Biology_of_Turtles

Figure 3.32  Bony plastron of a young green sea turtle, Chelonia mydas.

to the viscera, the plastron remains flexible along the midline and at the bridges throughout life, as it does in trionychids. This feature allows the chest to be pressed inward or distended in response to rapid inhalation of air during brief surfacings, or as a result of exposure to high pressure during deep dives.

In juvenile cheloniids the hyoplastra and hypoplastra have a “sunburst” appearance with numerous long spines, especially on the proximal edge. The epiplastra and xiphiplastra are somewhat curved and rather narrow, and they make mutual contact only at the extreme front and rear of the plastron. The sutural line between hyoplastra and hypoplastron on each side is tight and well sutured but is relatively short because of extensive fontanel development both mesially and laterally. The entoplastron is dagger-shaped, only the “handle” making (non-sutural) contact with other bones (i.e., the epiplastra).

Changes with ontogeny include progressive spreading of the bone in the hyoplastra and hypoplastra, so that the transverse sutures become relatively longer as the fontanels close, and the long radiating spines become progressively buried in a bony matrix and eventually may (almost) disappear. Shell ossification and fontanel closure is most complete in the two species of Lepidochelys— the smallest cheloniid turtles, fully adult specimens showing enlargement of the xiphiplastra and progressive spread of the hyoplastra and hypoplastra, finally eliminating all fontanelles and attainment of a fully ossified plastron. The plastron of Natator depressus is unique among both living and fossil cheloniids in retaining large medial fontanels into full maturity while completely closing the lateral ones (Zangerl et al., 1988).

In the carapace, intercostal fontanels are well developed in young and half-grown specimens. In Caretta and Lepidochelys, the fontanels may achieve full closure in old adults. In adult Chelonia,

the anterior and posterior fontanels may progressively close but almost always some trace of fontanels is present toward the middle of the series. In Eretmochelys, fontanel development is variable; in some adults, there may be retention of extensive intercostal fontanels but in others, these become very narrow, although still open.

3.9.2Dermochelyidae

The single living dermochelyid species, the enormous Dermochelys coriacea, displays a degree of skeletal neotony more advanced than that of any other turtle. Only the nuchal bone, which has no sutural contacts with any other bones, is relatively intact. The costal bones, each of which, in other turtles, generally forms together with a rib in a fused composite bony structure, are absent. The ribs are thin, even flimsy, and have the appearance of those of an embryonic hard-shelled turtle. The peripheral bones are also absent. In the plastron, only the entoplastron is actually lacking, but the remaining elements (epiplastra, hyoplastra, hypoplastra, and xipihiplastra) are reduced to narrow, splint-like structures that together form a ring around an enormous central fontanel, open throughout life.

The shell thus derives very little rigidity from its core bony structure, but this is not the important point. Dermochelys has unusual habits for any turtle: it undertakes rapid, deep dives to depths of over 1000 m, and is able to survive—even thrive—in sub-polar waters. It appears that the shell, and especially the plastron, in which ossification is reduced to vestiges, can deform substantially to accommodate the extreme pressures encountered at great depths. Additionally, the advanced degree of homeothermy shown by adult leatherbacks is made possible not only by gigantothermy but also by various structural modifications such as the development of counter-current heat exchangers in the bases of the limbs. In addition, both the diving and the insulation functions are facilitated by the same thick layer of oily connective or fibrous tissue that forms over both the plastral and the carapacial bones, constituting a sort of stiff insulating blubber layer. Finally, the animal is able to achieve some sort of stability in its overall shape by means of a continuous, neomorphic layer of mosaic bones just beneath the skin of the carapace. These bones are irregular and number in the thousands, and form a sort of pseudo-carapace that has significant flexibility but which is stabilized by the formation of seven longitudinal ridges in which the individual mosaic elements are markedly enlarged.

3.9.3Trionychidae

The Trionychidae are an ancient (Cretaceous to Recent) family with extremely derived shell con- figurations—e.g., the loss of scutes (replaced by soft or leathery skin)—as well as certain non-shell features such as three claws on each limb, a nasal tube that may be long or short, and very elongated cervical vertebrae. The unique shell features include elimination of the peripheral and suprapygal bones, although neomorphic analogs of the posterior peripheral elements are present in one genus (Lissemys). The plastron includes many oddities, including frequent fusion of the hyoplastra and hypoplastra on each side, the characteristically boomerang-shaped entoplastron, and the elongate, narrow epiplastra. Loveridge and Williams (1957) proposed a different interpretation of the softshell plastron, the boomerang-shaped bone being identified as the fused epiplastra, the entoplastron being missing, and the “epiplastra” of other authors being neomorphic “preplastra.” However, Cherepanov (1995) has indicated that the entoplastron is formed from a pair of bony anlages in all turtles, and the only way in which the trionychid entoplastron is unusual is the lack of the posterior spike. Loveridge and Williams’ interpretation was also rejected by Meylan (1987).

