Medios aéreos

Th e diff erence between water and air profoundly af-fects feeding by tetrapods. In water, most food items are nearly weightless, and aquatic animals can suck the food into their mouth and move it within the mouth by creating currents of water. Aquatic vertebrates, from the tiniest tadpoles to the largest whales, use suc-tion feeding to capture food items suspended in the water column. In contrast, terrestrial animals use their

jaws, tongues, and teeth to seize food items and to ma-nipulate items in the mouth.

Th e skull of early tetrapods is much like that of early bony fi shes, with extensive dermal skull bones. (Th ese bones are retained in most extant tetrapods—most of the bones of the human skull represent the legacy of this bony fi sh dermatocranium.) However, the gill skeleton and the bones connecting the pectoral girdle to the head have been lost in all but the very earliest known tetrapods, and no tetrapod has retained the operculum.

Th e skull of bony fi shes has a short snout; movements of the jaws, hyoid apparatus (the lower part of the hyoid arch), and operculum cause water to be sucked into the mouth for both gill ventilation and feeding.

Early tetrapods had wide, fl at skulls and longer snouts than their fi sh ancestors so that most of the tooth row was now in front of the eye. Th eir fl at heads and long snouts combined the functions of feeding and breathing, as do the heads of extant amphibians, which use movements of the hyoid apparatus to venti-late the lungs. Th is method of lung ventilation is called a positive-pressure mechanism, or buccal pumping.

Th e same expansion of the buccal cavity is used for suction feeding in water. Suction feeding is not an op-tion on land, however, because air is much less dense than the food particles. (You can suck up the noodles in soup along with the liquid, but you cannot suck up the same noodles if you put them on a plate.)

Th e tongue of jawed fi shes is small and bony, whereas the tongue of tetrapods is large and muscu-lar. (Th e muscular tongues of lampreys and hagfi shes are not homologous with the tongues of tetrapods, and they are innervated by diff erent cranial nerves.) Th e tetrapod tongue works in concert with the hyoid apparatus and is probably a key innovation for feeding on land. Most tetrapods use the tongue to manipulate food in the mouth and transport it to the pharynx.

Most terrestrial salamanders and lizards have sticky tongues that help to capture prey and transport it into the mouth—a phenomenon called “prehension.” In ad-dition, some tetrapods—such as frogs, salamanders, and the true chameleon lizards—can project their tongue to capture prey. (Th e mechanism of tongue pro-jection is diff erent in each group; tongue propro-jection is an example of convergent evolution.)

Salivary glands are known only in terrestrial verte-brates, probably because lubrication is not required to swallow food in water. Saliva also contains enzymes that begin the chemical digestion of food while it is still in the mouth. Some insectivorous mammals, two spe-cies of lizards, and several lineages of snakes have elab-orated salivary secretions into venoms that kill prey.

(a)

(b) (c)

(d) (e)

(f)

(g) (h)

(j) (n)

(i)

(o) (p)

(l)

(m)

(k)

Figure 8–8 Phylogenetic view of tetrapod terrestrial stance and locomotion. (a) Ancestral tetrapod condition, retained today in salamanders: movement mainly via axial movements of the body, limbs moved in diagonal pairs (basic walking-trot gait).

(b) Derived jumping form of locomotion in the frog, with highly specialized hind limbs. (c) Ancestral amniote condition, seen in many extant lizards: limbs used more for propulsion, with development of the walk gait (limbs moved one at a time/independently).

(d) Diapsid amniote condition with hind limbs longer than forelimbs; tendency for bipedal running, seen in some extant lizards.

(e) Derived limbless condition with snakelike locomotion; evolved convergently several times among early tetrapods (e.g., several types of lepospondyls), lissamphibians (caecilians and salamanders), and lepidosaurs (many lizards, snakes, amphisbaenians). (f) Ancestral archosaur condition, with upright posture and tendency to bipedalism. (g) Secondary return to sprawling posture and quadrupedalism in crocodilians. (h) Obligate bipedality in early dinosaurs and (i) birds. (j) Return to quadrupedality several times within dinosaurs. (k) Ancestral mammalian condition: upright posture and the use of the bound as a fast gait with dorsoventral fl exion of the vertebral column (all mammals use the walk as a slow gait). In the bound the animal jumps off the two hind limbs, fl ies through the air with limbs outstretched, and lands on the two forelimbs (or on one forelimb and then the other, as in this half-bounding cat). (l) Condition in larger mammals where the bound is turned into the gallop: the limbs move one at a time, and the period of suspension when all four feet are in the air occurs when the legs are bunched up, as shown. (m) Th e trot is used at inter-mediate speeds between the walk and the gallop. Th e canter is essentially a slower version of the gallop. (n) Th e ricochet, a derived hopping gait of kangaroos and some rodents. (o) Th e amble, a speeded-up walk gait seen in the fast gait of elephants and in some horses (e.g., Paso Finos and Icelandic ponies). (p) Th e unique human condition of upright bipedal striding. Penguins can also walk with an upright trunk, but they waddle rather than stride.

Thomson's gazelle (15 kg), bounding

Eland (250 kg), galloping

Figure 8–9 Bounding and galloping. Small species of antelopes, such as the Th omson’s gazelle, bound, whereas larger species, represented by the eland, gallop. Th e dark bars represent the vertebral column.

With the loss of gills in tetrapods, much of the as-sociated branchiomeric musculature was also lost, but the gill levators are a prominent exception. In fi shes, these muscles are combined into a single unit, the cucullaris, and this muscle in tetrapods becomes the tra-pezius, which runs from the top of the neck and shoul-ders to the shoulder girdle. In mammals, this muscle helps to rotate and stabilize the scapulae (shoulder blades) in locomotion, and we use it when we shrug our shoulders.

Understanding the original homologies of the trape-zius muscle explains an interesting fact about human spinal injuries. Because the trapezius is an old branchio-meric muscle, it is innervated directly from the brain by cranial nerves (cranial nerve XI, which is actually part of nerve X), not by the nerves exiting from the spinal cord in the neck. Th us people who are paralyzed from the neck down by a spinal injury can still shrug their shoul-ders. Small muscles in the throat—for example, those powering the larynx and the vocal cords—are other remnants of the branchiomeric muscles associated with the gill arches. Ingenious biomedical engineering allows quadriplegic individuals to use this remaining muscle function to control prosthetic devices.

Th e major branchiomeric muscles that are retained in tetrapods are associated with the mandibular and hyoid arches and are solely involved in feeding ( Figure 8–10 ). Th e adductor mandibulae remains the major jaw-closing muscle, and it becomes increasingly complex in more derived tetrapods. Th e hyoid musculature forms two

Adductor

mandibulae (deep part)

Adductor mandibulae (superficial part) Mylohyoid

Eardrum

Trapezius

Depressor mandibulae

Sphincter colli

Sternocleidomastoid

Figure 8–10 Head and neck musculature. _Th is is the gen-eralized tetrapod condition as seen in a tuatara ( Sphenodon ).

new important muscles in tetrapods. One is the de-pressor mandibulae, running from the back of the jaw to the skull and helping the hypobranchial muscles to open the mouth. Th e other is the sphincter colli that surrounds the neck and aids in swallowing food. In mammals the sphincter colli has become the muscles of facial expression.