Fig. 14. Extent of closure of the costoperipheral fontanelles in relation to length of plastron in 17 skeletons of T. o. ornata from eastern Kansas. Extent of closure is expressed as an estimated percentage of total closure of all the costoperipheral fontanelles, even though some of them close sooner than others. Closure is usually complete by the time sexual maturity is attained.
Carapace
1.) Costoperipheral. Openings between the free ends of developing ribs, between nuchal bone and first rib, and, between pygal bone and last rib; limited laterally by peripheral bones; variable in shape.
2.) Costoneural. Triangular openings on either side of middorsal line between proximal ends of costal plates and developing neural plates.
The costoneural fontanelles are nearly closed in individuals of the 70 millimeter (plastron length) class and seldom remain open after a length of 80 millimeters is attained ([Fig. 14]). Of the costoperipheral fontanelles, the anterior one (between first rib and nuchal bone) closes first and the posterior one (between last rib and pygal bone) last. It remains open in some turtles in which the plastron is longer than 100 millimeters. The remaining costoperipheral fontanelles close in varying sequence but those in the area of the bridge (nos. 2 to 5), where there is presumably greater stress on the shell, close sooner than the others.
The plastral fontanelles are closed in most specimens of the 90 millimeter (plastron length) class; the anteromedian fontanelle closes first.
The meager covering of the fontanelles makes juvenal turtles more susceptible than adults to many kinds of injuries and to predation.
Movable Parts of the Shell
Parts of the shell that are more or less movable upon one another and that function in closing the shell are found in several families of Recent turtles. African side-necked terrapins of the genus Pelusios have a movable forelobe on the plastron. Kinosternids have one or two flexible transverse hinges on the plastron. In the Testudinidae the African Kinixys has a movable hinge on the posterior part of the carapace and Pyxis arachnoides of Madagascar has a short, hinged, anterior plastral lobe. Certain trionychid turtles, such as Lissemys, utilize the flexible flaps of the carapace (the flaps of some species are reinforced with peripheral bones) to close the shell.
Movable shell-parts of turtles are, in general, protective in function; they cover parts of the soft anatomy that would otherwise be exposed.