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.

Fieldwork Equations

I’m wrestling with two realities about paleontology.  The first involves paleontological fieldwork; the second, fieldwork’s aftermath.

To begin with, take fieldwork, a challenging and time consuming endeavor with no assured success.  In an essay on his own fieldwork, evolutionary biologist Stephen Jay Gould erected a straw man argument denigrating fieldwork – “An eminent colleague, a fine theoretician who has paid his dues in the field, once said to me, only partly facetiously, that fieldwork is one hell of a way to get information.  All that time, effort, and money, often for comparatively little when measured against the hours invested.”  (Opus 100, The Flamingo’s Smile:  Reflections in Natural History, 1985, p. 183-184.)  Responding with his characteristically affirming enthusiasm, Gould said that’s not the reality of fieldwork because the rewards are so great and the enjoyment abundant.  “[A]ll the frustration and dull, repetitive effort vanish to insignificance before the unalloyed joy of finding something new – and this pleasure can be savored nearly every day if one loves the little things as well.”  (p. 184)

Paleontologist Peter Vaughn turned fieldwork into an equation.  According to paleontologist Michael Novacek in his book Time Traveler:  In Search of Dinosaurs and Ancient Mammals from Montana to Mongolia (2002), Vaughn “described paleontological fieldwork as 49 percent anticipation, 49 percent recollection, and 2 percent success.”  (p. 49.)  Certainly, Vaughn was not railing against fieldwork; he was simply being realistic that a lot goes into it and the results are not assured.  Regardless of how puzzling this equation is (e.g., I’m really not sure what “recollection” is and wonder if he’s mixing apples and oranges), that two percent success is pretty slim.  That, asserted Vaughn, is the reality.

Given my last couple of experiences of carefully planning and carrying out forays into the field looking for the fossilized calcium carbonate shells of foraminifera and ostracodes with absolutely nothing to show for the effort, I decided, for just a moment, to ward off Gould’s zeal and buy into his straw man argument because, right then, I concluded a two percent solution isn’t a strong enough allure.  I eschewed fieldwork (my half-assed version of it) and bought (oh, the horror) some Cretaceous matrix online – yes, I let someone else do the fieldwork for me.

Sight unseen, I purchased slightly more than two and a half pounds of purportedly unsorted matrix collected from a Cretaceous formation from which I hoped to extract microfossil shells.  It didn’t start off well.  What tumbled out of the box that came through the mail was some pretty dispiriting rock.

There’s another paleontological reality which came into play at this point – fieldwork engenders tedious, painstaking, seemingly never ending lab work.  When Peter Makovicky, associate curator at the Field Museum, described the work on the fossil bones of the dinosaur Siats meekerorum, an apex predator, he concluded that every hour of fieldwork generated 100 hours of lab work (and, for the S. meekerorum, there was a lot of fieldwork – a summer of exploring and finding, and two summers of excavating).  (Steve Johnson, Dinosaur Discovered by Field Expedition Rivaled T. rex, Chicago Tribune, November 22, 2013.)

I am no stranger to that reality. Take my recent, unsuccessful efforts in the field, they generated many hours of soaking and screening matrix, and days spent hunched over the microscope.  In fact, for microfossils, it’s lab work itself that determines whether the fieldwork is successful or not.  In many ways, the lab work under those circumstances becomes a continuation of the searching and exploration in the field.

The mail-order Cretaceous material came with a mix of unknowns and knowns.  It was collected, at some unknown point in the past, at a construction site somewhere near the Texas town of Prosper (about 35 miles north of Dallas and marked in the geologic map below), by someone, whom I don’t know, who, for a small price, mailed it to me.

The polka-dotted green area in this map shows the presence of rocks of the Austin Group.  (This map is a tiny excerpt from the Sherman sheet of the geologic atlas of Texas.)   Somewhere in that green near Prosper was the construction site where the Atco Formation (part of the Austin Group) outcropped.  This material, averred the seller, came from this formation which dates to the Coniacian Age (89.8 to 86.3 million years ago) during the Upper Cretaceous Period.  One could be skeptical that the seller had it right about the formation, but, for better or worse (mostly worse), much about the matrix screams “Atco Formation!”

The rocks of the Atco Formation are dominated by, and bound together by, calcium carbonate. It has been described as composed of “thin-bedded calcisiltite and indurated chalky limestone interbedded with gray calcareous shale and mudstone.”  (Charles C. Smith, Calcareous Nannoplankton and Stratigraphy of Late Turonian, Coniacian, and Early Santonian Age of the Eagle Ford and Austin Groups of Texas, Geological Survey Professional Paper 1075, U.S. Geological Survey, 1981, p. 1)  The key word is “indurated.”  Porous rock is considered indurated if it has been “cemented” together through mineral deposition.  (John O. E. Clark and Stella Stiegeler, The Facts on File Dictionary of Earth Science, 2000, p. 172.)  In other words, it becomes very solid, resistant to pressure and scraping.  Appropriately enough, the formation is named after the company whose quarry is the type locality – the Universal Atlas Cement Company (of McLennan County, Texas).

