On The Cutting Edge

The entire ocean is a tomb and I a tooth. ~ from the poem “Shark” by Richard Grossman

Playing Around How to capture the image of a fossil continues to frustrate me. Would that I could draw like those 19th Century artists who specialized in fossils, artists like Joseph Dinkel. But, absent the necessary talent, I rely on digital photography supplemented on occasion by amateurish sketches. Lately, though, I’ve been playing around with Inkscape, a free vector graphics drawing software program. Working from photographs, I created the two profile images (below) of a fossil tooth from a sand tiger (Striatolamia striata, Paleocene epoch, about 60 million years ago).These images precipitated an exploration of the cutting edges of fossil shark teeth – certainly nothing avant-garde or truly cutting edge about my efforts. As will become evident, this exploration triggered mixed emotions. (In these drawings, the cutting edge on either side of the tooth is the middle dark line that tracks up the crown, beginning about a third of the way up from the root. The tooth is approximately 1 7/16 inches long and somewhat worn.) [Later edit: I should have noted that the drawings don't show how the lower two-thirds of the crown is striated, a hallmark of teeth from this species. The striations are faint, probably due to wearing, and mostly do not appear in the photos upon which the drawings are based.] Cutting Edges of the Shark Kind The cutting edge on a shark tooth is a more or less prominent ridge that can run down either margin of the crown and was “developed for slicing flesh” (Kent, p. 92 – full citation below). As best I can figure, there are two basic types of cutting edges on shark teeth – smooth and serrated. Each type comes in various flavors. For example, cutting edges may extend the complete length of the crown margins or just some portion of them. Serrations, described by Frazzetta as “a series of small projections or scallops” that run down side of the crown, may be simple or complex (p. 97 – full citation below). The latter are serrated serrations. The picture on the left below is of a fossilized tooth from a snaggletooth shark (Hemipristis serra) with simple serrations that stop before reaching the top of the crown, conveniently leaving a point for puncturing flesh (the serrations are then in a great place for major cutting). The picture on the right shows a fossilized tooth from a tiger shark (Galeocerdo cuvier) with a closeup of the serrations on the shoulders of the crown. Examined closely and it’s evident that the serrations are serrated. Both of these specimens are from the late Miocene epoch, about 5 mya. The third picture below is of a modern Galeocerdo cuvier – the serrated serrations are much clearer on this specimen. S. striata With A Twist There is a complex diversity in cutting edges, influenced greatly by overall tooth shape. I wont go into that here, other than to return the two drawings above of the S. striata tooth. These drawings highlight a feature of the shape of the teeth (particularly the anterior ones) from this sand tiger species and others – they can be gracefully sinuous – often described as sigmoidal. From the root, the crown first curves out, away from the tongue (the prominent protrusion in the root points toward the tongue), then arches back and, finally, at the apex, bends toward the lip. Frazzetta observed this “reverse curvature” at the tip and likened it to the shape he saw in the teeth of pythons and boas, and some viper fangs. The general shape, apparently, is useful for penetrating and holding. I wonder if there aren’t some interesting tradeoffs in developing these curved teeth, beyond the mechanics of their use. Presumably, the curvature allows for a longer cutting edge on a tooth that doesn’t take up as much vertical space, perhaps allowing them to fit in a smaller mouth. More bang for the buck? Just a thought. The very process of capturing an image of a fossil, whether it’s by drawing or photographing, sometimes forces me to register features for the first time. In this case, it was the puzzling aspect of the S. striata tooth – a difference in the cutting edges on either side of the crown. In the drawing on the left above, the cutting edge has a final, little twist at the apex, seemingly joining the lingual side of the crown; while, in the drawing on the right above, the cutting edge appears to become one with the labial, relatively flat side of the crown. I’ve reproduced the first drawing below with the twist circled. It’s not a function of the drawings; it’s really in the tooth and, in my limited experience, this twist isn’t unique to this particular specimen of S. striata, nor to just this particular species of sand tiger. There are two issues actually – why does the cutting edge do this little flip or twist only on one side of the crown, and why have I not seen it described in the literature? Maybe I haven’t looked hard enough or smart enough. My tried and true references appear to be silent on it. I had high hopes for Cunningham’s article on the dentition of Striatolamia macrota (full citation below – it’s relevant since some folks argue that S. striata is the same species), but, the twist doesn’t appear. Surrounded by Cutting Edges I mentioned Frazzetta’s work earlier. It’s intriguing as much for the methodology as for the analysis. He found no available literature on how cutting edges rend materials that are flexible, that is, have give to them, materials such as, oh, say, . . . flesh. So, to explore how shark teeth do their deadly deed, he gathered various smooth and serrated metal blades, including saws, and then closely observed the mechanics of what happened when the cutting edges were brought to bear on such organic materials as fish, cow leather, and shark skin, and on some inorganic materials including paper and cloth. His article triggered the sickening realization that I am surrounded by cutting edges. At this juncture in this Mid-Atlantic winter of our discontent, the cutting edge on the snow shovel I’ve been using has been worn dull. Given that it’s plastic, it has suffered from encounters with ice, concrete, and asphalt. The other morning, as I walked through my garage to get the shovel, I passed two band saws hanging on the wall. They’d been doing their job as recently as a couple of weeks ago, cutting up some of the limbs and branches from three (I think) trees that came down in the farthest reaches of my backyard during a day of high wind (not knowing for sure how many trees fell suggests the extent of the mess and, of course, all parts of said backyard are buried under more than three feet of snow). Back in those pre-snow days, the jaggedly notched cutting edges of those band saws tore through pine and locust, scattering sawdust, and on occasion catching on my gloves. At that point, hidden and protected in my right glove was a bandaged finger, nicked as I prepared dinner a few days earlier, a thin serrated knife failing to distinguish chicken from human flesh. Just consider the myriad utensils in the kitchen drawers, the edges of the boxes in the pantry that hold plastic wrap or wax paper, the razors, scissors, and clippers waiting in the bathroom. Wander through the house, they’re everywhere. A mouth full of teeth, probably the most varied collection of different cutting edges each of us typically has. All of this designed to puncture, tear, rip, break up (and down) . . . . Nasty, brutish. It is extremely absurd that I hunt for, and have drawers full of, fossil shark teeth, objects with wonderfully efficient cutting edges, cutting edges that I find quite beautiful . . . on shark teeth. To be honest, cutting edges and I should not mix. I do very poorly at the sight of blood, I even blanch at reading those poems by Sylvia Plath (“Cut”) and Robert Frost (“Out, Out – ”), and . . . point made. I’m already feeling faint.

Sources

Henri Cappetta, Chondrichthyes II: Mesozoic and Cenozoic Elasmobranchii (1987)

Stephen B. Cunningham, A Comparison of Isolated Teeth of Early Eocene Striatolamia microta (Chrondrichthyes, Laminformes), With Those of a Recent Sand Shark, Carcharias taurus, Tertiary Research, June 2000

T.H. Frazzetta, The mechanics of cutting and the form of shark teeth (Chrondrichthyes, Elasmobranchii), Zoomorphology, Vol. 108, p. 93-107, 1988

Bretton W. Kent, Fossil Sharks of the Chesapeake Bay Region (1994)