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Anatomy.

All known Nectrideans were small, with the largest species having a maximum length of 500mm. All had short barrelled trunks and long, deep tails, making up about two-thirds their total length. Limbs were small but well developed. Some early forms had five fingers, where as in the later forms some had just four.

Land marks of the Nectridean vertebrae

Figure 2. Landmarks of the Nectridean vertebrae. With permission from Verlag Dr. Friedrich Pfeil 2006.

The characteristics of the vertebra are what define Nectridea as a group (see figure 2). All Nectridean vertebra have a spool-shaped centrum with a fused neural arch. Each neural arches bears an expanded, fan-shaped, neural spine, which is grooved and crenulated along its distal (upper) border. In the caudal (tail) vertebrae these spines are opposed ventrally by similar, expanded haemal spines. Additional accessory articulations above the normal zygapophyses kept the vertebral column rigid in the dorso-ventral plane but allowed for greater lateral (side to side) movement. The numbers and morphology of the trunk and tail vertebrae varied within the order but all shared the above basic characters.

Within the skull there was no otic notch; a small depression that can be found in the back of the skull of some modern amphibians. Further no stapes (a part of the ear) has been described in any Nectridean and they may well have lacked this structure.

Diplocaulids (members of the Nectridean family Diplocaulidea) had skulls that were broad, flattened and solidly built, with short-snouts. They also had characteristic long, posteriorly (backward) directed horn-like extensions. These were taken to an extreme in the “long-horned” genera Diploceraspis and Diplocaulus where they developed in to massive posterolateral extensions (Milner 1980) (see figure 3). Such an adaption may have evolved in a flowing-water environment (Cruickshank & Skews 1980) and helped in feeding (see below). These extension form out of the squamosal and tabular bones – which normally are only small parts of the skull (Benton 2005).

Short horn and long horn Diplocaulids

Figure 3. Comparison of Diplocaulid skulls. A. dorsal view and B. lateral view of Keraterpeton. C. dorsal view of Diploceraspis. Skull bones are; a, articular; d, dentary; f, frontal; j, jurgal; l, lacrimal; n, nasal; p, parietal; pf, post-frontal; pm, premaxilla; po, post-orbital; pp, post-parietal; prf, pre-frontal; qj, quadratojurgal; sa, surangular; sp, splenial; sq, squamosal; st, supratemporal; t, tabular. With permission from Verlag Dr. Friedrich Pfeil 2006.

The jaw articulations in this family of Nectrideans is level with, or anterior (in front) to the occiput (esp in later forms). The positioning of the jaw articulation is an important feature in distinguishing between the three families. In the mandible teeth are conical, angled slightly outwards with incurved tips.

Diplocaulids were shorter bodied than other Nectrideans with a maximum of 17 presacral vertebrae. Their tail was less deep and relatively shorter than that of Urocordylids.

Urocordylids

Figure 4. Comparison of Urocordylid skulls. A. dorsal view and B. lateral view of Sauropleura scalaris. Skull bones are; a, articular; d, dentary; f, frontal; j, jurgal; l, lacrimal; m, maxilla; n, nasal; p, parietal; pf, post-frontal; pm, premaxilla; po, post-orbital; pp, post-parietal; prf, pre-frontal; q, quadrate; qj, quadratojurgal; sa, surangular; sp, splenial; sq, squamosal; st, supratemporal; t, tabular. With permission from Verlag Dr. Friedrich Pfeil 2006.

The Urocordylid (of the family Urocordylidea) skull is elongated and arrow-shaped, but high-sided (figure 4). Their jaw articulation is level to, or posterior (behind) to the occiput. A slender mandible contained long and conical teeth, widely spaced (Carlson 1999).

The most complete Urocordylid  genera, Ptyonius and Urocordylus, both had 20 to 22 presacral vertebrae. They also possessed and a very deep tail of  a constant depth for two-thirds its length which then diminishing rapidly towards it tip (Bossy & Milner 1998) (figure 5).

Reconstruction Ptyonius

Figure 5. Reconstruction of the skeleton for the genus Ptyonius. Note its long tail relative to its body and the distinct caudal vertebrae (magnified); a feature of all Nectridean species. With permission from Verlag Dr. Friedrich Pfeil 2006.

Scincosaurids (of the family Scincosauridea) did not share any of the cranial features of the other two families (figure 6). Their skulls were box-like and this may reflect the primitive, ancestral, condition for the Nectridean order (Bossy & Milner 1998).

Scincosaurus

Figure 6. Scincosaurus. A. dorsal aspect of the skull. B. reconstruction of Scincosaurus. Compare the difference between the positions of the skull bones in the other two families. Skull bones are; f, frontal; j, jurgal; l, lacrimal; m, maxilla; n, nasal; p, parietal; pf, post-frontal; pm, premaxilla; po, post-orbital;  prf, pre-frontal; sq, squamosal; t, tabular. With permission from Verlag Dr. Friedrich Pfeil 2006.

Arizonerpeton shows only one of the diagnostic characteristics of the Nectirdeans; the accessory zygapophyses. It does though share other nectridean-like characteristics such as the tall, flat vertebrae and the spool-like centrum (Thayer 1985).


Back to Anatomy and Behaviour
Back to the Nectridean index.
Major Subgroups of the Nectrideans.
Fossil Record.
Modern Forms.
Literature and Web Links.


Author: Iain McIntyre
Last updated: 19/11/2006
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