Implications of Spare Parts

A fossil skeleton cast of the house-cat-sized mammal Didelphodon vorax graces the new exhibit at the Smithsonian’s National Museum of Natural History.

The subject of a previous post, this exhibit, titled The Last American Dinosaurs, offers a glimpse of the flora and fauna that were part of the complex ecosystem of the Hell Creek Formation during the last couple of million years of the Cretaceous period.  D. vorax, the largest mammal living at the time, was a member of the most diverse and abundant of mammalian groups in Late Cretaceous North America – the metatherians.  This group, originally named by Thomas Huxley in 1880, includes “all mammals more closely related to living marsupials (such as kangaroos and opossums) than to living placentals (such as humans and hedgehogs) and monotremes [egg-laying mammals].”  (Thomas E. Williamson et al., The Origin and Early Evolution of Metatherian Mammals:  The Cretaceous Record, ZooKeys, Volume 465, 2014, page 8.)

As I’ve learned about metatherians in the Cretaceous, some questions about this skeleton cast and, by extension, all such casts, have bubbled to the surface.  More on that in a bit.

Paleontologist Thomas Williamson and his colleagues have written a comprehensive article on metatherians in the Cretaceous (see citation above).  They consider in detail the evolutionary development of the metatherians, review the fossil record of these animals, describe the ecological environment within which they lived, and hypothesize about the impact of the extinction event at the end of the Cretaceous Period on these organisms.  Their treatment of these subjects is surprisingly accessible to a general reader, though, certainly, my interest in one of the metatherian species helps.

Blogger Brian Switek (Laelaps) posted a good overview of the main findings of Williamson’s piece.  Short of reading the article itself, his post serves nicely.

Perhaps the key aspect of the article for me, and the one that started me wondering about the Didelphodon skeleton cast, is how often Williamson and his colleagues come back to that fundamental reality for mammalian fossils in the Cretaceous and earlier, and for the metatherians in particular:  their fossils are few and very fragmentary, nearly always just isolated teeth and pieces of jaws.

Here are but two of the many instances in the article when they allude to the nature of these fossils:

Most Mesozoic mammal fossils consist of fragmentary jaws and teeth, which largely explains the intense emphasis that paleontologists place on the evolution of the mammalian dentition.  (p.6)

The crania and postcrania of Cretaceous metatherians are poorly known.  Most taxa are represented only by teeth and jaw fragments.  (p. 24)

Coincidentally, in recent weeks, I’ve been engrossed in paleontologist Michael Novacek’s memoir titled Time Traveler:  In Search of Dinosaurs and Ancient Mammals from Montana to Mongolia (2002).  It’s quite a rip-roaring account of one paleontologist’s life (including serious injuries and illnesses) in the field, from New Mexico to Montana to Chile to Mongolia – with some additional stops.  All in all, a great read.

At one juncture in a description of his paleontological work in the Gobi desert, Novacek observes that the truly important finds by the paleontologists there might not have been dinosaur remains, but small mammal skulls.

Some of these skulls were no bigger than an almond, but they were extremely important. Up to that time very little was known about mammals of the Cretaceous that lived alongside dinosaurs, and most of the evidence hailed from the American West, from Wyoming and Montana, where these tiny creatures left their remains as enigmatic fragments of teeth and jaws – as we call them, “spare parts.” (p. 283)

Spare parts!  What a brilliant characterization, but what a sobering one as well.  There is precious little beyond “spare parts” for Cretaceous metatherians.  Williamson et al. caution that in all of the research literature, “[o]nly two specimens that include articulated and partial skeletons have been described:  the stem marsupialiform Asiatherium from the Campanian of Mongolia and the early-diverging metatherian Sinodelphys from the Early Cretaceous (Barremian) of China.”  (p. 24)

The implications of this reality of metatherian fossils?  Paleontologists have to rely almost exclusively on metatherian teeth to glean insight into the shape, size, and life of these creatures.  Williamson and colleagues stress, “Reconstructing the postures, locomotor abilities, and habitat preferences of most fossil metatherians is exceedingly difficult, because most extinct taxa are known only from isolated jaws and teeth.”  (p. 45)

But wait, there’s a very specific implication of this dearth of articulated fossils . . . what about that beautiful Didelphodon skeleton cast?  Fabricated solely on the basis of just teeth and fragments of jaws?

