Section I—Skeleton

Cranial Skeleton

Skull

Examine the skeleton of the head on a mounted speci­men (Figure 5.1) to identify the skull, mandible, and hyoid apparatus, but examine these cranial skeletal components on isolated specimens as well.

The dermal skull roof covers the brain and major sense organs dorsally. The ventral surface, or underside, of the skull is formed mainly by the palatal complex. Between the dermal roof and palatal complex is the chondrocra­nium, which remains mainly cartilaginous and has only small exposures on the outside of the skull. In the shark (page 28), the chondrocranium forms a nearly complete enclosing and supportive structure for the brain and major sense organs. The chondrocranium is highly developed in the shark and other chondrichthyeans, but this is necessary because of the near-absence of bone in these vertebrates. In most other vertebrates, bone covers the brain dorsally and forms the floor of the skull, so the chondrocranium is a relatively minor structure. This is particularly true in Necturus, in which the chondro­cranium (Figure 5.3) retains larval characteristics, as do many other parts of the body, and is even less developed than in most vertebrates.

Examine an isolated skull. In dorsal view (Figure 5.2a) the paired premaxillae are seen anteriorly, followed by the paired frontals and parietals. Note the long slender anterior extension of the parietal that runs along the lateral margin of the frontal. The cartilaginous antor­bital processes, which are part of the chondrocranium (Figure 5.3), project laterally near the anterior end of the parietal bones, but these delicate elements are often missing in prepared specimens.

The otic capsules lie at the posterolateral corners of the skull (Figures 5.2a-c). The paired bones that contribute to each capsule are the prootics and the opisthotics. The prootic lies lateral to the parietal near the suture of the parietals and frontals, and the opisthotic lies posterior to the parietal. A cartilaginous region, containing the oval window, lies between the prootic and opisthotic. The squamosal extends anterolaterally from the opisthotic, forming the posterolateral margin of the skull. The quadrate articulates with the anterior end of

FIGURE 5.1 Skeleton of the mudpuppy in dorsal view.

the squamosal and inclines anteroventrally (Figure 5.2b). It contacts the pterygoid anteriorly and forms the jaw joint through an articulation with the mandible. The quadrate cartilage forms a slender bridge extending from the anterior end of the quadrate toward the lateral margin of the parietal.

Examine the skull in lateral view (Figure 5.2c) and re­identify the elements described above. Just anterior to the opisthotic, and thus essentially covering the carti­laginous region between the prootic and opisthotic, is the disc-shaped columella. The columella covers the oval window, mentioned above. A small stylus projects from its lateral surface. The columella, with its stylus, is a small delicate bone that transmits vibrations to the inner ear. It may be missing in your specimen, in which case the oval window should be clearly visible. The foramen magnum (Figures 5.2a, b) is the large opening on the posterior surface of the skull through which the spinal cord passes. The exoccipitals, each bearing an occipital condyle, form the skull lateral and ventral to the foramen magnum.

Examine the skull in ventral view (Figure 5.2b) and identify the premaxillae, vomers, and pterygoids.

The chondrocranium should be studied in a separate preparation (Figure 5.3), usually set in an acrylic block, even though many of its elements have already been observed. The posterior half of the chondrocranium consists of the otic capsules, with its cartilaginous portion set between the bony prootic and opisthotic, and the exoccipitals. Left and right otic capsules are connected dorsally by a delicate cartilaginous bridge, the synotic tectum. The basal plate forms a similar bridge between left and right exoccipitals. A para­chordal plate extends anteriorly from each otic capsule. Trabeculae are the slender rod-like elements extending anteriorly from the parachordals. The antorbital carti­lages (noted above) project laterally from each trabec­ula. Beyond the antorbital cartilages the trabeculae converge to form the ethmoid plate (noted above) and then send a pair of trabecular horns anteriorly between the nasal capsules, which are extremely delicate and usually not preserved.

Just anterior and laterally to each otic capsule, many preparations include the bony and cartilaginous com­ponents of the quadrate. These have already been observed, but remember that they are part of the palatal complex rather than the chondrocranium. They are in part homologous to the palatoquadrate cartilage, and thus belong to the mandibular arch, which is part of the splanchnocranium.

Mandible

In lateral view, most of the mandible is formed by the large dentary bone, which bears a long row of marginal teeth (Figures 5.2c, d). Posterior to these teeth is a short tooth row borne by the splenial bone.

FIGURE 5.2 Skull and mandible of the mudpuppy: (a) skull in dorsal view; (b) skull in ventral view; (c) skull and mandible in left lateral view; (d) left mandible in medial view.

