What is the shell?
The shell is a bony structure unique to order Chelonia. No other animal, living or extinct, has its body enclosed within a bony shell similarly constructed in its entirety (Boyer 2006, McArthur 2006, Rieppel 2009).
The turtle shell consists of flat, plate-like bones that surround spongy, cancellous tissue (McArthur 2006). These “sandwich bones” are derived from the ribs, vertebrae, clavicles, interclavicles, and gastralia or abdominal ribs. Much of this bone is membranous or dermal, meaning it is not preformed with cartilage (McArthur 2006). In most terrestrial vertebrates, dermal bone is retained only in structures like the cranium and scapulae. Another striking feature of turtle anatomy is that both the pectoral girdle and pelvis sit within the carapace or rib cage (Achrai 2013, Nagashima 2012). The vertical orientation of the pectoral and pelvic girdles reinforces the shell and provides strong ventral anchors for the humerus and femur (Boyer 2006). The shell is covered by epidermal tissue, usually in the form of flexible, keratinized plates known as scutes (Boyer 2006, Magwene 2013, McArthur 2006).
What function does the shell serve?
The turtle shell is an unusually strong and durable structure that provides variable degrees of protection against predator attack (Achrai 2013, Jackson 1997, Magwene 2013) (Table 1).
|Table 1. When turtles are attacked…|
Shell vocabulary terms
The dorsal part of the shell is the carapace, and the ventral shell is called the plastron. The bridge connects the upper and lower shell laterally (Table 2).
|Table 2. Common shell-related terms (Boyer 2006, McArthur 2006, Magwene 2013)|
|Bridge||Connects the upper and lower shell laterally|
|Scutes||Superficial keratin layer consisting of horny plates|
|Suture||Seam between bony plates|
|Sulcus||Seam between horny plates or scutes|
Although the terms suture, seam, and sulcus are used interchangeably in some sources, suture refers to the margins of the epidermal scutes while sutures represent the margins of the underlying bony plates. Scutes and bones are staggered so that horny plate sutures do not sit directly above bone sulci. Scute terminology is based upon anatomic location (Table 3).
|Table 3. Scute vocabulary (McArthur 2006)|
|Nuchal||Central carapacial scute above head (a type of marginal)|
|Marginal||Scutes along carapace edge (usually 11)|
|Vertebral||Central row of scutes along carapacial spine|
|Costal (pleural)||Scutes between vertebral and marginal|
|Supracaudal||Carapacial scute above tail|
|Inguinal||Small triangular scute cranial to hind limb|
|Gular||Plastral scute below head|
|Pectoral||Plastral scute behind gular scute|
|Abdominal||Plastral scute behind pectoral|
|Femoral||Plastral scute between anal and abdominal scutes|
|Anal||Last plastral scute, below tail|
The shape of the chelonian shell can vary dramatically with lifestyle (Boyer 2006):
- The scutes of some aquatic and semi-aquatic species such as leatherbacks (Dermochelys coriacea), sea turtles (superfamily Chelonioidea), softshells (Trionyx spp.), and the pig-nosed turtles (Carettochelys insculpta) are replaced with tough, leathery skin.
- The aquatic turtle shell, with its low-domed relatively wide shape, is also relatively reduced in size (Magwene 2013).
- The pancake tortoise (Malacochersus tornieri) has a flattened shell that allows it to escape predators and heat by squeezing into rocky crevices (Boyer 2006).
A small percentage of chelonians possess a modified suture or movable shell hinge that allows the animal to completely enclose its head and limbs within the shell (Boyer 2006, Magwene 2013) (Table 4). Most hinges are found on the plastron, however hinged-back tortoises (Kinixys spp.) possess a caudal carapacial hinge. Minor mobility of the caudal plastron is also observed in female Mediterranean tortoises (Testudo spp).
|Table 4. Hinged chelonians|
|Asian box turtle (Cuora sp.)||Plastron|
|Box turtle (Terrapene spp.)||Plastron|
|Mud turtle (Kinosternon spp.)||Plastron|
|Spider tortoise (Pyxis spp.)||Plastron|
|African hingeback tortoise (Kinixys spp.)||Caudal carapace|
The shell is a metabolically active structure capable of growth and change:
- At hatch, the carapace is no more than weakly ossified ribs and connective tissue overlaid by scutes. The plastron is slightly more developed but still provides only minor protection. The bony plates are relatively thin with large fontanelles or fenestrae between bones. The minimal defense value of the moderately flexible shell of the hatchling turtle is evidenced by the wide variety of animals that prey upon them (Magwene 2013).
As the chelonian grows, bones thicken, fenestrae fuse, and dermal plates ossify allowing the shell to quickly become rigid (Boyer 2006, Magwene 2013). A normal, physiologic exception to this rule of thumb can be found with pancake turtles and softshell turtles, which demonstrate reduced ossification at all ages (McArthur 2006). Soft or flexible shells can also be observed in metabolic bone disease due to a failure of fenestrae between bony plates to fuse (Boyer 2006).
- Scutes are also capable of growth. Turtles produce new scutes during each major growth period. Theoretically counting scute rings can be used to estimate age in some species, but this method is considered unreliable (Boyer 2006, McArthur 2006).
- Scutes are actually regularly shed in some semi-aquatic species. Shedding scutes is rarely observed in terrestrial chelonians (McArthur 2006).
There are a variety of studies that evaluate shell strength and the forces involved in compression and failure of the shell (McArthur 2006, Stayton 2011, Magwene 2013):
Small yet powerful: Given similar amounts of energy applied to the shell via both point and compressive loads, smaller shells can undergo relatively greater deformation before failure than larger shells. Whole shell loading experiments have also shown that smaller individuals are able to withstand more dorsoventral deformation before the shell fails (Magwene 2013).
Zones of weakness:
- Shell failures were much more likely to occur at sulci or the seams that mark the borders between epidermal scutes. Bony regions beneath sulci are somewhat thinner than surrounding bone, which may make these regions more susceptible to microdamage (Magwene 2013).
- Composed largely of ligament instead of bone, the box turtle bridge is another point of shell weakness (Stayton 2011, Magwene 2013).
- Sutures, or the seams between bones, are also relatively weak however research shows that few shell failures begin at sutural joints. Despite being relatively weaker in bending strength, sutures can deform more. This allows sutures to safely absorb similar amounts of energy when compared to surrounding bone (Magwene 2013).
Anomalous shell growth, commonly known as pyramiding, is believed to be related to excess dietary protein and rapid growth rates during the first few years of life. Pyramiding is often, but not always, correlated with irregular calcium metabolism (McArthur 2006). Unusual growth patterns can also result from improper incubation parameters such as excessive or suboptimal air temperature.
Conclusion & pop quiz
The chelonian shell is a complex system with interlocking elements of bone, sutures, and keratin that surround soft tissue (Magwene 2013). Are you now comfortable with basic turtle shell anatomy and physiology?