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.

A Different Way to See Fossil Skeleton Mounts

Memorandum
To:  Myself
Subject:  Fossil Skeleton Mounts

Next time you are in a natural history museum (one with its fossil halls not undergoing renovation, so, forget the Smithsonian’s National Museum of Natural History), approach a fossil skeleton mount (that sine qua non of natural history museums), and try your damnedest to get beyond that initial awe-inspiring view.  Move as close as you’re allowed and look for those telltale signs of how that skeleton is being held together, how it’s holding its pose.  Look for the armature, that construct of metal that keeps the bones in place.  Yes, some armatures are internal, but a glimpse of a bit of metal sneaking through joints, from inside one bone into another, might be had (unless the joints are filled in with plaster).  An external armature might be as obvious as that holding up the neck and torso of the Apatosaurus skeleton dominating the Peabody Museum of Natural History’s Great Hall,

or it might be mostly hidden on the backside of the bones.

Look carefully at the bones.  Try to identify which of the skeleton’s bones are real fossils and which are casts.  Museums seem to be doing a better job these days of ensuring that fossil bones and casts can be distinguished from each other.  On older mounts this may not be so easy given that fossils and casts were often painted the same color.

If fossil bones can be distinguished from casts, consider the fate of those real fossil bones in such a mount.


It’s not clear when I’ll be able to follow the guidance in this memo.  [Later edit:  I will be able to follow the memo a lot sooner than I thought.  In the original version of this post, I wrote that "there are no fossil skeleton mounts on display, at the moment, at the NMNH (my natural history museum home)."  That's wrong.  There's at least one on display in the Sant Ocean Hall, a Basilosaurus, which is an ancient whale.  See Addendum at end.]  “Hatcher,” the cast of a Triceratops, which had been the sole dinosaur standing publicly in the museum after the Fossil Halls shuttered their doors earlier this year, is now behind temporary walls on the third floor as a new Late Cretaceous display takes shape around it and “Stan,” the museum’s T. rex cast.  When it opens in late November, this display will be up for five years as the Fossil Halls undergo their total makeover.

It’s not a coincidence that my interest in the process and consequences of including real fossils in skeleton mounts has taken flight just when the NMNH Fossil Halls are off-limits.  Most folks would be interested in the construction of the new NMNH displays, but, in my contrarian (perhaps better read “perverse”) fashion, I have become focused on the flipside of that, that is, on the dismantling of the myriad mounts that graced the Fossil Halls for the past century.

Some wonderful pictures of aspects of this deconstruction can be found in various postings in the Smithsonian blog Digging the Fossil Record:  Paleobiology at the Smithsonian.  To date, primarily featured in these posts is the taking apart and moving of the casts of “Hatcher” and “Stan” (cast mounts really aren't my concern here).

Visitors to the NMNH can witness some of that fossil mount deconstruction if they visit the museum’s FossiLab on the second floor and look in through the windows and watch the volunteers at work.  A recent entry in the Digging the Fossil Record blog highlighted the reopening of the FossiLab (which had briefly been shielded from public view in its previous location down in the Fossil Halls) and noted that some volunteers in the lab will be engaged in taking mounts apart and creating storage containers for the disarticulated bones.  The blogger (sadly, anonymous) comments that:

Dismantling skeletons is challenging.   Some of the mounts were built a long time ago and, since mounting techniques have changed a lot over time, nearly every mount was constructed in a different way.  Add in the problem that some of the bones are extremely fragile, and it can take some puzzling to figure out how to tease the mounts apart safely.  Once the bones are free of the armature they are photographed, logged into our collections database, and placed in archival storage where they will be preserved for scientific study and, perhaps, remounting for a future exhibit.

What happens to fossil bones when they are included in such mounts, particularly those “built a long time ago”?  There is literature out there about that.  A particularly depressing account comes from Robert L. Evander, senior principal preparator at the American Museum of Natural History, in a 2009 piece titled Armature Damage to a Mounted Specimen (this full-text link is listed on the Society of Vertebrate Paleontology’s Preparator’s Resources site).

The fossil mount in question is the skeleton of a glyptodont, a large (some were car-sized), armored, armadillo-like mammal, that dates back to at least the Miocene (23 to 5 million years ago) and which went extinct about 10,000 years ago.  Evander basically reverse-engineered the mount because the American Museum of Natural History’s Adam Hermann, who prepared the mount from the fossil bones in 1916 – 1917, left no detailed account of how that particular mount was constructed.  This was the final fossil skeleton he mounted, so, it may reflect the best of Hermann’s work.  Writes Evander,

Adam Hermann’s free mounts are deceptive.  The bones appear to be gently mounted upon cleverly-hidden external armatures closely shaped to the bones.  The few visible drill holes seem to be attaching individual bones to this external armature.

How did he achieve this deception?  Well, he drilled some 100 holes lengthwise into articulated fossil bones in order to run wires through them as part of the internal armature.  Spaces in joints were filled with plaster.  The external armature was shaped on the basis of plaster casts of the bones and was then attached to the fossil bones by more drilling through the bones, and the insertion of many more nuts and bolts.  Here’s an illustration of parts of this method from a piece Hermann wrote explaining his fossil preparation and mounting techniques; the caption tells the story (Modern Laboratory Methods in Vertebrate Palæontology, Bulletin of the American Museum of Natural History, Volume XXVI, 1909, p. 318):

Evander discovered that, because the armatures are harder than the bones, the points at which the armatures are screwed or bolted to the fossil bones are places where cracks and breaks have developed.

