Fish and chips

As 1978 turned to 1979, a provocative and entertaining letter and reply were published in the scientific journal Nature, discussing the relationships of three gnathostomes: the salmon, the cow, and the lungfish.1 English paleontologist L. B. Halstead argued that, obviously, the two fish must be more closely related to each other than either is to a cow. After all, he argued, they're both fish! A coalition ofEuropean cladists disagreed, pointing out that, in an evolutionary sense, a lungfish is more closely related to a cow than to a salmon. In their view, if the lungfish and the salmon are both to be called "fish," then the cow must also be a fish. Can a cow be a fish?

The vast majority of vertebrates are what we call "fishes." They all make a living in either salt or fresh water and, consequently, have many features in common that relate to the business of getting around, feeding, and reproducing in a fluid environment more viscous than air. But, as it turns out, even if "fishes" describes creatures with gills and scales that swim, "fishes" is not an evolutionarily meaningful term because there are no shared, derived characters that unite all fishes that cannot also be applied to all non-fish gnathostomes. The characters that pertain to fishes are either characters present in all gnathostomes (that is, primitive in gnathostomes) or characters that evolved independently.

The cladogram in Figure B4.1.1 is universally regarded as correct for the salmon, the cow, and the lungfish. In light ofwhat we have discussed, this cladogram might look more familiar using groups to which these creatures belong: salmon are ray-finned fish, that is fish with long rays made of a distinct protein supporting their pectoral and pelvic fins; cows are tetrapods; and lungfishes are lobe-finned fishes. Clearly, lobe-finned fishes share more derived characters in common with tetrapods than they do with ray-finned fishes. Thus there are two clades on the cladogram:

1. lobe-finned fishes + tetrapods; and

2. lobe-finned fishes + tetrapods + ray-finned fishes.

1. Halstead, L. B. 1978. The cladistic revolution - can it make the grade? Nature, 276, 759-760. Gardiner, B. G., Janvier, P., Patterson, C., Forey, P. L., Greenwood, P. H., Mills, R. S. and Jeffries, R. P. S. 1979. The salmon, the cow, and the lungfish: a reply. Nature, 277, 175-176.

Salmon Cladogram
Figure B4.1.1. The cladistic relationships of a salmon, a cow, and a lungfish. The lungfish and the cow are more closely related to each other than either is to the salmon.

Clade 1 is familiar as Sarcopterygia. Clade 2 occurs at the level of all fish (and the descendants of fish) and looks like part ofthe cladogram presented in Figure 4.3 for gnathostome relationships. If only the organisms in question are considered, the only two monophyletic groups on the cladogram must be (1) lungfish + cow; and (2) lungfish + cow + salmon (that is, representatives of the sarcopterygians and Osteichthyes, respectively).

Which are the "fishes?" Clearly the lungfish and the salmon. But the lungfish and the salmon do not in themselves form a monophyletic group unless the cow is also included. The cladogram is telling us that the term "fishes" has phylogenetic significance only at the level of Osteichthyes (or even below). But we can and do use the term "fishes" informally. Fish and chips will never be burger and fries.

may stick out from each neural arch. These can be for muscle and/or ligament attachment, or they can be sites against which the ends of ribs can abut. The repetition of vertebral structures, a relic of the segmented condition that is primitive for chordates, allows flexibility along the length of the animal.

Girdles. Sandwiching the backbone are the pelvic and pectoral girdles (Figure 4.5). These are each sheets of bone (or bones) against which the limbs attach for the support of the body. The pelvic girdle is the attachment site of the hindlimbs; the pectoral girdle is the attachment site of the forelimbs.

Each side of the pelvic girdle is made up of three bones: (1) a flat sheet of bone, called the ilium (plural ilia), that is fused to the sacrum, which is a block of vertebrae between the iliac blades; (2) a piece that points forward and down, called the pubis; and (3) a piece that points backward and down, called the ischium. Primitively, the three bones come together in a depressed area of the pelvis called the acetabulum: the hip socket.

By contrast, the pectoral girdle consists of a flat sheet of bone, the scapula (shoulder blade), on each side of the body, attached to the outside of the ribs by ligaments and muscles.

Chest. Some chest elements deserve mention. The breastbone (sternum) is generally a flat or nearly flat sheet of bone that is locked into its position on the chest by the tips of the thoracic (or chest) ribs. The rib cage is supported at its front edge by the clavicles, themselves connected to the coracoids, a pair of shield-like bones that contact the scapula.

