Subject: Tiger shark again
Date: Wed, 20 Nov 1996 14:24:25 -0800
From: Bill Heim
To: SHARK-L@RAVEN.UTC.EDU

References: Erich Ritter wrote: "In the case of Tiger shark his niche is: Being best adapted towards decreasing of particular food sources and able to switch (tooth shape)...

I've been following this discussion for some time and I just have to comment. It's interesting that you picked the Tiger shark for an example of a shark which evolved non-specific teeth for generalized feeding. I say this because if I was to pick out a single example of a shark evolving teeth to fit a specific food item, I would probably use the tiger shark as my example. I consider it to have one of the most non-generalized dentitions in the shark world. It happens that it is an efficient and large dentition allowing it to feed on a variety of prey items but then the Great white which has a specialized dentition as well (marine mammals) also has a varied diet consisting of fish, sharks, squid, as well as mammals.

The Tiger shark whose dentition has undergone relatively little change (except a marked increase in size) since it evolved in the Eocene out of the Galeorhinus (tope shark) linage. If we look a tooth of the Paleocene praecursor of the tiger shark (Galeorhinus lefevrei) in many ways it appears to much like that of the modern tiger shark (Galeocerdo cuvier) except in minature. It lacks some of the details (the blade is not as recurved and there are no secondary serrations on the cusplets) but these are minor details. The following sharks (particularly Galeocerdo latidens and Galeocerdo eaglesomei) even more closely resemble teeth from the modern shark. The other tiger shark of this age eventually reached an evolutionary dead-end with G. contortus in the Miocene but even here the resemblence to teeth of the modern tiger is unmistakeable. In the Oligocene the last ancestor to the modern tiger shark evolved, G. aduncus. This shark's teeth are virtually identical to those of the modern shark except for size. If compared with teeth from a modern juvenile shark, they are indistinquishable. Finally in the late Miocene full scale teeth of G. cuvier are found.

G. aduncus	G. contortus	G. cuvier

So the point is to say that the tiger shark's teeth changed to fit a new cosmopolitan diet is not entirely correct. In fact, they have changed little except in size.

Now to say that the teeth are shaped for a cosmopolitan diet is also probably also incorrect. The teeth are in fact specialized for feeding on sea turtles. Unlike the vast majority of sharks in which the upper and lower teeth are noticeably different (typically - with many exceptions - narrow lower and wider upper teeth), tiger shark teeth are so alike from both jaws that it is difficult to tell uppers from lowers). The teeth are also shaped like the cutting edge of a round-saw. Thus when the shark clamps down on a turtle shell and begins to shake his head, his serrated, round-saw blade shaped teeth litterally begins sawing through the turtle's shell, both on the top and the bottom. Compare this with a more typical dentition such as that from a dusky shark (Carcharhinus obscurus). With curved, roughly triangular serrated upper teeth and narrow, pointy lower teeth, biting into a turtle presents a problem. While the upper teeth would have an effective sawing bite. The narrow, pointy lower teeth would not. Without an effective hold on the turtle, shaking its head might simply toss the turtle out of its mouth and in any event, the upper teeth are going to have to do virtually all of the cutting work.

With a large, efficient cutting dentition, the tiger shark can feed on a wide variety of prey, but it is adapted, designed, and specialized to feed on turtles. If you want an example of a less specialized dentition, I would use the one of the Carcharhinus species such as the bull or dusky sharks as examples. With large, roughly triangular upper teeth and narrow, pointy lower teeth, they feed on a wide variety of prey. In fact, so successful is this type of dentition that it or a close approximation is found in dozens of species of sharks.

One last note: In the late Cretaceous, a period famous for its large sea turtles, a genus of shark (Squalicorax - crow sharks) evolved teeth along the same pattern as the modern tiger shark. The teeth are of a remarkably similar size and shape as tiger shark teeth and even evolved larger as time went on. In fact, a few teeth of (Squalicorax pristodontus (the last and largest species) have fine serrations on the edges of the coarse serrations, a pattern found virtually no-where else except on tiger sharks. The crow sharks were from a completely different family of sharks than the tiger sharks and went extinct with dinosaurs, leaving no descendants.