A lot of what I bought is chunks of hard rock, studded with bits of pebbles and, on occasion, teeth; the rest is fairly friable, crumbling with manual pressure.  With some of former, I decided to sacrifice the microfossil shells by soaking these chunks in vinegar, letting the acetic acid interact with the calcium carbonate in the rock, turning it into calcium acetate which stays in solution and carbon dioxide which bubbles away.  Time consuming, sure, but I just let the acid do its thing.  The effort has been going on for some three weeks – I periodically refresh the vinegar, and poke at and otherwise abrade the surfaces of the soaking rocks  to expose more calcium carbonate.  I am encouraged – some of these pieces are actually breaking apart.

There are “little things” in the effort to dissolve the calcium carbonate that have exerted a bit of magic and kept me engaged, even opening up new areas of exploration (such as the chemistry of the interaction between calcium carbonate and acetic acid).  Just the visual display that is involved in the process is entertaining.  One piece of matrix, which gave off bubbles at a furious pace from several points on its surface, amused me for some time because of the organized array of bubbles that emanated from it and how they arranged themselves on the surface of the vinegar – miniature, carbon dioxide fireworks.

The material that could be crumbled into much smaller grains was soaked in water for several days and then screened.  The smallest of the particles captured by my screens might yield forams and ostracodes, if anything in this material will.  Afraid of the verdict, I haven’t yet examined this material for microfossils.  But, I have scrutinized the particles of one millimeter or slightly larger and, though few, there have been some interesting finds.  Here are a couple of them.

The first specimen is a tooth from a shark, a Squalicorax falcatus.  The tooth is certainly the worse for wear, but some of the serrations have come through those nearly 90 million years.  The second photo shows the occlusal (left in the photo) and lingual (right) sides of a tiny tooth from what I believe to be a Ptychotrygon triangularis, a ray.  Both are first time finds for me.  (These identifications are based on Bruce J. Welton and Roger F. Farish’s The Collector’s Guide to Fossil Sharks and Rays From the Cretaceous of Texas, 1993.)

Okay, it’s not much to show for three weeks and counting invested in two and half pounds of mercenary Cretaceous matrix – some bubbles and a few teeth.  The work has been tedious and tiring, but, I have to admit that Gould had it right – be open to enjoying and marveling in those “little things” experienced in the field and, I'd add, in some of the lab work as well.

When biologist and gardener Roger B. Swain penned a piece about Viktor Muhlenbach of the Missouri Botanical Garden, who spent years walking along the railroad tracks in St. Louis cataloguing the incredible variety of flora he found growing there, Swain reflected on the possible attraction such an activity might hold.  He likened walking the rails searching for plants to similar endeavors such as birding and beachcombing.  I’d insert paleontological fieldwork and, even, some lab work into his list.  Swain concluded that their allure arises from the very doubt about the outcome.  He wrote,

Such excursions are risky, of course.  There is always the chance that you’ll find nothing, but that chance makes the finding more momentous . . . .  That very uncertainty is more important than most of us realize.  God does play dice with the universe, and out of the uncertainty comes anticipation, and, in turn, hope.  (Field Days:  Journal of an Itinerant Biologist, 1983, p. 122.)

Mysterious Assassin at the Natural History Museum

The assassin moved slowly through the small lobby.  Reaching the sunlight that streamed through a set of locked glass doors, she (he?) paused.  For the moment, she was alone in this area open only to staff and volunteers of the Smithsonian’s National Museum of Natural History.  Later, we would wonder how she made it into this inner sanctum.  Did someone unknowingly give her access?  Certainly, not knowingly, because she was a dedicated and skilled assassin, sporting a fear-inducing piece of armor and able to dispatch her victims with the stab of her long, curved weapon.

As soon as I opened the stairway door to the lobby, I knew something was amiss.  It took me a moment to register what was definitely out of place here.  Ignorant of her lethal nature, I innocently approached her.  She went still at my approach and, when I dropped a tissue next to her, she crawled onto it.  Her size alone gave me pause.  Gingerly holding the tissue at one corner and then another as she moved inexorably up toward my hand, I hurried through a corridor, past museum offices, then into the public lobby, and finally out into the open air and the noises of Constitution Avenue.  I deposited the tissue in the grass behind a bush and the assassin slowly, very slowly walked off.

This is the large Arilus cristatus, a member of the group of bugs known as Assassin Bugs.  She bears the popular name of Wheel Bug in recognition of the extension of her thorax, a striking cogwheel structure.  This crest (cristatus is Latin for “crest”) is unique among U.S. insect species, but its function remains largely a mystery.  As zoologist Bob Thomas observes

There are no firm opinions on the purpose of the gear-like structure on the wheel bug’s thorax. It may serve them in species recognition, may help potential predators recognize them as dangerous, or the teeth on the gears may make them less palatable or more uncomfortable to eat. Since they have a ravenous appetite for agriculturally damaging insects, maybe the wheels are indeed a Rotary symbol of “service above self.”  (Wheel Bug, Arilus cristatus, Loyola Center for Environmental Communication, Loyola University of New Orleans, November 11, 2009.)

Her prey are a mixture of insect heroes and villains, among them caterpillars, Japanese Beetle larvae, wasps, lady bugs, and honey bees.  In the nymph stage, she feasts on aphids.

