Diagrams of turtle shells showing named units

The turtle shell is a highly complicated shield for the ventral and dorsal parts of the turtle, completely enclosing all the vital organs of the turtle and in some cases even the head. It is constructed of modified bony elements such as the ribs, parts of the pelvis and other bones found in most reptiles. The bone of the shell consists of both skeletal and dermal bone, showing that the complete enclosure of the shell probably evolved by including dermal armor into the rib cage.

The shell of the turtle is an important study, not just because of the obvious protection it provides for the animal, but also as an identification tool, in particular with fossils as the shell is one of the likely parts of a turtle to survive fossilization. Hence understanding the structure of the shell in living species gives us comparable material with fossils.

Shell nomenclature

The turtle shell is made up of numerous bony elements, generally named after similar bones in other vertebrates, and a series of keratinous scutes which are also uniquely named. The top of the shell is known as the carapace, and the ventral surface is called the plastron. These are joined by an area called the bridge. The actual suture between the bridge and the plastron is called the anterior bridge strut. In Pleurodires

Pleurodira

The Pleurodira are one of the two living suborders of turtles, the other being the Cryptodira. In many cases in the nomenclature of animals, ranks such as suborder are considered of little importance apart from nomenclatural or taxonomic reasons. However, this is not the case with the suborders of...

 the posterior pelvis is also part of the carapace, fully fused with it. This is not the case in Cryptodires

Cryptodira

Cryptodira is the taxonomic suborder of Testudines that includes most living tortoises and turtles. Cryptodira differ from Pleurodira in that they lower their necks and pull the heads straight back into the shells; instead of folding their necks sideways along the body under the shells' margins...

 which have a floating pelvis. The anterior bridge strut and posterior bridge strut are part of the plastron, on the carapace are the sutures into which they insert, known as the Bridge carapace suture.

The bones of the shell are named for standard vertebrate elements. As such the carapace is made up of 8 pleurals on each side, these are a combination of the ribs and fused dermal bone. Outside of this at the anterior of the shell is the single nuchal bone, a series of 11 paired periphals then extend along each side. At the posterior of the shell is the pygal bone and in front of this nested behind the eighth pleurals is the suprapygal.
Between each of the pleurals are a series of neural bones, which although always present are not always visible, in many species of Pleurodire they are submerged below the pleurals. Beneath the neural bone is the Neural arch which forms the upper half of the encasement for the spinal chord. Below this the rest of the vertebral column. Some species of turtles have some extra bones called mesoplastra, these are located between the carapace and plastron in the bridge area. They are present in most Pelomedusid

Pelomedusidae

Pelomedusidae is a family of freshwater turtles, native to eastern and southern Africa. They range in size from to in shell length, and are generally roundish in shape...

 turtles.

The skeletal elements of the plastron are also largely in pairs. Anteriorly there are two epiplastra, with the hyoplastra behind them. These enclose the singuar entoplastron. These make up the front half of the plastron and the hyoplastron contains the anterior bridge strut. The posterior half is made up of two hypoplastra (containing the posterior bridge strut) and the rear is a pair of xiphiplastra.

Overlying the boney elements are a series of scutes, these are made of keratin

Keratin

Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...

 and are a lot like horn or nail tissue. In the center of the carapace are 5 vertebral scutes and out from these are 4 pairs of costal scutes. Around the edge of the shell are 12 pairs of marginal scutes. All these scutes are aligned so that for the most part the sutures between the bones are in the middle of the scutes above. At the anterior of the shell there may be a cervical scute (sometimes incorrectly called a nuchal scute) however the presence or absence of this scute is highly variable, even within species.

On the plastron there are two gular scutes at the front, followed by a pair of pectorals, then abdominals, femorals and lastly anals. A particular variation is the Pleurodiran turtles have an intergular scute between the gulars at the front, giving them a total of 13 plastral scutes. Compared to the 12 in all Cryptodiran turtles.

Carapace

The carapace is the dorsal

Dorsum (biology)

In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...

