Animal Diversity: Chordata

Bones seen on the dorsal surface:
Premaxillae, maxillae, quadratojugals, squamosals, septomaxillaries, nasals, fronto-parietals,
prootics, exoccipitals and occipital condyles
Bones seen on the ventral surface:
Premaxillae, maxillae, quadratojugals, vomers, palatines, sphenethmoids, parasphenoid,
pterygoids and exoccipitals
The skull of urodela differs from that of the frog in many ways: The trabeculae do not meet
either below the brain to form a basis cranii or above it to form a cranial roof. There is,
above, a huge superior cranial fontanelle, and below an equally large basicranial fontanelle.
The former is covered, in the complete skull, by the parietals and frontals, the latter by the
parasphenoid. The parietals and frontals are separate. The parasphenoid is not T-shaped. A
single bone, the vomeropalatine bearing teeth, represents the palatine and vomer. The hyoid
arch is large and its dorsal end may be separated as a hyomandibular. There are three or four
branchial arches. The stapes has no extra-columella and no tympanic cavity or membrane. In
some anuran species, there is the presence of small supra and basi-occipitals. In others,
investing bones of the roof are very strongly developed. In the apoda, the investing bones are
very large and form a substantial structure.
The vertebral column is divisible into: the cervical region, an abdominal / thoracolumbar
region, a sacral region and the caudal region. The total number of vertebrae in urodeles and
apoda may be as much as 250, while there are only 9 vertebrae and a single rod shaped
caudal bone called the urostyle in anurans. In the lower urodela the centra are biconcave as in
fishes. However, the neural arches are much better developed than in any fish and have well
developed zygapophyses. The apoda also have biconcave vertebrae but in the higher
urodeles, the anterior surface of the centrum has a convexity while the posterior surface
retains its concavity, thus forming a ball and socket joint and the condition is known as
opisthocoelous. In the anura the condition gets reversed with respect to the anterior and
posterior surfaces and this condition is called as procoelous. A frog’s vertebral column
includes the following bones:
Vertebrae of frog (Figure 13)
35
Atlas vertebra
1. The first vertebra is called the atlas.
2. It is small and ring-like in form.
3. Centrum and neural spine are
reduced.
4. Transverse processes and
prezygapophysis are absent.
5. The neural arch is large.
6. The anterior face of centrum
possesses a pair of concave facets for
the articulation with the occipital
condyles of the skull.
7. The posterior margin of the neural
arch bears a pair of
postzygapophyses.