Well, it turns out, no.  Some exploration of the information presented in the label for the skeleton cast makes that clear.

(Note that it's the cast that has had a Natural History Museum identification number assigned to it.)

The collector, Mike Triebold, is a commercial fossil hunter and preparator.  He is founder of the Rocky Mountain Dinosaur Resource Center and president of Triebold Paleontology, Incorporated.  According to the description on its website, TPI is a full-service company for vertebrate fossils, from collecting to preparing to mounting fossil skeletons.  I take it that the Smithsonian’s cast is one made by TPI, and based on a specimen found by Triebold and held in the RMDRC’s collection.

This specimen is more than teeth and jaw fragments.  RMDRC’s description states that the fossil discovered by Triebold constitutes some 30 percent of the entire skeleton.  In this context, that’s a remarkably large portion.  Indeed, RMDRC asserts, “This is the only North American mammal skeleton ever found from the late Cretaceous period.”

But, to date, as far as I can determine, this D. vorax skeleton has not yet been the subject of a formal published description.  Williamson et al., as quoted above, observe that there are only two articulated, partial, metatherian skeletons described in the literature, neither of them a Didelphodon.

So, the skeleton standing in the case at the Natural History Museum’s exhibit is based on a specimen that is about a third complete.  What are the implications of that?  For me, it reinforces my inclination to approach this cast, and, by extension, all casts, with a questioning attitude.  For this specific skeleton, I’d really like to know how the full skeleton was “fleshed out.”  Did the preparators have access to enough isolated fossil bones from other disparate D. vorax individuals to assemble an accurate full skeleton?  A reading of Williamson et al. would suggest they perhaps did not, but I don’t know.  To what extent did extinct or living analogues serve as models for the casts of specific bones missing from the fossil record?  I assume that occurred.  When did informed and educated assumptions about aspects of the skeleton enter into the equation?

Further, when the label (as in this case) for a skeleton cast identifies who collected the original specimen (a nice piece of information), would it not make sense to state what portion of the displayed skeleton is actually based on that fossil find?

These aren’t questions intended to convey skepticism of the validity and value of casts.  I embrace their use.  Rather, I am in search of a fuller picture of how any skeleton cast reflects the reality of its fossil record, particularly when that record is composed primarily of spare parts.

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.

Holes Held Together

I have been wrestling with the poem Mesh by Maureen N. McLane.  That I have also been trying to identify a microfossil from the Cretaceous has given the poem particular salience.  The poem and poet are admonishing me directly it seems, warning me against the very act of learning the name of the foraminifera species whose shell I have.  Naming is divisive, serving to separate into pieces that which is fundamentally one.

Mesh appeared in August 12 & 19, 2013, issue of The New Yorker and can be found, in its entirety, here.  McLane also reads the poem and provides some commentary in the latter half of a New Yorker poetry podcast.

In a recent review of This Blue, a new collection of McLane’s poems, New York Times reporter Jeff Gordinier writes that, although the natural world figures prominently in many of her poems, “Calling her a nature poet would be inaccurate, and unfairly limiting, . . . .”  He writes that she is bringing a focus on nature at the moment when “nature itself appears to be going haywire,” and she responds unromantically, offering sharp thrusts that puncture our complacency and give rise to a tone of “elegant unease.”  Apologies to Gordinier, but I feel that McLane is, indeed, a nature poet, though of an unconventional stripe.

I first came upon Mesh some time ago as I carelessly skimmed through a copy of the New Yorker in pursuit of the cartoons.  Wait, I thought (the page arched in mid-flip), is that poem about taxonomy?  It opens:

Everything in the world
has a name
if you know it.
You know that.

I have used this verse as an epigraph to a blog post, but I now think I used it inappropriately.  I am coming to understand that, though this naming of things is inherently human, it is, to the poet, also an action hostile to nature.  This changes the way I hear – “You know that.”  More sarcastic, more dismissive.

