Animal Information

SHELL YEAH! Turtle and Tortoise SHELL Anatomy.

The tortoiseshell is the primary defense device against would-be predators of turtle and tortoises'. The shell is so effective that it has remained almost uninfluenced by two hundred million years of evolution.

The shell is an extension of the rib cage, which unlike most vertebrates is housed on the “outside” rather than inside the body.

The shell is made up of two halves, the underneath known as the plastron and the top known as the carapace. Both parts are fused at the sides by what is called a “bridge”.

The whole shell of the tortoise is made up of numerous small bones which are covered by separate plates of keratin called scutes. As a tortoise grows, extra layers of keratin are added underneath the existing layer, causing “growth rings”. Contrary to popular belief, a tortoise cannot be accurately aged by counting these rings. 

However, they can tell us approximately how many spurts of growth the tortoise has had. With this visual, you can sort of gauge what type of seasonal changes the tortoise has in its natural environment.

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Abundant vegetation means more food, which relates to more growth. Whereas sparse vegetation due to extreme climatic conditions would mean little food would be available.

This lack of nutrients causes little or no keratin growth. Very old tortoises often have extremely worn scutes, giving their shells an almost completely smooth appearance.

There is a study that has identified the nucleotide and deduced amino-acid sequences of 17 unique beta-keratins in turtles and lizards.

The 17 turtle beta-keratin sequences are rich in glycine, proline, serine, and tyrosines.

Scute, Scute, and Boogie

These are the names of some scutes
  • The Nuchal – the scute directly above the head
  • The Supracaudal – the scute directly above the tail
  • The Vertebral – a single line of scutes centrally from the head to the tail
  • The Costals – run parallel to, and at either side of, the Vertebral
  • The Marginals – flank the Costals and attach to the “bridge”
  • Click to see more photos of these different scutes.

In some species such as sea turtles, whether or not the first costal scute makes contact with the nuchal scute can help with identification. And oddly in some species such as the Leopard Tortoise, the nuchal scute is missing.

The Marginal scutes have a large influence on the overall shape of a tortoise’s shell. In some species, most noticeably Testudo Marginata, the Marginal scutes are extremely flared.

The scutes of the plastron are also separately categorized, of which there are two scutes in each category. Starting from the head moving down to the tail, you will find:

  • The Gular
  • The Humeral
  • The Pectoral
  • The Abdominal
  • The Femoral
  • The Anal

Some turtles have a flexible “hinge” on their plastron which they can use for extra protection from predators by clamping the carapace and plastron firmly shut.

Some females of other species have a much less flexible plastron, but flexible enough to move slightly to aid her egg-laying duties.

The skeleton of turtles is made up of two parts which are the exoskeleton (carapace and plastron) and the endoskeleton (internal bones).

The endoskeleton consists of two main groups; the appendicular skeleton (limb bones and girdles) and the axial skeleton (ribs, vertebrae, and skull).

Anoxic turtles accumulate high levels of lactate in the blood. To avoid fatal acidosis, turtles exploit buffer reserves in their large mineralized shell.

The shell acts by delivering calcium and magnesium carbonates and by storing and buffering lactic acid. Collectively with profound metabolic depression, shell buffering favors survival without oxygen for several months even when the temps are as low as 3 degrees Celcius.

Turtle and Tortoises Are “Boneheads”

Here are thet types of boens in turtkes and tortosies
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  • Skull and lower Jaw Mandible – consisting of many small bones fused
  • Cervical Vertebrae – neck bones
  • Dorsal Vertebrae – a rib branches off each dorsal vertebrae, which are fused to the carapace
  • Humerus – upper foreleg bones
  • Radius and Ulna – lower foreleg bones
  • Carpals – wrist bones of front legs
  • Phalanges – digit bones
  • Scapula and Coracoid – bones of the pectoral girdle
  • Femur – upper rear leg bones
  • Fibula and Tibia – lower rear leg bones
  • Tarsals – ankle bones of rear legs
  • Metatarsals – bones of the feet

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