Here are two more photos from my immensely edifying visit to Jim Krupa’s lab at The University of Kentucky. They show the extreme diversity of morphology that evolution can produce in a single group.
The first shows the jaw of what I remember as a tiger shark, Galeocerdo cuvier. National Geographic notes that “They have sharp, highly serrated teeth and powerful jaws that allow them to crack the shells of sea turtles and clams. The stomach contents of captured tiger sharks have included stingrays, sea snakes, seals, birds, squids, and even license plates and old tires.”
The rows of teeth are lined up, waiting in the wings, to be replaced after one on duty is lost. The teeth aren’t embedded in the jaw, but merely in the gum tissue. Wikipedia has a good article on them.
Sharks are in the class Chondrichthyes: they have cartilage rather than bone. The subclass Elasmobranchii includes sharks, skates and rays. And the tiger shark is in the largest order of elasmobranchs, the Carcharhiniformes.
And here’s one of the weirdest elasmobranchs—the jaw of the Port Jackson shark (Heterodontus portusjacksoni), endemic to Australian waters. It’s in the order Heterodontiformes (“bull sharks”), distinguished, among other things, by having the mouth completely in front of the eyes. The “Heterodontus” part of the genus name means “different teeth,” and that’s indeed what you see, spectacularly, in the jaw. Having differentiated teeth in the jaw is very rare in sharks:
The small teeth in front are for grabbing and piercing, the ones at the rear for grinding up stuff, especially molluscs. The Florida Museum of Natural History site notes:
This species feeds primarily on echinoderms, crustaceans, molluscs, and some small fish. Sea urchins and large gastropod molluscs are noted in almost every study on the diets of Port Jackson sharks. Stomach contents are typically ground up too small for full identification, thus leading researchers to believe Port Jackson sharks grind their food thoroughly before swallowing. This is also supported with juvenile diets, since it has been noted that juveniles eat more soft-bodied animals, and contain less molar-like teeth.
Here’s what the jaws look like in situ:
For $750 you can actually buy a Port Jackson shark for your aquarium, but I’m not sure why anyone would do that, as they grow over five feet long.