Section I—Skeleton

Skull, Mandible, and Hyoid Apparatus

The skeleton of anurans demonstrates quite dramati­cally the misleading assumption that many beginning students have about amphibians—that they are inter­mediate between fishes and higher tetrapods, and so are simpler versions of reptiles and mammals.

FIGURE 6.1 Dorsal view of the skeleton of the frog.

Living amphibians generally tend to have specialized and reduced (and in some cases largely cartilaginous) skeletons, and that of frogs is no exception (Figure 6.1). There has indeed been considerable loss of bone and decrease in ossification over basal tetrapods (as well as higher tetrapods), which is clearly evident in the broad, flattened, and fenestrated skull (Figures 6.1 and 6.2). Two particularly large openings are the orbits dor- sally and the interpterygoid vacuities ventrally on the palate.

Examine the skull in dorsal view (Figure 6.2a). Its margin is approximately parabolic. On each side, this margin is composed of the small median premaxilla, the long maxilla, and the shorter quadratojugal, in anterior to posterior order. Ventrally, the premaxilla and maxilla bear a single row of small teeth, the premaxillary and maxillary teeth, respectively (Figures 6.2b-d). The vomers lie anteriorly just behind the premaxillae. They bear vomerine teeth.

The paired nasals are broad and flattened medially and contact each other on the dorsal midline (Figures 6.2a, c, d). Each has a narrow process that extends lat- eroventrally, forming the anterior margin of the large orbits, and contacts an ascending process of the maxilla.

(b) Skull, ventral view

(c) Skull and mandible, left lateral view

(e) Skull, left dorsolateral view

(d) Skull and mandible, posterior view

(f) Mandible, left medial view

FIGURE 6.2 Skull and mandible of the frog. (a) dorsal and (b) ventral views of the skull; (c) and (d) left lateral and posterior views of the skull and mandible; (e) left dorsolateral view of the skull; (f) dorsal view of the mandible.

that helps form the roof of the braincase is exposed dor- sally, wedged between the nasal and frontoparietal bones.

The parasphenoid is approximately cruciate or “t”- shaped (Figure 6.2b). The anteriorly tapering stem of the “t” covers the sphenethmoid ventrally. The top part of the stem projects posteriorly toward the occipital bones. Each transverse arm of the “t” extends laterally toward a pterygoid bone. The paired occipital bones form the posterior part of the skull (Figure 6.2e). They enclose the foramen magnum, the large opening for passage of the spinal cord.

Examine the posterior end of the skull in lateral view and identify the “T”-shaped squamosal (Figures 6.2c, d). Its long stem is oriented posteroventrally toward the angle of the jaw. The top of the “T” is curved, with one arm extending anteroventrally, the other posterodor­sally. Some prepared specimens provide an unobstructed view of the squamosal (Figure 6.2c), due to the absence of the annular cartilage that supports the tympanic membrane. Other specimens retain the cartilage, and the distal end of the columella (Figure 6.2c; see below) can be seen within the area it circumscribes.

Examine the skull in dorsal view. The irregularly shaped prootics contain the inner ear. The prootic extends between the squamosal laterally, and the exoccipital and frontoparietal medially. Anteriorly it helps form the pos­terior wall of the orbit and contains a large opening, the trigeminal foramen, through which the trigeminal nerve (Cranial Nerve V) passes. Posteriorly the prootic and exoccipital form the foramen ovale, the opening just beside the occipital condyle, for passage of the glos­sopharyngeal and vagus nerves (Cranial Nerves IX and X, respectively).

The mandible is a slender, parabolic, edentulous struc­ture. Each half of the mandible is composed of a carti­laginous element and two bony elements. The largest element is the angulare, which forms most of the demi­mandible. Its posteromedial surface bears a medially directed flange, the coronoid process. The angulare has a long, trough-like groove on its dorsal surface.

