_Fontanelles_
At the time of hatching, fontanelles remain where bones of the shell
have not yet articulated with their neighbors. In general, the
fontanelles of the shell are closed by the time sexual maturity is
attained, but some remain open a year or two longer.
The fontanelles of the shell are classified as follows (see Figs. 14
to 16 and 18 to 19):
_Plastron_
1.) _Anteromedian._ Rhomboidal; limited anteriorly by hyoplastral
bones and posteriorly by hypoplastral bones; posterior tip of
entoplastral bone may project into this fontanelle.
2.) _Posteromedian._ Limited anteriorly by hypoplastral bones and
posteriorly by xiphyplastral bones (since hypoplastral bones do not
articulate medially in hatchlings, anteromedian and posteromedian
fontanelles form a single, more or less dumbbell-shaped opening).
[Illustration: 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.
A hinged plastron, capable of wholly or partly closing the shell,
occurs in six genera of the family Emyidae (see introduction). In
these emyids the plastron is divided into two lobes, which are joined
to each other by ligamentous tissue at the junction of the hyoplastral
and hypoplastral bones; externally, the hinge occurs along the seam
between the pectoral and abdominal laminae. This junction forms a more
or less freely movable hinge in adults. The plastron is attached to
the carapace by ligamentous tissue. Both lobes of the plastron or only
the buttresses of the hind lobe may articulate with the carapace. The
former condition obtains in _Emys_ and _Emydoidea_; the latter more
specialized condition is found in _Terrapene_.
In generalized emyid turtles such as _Clemmys_ there are no movable
shell parts. The plastron is joined to the carapace by the sutures of
the bridge. A long stout process, the axillary buttress, arises on
each side from the hyoplastron and articulates with the tip of the
first costal. A similar process, the inguinal buttress, arises from
the anterior part of each of the hypoplastral elements and meets the
sixth costal on each side. The buttresses form the anterior and
posterior margins of the bridge. It is clear that movement of the
plastron in many emyids is mechanically impossible because of the
bracing effect of the buttresses.
In _Terrapene_ the movable articulations of the shell are neither
structurally nor functionally developed in juveniles. Adults of _T.
ornata_ have highly modified bony buttresses on the plastron that are
homologous with those in more generalized emyids. The inguinal
buttresses are low and wide, and have a sheer lateral surface forming
a sliding articulation with the fifth and sixth peripheral bones of
the carapace. The axillary buttresses are reduced to mere bony points
near the posterolateral corners of the forelobe and do not articulate
directly with the carapace (Figs. 15 and 16).
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.
[Illustration: FIG. 15. Lateral view of adult shell (A-- A3/4),
showing movable parts with anterior portion at left.
(Abbreviations are as follows: ab, axillary buttress;
hp, hypoplastron; hy, hyoplastron; ib, inguinal buttress;
p5, fifth peripheral bone; th, transverse hinge).]
[Illustration: FIG. 16. Medial view of adult shell (A-- A3/4),
showing movable parts with anterior portion at left.
(Abbreviations as in fig. 15).]
[Illustration: FIG. 17. Lateral view of adult shell (A-- A3/4),
showing scutellation of movable parts with anterior portion at
left. (Abbreviations are as follows: ap, apical scale; ax,
axillary scale; m5, fifth marginal scale; pl, pectoral
lamina.)]
The external scutellation of the plastral hinge in adults also differs
from that in juveniles. In adults (Fig. 17 and Pl. 8) 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.
[Illustration: FIG. 18. Plastron of hatchling (A-- 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.)]
[Illustration: FIG. 19. Carapace of hatchling (A-- 1A1/2),
cleared and stained to show bony structure; lateral view;
anterior end at left. (Abbreviations as in Fig. 15.)]
[Illustration: FIG. 20. Lateral view of hatchling (A-- 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.