Her modus operandi is quite dramatic.  With that long, curved, red proboscis, she injects her victims with a paralyzing mixture of enzymes that quickly dissolves the internal organs which are then sucked out.

Much of what I’ve read emphasizes how slowly this insect moves, whether walking or flying.  Coming in at between some 1 and 1 1/4th inches long, it’s probably just as well for the rest of us that the adults are not in a hurry.

Even more disconcerting, particularly given my maneuver through the museum’s corridors with the Wheel Bug-bearing tissue, is that, when disturbed, she is known to use her sharp proboscis on humans, delivering a deeply painful and enduring bite.  As described in the online Wheel Bug profile prepared by the Entomology and Nematology Department of the University of Florida:

This bite has been described variously as worse than stings from bees, wasps, or hornets. Barber (1919) and Hall (1924) described in detail the effects of such bites. In general, initial pain often is followed by numbness for several days. The afflicted area often becomes reddened and hot to the touch, but later may become white and hardened at the puncture area. Occasionally, a hard core may slough off, leaving a small hole at the puncture site. Healing time varies but usually takes two weeks.

(The two citations in the preceding description of the effect of a Wheel Bug’s bite are: G.W. Barber, On the bite of Arilus cristatus, Journal of Economic Entomology, Volume 12, 1919; and M.C. Hall, Lesions due to the bite of the wheel-bug, Arilus cristatus (Hemiptera; Reduviidae), Journal of the Washington Academy of Science, Volume 14, 1924.)

Wheel Bugs have a single generation during the year, largely spending the spring as nymphs, the summer as adults, and the winter as eggs (which, incidentally, look much like a compact collection of bullet cartridges).

Now, I have to admit that, not knowing what insect this was when I encountered it on the sunlit floor, my instinctive action of removing her to the great outdoors was perhaps somewhat misguided.  Afterwards, before heading to the literature, I did wonder if she could have been some rare, tropical insect escaped from the museum’s Entomology Department and if what I’d done was such a good thing.  Was this an invasive that would soon disturb the ecology of the Washington, D.C. area?  Luckily, no.  I assume she came in from the grassy area outside the glass doors; though, how this slow-moving assassin accomplished that trick remains a mystery.

Dinosaurs: Surprising Candor at Discovery

In which the blogger discovers that the Discovery Communications Corporation exhibits surprising candor about its dinosaurs, despite what one might expect given much of the TV programming on its myriad networks.

These days, a visitor in search of mounted dinosaur skeletons will come away disappointed by the Smithsonian’s National Museum of Natural History.  As I’ve noted previously, the Fossil Hall is closed for renovation and the interim display, The Last American Dinosaurs display, which will feature a few dinosaur skeletons, wont be unveiled until this November 25th.  For dinosaur lovers, the couple of dinosaur skulls one can find in public nooks and crannies of the museum really don’t count or satisfy.  So, what’s a person in the D.C. area to do?

Well, if you define “nearby” relatively broadly, there are a number of great sites nearby to satisfy that dinosaur itch.  One might start with the places identified by Joe Bruns in an article in the Washington Post.  (Joe Bruns, You Don’t Have to Roam For More Dinosaurs, Washington Post, April 19, 2014.)  His list includes the following:

• the Maryland Science Center in Baltimore (for a great review of this exhibit, I would recommend the one Ben wrote for his Dinosours! blog),
• the Delaware Museum of Natural History in Wilmington,
• the Academy of Natural Sciences of Drexel University in Philadelphia,
• the Carnegie Museum of Natural History in Pittsburgh, and
• the American Museum of Natural History in New York City (I recently reviewed it in my usual idiosyncratic way).  

I have a quibble with Bruns’ list because he clearly assumes that, in the quest for these beasts, all roads lead north from D.C., which is wrong.  For example, one can head south to the Virginia Museum of Natural History (in Martinsville, Virginia) which offers dinosaurs in its Hall of Ancient Life.

Also, more recently, a Spinosaurus skeleton mount went on display in the National Geographic’s Museum in D.C.  According to the review Ben posted on his blog, this is a worthwhile exhibit describing the historical context for the dinosaur and explaining quite frankly the nature of the skeleton on display (a cast of bones from several individuals).

If an isolated, fossil skeleton mount of a dinosaur does the trick, then there’s the lobby of the Discovery Communications’ world headquarters in Silver Spring, Maryland.

Perhaps it’s not surprising or illogical for Discovery Communications to have dinosaurs in the lobby.  After all, this global, pay-television-programming behemoth claims to be guided by “passionate curiosity.”  (2013 Annual Report.)  Of its extensive TV programming, Discovery avers:

Our content spans genres including science, exploration, survival, natural history, technology, docu-series, anthropology, heroes, paleontology, history, space, archeology, heath and wellness, engineering, adventure, lifestyles, crime and investigation, civilizations, current events and kids.

That does give me pause, though.  What a curiously constructed list of “genres.”  It’s a hopeless mélange of fields of science and elements of pop culture.  Frankly, that list isn’t surprising if one considers the programming that the Discovery Communication’s various networks spew forth.  A reasonable conclusion is that, for Discovery, “passionate curiosity” is joined (or perhaps displaced) by a passionate search for ratings and earnings.

