Nomenclature of the chelonian shell

1. Bony shell

1.1. Carapace 

·      Prenuchal – a small oval bone at the front of the carapace in some trionychoids - cyclanorbids (Lissemys spp., Cyclanorbis senegalensis) and trionychids (Dogania subplana, plastomenusines, etc.). It occurs sporadically in old specimens of the trionychine Platypeltis ferox.

·      Nuchal - a large, wide bone at the front of the carapace. This name is nearly universal  although Carr (1952) called it the “proneural”.

·      Neurals – the series of bones behind the nuchal outgrowths of the neural arches of the thoracic vertebrae (see discussion in Kordikova (2000).  The term is universally accepted in spite of some authors also using “vertebral plates”. Besides, many researchers use the term „preneural“ for the most anterior neural if it is highly differentiated in form or “extra”. However, this term just demonstrates the specific status and position of the plate. It is not so informative because it does not facilitate homologization of thoracic vertebrae of turtles with those of other tetrapods. A number of authors (Hasan 1941, Carpenter (1981, Meylan 1984, Kordikova 1993b, 2000) call the most anterior neural („preneural“) „the first neural“. Two neurals between a first pair of costals occur as a norm in some trionychoids (Lissemys spp., cyclanorbines, aspideretines, Late Cretaceous paraplastomenusines, Late Cretaceous and Eocene ulutrionychines), baenids, etc. This bone in above-mentioned turtles is not sutured to the neural arch of the thoracic vertebra I but appears to be formed as an expansion of perichondral ossifications of the neural arch of the thoracic vertebra I (Kordikova 1993b, 2000; compare with Cherepanov 1995). The neural I is reduced in most chelonians but it is not fused with next to it neural as Webb (1962), Gaffney (1979) and Meylan (1987) suggested. I also prefer to call „first neural“ the most anterior neural, taking into account its formation as a modified element of the neural arch of the thoracic vertebra I (Kordikova 1991, 1993b, 2000). The following neurals after the neural I have corresponding numeration: the neural of the thoracic vertebra II is the neural II, the neural of the vertebra III is the neural III, etc.

Neurals are often reduced or missing in some chelids, cyclanorbines, etc.

·      Costals – the series of bones on each side of the neurals. Many authorities use this term, although some authors use "pleurals" (Meylan 1987, Gaffney 1990, Pritchard & Trebbau 1984, etc.). Pritchard & Trebbau (1984) take into consideration the position of bones, which are more superficial structures, external to the lungs or pleura and argue that it is preferable to use an exclusively Greek nomenclature for the carapace bones and Latin nomenclature for the scutes. However, I prefer to use „costals“ because this term comes from the ribs or costae and points to the origin of the plates, which form from outgrowths of corresponding ribs. Moreover, costals are modified ribs of the corresponding vertebrae and, consequently, the rib of the thoracic vertebra I forms its own costal I, the rib of the vertebra II forms the costal II, etc. (Kordikova 2000). Usage of either Greek or Latin term is inessential if it does not reflect homology of bony elements and confuses a number of researchers. Some authors (Siebenrock 1902, Meylan 1984, 1987) use a term „callosities“ for the carapace and plastral ossifications that refers to the dermal origin of the costals.

·      Peripherals - bones forming the edge of the carapace, a nearly universal term. However, they are sometimes termed "marginals." Peripherals are formed by respective myosepta, which also take part in the formation of corresponding ribs and vertebrae (Cherepanov 1989). Thus, the numbering of peripherals corresponds to that of the myosepta and, consequently, the ribs and vertebrae: the myoseptum number I is that from which the thoracic rib (I) and peripheral (I) originates, the myoseptum II is that from which the thoracic rib (II) and peripheral (II) originates, etc. The nuchal and the nuchal peripherals also originate in the same myoseptum (Kordikova 2000).

·      Pygal is the most posterior bone of the carapace forming together with peripherals  the edge of the carapace; a term universally used.

