Gills and their supporting structures (the gill bars) are prominent features of cephalochordates and fishes. Though the gill bars are simple in structure and function, they have played an important role in the evolution of vertebrates. You will study the homologies of gill bars in depth, because of the importance of some gill-bar derivatives and because they illustrate some important points about the evolution of structures.

(1) Lancelet (Branchiostoma )

One of the striking features of the lancelet is its pharynx, with long, parallel gill slits alternating with supporting gill bars. Reexamine the slide of Branchiostoma to refresh your memory, if necessary. The lancelet, as noted above, is a filter feeder, maintaining a flow of water through its pharynx by the beating of cilia. It has no gill filaments. (Gill filaments are lateral extensions of the pharynx, and are the red, feathery structures seen in fish.)

(2) Lamprey

The lamprey larva moves water into its pharynx by muscular contractions. It has fewer gill slits than the lancelet (seven on each side), and each slit is located on an outpocketing, the gill pouch. Each gill pouch has a set of gill filaments. Thus, the lamprey larva uses its pharynx for respiration as well as filter feeding. Adult lampreys are highly specialized parasitic feeders, with a dramatically different morphology (so much so that the larvae were initially thought to be a completely different species). But the adults retain gill pouches with filaments that function in respiration.

Lancelets and agnathans have no jaws, but the jaws of gnathostomes probably arose as modified gill bars.

(3) Shark

Examine the cartilaginous jaws and gill bars of a shark, and note the general similarity in appearance and position of the jaws and gill bars. All are suspended below the braincase, and consist of a jointed series of cartilages. The joints in gill bars allow the pharyngeal cavity to be flexed during swallowing and water to be pumped over the gills. Based on the structural and positional similarity of gill bars and jaws, it is very likely that jaws were derived from the anterior most gill bars of ancestral agnathans. The existence of solid, jointed structures near the mouth made possible, in a sense, the evolution of jaws. This is the first of several cases you will see in which the simple location of suitable structures made these structures useful for different functions.

The first gill arch is known as the mandibular arch, in honor of its function in most vertebrates. The dorsal part of this arch (the upper jaw in sharks) is the palatoquadrate cartilage, because of some bones that develop from it in bony vertebrates (the palatine and quadrate bones). The lower part of the mandibular arch is known as Meckel's cartilage. It forms all or part of the lower jaw in all gnathostomes.

The second gill arch is called the hyoid arch, and is modified in many gnathostomes for support of the jaws. Examine the shark skull again, and note the strut that extends from the corner of the jaw to the otic region of the skull. This is the upper part of the hyoid arch, and is known as the hyomandibula (or hyomandibular bone, when ossified). It serves to brace the rear of the jaws against the skull, permitting a strong bite while still allowing lateral flexibility. The value of a strong bite is obvious, and lateral flexibility is valuable in allowing the expansion and contraction of the oral cavity during respiration. This flexibility would not be possible if the upper jaw was fused to the skull. The lower part of the hyoid arch forms the floor of the mouth in sharks and in all other gnathostomes. Note the small hole behind the eye in a whole shark. This hole, known as the spiracle, is the remnant of the gill slit between the mandibular and hyoid arches.

(4) Teleost Fish

Now examine the skull of a teleost fish and review the skull of the bowfin fish. The supports for the gills should still be evident, even though they are now covered by a series of modified dermal bones, the opercular bones; however, major changes have taken place in the jaws and jaw supports of bony fishes. These changes are related to the increasing importance of dermal bones in the skull. The dermal bones located around the mouth come to bear teeth, as you noticed in the bowfin fish, and largely replace the mandibular arch elements as the functional jaws. Meckel's cartilage remains in the lower jaw, surrounded by dermal bones, but the palatoquadrate cartilage (the upper jaw of sharks) is replaced (as the upper jaw) by the maxilla and premaxilla, which are also dermal bones. The palatoquadrate is not lost, however. In bony fishes it combines with the hyomandibula to support the jaws. Examine the head of a teleost fish, noting the jaws and the triangular series of bones that support them. The anterior part of this supporting piece is derived from the palatoquadrate cartilage, and the most posterior is the hyomandibula. The hyomandibula again braces against the otic region of the braincase, and the anterior part of the palatoquadrate articulates with the anterior part of the skull. The lower part of this triangular piece forms the joint with the lower jaw. (For later reference, the quadrate bone of the palatoquadrate is the joint forming bone.) This arrangement is useful in fish, in that it allows the mouth cavity to expand and contract as the palatoquadrate/hyomandibula swings out and in.

The remaining gills and their supports retain their respiratory function in fishes, but the situation changes radically in land vertebrates. Tetrapods, of course, normally lack gills as adults, and gill pouches appear only briefly during development. However, rather than disappearing altogether, gill bars and pouches develop into other structures. By following their development, it seems that the gill pouches have become the small glands of the neck region: the thymus, thyroid, and parathyroids. The supporting elements of the gills also develop into new features: the cartilages of the trachea and larynx. These transformations in function are remarkable, but again illustrate how the simple location of a structure may make it available for a different function.

(5) Ear bones in Mammals

Another instance in which location seems to have played an important role in the evolution of structures is the evolution of the middle ear ossicles of tetrapods. The stapes, incus, and malleus of mammals are homologous to parts of the shark jaw and its support. You will learn some of the details of this transition in a moment, but to set the stage first note how close your ear is to the joint of your jaw. The jaw joint in vertebrates usually lies near the auditory region of the skull, just because that's where jaws are likely to reach.

Trace the origin of the stapes first. Find the stapes (or columella, as it is often called in reptiles) in the skull of the turtle and alligator. Again, note its proximity to the jaw joint. Now recall the location of the hyomandibula in the bony fish, and note how it abuts against the otic region of the skull. Reexamine the fish skull, if necessary. Also recall how the hyomandibula partly braces the quadrate bone (which forms the joint with the lower jaw). In tetrapods, the quadrate bone and its associates fuse with the skull to form a firmer bite. Lateral expansion of the mouth cavity is less important in tetrapods than in fish, since feeding and respiration are accomplished differently. This apparently left the hyomandibula with little function in jaw support. The location of the hyomandibular bone near the ear seems to have made possible another function, however: transmitting vibrations to the inner ear. Thus the stapes of reptiles and their descendants is homologous with the hyomandibular bone of fishes (and ultimately the second gill arch of jawless vertebrates).

The remaining middle ear ossicles of mammals, the incus and malleus, are also derived from the jaw apparatus. Re-examine the skulls of the turtle and alligator, and find the quadrate and articular bones. These two bones form the joint of the lower jaw, the quadrate being attached to the skull, and the articular being the joint-forming bone of the lower jaw. Note again their proximity to the ear and to the stapes. The stapes (or properly, the hyomandibula) remained braced against the quadrate and articular in the early mammal-like reptiles. The stapes picked up vibrations from the jaw and transmitted them to the ear. With time, the quadrate and articular became progressively smaller, and finally (in an event that separates reptiles from mammals) left the jaw joint completely. Retaining their ancestral connection with the hyomandibula (now stapes), they became the two remaining ossicles of the middle ear. The incus is homologous with the quadrate, and the malleus homologous with the articular. Examine the skull of a mammal, and note the auditory bulla, an enlargement of the skull near the ear, which contains the ossicles. A model of the human ear will show the location of the ossicles within the bulla. The evolution of these structures supports the idea that early mammals (much like most of their contemporary relatives) were active at night (nocturnal) and depended on hearing, smelling, and feeling as their main sensory modalities. Diurnal (day-active) reptiles depend more on vision.