Among the many, many programs broadcast by Discovery’s various networks are numerous so-called “reality TV” shows, including such gems as Naked and Afraid in which, for each episode, two stark naked strangers (a male and a female, of course) are dropped into some wild area with minimal equipment and followed as they try to survive for 21 days, or Amish Mafia which features the doings of a handful of Amish who purport to be the local enforcers in the Amish community of Lancaster County, Pennsylvania.

(In the interest of full disclosure, I’m not opposed to reality TV per se.  I’ve been known to watch a few reality TV shows, such as Top Chef and Storage Wars (neither of which is part of the Discovery family).)

Discovery Communications’ programs have often taken their lumps from the critics.  Though the premises may be laughable or cringe-worthy, it’s Discovery’s lack of candor about the reality of reality TV that troubles me the most.  For instance, the New York Times reviewer of the Amish Mafia when it debuted, observed, “An early credit warns of ‘select re-enactments,’ and since we’re never later told whether we’re watching staged scenes, it’s fairly safe to assume that everything is staged. (A closing credit clarifies that ‘re-creations are based on eyewitness accounts, testimonials and the legend of the Amish Mafia.’)”  (Mike Hale, The Dirty Work for the Clean, New York Times, December 11, 2012.)

And, hey, when Discovery’s annual TV feast known as Shark Week can be taken to task for lying to its audience by Wil Wheaton (yes, that Wil Wheaton of Wesley Crusher fame), among others, one has to believe ratings may not have just displaced passionate curiosity, but honesty as well.  Here’s Wheaton on the opening episode of Shark Week in 2013:

Discovery Channel started Shark Week with a completely fake, completely made-up, completely bullshit “documentary” and they lied to their audience about it.  They presented it as real.  (Wil Wheaton dot Net, August 5, 2013.)

Joining in, science writer George Johnson dismissed the “science” content on the Discovery Channel as not even “good fiction.”  (Not Just Sharks – The Junk That Passes For Science on the Discovery Channel, Fire in the Mind blog on Discover Magazine’s website, August 6, 2013.)

So, with that as context, it wasn't unreasonable to be curious about how Discovery would present the dinosaurs housed in the lobby of its world headquarters.  First things first:  the hours that it's open to the public aren't obviously displayed anywhere.  I gained admission during the work week, after having failed on a Saturday afternoon.

Once inside the lobby, I found it hard to draw myself away from the fascinating piece of kinetic artwork by George Rhoads which greeted me (it was operating when I entered).  But, yes, there are dinosaurs in the lobby:  a mounted Tyrannosaurus rex skeleton, a very small mounted Bambiraptor (the creature was small in life; indeed, the holotype is probably a juvenile), and a Triceratops skull.

I was surprised and impressed that, for the most part, the signs for each are richly detailed and informative.  In general, they describe with refreshing candor what is being shown to visitors, including background information on how these display specimens came to be.

For example, though its wording is convoluted and unfortunately conflates casts and molds, the display sign for the T. rex makes it clear that the visitor is not seeing fossil bones:

Discovery Rex is not an actual skeleton. . . . What you see here is a mold of the bones, captured in painstaking detail by a process called casting. . . . Institutions are using a variety of silicon rubber products to produce scientifically accurate molds that can capture such minute detail as the serration in a T. rex tooth.

The sign also notes that the actual fossil skeleton is nicknamed “Stan” after its discoverer and that it resides at the Black Hills Institute of Geological Research.

Hmmm, that means that this Stan has a twin that will grace the Last American Dinosaur exhibit which will open shortly at the Natural History Museum.  Perhaps it was just the lighting, but I found the Discovery cast of Stan to be more impressive than the Smithsonian’s cast of Stan, at least as it was displayed for years in the Fossil Hall.  Somehow Smithsonian Stan is no longer so special.

[Later edit:  Casts of Stan are actually widespread.  In a post titled "The Stan Gallery" on Dinosours!, Ben explores the phenomenon, illustrating it with pictures of a subset now on display in 14 different venues, mostly museums, but including the Discovery headquarters and a Best Western.]

The Triceratops skull is very nice and open to close inspection.  It was found on a private ranch in North Dakota in 1994, and, if I interpret the sign correctly, what is on display is some 85 percent fossil bone.  The cast material appears pretty obvious on the skull.

The Triceratops sign notes that Geolinea Paleontological Laboratories handled the prep of the remainder of the skeleton.

Well, the mention of Geolinea undoes a bit of the good vibe I had from this dinosaur display.  Geolinea, now known as Geoworld, was the source for the fossils that were auctioned off in August, 1999, through a collaboration between Discovery Communications and Amazon.  (Judith Graham, Scientists Harden Position on Selling Fossils, Chicago Tribune, October 24, 1999; Keay Davidson, Online Fossil Sales Worry Scientists, San Francisco Examiner, October 4, 1999.)  The whole thing greatly stirred up the paleontological world, prompting the Society of Vertebrate Paleontology to pass a resolution in October, 1999, expressing alarm about the auction:

We are deeply concerned by the on-line, auction sale of vertebrate fossils, as the nature of the process cannot assure that scientifically significant fossils are deposited into not-for-profit scientific and educational institutions.

