-sized marsupial, both because it’s bigger than I, in my ignorance, believed mammals to have become when they co-existed with dinosaurs, and because it disappeared in the end-Cretaceous extinction event, along with some other mammal taxa.  This extinction may have opened a door to mammal diversification and size increase, but it came with a mammalian cost.

Parrish carefully places one in her mural.  Seen here, clinging to a limb, beneath a philodendron leaf.

Plants, plants, and more plants.  The mural is awash in vegetation.  Displayed immediately before the mural are fossil impressions of leaves of plants that lived then.

Further, a slab of leaf impressions suggests how abundant the vegetation was.

Of the many other aspects of the story of this lost ecosystem that merit attention, there is one I will mention in closing – what message it might hold for us now.  Given the contemporary demise of many species, the exhibit asks whether we are in the midst of the sixth mass extinction.

There’s a great deal to like about the exhibit, and, happily, I think it’s a harbinger of what we will experience when the renovated fossil hall opens in 2019.

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