As years pass, calcium in the center of a turtle's bone is resorbed, erasing earlier growth layers. They've been replaced by more porous bone. Zug and collaborators have devised a range of methods for estimating the number of lost growth layers that once filled the bone core. The fourth species Zug studied was the largest sea turtle. In fact, at recorded weights of well over 1,000 pounds, the leatherback sea turtle may be the largest living reptile.
Leatherbacks have an un- reptilelike, more mammalian pattern of growth in their limb bones.
The leather-back's leg bone lacked growth layers that Zug could count in limbs of other sea turtles. Measuring those layers allowed him to come up with age estimates for leatherbacks. Zug's turtle salvagers provided him not only with bones but also with carapace-lengths for the hundreds of stranded animals. With that data, he has plotted the age estimates against lengths for the four sea turtle species. He also has predicted the average age of sexual maturity for each species by comparing his age estimates with data from field researchers who, over the decades, have recorded average shell lengths of thousands of female turtles on nesting beaches.