The largest freshwater turtle in the world, the alligator snapping turtle (Macroclemys temminckii) is native to the southeastern region of the United States. Like its distant relative, the common snapper (Chelydra serpentina), the alligator snapper has a large head and powerful jaws. However, the alligator snapper differs quite a bit from the common snapper, both in the way it looks and in the way it hunts and eats.

Legend has it that a 403-pound Macroclemys was found in the Neosho River, Cherokee County, Kansas in 1937. However, the size of this specimen cannot be verified. The largest individual on record is a 236-pounder in the Brookfield Zoo, Chicago. A snapper skeleton which Peter Pritchard viewed in White Springs, Florida, had a carapace length of 31 inches and weighed over 200 pounds in life. These record-breaking individuals aside, Ernst and Barbour report that generally they can grow to a carapace length of 26 inches (66 cm) and weigh up to 176 pounds (80 kg).

While the common snapper (Chelydra) has a rather rounded smooth carapace, the alligator snapper's shell has three large, pronounced ridges running front to back across its massive shell. These ridges are very pronounced even in hatchling specimens, which also have a tail that is almost as long as the carapace itself. Quoting from Pritchard, "The snout is more pointed, when viewed from above, than that of Chelydra, and the eyes are placed on the sides of the head, rather than toward the top as in Chelydra. The eye is surrounded by a star-shaped arrangement of fleshy filamentous "eyelashes", and the plastral scutes often become so complicated and subdivided that it is wise to give up the task of trying to name or homologize them."

The large ridges, the large, coarse neck and head, and the huge size of the alligator snapper all contribute to its primitive look and its reputation of being the dinosaur of the turtle world.

All snapping turtles are both scavengers and active hunters to some degree. However, Macroclemys is unique in that it has a small pink worm-like lure in the bottom of its mouth. It lies quietly on the bottom of the dark, slow moving body of water, with jaws wide open, wiggling the lure so as to entice unwary fish to investigate. The alligator snapper is so sedentary on the bottom of swamps and bayou that algae covers its rough, irregular carapace making it almost invisible to fish. Furthermore the lining of the mouth is gray and black. When a fish comes close, the massive jaws close quickly on the prey, netting a meal (Pritchard, page 496).