An Overview of the Turtle skeleton
A preserved turtle skeleton revealing the manner in which the plastron and scutes connect to the rest of the turtle’s body to form a complete shell surrounding the head. Scales (upper) and abdominal elements (lower) of a turtle’s abdominal carapace. The turtle shell itself is a tough outer protective shield for all the internal organs of the turtle, [including the eye, head, and teeth] all completely enclosing all of the vital internal organs. The head of a turtle is covered with a hard plastron, or breastbone, held together by a series of plates called plastrons.
An illustration showing the gradual formation of a turtle shell through the million years or more since the beginning of aquatic turtles. The outer covering – the plastron – is continuously becoming thinner over the million years. At the same time the internal (or “sac” or “earth”) layer becomes thicker and more viscous, protecting the interior from further stresses.
A close-up of a modern-day tortoise shows the thin line between the plastrons and the shell margin along the side of the neck. The line is also present along the flanks of both legs. Modern-day turtles are closely related to all other reptiles (all reptile). All reptiles have a soft-plastic egg shell, held together by a series of hard collagen layers. This egg is surrounded by a sheath that is shed on a periodically basis after a turtle has hatched.
An illustration showing the evolution of various reptile and amphibian taxa over the last two million years or so. Note that most (if not all) representative species are found only in the coastal plain of what was previously recognized as the Jurassic period. The majority of sea turtles have originated from the later Permian or Triassic period of prehistoric geology. Most of the various iguanodon and megalosaurus genera which have been discovered to belong to the latter period.
The complete and total evolution of a turtle skeleton should also take into account the loss or shedding of their carapace (or shell) during the process of metamorphosis. The loss of their carapace is strongly correlated with the evolution of a more specialized body plan. The cladocarp of a turtle skeleton provides good information about the changes that led to the development of its legs and tail. The cladocarp is the solid forelimb bone of a turtle. It is present in the external part of the shell and is composed of two parts: the anterior (or sternum) and the posterior (or tail) parts.
The shape and size of a turtle skeleton can be compared with that of a hand or a foot. A well-developed plastron has a definite line that separates it from the surrounding smooth area inside the shell. It is possible to identify a single plastron and determine whether or not it belongs to the metamorphosis or the non-metamorphosis shell. The Plastron of a fully-grown turtle is almost circular, oval or sometimes triangular shaped and has a broad base, gradually tapering to a point at its end.
In contrast to the reptile and amphibians, the osteoderms have evolved from a bony tissue (ceris) rich environment to hard skeletal bones. The very long period of evolution of reptile and amphibian skeletons (roughly half to 2 million years) proves the importance of steeds in their evolution. Osteoderm development began in the Ordovician or Cretaceous geological eras. It is estimated that more than half of all dinosaurs may have come from osteoderms.
The complete lack of visible external bony parts in the juvenile turtles (including the eyes, head, and teeth) led to the widely held misconception that these are not developed bone parts. However, modern genetic analysis of modern turtles revealed that these are indeed bony prominences. Recent studies showed that the greatest volume of osteoid tissue is located in the ventral part of the turtle shell, which is centrally located. The remaining solidus is composed exclusively of an exoskeleton. Its purpose is to protect the more fragile bony parts of the turtle shell.
