Turtle Shell - Shell Nomenclature

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