·    Suprapygals - the bones between the neurals and the pygal, of which suprapygals may be one or more. In comparison with neurals suprapygals are formed in the dermis independently but not as outgrowths of the neural arches of the thoracic vertebrae. They have a sutured contact with vertebrae secondarily (Cherepanov 1989). Boulenger (1889) called these bone the “pygals”, but otherwise “suprapygal” is virtually unanimous.

1.2. Plastron

The paired bones of the plastron are, from anterior to posterior, the epiplastra, 

hyoplastra, hypoplastra, and xiphiplastra.  A single bone encircled by the epiplastra and hyoplastra is the entoplastron (Smith & Smith 1980, preferred the spelling "endoplastron"). The additional bones between the hyoplastra and hypoplastra in proganochelyids, platypeltids, baenids, desmostylids, pelomedusids, etc. are the mesoplastra.  Sometimes they are called interplastra (Sukhanov 1964). An additional pair of epiplastral bones – preepiplastra can form in the cyclanorbid Cyclanorbis senegalensis.

There are many authors (Siebenrock 1902, Romer 1956, Zangerl 1969, Meylan 1984, 1987, etc.) who distinguish two kinds of plastral bones in the chelonian shell and use a term „callosities“ for plastral ossifications. However, such viewpoint is not confirmed by morphogenesis - there are no independent superficial ossicles over the plastral bony primordial (see wide discussion on this question in Cherepanov 1997 as well as in Kordikova 2000). Here, I prefer to use plastral bones or plastral ossifications for the epi-, hyo-, hypo-, xiphiplastra and entoplastron.

2. Horny shell

2.1 Carapace  

I use terms “scale” and “scute” for shell integumentary structures: “scales” for tiny horny formations distributed on the whole body, including shell, and “scutes” for the wide horny plates forming the surface of the shell. Scales are present in all turtles (they are ossified in dermochelyids) while scutes are present in most chelonians except trionychoids and dermochelyids (scutes are usually reduced in chelonioids and carettochelyids). Carr (1952) proposed that scutes be termed "laminae" instead, but Zangerl (1969) observed correctly that this term is ambiguous, having an established usage in osteology, and should not be used.  The old term “corneoscutes” is accurate but laborious and is little used today (Pritchard & Trebbau 1984).

·      Nuchal scute - a median scute that lies over the nuchal bone anterior to the vertebral scutes in many turtles (e.g., Pritchard & Trebbau 1984). Carr (1952) called it the “precentral” but this has not been accepted by others authors. Zangerl (1969) proposed the term "cervical scute" but no previous authors and only a few subsequent ones (e.g., Ernst & Barbour 1989) have used this term. Nuchal scute is usually small or absent in pleurodires (it is wide in Hydromedusa).

·      Nuchaloid scute – the small anterior most marginal scute that lies over the lateral part of the nuchal bone and anterolateral part of the nuchal peripheral bone between the nuchal scute and the marginal I. This term was proposed by Chkhikvadze (1973) but has not been used by others, who consider this scute to be the marginal I. However, the nuchaloid scutes distinguish from other marginal scutes by size and  anteromedial expansion in many pleurodires.

·      Vertebral scutes are the (usually five) large scutes along the midline of the carapace. This term is nearly unanimous although Carr (1952) used “centrals”.

·      Pleural scutes or pleurals are the large scutes forming a longitudinal series on each side of the vertebrals. This term was proposed by Zangerl (1969) and a number of authors have subsequently adopted Zangerl's usage. Carr (1952) used “lateral”. These scutes are termed “costals” by Pritchard & Trebbau (1984). This has also been accepted by other authors. However, this term is more often used for the underlying bones than for the scutes (see the discussion under costals justifying use of this term). I prefer to avoid usage “costals” for scutes because they have different origin and it is necessary to distinguish between the underlying bone of the axial skeleton and the superficial horny scute. In comparison with the costal bones and pleural scutes both the nuchal scute and nuchal bone have dermal origin.