The softshell carapace is only “soft” in the posterior flap (which sometimes extends to 50% of the bony carapace length) and to a lesser extent at the sides, although these are generally supported by the tips of the ribs extending into the leathery margin. Indeed, in a large softshell, the bony carapace may be very thick and offers substantial protection for the viscera below. The ratio between the size of the bony hard carapace disk and the soft “floppy” disk is variable; for example, in Cycloderma frenatum and Cyclanorbis aubryi the soft disk is only slightly larger than the hard

disk, which in turn is sufficiently developed in adults for the (usually exposed) rib tips to be concealed. Alternately, in many other forms (Apalone, Dogania, and others) the flexible flap of the soft disk extends considerably beyond the edges of the bony shell. The surface of the carapace bone in trionychids is extensively pocked with hundreds or thousands of shallow pits (or even finer shagreen-like asperities), sometimes coalescing into vermiform ramblings, a feature that probably corresponds to the requirements for attachment of the smooth epidermis that replaces the scutes of other turtles. In Rafetus, the eighth pleural bones are reduced, and in Apalone, the eighth pleurals may be small or even lost on one or both sides, although the eighth rib on each side is retained as a elongate, thin, flexible structure that extends posterolaterally and that may serve as a partial “stiffener” for the posterior flap.

However, the most remarkable feature of the trionychid plastron is the presence of bony callosities (the “epithecal skeleton,” or intermembranous sesamoid bones) in addition to the nine standard plastral bones. These callosities are astonishingly variable in both degree of development and actual deployment over the plastron, and they often show striking ontogenetic change. It has been argued that the callosities are merely roughened areas on the dermal plastral bones, and this may indeed sometimes be the case. For example, in Rafetus and in Dogania, as well as in Cyclanorbis elegans, the callosities are very feebly developed throughout life, consisting at most of a slightly roughened area on each hyoplastral-hypoplastral area. But callosities may not correspond on a one-to-one basis with the dermal bones, and neomorphic, free-standing callosities (the “prenuchal” or the “peripherals” of certain cyclanorbines, or the anterior plastral elements of Cyclanorbis senegalensis), unsupported by dermal bones, may develop. The latter species is especially unusual in having the anterior part of the plastron heavily festooned with callosities whereas the xiphiplastra show no trace of callosity formation. A more standard configuration, such as is seen in the giant softshells Chitra or Pelochelys, offers substantial callosities on each hyoplastral-hypoplastral combination as well as extensive development of a xiphiplastral callosity on each side but with the entoplastron and epiplastron remaining simple, narrow, and without callosities throughout life. Such a configuration is also seen in much smaller forms (Pelodiscus, Palea).

The previously mentioned genera, large and small, show full development of the callosities very early in life, indeed shortly after the hatchling stage is passed. Alternatively, species such as Apalone ferox hatch without traces of callosities, although early in development the hyoplastra and hypoplastra become completely fused. Initially, the hyoplastra, hypoplastra, and xiphiplastral elements are narrow and show elaborate elongate spikes (some sharp, some blunt) extending from the corners and extremities. However, early in life a superficial callosity develops on each of these bones and spreads like thick molasses until it reaches and indeed surpasses the edges of the original bones; eventually, the bones have a plate-like shape with the elaborate outline of the original bone completely concealed (except as a sort of shadow or “ghost” in visceral view). In extreme old age, Apalone ferox may show a single massive plastral bone on each side corresponding to the hyoplastron, hypoplastron, and xphiplastron, and a deep groove forms along the distal edge of the hypoplastron as growth of the edges of the bone and of the attached callosity extend in divergent planes. Despite these elaborate changes, the entoplastron and epiplastra remain simple and smoothsurfaced throughout life.

In some cyclanorbines, the plastral callosities may show remarkable development. In Lissemys scutata, the initially small, circular entoplastral callosity shows spectacular enlargement in adulthood, and in Cycloderma aubryi the plastral callosities become so huge that they virtually constitute a “recovered” complete bony plastron (Figure 3.33). The situation is even more extreme in Lissemys punctata, in which the hyoplastra-hypoplastral callosities extend dorsolaterally so that they offer an entire bony bridge on each side, and continue dorsally to ossify the lateral edges of the carapace in the areas where peripheral bones would be present in hard-shell turtle species—a classic case of evolution reversing itself to create analogous but not homologous structural forms. By contrast, in the two largest cyclanorbine species, Cycloderma frenatum (Figure 3.34) and Cyclanorbis elegans (Figure 3.35 and Figure 3.36), callosity development remains minimal throughout life.

Figure 3.38  The bony carapace of a juvenile Trionyx triunguis (CL = 21 cm) shows complete ankylosis of costal I through costal IV on each side, as well as neural 1 to neural IV as a single block. The nuchal bone and the posterior half of the carapace retain all of their sutures.

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