Sea turtles in some form have existed on Earth for as long as 250 million years. Evidence of their existence can be found in fossils from the Jurassic Era.
There are seven species of sea turtles living today. The smallest is Kemp's Ridley at 30 inches and 100 pounds, and the largest is the leatherback at over 6 feet and 2,000 pounds. Sea turtles spend most of their lifespan in the water, making it difficult to determine actual populations or observe behavior. Their shell, or carapace, is streamlined to move smoothly through water and, unlike other turtles, they cannot retract their legs and head. Sea turtles prefer tropical and subtropical waters.