·      Marginals are the numerous small scutes around the edge of the shell, except those on the anterior (the nuchal and nuchaloid scutes) and posterior (a supracaudal scute) midline.

·      Supracaudals are represented the two rearmost scutes of the carapace (following Pritchard & Trebbau 1984). In some turtles they are fused into a single scute. Carr (1952) proposed the alternative term “postcentrals”. Some authors, including Hay (1908) and Zangerl (1969), have simply considered these scutes to be the posteriormost (normally the twelfth) pair of marginals, and this is indeed how they appear when they are paired. However, the single large scute in this position (typical of most tortoises, for example) has a different appearance and according to Pritchard & Trebbau (1984) justifies a different name.

2.2. Plastron 

Scutes of the plastron (not the intergular, single gular, interanal, undivided anal, etc.) are divided into pairs by a median longitudinal seam. Anteriorly there is a pair of gular scutes. In some families (notably plesiochelyids, cheloniids, most kinosternids, dermatemydids, pleurodires) an intergular scute or pair of intergular scutes is also present. In kinosternids Hutchison & Bramble (1981) homologizate the intergular scute(s) with the gulars, and the posterior humerals with the abdominals of other turtles (see discussion in Kordikova 2000, 2002). Paired humerals, pectorals, abdominals, femorals, and anals follow, respectively, and in the Cheloniidae an interanal is sometimes present. Some tortoises have two additional pairs of plastral scutes – pregulars and preintergulars anterior to the gulars and intergulars.

·      Inframarginals - scutes between the main plastral scutes and the ventral edge of the marginal scutes. They are mainly present in Cheloniidae, Chelydridae, Dermatemydidae, Kinosternidae and Platysternidae. Inframarginals are often reduced to just the anteriormost and posteriormost members, known as the axillaries and inguinals, respectively. Some authors (Chkhikvadze 1973) use the term latiplastral.

3. Fontanelles

These structures are not named by all authors even though fontanelles are present at least in early stages of morphogeny in almost all Testudinata. I distinguish the following fontanelles.

3.1. Carapace

·      Postnuchal fontanelle - fontanelle between the nuchal and anterior pair of costals. 

It can be unpaired or paired. Some authors (Meylan 1987, Pritchard 1993) call it suprascapular one.

·      Neurocostal fontanelles - fontanelles between costals and neurals.

·      Intercostal fontanelles - fontanelles between neighboring thoracic ribs or costals.

·      Costoperipheral fontanelles – lateral fontanelles between costals and peripherals.

3.2. Plastron

·      Epihyohypoxiphiplastral fontanelle – central fontanelle between epi-, hyo-, hypo- and xiphiplastra. It is present in Dermochelys coriacea.

·      Entohyoplastral fontanelle(s) - unpaired or paired ventral fontanelles between entoplastron and hyoplastra.   

·      Entohyohypoplastral fontanelle(s) - unpaired fontanelle between entoplastron, hyo- and hypoplastra.   

·      Hyohypoplastral, or central fontanelle - fontanelle between hyo- and hypoplastra. It is as a rule unpaired.

·      Hypoxiphiplastral fontanelle or hyohypoxiphiplastral fontanelles - central fontanelle between hypo- and xiphiplastra or between hyo-, hypo- and xiphiplastra. It is unpaired. This fontanelle is very often present in some pelomedusids (e.g. Pelomedusa subrufa) or adult cheloniids and trionychoids.

·      Xiphiplastral fontanelle - unpaired fontanelle between xiphiplastra.

·      Plastroperipheral fontanelles - paired lateral fontanelles between hyo-hypoplastra and peripherals.

