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