Saturday, September 27, 2014

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.

No comments:

Post a Comment

Nature Blog Network