www.ck12.org C HAPTER Chapter 1. Fish Evolution - Advanced 1 Fish Evolution - Advanced • Understand the major evolutionary stages in fish evolution. What fish evolved first? Not these. But the coelacanth fish, shown here, do follow the oldest known living lineage of Sarcopterygii (lobefinned fish and tetrapods), which means they are more closely related to lungfish, reptiles, and mammals than to the common ray-finned fishes. The coelacanth fish was believed to be extinct but were discovered in 1938 to still be living. Evolution of Fishes The early evolution of fishes is marked by the switch from using their gills as organs for filter feeding to using them as respiratory organs to absorb oxygen. Non-vertebrate chordates, such as lancelets, absorb oxygen from their aquatic environments through the surface of their bodies and use their gill slits for filter feeding. This type of respiration was adequate for these smaller organisms, but, as the vertebrates evolved, their body sizes increased, and they required more efficient oxygen systems. The filaments of fish gills provide a large surface area to allow increased oxygen uptake. The transition from filter feeding gills to oxygen transporting gills occurred with the evolution of the jawless fish. The most primitive species of jawless fish alive today are the hagfish. The fossil record of hagfish is very sparce and traces back to the late Carboniferous period, about 330 million years ago. Fossilized species bear a striking resemblance to modern hagfish, indicating that they have remained relatively unchanged over a long period of evolutionary time. Even though the fossil record only dates back 330 million years ago, most scientists consider hagfish to be one of the earliest fish species to have evolved. The structural features of hagfish are likely very similar to those of the most recent common ancestor of all vertebrates. Until 1999, the earliest known fish fossil was of a jawless species, Arandaspis prionotolepis, which was a close relative of the hagfish. This fossil dates back 480 million years to the Ordovician period. Arandaspis prionotolepis was jawless, finless, and had armored plates covering the surface of its body. In 1999, Chinese paleontologists discovered 530 million year old fossils of two jawless fish species, which they named Myllokunmingia and Haikouichthys. These species appear to have had cartilaginous skulls. 1 www.ck12.org FIGURE 1.1 Fossilized fish. The case of the hagfish fossil record demonstrates the problem with tracing the evolutionary history of all cartilaginous vertebrates - the challenge of determining, with only a limited fossil record, when they arose and how they changed through time. Cartilaginous endoskeletons do not yield good fossils the way that bony skeletons do. Despite the sparse fossil record, scientists have been able to piece together a fairly clear outline of early vertebrate evolution, although there are regular revisions to this outline that are made as new fossils are continuously discovered. There are only a few reliable fossils of lampreys. Three of these fossils date back to the late Carboniferous period (330 million years ago), but a new fossil was discovered in 2006 that dates back to the Devonian period (360 million years ago). Structural features of lampreys, such as a notochord with neural arches, suggest that lampreys represent an evolutionary transition between non-vertebrate and vertebrate chordates. Recall that neural arches are a component of vertebrae. Cartilaginous fish fossils date back 450 million years. This establishes their evolutionary appearance to be prior to that of terrestrial vertebrates, which appeared around 370 million years ago. Earlier cartilaginous species were smaller than many modern species. Fossils of modern shark species date back about 100 million years. One possible factor that selected for the evolution of larger shark species may be the concurrent evolution of large aquatic mammals, such as whales and seals, which could serve as ideal prey for larger sharks. Jawless and cartilaginous, jawed and cartilaginous, and lobe-finned, bony fish collectively make up about less than one-third of all living fish species today. However, this was not always the case. In previous time periods, many of these groups flourished and included a large number of species. For example, during the Devonian period, the lobefinned fish were more diverse in number of species than the ray-finned fish that dominate today. Other examples include the many extinct fish classes within the vertebrate subphylum such as the thelodonts (jawless fish), anaspids (jawless fish that likely gave rise to lampreys), placoderms (early jawed, armored fish from 395-345 million years ago), acanthodians (spiny sharks, which were likely to be the first jawed fish, that shared features of Chondrichthyes and bony fish and date back 410-250 million years ago), and several others. Following extinction events, relative populations shifted, and, by the end of the Paleozoic era (250 million years ago), bony, ray-finned fish dominated vertebrate populations in the aquatic regions of the world. The relative numbers of species within several of these classes that existed between the Cambrian and Cenozoic periods are shown in Figure 1.2. The fossil record of bony fishes is generally rich and informative. From this fossil record, we find that bony, rayfinned fish of the class Actinopterygii likely evolved in freshwater habitats. This conclusion is based on the fact that there are no marine fossils from this class that date back earlier than the Triassic period, roughly 230 million years ago. The earliest freshwater actinopterygians date back to the Lower Devonian period, about 395 million years ago. Lobe-finned fish, the sarcopterygians, are most closely related to actinopterygians. Their fossils can be found from 2 www.ck12.org Chapter 1. Fish Evolution - Advanced FIGURE 1.2 This time line shows the approximate time periods that each of the major vertebrate subdivisions arose and, in some cases, became extinct. The width of each shape indicates the relative number of species contained within that group during the corresponding time in their evolutionary history. Notice how the ray-finned and tetrapod groups flourished and became the dominant species over time. the Lower Devonian period (roughly 410 million years ago) onward. During the Devonian, most species of lobefinned fish appear, and they consist of two main groups: the coelacanths and the Rhipidistia. Much of coelacanth evolution continued in the ocean, where they still exist today in small numbers. Coelacanths were nearly wiped out by the massive Permian-Triassic extinction event roughly 250 million years ago. Rhipidistia evolution gave rise to two additional groups: the lungfish and the tetrapodomorphs. In turn, the tetrapodomorphs gave rise to the tetrapods in the late Devonian period. The lungfish are very similar in morphology to the tetrapodomorphs. Summary • The early evolution of fishes is marked by the switch from using their gills as organs for filter feeding to using them as respiratory organs to absorb oxygen. • Cartilaginous fish fossils date back 450 million years. • Following extinction events, relative populations shifted, and, by the end of the Paleozoic era (250 million years ago), bony, ray-finned fish dominated vertebrate populations in the aquatic regions of the world. Review 1. the What was below: a major Answer questions evolutionary adaptation that allowed fish to grow larger bodies? 2. How does the hagfish fossil record demonstrate problems with the evolutionary record of cartilaginous verte1. What was a major evolutionary adaptation that allowed fish to grow larger bodies? brates? 3. How old are cartilaginous fish? 4. Where did bony fish evolve? How do we know this? 2. How does the hagfish fossil record demonstrate problems with the evolutionary record of cartilaginous vertebrates? Explore More 3. How old are cartilaginous fish? MEDIA 4. Where did bony fish evolve? How do we know this? Click image to the left or use the URL below. URL: https://www.ck12.org/flx/render/embeddedobject/186788 3