 I don’t know how this specific story played out in the intervening 15 years with these players.  A brief search didn’t turn up anything.  But, getting into bed with Amazon to auction off fossils doesn’t seem out of character for Discovery Communications (however scary it seems to me).  And, even if the Geolinea reference hadn’t been enough to remind me of the nature of the corporation in question, what appears in a corner of the lobby, behind Stan, would have.  There they are:  real (I assume) motorcycles and cardboard cutouts of stars of the now defunct Discovery Channel’s reality TV show, American Chopper.

Apart from the dinosaurs, I was quite taken with the mammoth material that is also on display in the lobby – real teeth and hair, a model of a baby, and a quite beautiful, very real tusk with blue-green stains from the cobalt and copper in the soil which held it all these years.  Some 15,000 to 18,000 years old, it was found in Alaska in 1996.

There are, I think, at least two kinds of truth one might look for in fossil skeleton displays.  One, and perhaps the more important, is whether aspects of the complex reality of where, when, and how that ancient life existed are represented, based on current scientific understanding.  Mounted skeletons in isolation like those on display at Discovery Communications don’t demonstrate that truth, nor do they seek to.

Another truth comes from the disclosure of whether what’s on display is fossil bone, a cast, some combination of the two, and whether this display comes from a single specimen or is some amalgam of pieces from different individuals.  Discovery Communications' lobby display seems to me to handle that truth quite well.  It’s a matter of candor.

One final note (and a segue from the Discovery mammoth material), the Natural History Museum is apparently just learning the truth about the mammoth skeleton that had been on display in its now-closed Fossil Hall.  As the skeleton is being “deconstructed” by a team of experts, we are now coming to understand that this skeleton is a composite of skeletal pieces from perhaps as many as 70 individuals and not necessarily all from the same species of mammoth.  (A Mammoth, But Careful Restoration At Smithsonian, Associated Press, Washington Post, October 21, 2014.)  No, it’s apparently not what would be done today - one Smithsonian scientist is quoted as saying, “Mixing species is a strange thing to do in a modern exhibit.”  Well, if such a skeleton mount were to be on display today, one hopes for some candor.

A Buggy Universe ~ A Review of Planet of the Bugs

After a quick break to take the dog for a walk (“Not a euphemism,” as British comedian Miranda Hart would add), we had resumed our journey south on the New Jersey Turnpike when something on the car’s stick shift caught my eye – a cricket.  A jumping bush cricket (Orocharis saltator), if I’m not mistaken (though I may well be – mixing uninformed amateurs and identification guides, such as Thomas Walker’s fun and useful Singing Insects of North America, often leads to foolishness).  I suggested that my wife take a look; she did, offering only a mild sound of disapproval.  With a tissue, she gingerly pried the little beast off the stick shift and tried to shake it out of an open window.  Of course, given the air flow around the car, the cricket headed straight for the back seat where it rode in some comfort (I assume) for another two hundred miles (though part of a leg had been lost in the encounter with the tissue).

Given this insect’s body plan and structure, even the alternative of being blown out of the car into the wilds of the Turnpike and the rest of New Jersey was not necessarily an immediate death sentence.  Small size, exoskeleton (keeping the necessary, good parts encased in a skeleton), and wings spelled odds of survival that were not too bad.  Insects are ancient survivors for some very good reasons.

My current thinking about insects – their shapes, structures, behaviors, evolution, complexity, and beauty – has been informed by entomologist Scott Richard Shaw’s engrossing new book, Planet of the Bugs:  Evolution and the Rise of Insects (2014).  The prose in this book is graceful and the scientific content accessible, though still substantive.  All in all, a pleasure.

For me, by far the most exciting and fulfilling aspect of the book has to do with a simple, though fundamental question about insects, a question I’d never been smart enough to formulate.  Think on the earthbound caterpillar munching on a milkweed plant and the highflying Monarch Butterfly that it will become, and ask, “Why does that happen?”  Indeed, why do more than three-quarters of all modern insects undergo such a complete and complex metamorphosis in their lives?

Because it meant and means survival.  Complete metamorphosis, which first arose among insects in the Permian Period (299 – 252 million years ago), allows the young larvae to be “stunningly different from adults.”  Shaw asserts it “may arguably be the single more important factor in the insects’ long-term success . . . .”  (p. 104)  It accomplishes many different things.  First, this “remarkable innovation . . . allowed adult insects to avoid competing with their own offspring for food.”  (p. 14)  The animals’ tasks at these stages are dramatically different – the larvae eat and grow, the adults mate and reproduce.  It may have been prompted by the different species’ need to protect their wings.  This kind of metamorphosis allows for the development and growth of these crucial and delicate appendages with some protection during the transitional stage from larva to adult.  It also provides “diverse resting stages [during the life cycle] for escaping difficult environmental conditions.”  (p. 105)  That last may have played a key role in enabling insects to shrug off most past mass extinctions.  Insects took their most significant hit in the mass extinction at the end of the Permian, some 252 million years ago, but “virtually all the orders with complete metamorphosis survived . . . .”  Many others with less complete metamorphosis did as well.  (p. 111)

Shaw tells the tale of the evolution of life on this planet from a markedly distinct point of view, that of the insect.  In doing so, he stresses that he is offering a necessary counterpoint to the human-centric way in which the evolutionary story is often told.  (Indeed, we humans are displaced, though not missing, in this account.)  It’s largely a chronological telling, a journey from the Cambrian to the present, in which he describes the evolutionary path that insects have followed.  This is a story he tells well, covering the ground with a sure hand, freshness, and a sense of humor.