FIGURE 6.3 Hyoid of the frog in dorsal view. The anterior portion of the right side is missing.

terior end of the trough, where Meckel’s cartilage forms the articulation with the upper jaw. The cartilage extends to the anterior end of the demimandible and is ossified as a recognizable element, the mentomeckalian. The right and left mentomeckalians are connected by a ligamentous attachment. The dentary is a thin flange of bone that covers the anterolateral surface of the demimandible.

The hyoid apparatus is mainly a thin, broad cartilagi­nous plate, or body, on the floor of the oral cavity that supports the tongue and larynx (Figure 6.3). Several processes project from it. The anterior cornu initially extends anteriorly, but curves sharply posterodorsally to attach to the skull. In Figure 6.3 only part of the left anterior cornu is preserved. The posterior cornua are bony rods extending from the posterior margin. They begin medially but diverge to contact the larynx.

Key Terms: Skull, Mandible, and Hyoid Apparatus

pterygoid quadratojugal skull sphenethmoid

squamosal trigeminal foramen tympanic membrane vomers

Postcranial Skeleton

The postcranial skeleton also shows evidence of extreme modification, associated mainly with the highly special­ized locomotor mechanism characteristic of most anurans. The vertebral column is reduced, with only nine free vertebrae (Figure 6.4).

(a) Vertebral column and pelvic girdle, dorsal view

(b) Vertebral column and pelvic girdle, right lateral view

FIGURE 6.4 Vertebral column and pelvis of the frog in dorsal and right lateral views.

elongated, rod-like urostyle, which is formed by the fusion of several vertebrae. The vertebrae have promi­nent transverse processes, but ribs are lacking.

The appendicular skeleton is well developed. The pec­toral girdle, largely ossified, contains several elements (Figures 6.1 and 6.5). The scapula is a nearly vertical

(a) Pectoral girdle, dorsal view

(b) Pectoral girdle, ventral view

(c) Pectoral girdle, left lateral view

FIGURE 6.5 Pectoral girdle of the frog in (a) dorsal, (b) ventral, and (c) left lateral views.

Figure 6.6 Right antebrachium and manus of the frog in dorsal view.

plate-like structure. Extending dorsomedially from it is the suprascapula, which has a prominent and usually calcified cartilaginous portion medially. Ventrally, there are two large paired elements. The more anterior and slender paired clavicle, a dermal element, extends almost directly medially from the scapula. The larger, more posterior procoracoids form a plate-like base to the girdle.

The forelimb includes three segments, the most proxi­mal of which is the humerus, extending laterally from the glenoid fossa (Figures 6.1 and 6.5). The next segment includes the radius and ulna, fused to form a radio-ulna (Figures 6.1 and 6.6). The manus includes a proximal series of small, nodular carpals, followed by four complete digits, including metacarpals II-V and phalanges. The two medial digits each bear two pha­langes, while the lateral two bear three phalanges each. A small prepollex extends medially from the carpals and may represent a reduced metacarpal.

In the pelvic girdle, the pelvis is formed on each side by the ilium, ischium, and pubis (Figures 6.1 and 6.4). The slender ilium is an elongated, anteriorly directed element, with a well developed iliac crest. The ischium and pubis together outline a semicircle in lateral view, the ischium forming the more posterior portion. The acetabulum is a conspicuous depression for articulation with the femur (see below). The hind limb consists of the proximal femur, followed by the tibia and fibula, fused to form the tibio-fibula. The pes has rather typical metatarsals and phalanges, but a modification in the tarsal region produces another functional segment to the hind limb, a feature that is related to the saltatory loco­motion of frogs. Here, the two proximal tarsals are elongated to form a medial tibiale (= astragalus) and lateral fibulare (= calcaneum) that are partly fused at their ends. The distal tarsals have the more typical nodular form. There are five digits, with the first being the shortest and the fourth longest. Digits I and II each bear two phalanges, III and V three, and IV four. A small prehallux, simlar to the prepollex, extends medially from the tarsal region.

Key Terms: Postcranial Skeleton