 (back), convex part of the shell structure of a turtle

Turtle

Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

, consisting of the animal's rib

Rib

In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...

cage combined with dermal bone. The spine

Vertebral column

In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

 and ribs are fused to dermal plates beneath the skin which interlock to form a hard shell. Exterior to the skin the shell is covered by scutes, which are horny plates made of keratin

Keratin

Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...

 that protect the shell from scrapes and bruises. A keel, a ridge that runs from front to the back of the animal is present in some species, these may be single, paired or even three rows of them. In most turtles the shell is relatively uniform in structure, species variation in general shape and color being the main differences. However the soft shell

Trionychidae

Trionychidae is a taxonomic family which comprises a number of turtle genera commonly known as softshells. They are also sometimes called pancake turtles. Softshells consist of some of the world's largest fresh water turtles, though many can adapt to living in highly brackish areas. Members of this...

 turtles, pig-nose turtles and the leatherback sea turtle

Leatherback Sea Turtle

The leatherback sea turtle is the largest of all living sea turtles and the fourth largest modern reptile behind three crocodilians. It is the only living species in the genus Dermochelys. It can easily be differentiated from other modern sea turtles by its lack of a bony shell. Instead, its...

 have lost the scutes and reduced the ossification of the shell. This leaves the shell covered only by skin. These are all highly aquatic forms.

In the figure to the right, the exploded shell view, the roman numerals and other notations are as follows:

(i) Neural 1, (ii) Neural 2, (iii) Neural 3, (iv) Neural 4, (v) Neural 5, (vi) Neural 6, (vii) Neural 7, (viii) Neural 8, (ix) extra neural, divided, (x) suprapygal, (xi) nuchal, (xii) right peripheral 1, (xiii) right peripheral 2,
(xiv) right peripheral 3, (xv) right peripheral 4, (xvi) right peripheral 5, (xvii) right peripheral 6, (xviii) right peripheral 7, (xix) right peripheral 8, (xx) right peripheral 9, (xxi) right peripheral 10, (xxii) right peripheral 11, (xxiii) pygal, (xxiv) left peripheral 11, (xxv) left peripheral 10, (xxvi) left peripheral 9, (xxvii) left peripheral 8, (xxviii) left peripheral 7, (xxix) left peripheral 6, xxx left peripheral 5, xxxi left peripheral 4, (xxxii) left peripheral 3, (xxxiii) left peripheral 2, (xxxiv) left peripheral 1, (xxxv) right 1st rib, (xxxvi) right pleural 1, (xxxvii) right pleural 2, (xxxviii) right pleural 3, (xxxix) right pleural 4, (xl) right pleural 5, (xli) right pleural 6, (xlii) right pleural 7, (xliii) right pleural 8, (xliv) right 10th rib, (xlv) left 1st rib, (xlvi) left pleural 1, (xlvii) left pleural 2, (xlviii) left pleural 3, (xlix) left pleural 4, (l) left pleural 5, (li) left pleural 6, (lii) left pleural 7, (liii) left pleural 8, (liv) left 10th rib, (9-18) centrums.

Plastron

Comparison of plastrons of a Cryptodire (Chrysemys picta marginata) and a Pleurodire (Chelodina canni

Chelodina canni

Cann's snake-neck turtle is an Australian species found in the northern and north eastern parts of the continent. It has a narrow zone of hybridization with its related species the Eastern Snake Neck Turtle Chelodina longicollis...

)

The plastron is the nearly flat part of the shell structure of a turtle

Turtle

Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

, what one would call the belly or ventral surface of the shell. It also includes within its structure the anterior and posterior bride struts and the bridge of the shell.

In certain families there is a hinge between the pectoral

Pectoral

Pectoral may refer to:* Pectoral cross, a cross worn on the chest* a decorative, usually jeweled version of a gorget* Pectoral , often iconographic with hieroglyphs* Pectoralis major muscle, commonly referred to as "pectorals" or "pecs"...

 and abdominal scutes allowing the turtle to almost completely enclose itself. In certain species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 the sex of a testudine can be told by whether the plastron is concave, male or convex, female. This is because of the mating position.