His insect-centric view point is totally appropriate.  The class Insecta is, after all, one of the planet’s greatest success stories with nearly a million known and named species, and many millions more unknown. Insects clearly have done well over hundreds of millions of years of evolution.  The first true insect appeared on land during the Devonian, some 400 million years ago.  The basic insect body plan apparently leaves little to be desired – three-part body with a head (housing brain, eyes, antennae, and mouth), thorax from which six legs extend (a pair from each thoracic segment, and wings, if appropriate), and abdomen (everything else is found there, most of the bodily systems – no wonder a squashed insect abdomen is mostly goo).  Having the skeleton on the outside offers protection for precious organs.  In addition, small size is an insect virtue, the truly big insects have gone extinct.  Such smallness “allows bugs to divide the world into exceedingly small niches (p. 12),” and weather many storms that have taken out other animals.

Focus for a moment on that hallmark of the prototypical insect – six legs.  Reflective of his approach to this story, Shaw posits that six-legged locomotion is the best of all possible arrangements.  In a section he labels “Two Legs Bad, Six Legs Good,” he lays out his argument.  It proceeds partly from numbers:  the many millions of insect species with hundreds of millions of years to experiment with alternatives are, almost without exception, hexapods.  “Six-legged form is sublime.  Fifty million insect species can’t possibly have it wrong.”  (p. 62)  And, lest we think that humans might have gotten onto something good with bipedalism, he concludes, “Two-legged bipedal locomotion is so unstable and difficult to master that it seems highly improbable and almost pointless.”  (p. 61)

Among insects’ great accomplishments is flight.  They were the first organisms to take to the air, stretching their wings initially in the early Carboniferous, some 327 million years ago, and monopolizing flight for the ensuing 150 million years.  Reflective of Shaw’s informative consideration of the hows and whys of insect evolution is his treatment of the development of wings and flight.  He’s not fond of the hypothesis that the attraction of tastier parts higher up plants got insects out of the soil to a jumping off point for flight.  Rather, he offers an array of other possibilities:  insects may have first climbed plants to gain some warmth with wings themselves favored by selection because they acted like “little solar panels” to warm these cold-blooded organisms, and, also, wings offered a canvas for colors and patterns to fuel courtship and mating, to camouflage their bearers, or to warn off predators.  Speaking of predators, flight itself might have been selected for because it offered a means of escape from predators (a recurrent theme in any evolutionary story), or a way to spread the wealth and colonize large areas.

In Shaw’s capable hands, even the kind of parasitism practiced by many wasp and fly species, a particularly nasty behavior that first appeared in the Jurassic Period (201 - 145 million years ago) and earns its practitioners the label “parasitoid” (that is, an organism which ultimately kills its host), becomes a source of wonder at the inventiveness of the evolutionary process.  Shaw actually approaches the subject with a degree of humor, labelling the main discussion “Which Way to Eat an Oreo:  Two Kinds of Parasitism”.  Of these two approaches, external and internal parasitism, the latter is practiced with an astoundingly rich array of lethal variations across species.  I wont get into the details of any of these many fascinating ways to slowly kill a host, but I am reminded of how the parasitic behavior of the Ichneumonidæ family of wasps led Charles Darwin to write,

I cannot persuade myself that a beneficent & omnipotent God would have designedly created the Ichneumonidæ with the express intention of their feeding within the living bodies of caterpillars, or that a cat should play with mice.  (Letter to botanist Asa Gray written on May 22, 1860.)

Nevertheless, when Shaw puts the parasitoids into an evolutionary context, one can appreciate the behavior.

The Jurassic parasitoids didn’t just find a new protein-rich meal, they narrowed their ecological niches to smaller dimensions than those of any previous predatory animals and in doing so allowed their descendants to live in a multitude of previously unoccupied microscopic niches.  From that time onward, parasitoids dominated the diversity of terrestrial communities, and by their selective killing behaviors they shaped the richness and abundance of both the insect and plant communities.  (p. 136)

The preeminence of insects on this planet is clear.  As Shaw puts it, “Whether or not they rule the planet, insects certainly have largely overrun it.”  (p. 3)  He reaches somewhat further afield when he considers the possibility of complex forms of life elsewhere in the universe.  Very likely to be insect-based, of course.  As he puts it,

The buggy universe hypothesis is verifiable and has already passed one test:  this planet is observed to be astronomically full of bugs.  We can easily image other pathways by which life on earth might have evolved without any humans, or even without any mammals or dinosaurs, but given the unfolding of the earth’s history as we understand it, it’s difficult to imagine how terrestrial ecosystems could have evolved without insects or insectlike creatures.  (p. 191)

He takes seriously the comment ascribed to biologist J.B.S. Haldane that, considering nature, one has to conclude that the Creator has, “[a] n inordinate fondness for beetles.”  For “beetles,” think “insects” and, for Shaw, the species numbers come up trumps again.  For believers in a Creator, the logic holds that the creation of one “buggy planet” (Earth) probably means “he would have made other planets buggy as well.”  (p. 193)

Finally, I have to add that, thankfully, Shaw avoids the trap that ensnares many writer of popular natural history – the first person narrative in which the author becomes the story’s hero.  Only on occasion for me does such a book succeed, more often not.  Shaw interjects himself into the narrative only sporadically and mostly to good effect.