4. Hinges

Many turtles have carapacial or plastral hinges (Borkin 1973, Bramble 1974, Bramble & Hutchison 1981, Brumble et al. 1984, Pritchard & Trebbau 1984, Ernst & Barbour 1989, Broadley 1993, Pritchard 1993, etc.). These hinges allow the carapace or plastron to be closed to protect head and limbs if they are large and mobile enough. Hinges in chelonians can be classified as:

4.1. Carapace

·      Costoperipheral hinge - a transverse carapacial hinge that always lies between the 5^th and 6^th costals and peripherals or, sometimes, between the 6^th and 7^th costals and peripherals. Most species of the testudinid genus Kinixys only has such a hinge.

·      Peripheral hinge - a transverse carapacial hinge between the 5^th and 6^th peripherals in Kinixys natalensis.

·      Carapacial and plastral pankinesis (Pritchard 1993) – kinesis between all elements of the carapace and plastron – is characteristic for chelonians having paedomorphic features. Among living species carapacial and plastral pankinesis is present in most trionychoids as well as in the cheloniid Eretmochelys imbricata. Plastral pankinesis is characteristic for cheloniids and some other chelonioids. In sea turtles the plastron has lines of flexibility at the bridge and the midline. Carettochelys insculpta has costoperipheral and plastroperipheral kinesis as well as the kinesis between the epi- and entoplastra, on the one hand, and hypoplastra, on the other hand. There is also a movable link between the hyo-, hypo- and xiphiplastra of the left and right sides. In most trionychoids the bony carapace is completely rigid, as is the hyohypoplastral suture line.

The carapacial and plastral pankinesis is associated with changes in shell depth and is the result of adaptive evolution within different groups.

4.2. Plastron

·      Epihyoplastral hinge - a movable plastral hinge, which lies between the epiplastra and hyoplastra - between the humeral and pectoral scutes, either, crossing the posterior portion of the entoplastron or behind it. It is present in Pyxis (Testudinidae). In kinosternids this hinge lies across the humeral scute (following Bramle et al. 1984) or between the humeral and abdominal scutes because of reduction of the pectoral scute.  

·      Hyomesoplastral hinge – a transverse plastral hinge placed between hyo- and mesoplastra. It is present in Pelusios (Pelomedusidae) but it is rather rigid in P. broadleyi and P. adansoni.

·      Hyohypoplastral hinge – a transverse plastral hinge placed between the hyo- and hypoplastra - between pectoral and abdominal scutes. It occurs in many genera of emydids such as Emys, Emydoidea, Terrapene, Cuora, Cyclemys, Pyxidea, Notochelys and Rhinoclemmys.

·      Interhyo- and interhypoplastral hinge – a longitudinal plastral hinge along the hyo- and hypoplastral sutures (in Sternotherus spp.).

·      Hypoxiphiplastral hinge – a transverse plastral hinge between hypo- and xiphiplastra - between the abdominal and femoral scutes. Hypoxiphiplastral hinge is well developed in kinosternids. The plastron of kinosternids may contain either an inconspicuous single epihyoplastral hinge (e.g. Claudius, Staurotypus) or two transverse movable hinges - epihyoplastral and hypoxiphiplastral hinges (in most species of Kinosternon). Most species of Testudo and Gopherus (Testudinidae) have a weak hypoxiphiplastral hinge.

·      Interxiphiplastral hinge - a longitudinal plastral hinge between the xiphiplastra (in Kinosternon scorpioides).

·      Plastroperipheral hinge - a longitudinal hinge between the hyohypoplastra and peripherals of the carapace (in Claudius angustatus and in emydids - Emys orbicularis, Emydoidea spp. and Terrapene spp., etc.)

Though this shell nomenclature proposed here and earlier (Kordikova 2000) represents a relatively recent innovation it seems to be logical and it has an embryological and comparative morphological (homological) basis for evolutionary applications.

5. Other terms

·      Seam - a term for the line of connection between two scutes.

·      Suture - a term for the connection between two bones.

·      Sulcus - a term for the impression that a seam leaves on the underlying bone, are of universal usage.