Craniata include the more familiar chordates: fishes, amphibians, reptiles, birds, and mammals. Most of the extant members of this clade belong to the Vertebrata, and only recently was the new name Craniata applied to this group. They are now called the Craniata, instead of the Vertebrata, because not all of the animals previously known as vertebrates actually had vertebrae! A synapomorphy for the Craniata, however, is the presence of a distinct and characteristic head, unlike the Cephalochordates. Within Craniates are the true vertebrates, which really do have backbones. The bony or cartilaginous vertebrae replace the notochord as the primary support for the body. Vertebrates are quite diverse in form, and have come to be the predominant large animals on earth.
As you proceed through the remainder of this exercise, use the cladograms on pages 679 and 684 of Campbell, plus the classification given in the table of contents of this exercise, to help you learn the relationships among the groups of craniates. You should write down major synapomorphies for each clade.
Class Myxinia (hagfishes)
Hagfishes are the only extant Craniates without a backbone, although extinct groups also lacked this structure. Hagfish, along with lampreys, also lack jaws and paired fins. The lack of jaws in hagfish and lampreys led zoologists to classify them both as Agnatha (as opposed to Gnathostomes), but the “agnatha” were defined on the basis symplesiomorphies (shared primitive characters; in this case the lack of jaws and paired fins), instead of synapomorphies. The new classification now organizes the Craniata into more monophyletic groups.
Hagfish live in the deep sea, where they act as scavengers, feeding on corpses of animals that die at the surface and sink to the seafloor. To fishermen who sometimes catch them in traps, they are known as “slime eels,” because they secrete copious amounts of mucous from their skins. In nature, this mucous serves to clog the gills of crustaceans and other fish that come to feed on carcasses on the sea floor.
We have not given you a rank for this taxon, because the overall classification is in flux. However, this clade includes all of the Craniates with a backbone.
Lampreys (the real name for this group is unpronounceable)
The lampreys (V & C Figs. 7.167-7.174) are true vertebrates, but they share ancestral characters with hagfish: lack of jaws, and lack of paired fins. Many species of lampreys are parasitic as adults, attaching themselves to the sides of fish and sucking their body fluids. Most species breed in fresh water and migrate to sea as adults; you might find the larval “eels” in local streams in the Bay Area.
Study a slide of the lamprey larva ammocoete, and find the tail with its segmental muscles (myotomes), the stiffening notochord, and the dorsal nerve tube. Observe the pharynx with gill slits separated by gill bars. Compare the ammocoete to Amphioxus and note the general similarity in shape.
Superclass Gnathostomata (Gr: stoma=mouth)
The remaining classes are often placed in the superclass Gnathostomata, the vertebrates with jaws and paired fins.
Among the superclass Gnathostomata, class Chondrichthyes (Gr: chondrus=cartilage; ichthys=fish) (V & C fig. 7.175-7.177) is distinguished by (among other things) a cartilaginous skeleton. The cartilaginous skeleton seems to be a derived characteristic in the Chondrichthyes, evolved from a bony skeleton as a way of reducing weight. Sharks and rays (subclass Elasmobranchii) and a lesser known group called chimaeras, or ratfish, (subclass Holocephali) comprise the extant members of this class. Chondrichthyans share a distant ancestry with other jawed vertebrates, and retain some characteristics that may have been typical of early gnathostomes, such as separate gill slits and the lack of a lung or swimbladder. The elasmobranchs are one of the more diverse groups within the fishes.
Subclass Sarcopterygii - fleshy-fin fishes
Subclass Actinopterygii- ray-fin fishes
Extant members of the other gnathostome classes usually have bony skeletons, lungs or their derivative, a swimbladder, and other features that suggest they form a lineage that is separate from the sharks and rays. The class Osteichthyes (Gr: osteum=bone) (V & C fig. 7.178-7.180), the bony fishes, contains most of the species we usually think of as “fish.”. It is usually divided into two groups: the ray-fin fishes (subclass Actinopterygii), the fleshy-fin fishes (subclass Sarcopterygii).
(1) Subclass Sarcopterygii
The Sarcopterygii are known as “fleshy-finned” fish because the skeletal support for their paired fins extends outside of the body wall. The entire group is often called “lobe-finned” as well, making for some confusion between the group as a whole and the subgroup Actinistia.
The Actinistia are represented by only one extant form, a "living fossil" called the coelacanth from the Indian Ocean. Many extinct species of Actninistia lived in shallow-, fresh-water environments, however.
Lung-Fishes. Six extant species of lungfish inhabit the Southern Hemisphere, usually living in stagnant, swampy areas gulping surface air. When water levels drop, they are capable of surviving burrowed in the mud in a state of torpor. Among all fishes, the lungfishes are the group that is most closely related to the tetrapods.
(2) Subclass Actinopterygii- the ray-finned fishes
In the ray-finned fishes, the skeletal support for the paired fins is inside the body wall, so that all you see of the fins externally are the ray-like structures in the webbing of the fins themselves. The ray-fins have a long evolutionary history culminating in our most familiar fishes. Past adaptive radiations have left only a few survivors, like sturgeons and garfish. The most recent adaptive radiation consists of the group Teleostei, which includes trout, bass, perch, goldfish, tunas, butterfly fish, and most of the fish with which we are familiar. They represent over 95% of all extant species of fish.
Examine the fishes on display, noting the variation in body form among them as well as the features that are common to the group as a whole.
Here are some orders representing the diversity of Teleost fishes. You lab instructor may provide you with an opportunity to make a report about one of these orders.