So, yes, I'd say I like the book.

Pleasure and Guilt from the Unexpected

How many things have I looked up
in a lifetime of looking things up?
          ~ Billy Collins, from the poem The Literary Life

I’ve been grappling with guilt over a “treasure” I found in an unexpected place.  Perhaps it was the very fact that it was a surprise, so out-of-place, that I lowered my defenses and bought it.

Late this summer, as I wandered through an antique dealer’s collection of nautically-related antiques (such as ship’s clocks, old and detailed ship models, and pieces of a ship’s figurehead), I unexpectedly came across, way at the back of his displays, the upper and lower jaws of a modern shark.  No antique this.  These cartilage jaws are complete with several rows of beautiful, very sharp, white teeth.  Its dimensions are approximately 9 ½ inches from side to side, and about 4 ¼ inches from top to bottom (measured in the very center of the jaws).

The Pleasure

There was that first rush of pleasure at the find, the difficult effort to mask my disbelief at the negligible price, the hurried handing over of some cash, and the quick retreat to a private place to examine and enjoy the prize.

I need to put this psychic rush into perspective.  Fossil collectors in my neck of the woods – the coastal area of the Mid-Atlantic states – have little choice but to come to know fossil fish teeth, principally those from sharks.  Such fossils are much of what can be discovered at the best collecting sites available to us.  So, teeth are typically our introduction to paleontology.  For those of us who choose to study what we find, shark dentition often morphs into an obsession.

There is a fundamental reason for that obsession – most shark families in ancient times and now exhibit heterodonty, the condition of an individual animal having multiple types of teeth (each type serving a different function, such as cutting or grasping).  Depending upon the species, a shark taxon exhibiting heterodonty may be:  (1) monognathic, that is, the array of teeth in one jaw exhibits more than one functional shape and the pattern is replicated in the other jaw, or (2) their dentition may be dignathic, that is, the functional types differ between upper and lower jaws.  Some shark groups have dentition that is both monognathic and dignathic.  According to ichthyologist Henri Cappetta, dignathic heterodonty (the functional types differ from upper to low jaw) “characterizes practically all sharks apart from some rare exceptions.”  (Chondrichthyes II:  Mesozoic and Cenozoic Elasmobranchii (1987, p. 12.)  (Though Cappetta discusses shark dentition, for this brief overview of shark dentition I have relied primarily on Bretton W. Kent’s Fossil Sharks of the Chesapeake Bay Region, 1994, Appendix B, p. 99 et seq.)

Heterodonty, alone, doesn’t account for all of the differences in tooth morphology within the same species.  Teeth can differ significantly between males and females (sexual dimorphism) and between young and old.

So, given that most extinct sharks are known primarily from their fossil teeth alone and, often, by isolated teeth, not complete arrays of teeth, a collector should develop some understanding of shark dentition, if not some degree of expertise.  Indeed, heterodonty, sexual dimorphism, and age differences have confused, confounded, and misled professional and amateur paleontologists from the beginning, giving rise to many different, extinct shark “species.”

That’s why coming upon the jaws of an extant shark among the antique dealer’s wares boosted my heart rate.  This is not a Rosetta Stone enabling me to decipher the extinct species represented by my myriad fossil shark teeth.  No, it is something perhaps even more important, a touchstone showing and reminding me how much shark teeth (extinct or extant) can differ within the jaws of the same individual.  Believe me, a collector of fossil shark teeth needs that reminder, at hand and in hand, to touch and to study.  Yes, I could look it up or see it on exhibit at a museum, but it's not the same.

The Specimen

These jaws, I believe, are from a young Carcharhinus falciformis, commonly known as a Silky Shark (because its dermal denticles give its skin a distinct smoothness).  The C. falciformis is a large (maximum length of 10 feet), ocean-going shark that can be found in the tropical portions of the Atlantic, Pacific, and Indian Oceans.  (According to the antique dealer, these jaws were sent to him by someone in Florida; so, the location fits.)  Females are larger than males, the former mature at 12 years of age, the latter at 9 or 10.  They live up to some 22 years.  Gestation is about a year and females mate in alternate years.  (The Florida Museum of Natural History offers very informative profiles of various fish on its website.  Much of the material cited here is based on the Biological Profiles:  Silky Shark.)

(This drawing is from page 471 in Sharks of the World:  An Annotated and Illustrated Catalogue of Shark Species Known to Date. Part 2, 1984, by Leonard J.V. Compagno.  This is volume 4 of the FAO Species Catalogue, issued by the Food and Agriculture Organization of the United Nations.)

The C. falciformis, along with the Prionace glauca (Blue Shark) and the C. longimanus (Oceanic Whitetip Shark) are the most common sharks in the world.  My assumption that the shark whose jaws I have was young is based on, to my eye, the relatively small size of these jaws.

