The age of a snapping turtle is determined by counting the number of rings on its shell, or carapace. The shell is living bone that is covered with keratin, the same material found in human hair and fingernails. As the snapping turtle grows, the keratin expands, creating growth lines.
Modern-day snapping turtles look much like Proganochelys, a primitive species that lived roughly 215 million years ago and predated the dinosaurs. Like their ancestors, snapping turtles lay eggs. The sex of each egg is determined by the incubation temperature. Male or female, the tiny hatchlings are about the size of a quarter when they emerge.
Growth is also determined by the outside temperature. Snapping turtles in northern climates grow slower but tend to get larger than their southern cousins. The largest snapping turtle found had a carapace that measured 18.5 inches. The heaviest wild snapping turtle weighed in at 68 pounds. Average recorded length, for both male and female turtles, measures 8 to 14 inches, with a weight of between 35 and 45 pounds.
As the turtles age, their growth rate decreases but never stops. The rings continue to form.
Some snapping turtles in captivity are more than 75 years old. The oldest turtle found, based on the shell ring count, was 79 years old.