The plastral scutes join along a central seam down the middle of the plastron. The relative lengths of the seam segments can be used to help identify a species of turtle

Turtle

Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

. There are six laterally symmetric pairs of scutes on the plastron: gular, humeral, pectoral, abdominal, femoral, and anal (going from the head to the tail down the seam); the abdominal and gular scute seams are approximately the same length, and the femoral and pectoral seams are approximately the same length.

The gular scute or gular projection on a turtle

Turtle

Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

 is the most anterior part of the plastron, the underside of the shell. Some tortoises have paired gular scute

Scute

A scute or scutum is a bony external plate or scale, as on the shell of a turtle, the skin of crocodilians, the feet of some birds or the anterior portion of the mesonotum in insects.-Properties:...

s, while others have a single undivided gular scute. The gular scutes may be referred to as a gular projection if they stick out like a trowel

Trowel

A trowel is one of several similar hand tools used for digging, smoothing, or otherwise moving around small amounts of viscous or particulate material.-Hand tools:...

.

The gular scutes or gular projection

• Gular (disambiguation), gular anatomical formations in other species

Plastral formula

The plastral formula is used to compare the sizes of the individual plastral scutes (measured along the midseam). The following flastral scutes are often distinguished (with their abbreviation):

• intergular = intergul
• gular = gul
• humeral = hum
• pectoral = pect
• abdominal = abd
• femoral = fem
• anal = an

Comparison of the plastral formulas provides distinction between the two species. For example for the eastern box turtle, the plastral formula is: an > abd > gul > pect > hum >< fem

Turtle plastrons were used by the ancient Chinese in a type of divination

Divination

Divination is the attempt to gain insight into a question or situation by way of an occultic standardized process or ritual...

 called plastromancy. See also oracle bone

Oracle bone

Oracle bones are pieces of bone normally from ox scapula or turtle plastron which were used for divination chiefly during the late Shang Dynasty. The bones were first inscribed with divination in oracle bone script by using a bronze pin, and then heated until crack lines appeared in which the...

s.

Scutes

The turtle's shell is covered in scutes that are made of keratin. The individual scutes as shown above have specific names and are generally consistent across the various species of turtles. The scutes are regularly shed individually, they will also fall off due to a number of disease processes. The scute effectively forms the skin over the underlying bony structures there is a very thin layer of subcutaneous tissue between the scute and the skeleton. The scutes can be brightly colored in some species but the basal color is a grey to dark brown color dorsally, the plastral scutes are often white to yellow in base color.

Septicemic cutaneous ulcerative disease (SCUD)

Originally described by Kaplan (1957) is often known by its acronym SCUD. This disease is known to be caused by a variety of bacteria entering through some sort of abrasion, combined with poor animal husbandry. The disease is identified by its progression and what starts as ulcerative lesions of the plastron leads to a septacemic infection causing the degradation of the liver and other organs. This will lead to death without treatment.

Pyramiding

Pyramiding is a shell deformity commonly found in captive turtle

Turtle

Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

s, in which the shell grows unevenly resulting in a "pyramid" shape underlying each scute. This deformity can vary in severity from barely noticeable to life-threatening. Note that Indian star tortoise

Indian Star Tortoise

The Indian star tortoise is a species of tortoise found in dry areas and scrub forest in India and Sri Lanka. This species is quite popular in the exotic pet trade.-Anatomy and morphology:...

s and some other species display this morphology naturally.

Several causes can lead to pyramiding, including excessive animal or vegetable protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

, inadequate calcium

Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

, sunlight and/or vitamin D3, and excessive moisture / poor nutrition foods. Once pyramiding has occurred, it cannot be reversed, though if the underlying problem is corrected, the animal may still lead a long, healthy life.

Further information on pyramiding can be obtained at Turtlestuff.com and the African Tortoise Website.