As always, I worry about my identification.  But, the dentition of my specimen matches the morphologies and numbers in each jaw cited in the literature for C. falciformis.  J.A. Garrick includes drawings of the teeth in the upper and lower jaws of each of the Carcharhinus sharks he describes in Sharks of the Genus Carcharhinus (NOAA Technical Report NMFS Circular 445, Department of Commerce, May 1982).  Though the quality of the scanned copy put online by NOAA is laughable (in fact, I did laugh when, on one page, the scanner’s hand makes a dramatic appearance), enough can be made out to match the teeth.  Here is the Silky Shark dentition he provides (for the right half of the upper and lower jaws), and close-ups of the same for my specimen.  (Right and left orientation is that of the shark's.)

The photograph of C. falciformis teeth in Sharks of the East Coast of Southern Africa (Volume I, The Genus Carcharhinus, by A.J. Bass, et al., South African Association for Marine Biological Research, 1973, p. 157) adds more weight to this identification.

Yes, there are multiple teeth lined up behind each tooth at the front of the jaw.  Sharks are constantly replacing teeth, potentially losing thousands over the course of their lifetimes.  Here's a picture of some of these replacement teeth lying in wait (the front of the jaw is at the bottom).

The two isolated teeth in the middle of Garrick’s drawing (the lousy image above) are the fifth ones in both jaws, counting from the middle of the jaw line (excluding the symphysis teeth – those small, unique ones in the very middle of each jaw line).

The picture below shows teeth one through six on the right of my specimen’s upper jaw (the little symphysis teeth are visible at the far right); I’ve marked numbers two and five.

The one marked five matches nicely the one Garrick includes in his drawing.  (I’ll get to number two in a moment.)  I cannot match the isolated bottom tooth in the drawing because the tip of the fifth tooth on the right side in the lower jaw of my specimen is broken off.  The picture below is of number three instead, but there’s little to choose between the drawing and the actual tooth shown because, as shown by my specimen, teeth in the lower jaw vary only by size.

If I understand the various types of dentition correctly, C. falciformis exhibits dignathic heterodonty.  In fact, Cappetta cites the genus Carcharhinus as a specific example of a taxon with such dentition.  The teeth in the top jaw of my specimen are wonderfully serrated, nearly all with angled edges, and sharp crown tips.  My sense is that each of these teeth serves a couple of functions.  They are nicely designed for cutting, which must be their primary function, but they would puncture and hold as well.  The sharp, pointed teeth in the bottom jaw seem to serve what is essentially a single function, to grasp and hold.

I labelled the number two tooth in the earlier photograph because it struck me that its triangular shape and erect cutting edges differ significantly from number five and most of the rest of the top jaw teeth which have an angled cutting edge.  I don't think the number two tooth serves a totally different function.  Rather, there’s some logic to having a couple of cutting and grasping teeth in the top jaw that point straight into the prey when it is initially encountered.  What really impressed me as I studied these jaws is how easy it would be to assume that the second and fifth teeth might have come from two different species of sharks, if one had to identify the species based solely on these teeth.  Add in the strikingly different teeth in the lower jaw and another species might well be named.  Clearly, with modern fish, one doesn’t confront such a challenge, but, with fossil teeth, it’s what has happened frequently.

The Guilt

Though I can build a case (and I have tried here to do so above) to justify acquiring these shark jaws, I think a more thoughtful and principled stand would have been to resist contributing in even the smallest way to the decimation of sharks in the world’s oceans.  Therein lies my guilt.

Humans catch and kill many C. falciformis.  The shark “is used for its meat, oil, and fins.”  (Florida Museum of Natural History, Biological Profiles:  Silky Shark.)  C. falciformis is targeted directly by shark fisheries and caught indirectly as a bycatch of tuna fishing.  It is a particular target of ire among tuna fishermen who have slapped it with the epithet “net-eater shark.”

The deeply offensive trade in shark fins cuts through Silky Shark ranks, if only because of the sheer numbers of these sharks.  According to the International Union for Conservation of Nature and Natural Resources (IUCN), “[The] Silky Shark ranks among the three most important sharks in the global shark fin trade, with between half a million and one and a half million Silky Sharks traded annually.”  Humans mutilate C. falciformis and leave it to die, or, they mutilate it, removing its fins for sale, and then extract its jaws to sell to people like me (yes, I was aiding and abetting, however indirect and at a distance).  The ReefQuest Centre for Shark Research sums it up in stark terms,

Thus, as a reflection of their abundance, Silky Sharks have the dubious distinction of being among the most abundantly represented species in Asian shark fin markets and are by far the most common source of cleaned and dried shark jaws sold to tourists in tropical countries.

In its most recent Red List assessment (2014.2), the IUCN concludes that, globally, the C. falciformis is “Near Threatened,” which means that this shark, though not considered endangered or vulnerable to extinction now, “is close to qualifying for a threatened category in the near future.”  But, in certain parts of the world’s oceans, its status is more dire.  In the Eastern Central and South Pacific, and the Northwest and Central Atlantic it is “vulnerable,” that is, it faces “a high risk of extinction in the wild.”  (Definitions of the Red List categories are provided in the Guidelines for Using the IUCN Red List Categories and Criteria, Version 11, February, 2014.)

The Silky Shark deserves better than this.