McLane then models the naming process:

The fungus
secreting itself
from the bark
is Colt’s Hoof.

In that single verse rests much of her argument as I understand it.  Scientifically, fungi are classified in their own kingdom, certainly separate from the plants.  Yet, she writes, that the fungus and the tree bark are one, the former “secreting itself” from the latter.  There is a unity in this natural relationship that the naming process would have us push aside.  Perhaps deliberately, McLane offers a common name (Colt’s Hoof) that, as far as I can tell, no fungus actually bears.  Is this misnaming evidence of how irresponsible applying names can be?

According to the poet, the taxonomist is being displaced (and proven wrong?) by the molecular biologist and geneticist who are using another tool to identify taxa and relationships among them – DNA.

The dignity
of cataloguers
bows before code.

Does the code signal how connected we all are or is it used to separate and segregate?  In fact, the poet writes, all of this may run counter to the reality of nature:

The thing
about elements –
they don’t want
to be split.

McLane then proffers several dualities that, upon reflection, dissolve (“I saw the world/ dissolve in waves”) into one entity:  the poet and her readers; trees and their shadows and their reflections (I would add the duality of wave and particle that is light);  the sharp sounds of the inanimate subway as it brakes being heard by its riders (becoming one with the machine); and hummingbirds and deer (bound in an endless diurnal cycle).

This poem about relationships and separations begins with naming, and ends with a call for us to join together (and with) what has been torn apart:

It turns out
the world was made for us
to mesh.

In the poetry podcast interview with Paul Muldoon, New Yorker poetry editor, McLane observes that the poem was influenced by her reading of ecological philosopher Timothy Morton.  She states that he, along with others, argue against the notion of species (which has served some horrific causes).  Indeed, the second of the epigraphs McLane placed in This Blue reads “Species means guilt,” title and first line of a piece by poet Bruce Andrews (second line of which references a slave ship, last line reads “Squirrels are happy without our help.” – the rest of Andrew’s piece?  beyond me).  McLane observes that these writers also, and more fundamentally, espouse the ultimate unity of the animate and inanimate.  We are seriously wrong, they warn, when we divide the universe into discrete parts.

During the poetry podcast, Muldoon asks McLane about how she decides what to leave out of a poem, a question he explains by citing the definition of a net which he attributes to Samuel Johnson:

a number of holes held together by string.

Wonderful.  Negative space – speaking of dualities that are irrevocably united.  Can one say that a net is composed of what it is and what it is not? Or is where it is not, part of where it is?  The mind reels.  Such a thought provoking definition, potentially changing how we look at many things.

Parenthetically, I have to note that I don’t think it was Johnson who defined a net as Muldoon would have it, though Johnson came close in A Dictionary of the English Language (volume II, sixth edition, 1785).  His second definition of net is:  “Any thing made with interstitial vacuities.”  An article titled A Strange Dictionary in the January 31, 1880, edition of The Saturday Review of Politics, Literature, Science, and Art attributed to “another lexicographer, whose name has not been preserved,” a definition nearly identical to the one put forward by Muldoon.

A net, defined this way by some unknown wag, is much like the fossil shells of foraminifera, those single-celled organisms dating back to the early Cambrian.  Their fossil shells are typically composed of calcium carbonate and include the empty chambers within which the organism once lived.  I offer this definition of a fossil foraminifera shell:

a series of holes often held together by calcium carbonate. 

And, back to the taxonomic endeavor which began this piece.  Below are views of two sides (spiral and umbilical) of the shell that I worked on for a couple of days before I could comfortably identify it as coming from Planulina texana, a foram originally named by preeminent paleontologist Joseph Cushman in 1938.  This specimen, found in material from the Atco Formation, is nearly 90 million years old.  (The spiral side view, on the left, shows the foram while wet in order to highlight the chambers.)

I have come to believe, contrary to where I think the poet would have me come down on naming, that her basic premise about the unity of all things is not undermined by the taxonomic enterprise.  The naming of species doesn’t separate us from that which we name.  Rather, it’s a part of the process by which we develop our own sense of interconnectedness with all objects, well, at least with the animate.  This act is a mark of engagement with the natural world of which we are a part, not one of estrangement.