Conclusion?  Disaster.  Writes Evander of this fossil skeleton and others mounted by Hermann a century ago,

In some measure, Hermann’s mounting has adversely affected scientific access to these specimens, which will never be freely available to study in the disarticulated state.  Individual postcranial elements cannot be extracted easily.  Realistically, these mounts are permanent.

It is ironic (from our perspective a century later) that in the same 1909 piece of his, Hermann cautioned,

I therefore say, that a beginner in a palæontological laboratory never ought to be trusted with a delicate and rare specimen.  In my experience, I have seen irremediable damage done on account of poor judgment.  (p. 331)

Because Hermann-like treatment of fossil bones in mounted skeletons continued well into the 20th century, Evander finishes with a word for us frequenters of natural history museums:

Thus the mounted specimens seen in the museums of the world may be more fragile than they appear.

Though the doors to the NMNH’s Fossil Halls are shut, I wondered whether photographs I’d taken previously of its fossil skeleton mounts might show something about the mounting process employed.  One entry in Digging the Fossil Record includes a couple of pictures of the work on dismantling the museum’s Irish Elk with its massive antlers.  This particular mount, which went on display in 1872, was the Smithsonian’s first mount utilizing real fossil bones.  (A post on the superb blog Dinosours! [sic] discusses this skeleton mount.)  That prompted me to look at pictures I’d taken of the Irish Elk for a previous post on my blog.  The picture below (looking head on) shows the animal’s left antler and the wiring that holds up.  A couple of seemingly benign metal supports appear to cradle the antler from its ventral side.

But, the following picture (taken from behind the animal) suggests that all is not so benign.  The “cradling” arms are apparently bolted through the antler to a metal plate on the backside of the antler.  Further, part of a metal armature can be seen coming from the skeleton up onto the antler.  This too is apparently bolted to the antler.  I have marked several of these bolt heads.

I found a bit more in my collection of photos.  A set of photos of the NMNH’s Brontotherium hatcheri (taken for a blog posting about this rhinoceros-like herbivore related to horses that lived some 40 million years ago) may also be revealing.  Unfortunately, I cannot be certain when this mount was created (the fossil bones were discovered by John Bell Hatcher in 1887) nor exactly which of the bones in this skeleton are real.  But, the Smithsonian’s Collections Search Center describes this specimen as including the following fossil bones:  “skull elements, lower jaw, Axial element, thoracic vertebra, Appendicular elements, pelvic girdle, phalange.”  Here’s a picture of the rump of the specimen.  I’ve marked two places that particularly drew my attention.

The lower arrow points to part of the external armature, where it is bolted onto the skeleton.

Do these go into fossil bone?  I don’t know.  I’d love to be able to see this skeleton again.

The upper arrow points to part of the pelvic girdle (which is listed as fossilized bone but perhaps not all of it is) where a bolt head is obvious.

I assume this bolt attaches to the external armature which runs behind the bone, and I believe that this bolt runs through fossil bone, not plaster.  Of interest is the crack that radiates from the bolt head.  Evander would probably not be surprised.

Despite all of this, I’m not prepared to let the other shoe drop, the one that signals opposition to inclusion of any actual fossils in free standing skeleton mounts.  That’s a major bone (pun intended) of contention in paleontological circles where there is concern, not just for the damage done in the mounting process, but the potential damage incurred by being in such a mount for the long term, subject to changes in temperature and humidity, vibrations, and other harmful stresses.  Though mounting techniques have changed for the better, I may still be heading in the direction of advocating for nothing real in free-standing mounts (I would not be alone in holding that position).

For the time being, memo writer, I will consider such mounts from that different perspective and with a great deal of concern.

ADDENDUM

Ben, author of the blog Dinosours!, commented on this posting (see below) and asked my reaction to more "gentle" mounting techniques.  In my response, I referred to Digging the Fossil Record and a post there by Matt Carrano, Smithsonian Curator of Dinosauria, in which he stated that mounts in which fossil bones were drilled into were a thing of the past.  He included a photo of some of the sternum bones of an ancient whale currently on display at the NMNH; these bones were cradled in metal supports.  When I went back to his post, I was dismayed on two counts:  first, I should have quoted Carrano in my original post (I do in my response to Ben), and, second, I had completely overlooked the fossil skeleton mount of the ancient whale in the Sant Ocean Hall (which allowed me to make the erroneous assertion that, with the closing of the Fossil Halls, there weren't any still on display - indeed, there may still be others, my search continues).  I've had a chance to take a look at that whale skeleton (which includes real bones) and, though I tried, I found it difficult to really observe how all of it was mounted (sorry, memo writer) because the whole thing floats high overhead, suspended from the ceiling (and accompanied by two cast mounts).  Clearly, the sternum bones are being supported without drilling, as apparently are the vertebrae.  Also, real bone and cast are easily distinguishable, a good practice.  But, I cannot be so sure that some bolts aren't still penetrating real bone.  Unfortunately, I cannot get close enough to be sure.  Here is a picture of a few of the ribs and a flipper (real bone is darker and rougher).  Perhaps I'm just inclined to see it, but it does look to me as though some real bone may have been drilled into.