Hyoid Apparatus

The hyoid apparatus (Figure 5.4) is large and composed of the hyoid arch and parts of the first three branchial arches. The hyoid arch is the largest and most anterior, and supports the tongue. It is composed on each side by the small hypohyal and, more laterally, the larger cera- tohyal. The median basibranchial 1 extends posteriorly from the hypohyals to the base of the first branchial arch, which is also composed on each side by two large elements, Ceratobrachial 1 and epibranchial 1. The more slender basibranchial 2 extends posteriorly from the left and right ceratobranchial 1. Ceratobranchial 2 is a tiny, nodular element at the base of epibranchial 2. Epi­branchial 3 lies posterior to epibranchial 2. The remain­ing branchial arches are apparently represented by tiny cartilaginous elements, but these are rarely preserved in prepared specimens.

Key Terms: Cranial Skeleton

FIGURE 5.3 Chondrocranium of the mudpuppy in dorsal view.

exoccipitals foramen magnum frontals hyoid apparatus hypohyal mandible Meckel’s cartilage

(mandibular cartilage)

occipital condyle opisthotics parachordal plate parasphenoid parietals

premaxillae prootics pterygoid quadrate quadrate cartilage skull splenial squamosal stylus synotic tectum trabeculae trabecular horns vomer

Postcranial Skeleton

Axial Skeleton

The axial skeleton consists of four types of vertebrae (Figure 5.1). Most anteriorly is the single cervical ver­tebra that articulates with the occipital condyles of the skull and with the first of the long series of trunk ver­tebrae (Figures 5.1 and 5.5). The latter bear transverse processes, which extend from the centrum and articu­late with small, Y-shaped, posterolaterally directed ribs. The neural canal passes dorsal to the centrum. The neural arch forms the roof of the canal and bears a neural process. Prezygapophyses and postzygapophyses are present for articulation with the preceding vertebra and succeeding vertebra, respectively. A single sacral vertebra (Figures 5.1 and 5.8) articulates with the pelvic girdle by way of its ribs. The caudal vertebrae (Figure 5.6), each bearing a hemal arch, follow the sacral ver­tebra. Most lack ribs, but bear transverse processes.

Appendicular Skeleton

The pectoral girdle (Figure 5.7) is composed of left and right halves that overlap ventrally but do not fuse. The ossified scapula is a short bone that dorsally bears the suprascapular cartilage. The glenoid fossa, the depres­sion that articulates with the forelimb, is ventral to the scapula. The slender, elongated procoracoid cartilage projects anteriorly in front of the glenoid fossa. The coracoid cartilage forms the broad, ventral, plate-like part of the girdle.

As is typical of tetrapods, the forelimb consists of the brachium, antebrachium, and manus in proximal to distal order. The brachium is supported by the humerus. In the anterbrachium the radius is the anteromedial bone, and the ulna is the posterolateral bone.

The pelvic girdle (Figure 5.8), like the pectoral, is mainly cartilaginous. The ilium is ossified and extends ventrally from its articulation with the rib of the sacral vertebra toward the acetabulum, the depression that articulates with the hindlimb. The girdle is formed ventrally by the expansive puboischiadic plate. Examine the plate in ventral view. The elongated, triangular pubic cartilage is the anterior portion. The posterior portion is the ischiadic cartilage, which contains a pair of ossifications termed ischia. Note the obturator foramen just anterior to each acetabulum.

The hindlimb is also formed of three segments, the thigh, crus, and pes. The femur is the single bone of the thigh. The tibia and fibula lie anteriorly and posteriorly, respectively, in the crus. The pes is formed proximally by six cartilaginous tarsals and distally by four ossified digits. The proximal bone of each digit is a metatarsal, while the distal elements are phalanges.

Key Terms: Postcranial Skeleton

acetabulum

carpals

caudal vertebrae

centrum (vertebral body)

cervical vertebra coracoid cartilage femur

fibula

glenoid fossa hemal arch humerus

ilium

ischium (pl., ischia) ischiadic cartilage metacarpal metatarsal

FIGURE 5.4 Skull, mandible, and hyoid apparatus of the mudpuppy in (a) left lateral and (b) ventral views.

neural arch (vertebral arch)

neural canal (vertebral canal)

neural process (spinous process)

obturator foramen pectoral girdle pelvic girdle phalanges postzygapophyses prezygapophyses procoracoid cartilage

pubic cartilage puboischiadic plate radius

ribs

sacral vertebra scapula

suprascapular cartilage tarsals

tibia

transverse processes trunk vertebrae

ulna

FIGURE 5.5 Trunk vertebra of the mudpuppy in (a) dorsal, (b) ventral, (c) right lateral, (d) anterior, and (e) posterior views.