Natural History of the Ornate Box Turtle, Terrapene ornata ornata Agassiz

by John M. Legler

The fifth peripheral bone, constituting the lowest point of the carapace, has a medial projection that acts as a pivoting point for both lobes of the plastron; the roughened anterior corners of the hind lobe articulate with these processes. The roughened posterior corners of the forelobe of the plastron likewise articulate with these processes. The posterior process or "tail" of the entoplastron extends to, or nearly to, the bony transverse hinge.

In juveniles that have been cleared and stained, the homologues of the parts that are movable in adults are easily identifiable; the proportions of these parts and their relations to one another are, however, much different.

In juveniles (Figs. [18] and [19]) the buttresses are relatively longer and narrower, and are distinct—more nearly like those of generalized emyids than those of adult T. ornata. The buttresses enclose a large open space, which in adults is filled by the fifth peripheral. The hyoplastral and hypoplastral bones are in contact only laterally. They are firmly joined by bony processes; the interdigitating nature of this articulation contrasts with its homologue in the adult, the point where the roughened corners of the forelobes and hind lobes meet. The fifth peripheral in juveniles ([Fig. 19]) lies dorsal to this articulation. The position of the future transverse hinge corresponds to a line passing through the articulations of the hyoplastra and hypoplastra. The tail of the entoplastron ordinarily extends posterior to this line in juveniles.

The external scutellation of the plastral hinge in adults also differs from that in juveniles. In adults ([Fig. 17] and [Pl. 22]) the transverse hinge is marked by ligamentous tissue between the pectoral and abdominal laminae; the forelobe of the plastron is distinctly narrower than the hind lobe. Two small scales lie near the corner of the hinge on each side. The larger and more anterior of these scales is the axillary; it is present in box turtles of all ages. The smaller scale ([Fig. 17]), to my knowledge, has never been named or mentioned in the literature; it is herein termed the apical scale. It is a constant feature in adults but is always lacking in hatchlings and small juveniles. Other scales, much smaller than the axillary and apical, occur on the ligamentous tissue of the hinge of some adults.

Fig. 18. Plastron of hatchling (× 2), cleared and stained to show bony structure. (Abbreviations not listed in legend for [Fig. 15] are as follows: af, anteromedian fontanelle; ep, epiplastron; pf, posteromedian fontanelle.)
Fig. 19. Carapace of hatchling (× 1½), cleared and stained to show bony structure; lateral view; anterior end at left. (Abbreviations as in [Fig. 15].)		Fig. 20. Lateral view of hatchling (× 1); note the lateral process of the pectoral lamina (pl) extending posterior to the axillary scale (ax) in a position corresponding to the apical scale of adults. There is no external indication of the transverse hinge in young individuals. The yolk sac of this individual has been retracted but the umbilicus (umb) has not yet closed.

In juveniles ([Fig. 20]) the pectoroabdominal seam contains no ligamentous tissue and is like the other interlaminal seams of the plastron. A lateral apex of the pectoral lamina projects upward behind the axillary scale on each side, in the position occupied by the apical scale of adults. Examination of a large series of specimens revealed that the apical scale of adults becomes separated from the lateral apex of the pectoral lamina at approximately the time when the hinge becomes functional as such.

Ontogenetic changes in the shell can be summarized as follows: 1) Buttresses become less distinct in the first two years of life (plastral lengths of 40 to 55 mm.); 2) Interdigitating processes of the forelobes and hind lobes become relatively shorter and wider, the entoplastron no longer projects posterior to the hinge, the lateral apex of the pectoral lamina becomes creased, and some movement of the plastron can take place between the second and third years (plastral lengths of 55 to 65 mm.); 3) Plastral lobes become freely movable upon one another and upon the carapace by the end of the fourth year (plastral length approximately 70 mm.) in most individuals.

The plastron of a juvenal box turtle is not completely immovable. The bones of the shell are flexible for a time after hatching and allow some movement of the plastron; but the relatively greater bulk of the body in young box turtles would prevent complete closure of the shell even if a functional hinge were present. Hatchlings can withdraw the head and forelegs only to a line running between the anterior edges of the shell. To do so the rear half of the shell is opened and the hind legs are extended. When the head and forelegs are retracted to the maximum, the elbow-joints are pressed against the tympanic region or behind the head; the fore-limbs cannot be drawn part way across the snout, as in adults. Hatchlings can elevate the plastron to an angle of approximately nine degrees; the plastron of an adult, with shell closed, is elevated about 50 degrees. Hatchlings flex the plastron chiefly in the region of the humeropectoral seam, rather than at the anlage of the transverse hinge.

Adult box turtles, when walking, characteristically carry the forelobe of the plastron slightly flexed. This flexion of the plastron, combined with its naturally up-turned anterior edge, cause it to function in the manner of a sled runner when the turtle is moving forward. A movable plastron, therefore, in addition to its primarily protective function, seems to aid the turtle in traveling through tall grass or over uneven ground. The gular scutes, on the anterior edge of the forelobe, become worn long before other plastral laminae do.

An adult female from Richland County, Illinois, had an abnormal but functional hinge on the humeropectoral seam in addition to a normal hinge on the pectoroabdominal seam. The abnormal hinge resulted from a transverse break in which ligamentous tissue later developed. The muscles closing the plastron moved the more anterior of the two hinges; the normal hinge was not functional.