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Fishes are aquatic vertebrates that have fins and gills throughout their
adult lives. They are a very heterogeneous assemblage of 4 distinct classes:
(1) jawless fishes (Agnatha); (2) extinct, armored Placodermi;
(3) shark-like or cartilaginous fishes (Chondrichthyes); and
(4) bony fishes (Osteichthyes). Amphibians are land vertebrates
(tetrapods) with aquatic, gill-breathing larvae and lung-breathing
adults that lay their eggs in water.
General characteristics of fishes:
The Devonian fossil Kiaeraspis was found abundantly in Sweden. Because he had hundreds of specimens, the Swedish paleontologist Erik Stensiö was willing to sacrifice one specimen to discover its internal anatomy: he ground down a small amount of the fossil, polished the surface, and took a photo; then he ground a little further and repeated the process in small increments until he had a complete photographic record of the interior anatomy of the fish. His reconstruction is shown in one of our illustrations, complete with details of the brain, the cranial nerves (designated by Roman numerals), and so on. Stensiö's study revealed a fish with a dome-shaped head and a flat bottom, indicating a bottom-feeder. The fish sucked in water and some sediment along the bottom and filter-fed by passing all the water through its many gill slits. Food that was trapped by the gills was then gathered (presumably by ciliary action) into the esophagus at the rear of the pharynx. The fish was heavily armored as a defense against predators. Many early fishes had heterocercal tail fins, meaning that the top half of the fin was much longer than and bottom half. With each swimming stroke, a heterocercal tail pushes the top half of the fish more forcefully forward than the bottom half, giving a slight downward component to every forward movement. This downward movement is adaptively favored in bottom-feeding fishes, and the frequent posession of heterocercal tails is evidence for bottom-feeding habits. (One group of early jawless fishes had a reversed heterocercal tail instead, longer on the bottom than on the top. This one group was probably surface-feeding, and their terminally located mouths, right in front instead of along the bottom surface, also shows adaptations for feeding on algae and other small plankton near the surface. Look at the illustration of Pharyngolepis to see these features.) Modern lampreys are degenerate parasites that attach to the outside of fishes (and occasionally large invertebrates) and suck their blood. Their circular mouth (the name of the order Cyclostomata translates as "circular mouth") has many rasping teeth embedded in it. The lamprey attaches to its victim by suction, then it sucks rhythmically. Each rhythmic suck draws the scraping teeth over the victim's body surface until the repeated scraping draws blood; then the lamprey continues to suck blood until there is none left. The lamprey then leaves the dead victim and goes off in search of another. Interestingly, the larval stage of the lamprey is a bottom-dwelling filter feeder, similar in habits to Kiaeraspis. The other living member of the Agnatha is the hagfish. It is also parasitic. It burrows into the flesh of its living or recently deceased victim and eats its way through all the flesh until nothing is left but skin and bones. Then, it borrows out and looks for another victim. Class Placodermi: An extinct group in which jaws first evolved. Paired fins also evolved in this group and are retained in all further vertebrate classes. Many placoderms were predators from 6 inches up to 50 feet long. Until placoderms evolved, the earliest vertebrates were small and ecologically unimportant, hiding on the bottom from large predators and unable to defend themselves (except passively, by bony armor). The evolution of jaws changed all that. Jaws, first developed in placoderms, allowed vertebrates to become important predators, occasionally large. Dunkleosteus (also known as Dinichthys, which translates as "terrible fish") was a ferocious predator that sometimes reached sizes of 50 feet (15 meters) long! Look at the illustrations, and imagine a large gaping mouth coming at you to attack! To make the gape ever wider, most placoderms had a hinge at the back of the head, allowing them to raise their head higher so that they could lower their jaw further and open their mouth even wider! (Study the illustration carefully to see this.) One interesting placoderm, Bothriolepis, was a small bottom-dwelling fish. Growing downward from the floor of its pharynx was a pair of sacs that were presumably capable of functioning as lungs! Why would an aquatic fish need lungs? Perhaps Bothriolepis lived in a desert habitat where the ponds would dry out seasonally— lungs would be useful in such a situation to permit the fish to survive and breathe air until the rainy season returned and the pond filled with water again. Modern lungfishes live in similar habitats and use their lungs to survive the dry season. Class Chondrichthyes: Cartilaginous fishes, including sharks, skates, and rays. Bone is reduced to a series of tooth-like denticles embedded in the skin. The rest of the skeleton is made of cartilage only. The sharks are streamlined predators with heterocercal tails and numerous sharp teeth. Related to the sharks are the skates and rays, bottom-dwelling fishes with greatly enlarged front (pectoral) fins. Skates and rays have blunt, crushing teeth that allow them to prey on clams and other hard-shelled invertebrates. Class Osteichthyes: Bony fishes, including the vast majority of fishes. Scales and internal skeleton are both usually bony. A wide variety of sizes, shapes, and habits occurs in this group. One great subgroup has fins with ray-like supports but no internal muscles; a much smaller subgroup has fleshy, lobe-like fins with internal muscles.
Origin of land vertebrates (tetrapods): The first tetrapods (amphibians) evolved from a group of bony fishes called Crossopterygians, who already had lungs and internal nostrils. The critical change transformed the fleshy fins into walking legs. Why would a fish ever evolve legs? Think back to the placoderm Bothriolepis, or to modern lungfishes living in dry, desert habitats. When a steam dries up in such a habitat, it starts by drying up partially, with dry portions strung between the wider and deeper ponds that may still contain water. A fish that is stranded in a dry portion of the stream bed may survive if it can push itself along the dry stream bed, hopefully far enough to reach a pond that still had water. (The water would become stagnant, so lungs would be helpful in rising to the surface to breathe air.) Natural selection would favor those fish whose fins were strong enough to allow them to push the fish along the stream bottom and hopefully find water. Over time, stronger fins evolved into legs capable of movement on land, but their original purpose had been getting the stranded fish back into the water! Fossil Crossopterygians are known that strong fins of this kind, similar to the legs of the earliest Amphibia. The Crossopterygii flourished in the Mesozoic era, and most of them are extinct. In the 1930s, a living crossopterygian, Latimeria, was discovered living in the Indian Ocean. Class Amphibia: Eggs are laid in contact with fresh water, then fertilized externally. Larvae ("tadpoles") breathe with gills, then undergo metamorphosis into an adult, usually with lungs and legs. Living species always have slippery, moist skin. Examples: salamanders, newts, frogs, toads, and extinct labyrinthodonts. In the illustration of the salamander, notice that the body still moves with fish-like undulations from side to side. Notice also that the legs are spaced apart one-half of a wave-length, so that the front feet are positioned with the right foot forward while the rear feet are positioned with the left foot forward. The salamander's next step will keep these two feet in place, while the diagonally opposite left front foot and right hind foot are moved forward, a so-called diagonal gait. Diagonal walking gaits are primitive for all four-footed land vertebrates, and are the preferred slow gaits for most mammals, including the moose shown in the illustration, crossing a road in Maine. |
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