Legs and arms. Limbs in tetrapods show the arrangement pioneered in their sarcopterygian ancestors (see Figure 4.4). All limbs, whether fore or hind, have a single upper bone connecting to a pair of lower bones. In a forelimb, the upper arm bone is the humerus, and the paired lower bones (forearms) are the radius and ulna. The joint in between is the elbow. In a hindlimb, the upper bone (thigh bone) is the femur, the joint is the knee, and the paired lower bones (shins) are the tibia and fibula.

Beyond the paired lower bones of the limbs are the wrist and ankle bones, termed carpals and tarsals, respectively. The bones in the palm of the hand are called metacarpals, the corresponding bones in the foot are called metatarsals and collectively they are termed metapodials. Finally, the small bones that allow flexibility in the digits of both the hands (fingers) and the feet (toes) are called phalanges (singular phalanx). At the tip of each digit, beyond the last joint, are the ungual phalanges.

Tetrapods primitively had as many as eight digits on each limb. Early in the evolutionary history of tetrapods, this number rapidly reduced to, and stabilized at, five digits on each limb, although many groups of tetrapods subsequently reduced that number even further (Figure 4.5).

Head. At the front end of the vertebral column of chordates are the bones of the head, composed, as we have seen, of the skull and mandible (Figure 4.6). Primitively, the skull has a distinctive arrangement: the braincase, a bone-covered box containing the brain, is located centrally and toward the back of the skull. At the back of the braincase is the occipital condyle, the knob of bone that connects the braincase (and hence the skull) to the vertebral column. A rear-facing opening in the braincase, the foramen magnum, allows the spinal cord to attach to the brain. Located on each side of the braincase are openings for the stapes, the bone that transmits sound from the tympanic membrane (ear drum) to the brain. Finally, covering the braincase and forming much of the upper rear part of the skull is a curved sheet of interlocking bones, the skull roof (inset to Figure 4.6).

The skull has two familiar pairs of openings. Located midway along each side of the skull is a large, round opening - the eye socket, or orbit. At the anterior tip of the skull is another pair of openings - the nares (singular, naris), or nostril openings. Finally, flooring the

Bird Occipital Bone

Occipital condyle

Figure 4.6. Skull and mandible of the primitive saurischian dinosaur Plateosaurus, exemplifying the general arrangement of bones in the skull and mandible. (a) Shull elements "exploded"; (b) cross-section through braincase; (c) "exploded" elements of mandible (lower jaw); (d) rear view of skull.

Occipital condyle

Figure 4.6. Skull and mandible of the primitive saurischian dinosaur Plateosaurus, exemplifying the general arrangement of bones in the skull and mandible. (a) Shull elements "exploded"; (b) cross-section through braincase; (c) "exploded" elements of mandible (lower jaw); (d) rear view of skull.

skull, above the mandible, is a paired series of bones, organized in a flat sheet, which forms the palate.4

Within Tetrapoda

Tetrapods share a variety of derived features (Figure 4.7). We have seen many of these in the tetrapod skeleton: the distinctive morphologies of the girdles and limbs, as well as the fixed patterns of skull roofing bones. The hypothesis that all of these shared similarities evolved separately in distantly related organisms is not parsimonious; for this reason, these

4. In mammals, a passage forms between the floor of the nasal cavity and the roof ofthe oral cavity (mouth), so that air breathed in through the nostrils is guided to the back of the throat, bypassing the mouth. As a result, it is possible for chewing and breathing to occur at the same time. Similar kinds ofpalates (called secondary palates) are known in other tetrapods besides mammals, but primitively the nostrils lead directly to the oral cavity. So, if food were to be extensively chewed in the mouth, it would quickly get mixed up with the air that is breathed in. For this reason, chewing is not a behavior of primitive tetrapods.

Diapsida

Figure 4.7. Cladogram of Tetrapoda. Derived characters include: at 1, the tetrapod skeleton (see Figure 4.5); at 2, a lower temporal fenestra (see Figure 4.9); at 3, the presence of an amnion (see Figure 4.8); at 4, lower and upper temporal fenestrae (see Figure 4.9); and at 5, an antorbital fenestra (see Figure 4.12). Lepidosauromorpha is a monophyletic group, the living members of which are snakes, lizards, and the tuatara. Chelonia - turtles -are reptiles whose primitive, completely roofed skulls place them near the base of Amniota.

Reptilia

Amniota

Tetrapoda

characters reaffirm Tetrapoda as a monophyletic group. Now we continue our journey to find out exactly what a dinosaur is.

Amniota

A subset of the tetrapods, Amniota, is characterized by the invention of a special membrane for the egg-bound, developing embryo called an amnion (Figure 4.8, and see below). The tetrapods without an amnion - anamniotes - are today represented only by frogs, salamanders,

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    Why is a lungfish closer related to a cow than a salmon?
    9 years ago

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