Understanding the Chelonian Shell

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).

Box turtle on grass

Shown here, box turtle (Terrapene sp.). Photo by Mike Comella

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).

Keratin scutes in a slider turtle

Close-up on keratin scutes in a slider turtle.
Photo by Jeanette Wyneken.

Arrow pointing to an area where keratin has sloughed off of the carapace in an ornate box turtle

Arrow pointing to an area where keratin has sloughed off of the carapace in an ornate box turtle (Terrapene ornata ornata). Image by Mike Comella.

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…

  • Small to medium-sized predators can gnaw at shell margins

  • Jaguars have been known to use their canines to break the adult turtle carapace

  • Birds can drop turtles from great heights or use their bills to peck at the shell

  • Large predators (e.g. adult alligators) simply bite the shell

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)
Vocabulary termDefinition
BridgeConnects the upper and lower shell laterally
CarapaceUpper shell
PlastronLower shell
PyramidingAbnormal growth
ScutesSuperficial keratin layer consisting of horny plates
SutureSeam between bony plates
SulcusSeam between horny plates or scutes

Egyptian tortoise (Testudo kleinmanni). Carapace (A), plastron (B), and connecting bridge (arrow)

Shown here, Egyptian tortoise (Testudo kleinmanni). Carapace (A), plastron (B), and connecting bridge (arrow). Photo by Florida’s Educational Technology Clearinghouse.

Turtle plastron or lower shell. Photo by Toby Otter.

Turtle plastron or lower shell. Photo by Toby Otter.

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)
TermDescription
NuchalCentral carapacial scute above head (a type of marginal)
MarginalScutes along carapace edge (usually 11)
VertebralCentral row of scutes along carapacial spine
Costal (pleural)Scutes between vertebral and marginal
SupracaudalCarapacial scute above tail
InguinalSmall triangular scute cranial to hind limb
GularPlastral scute below head
PectoralPlastral scute behind gular scute
AbdominalPlastral scute behind pectoral
FemoralPlastral scute between anal and abdominal scutes
AnalLast plastral scute, below tail
.

Cervical or nuchal scutes (1), vertebral scute (2), marginal scutes (3), pleural scutes (4) in a red-eared slider

Cervical or nuchal scutes (1), vertebral scute (2), marginal scutes (3), pleural scutes (4) in a red-eared slider (Trachemys scripta).

Shell morphology

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.
Leatherback or Tinglar sea turtle

Leatherback or Tinglar sea turtle (Dermochelys coriacea) US Virgin Islands. Photo by Claudia Lombard, USFWS/Southeast. Click image to enlarge.

  • The aquatic turtle shell, with its low-domed relatively wide shape, is also relatively reduced in size (Magwene 2013).
 Aquatic turtles tend to possess low-domed, relatively wide shells (left), while terrestrial species have high-domed shells.

Aquatic turtles tend to possess low-domed, relatively wide shells (left), while terrestrial species have high-domed shells. Photo on right by Mike Comella.

  • The pancake tortoise (Malacochersus tornieri) has a flattened shell that allows it to escape predators and heat by squeezing into rocky crevices (Boyer 2006).
Pancake tortoise (Malacochersus tornieri)

Shown here, pancake tortoise (Malacochersus tornieri). Photo by Jeremy Thompson.

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
SpeciesHinge location
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

Box turtles possess a modified hyo-hypoplastral suture that forms a movable hinge (arrow).

Box turtles possess a modified hyo-hypoplastral suture that forms a movable hinge (arrow).

Male chelonians possess a concave plastron that allows them to mount and breed the female.

This male turtle is recognizable by a depression in the plastron (large arrow).

This male turtle is recognizable by a depression in the plastron (large arrow). Also, note the hinge in the plastron (thin arrow) that allows the turtle to retract and seal itself tightly within its shell. Photo by Ineta McParland. Click image to enlarge.

Shell growth

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).
Hatchling red-eared sliders

Hatchling red-eared sliders. Photo by Mike Comella.

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).

Red-eared slider adult and hatchling.

Red-eared slider adult and hatchling.
Photo by Mike Comella.

  • 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).

Shell pathology

Shell failure

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).
Shell fracture involving the bridge (yellow circle).

Shell fracture involving the bridge (yellow circle). Photo by Mike Comella. Click image to enlarge.

Abnormal growth

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.

Pyramiding of a tortoise shell

Pyramiding of a tortoise shell. Note the bumpy, irregular surface. Photo by dawsonlm on Flickr Creative Commons.

 

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?

References