Jaws And Fish Evolution

The first jaws_

The basal vertebrates, including conodonts, lacked jaws, and jaws probably evolved during the Ordovician. Study of the anatomy of modern vertebrates suggests that jaws may have evolved from the strengthening bars of cartilage or bone between the gill slits, each of which consists of several elements, all linked by tiny muscles. The transition cannot be followed in fossils because the gill skeleton of jawless fishes was not mineralized. Molecular biologists have even suggested that the origin of jaws was so profound that it must have been associated with a dramatic genome duplication event - but the fossils say no (Box 16.4).

Some of the oldest jaw-bearing fishes were the placoderms, such as Coccosteus (Fig. 16.8a), which had an armor of large bony plates over the head and shoulder region, as in the ostracoderms, and a more lightly armored posterior region. They swam by beating this tail region from side to side. The edges of the jaws did not carry teeth, but instead sharp bony plates that would have been just as effective in snapping at prey.

Placoderms were fearsome predators, some of them, like Dunkleosteus from the Late Devonian of North America, reaching the impressive length of 10 m. This was the largest animal that had lived until then, and its size and fearsome jaws may explain why so many Devonian fishes were armored.

Other Devonian fishes were more modern in appearance. The first shark-like chondrich-thyans, or cartilaginous fishes, came on the scene during the Early Devonian. Acanthodi-ans were small fishes, mostly in the range 50-200 mm in length, and they bore numerous spines at the front of each fin and in

Box 16.4 Genome duplications and vertebrate evolution

Vertebrates have larger genomes than other animal groups. The genome is the entire sequence of genes contained on all the chromosomes within the nuclei of cells. Various worms and insects have around 15,000 genes in their genomes, while the figure is 31,000 for humans, 30,000 for the mouse and 38,000 for the pufferfish. However, vertebrates do not just have more genes than invertebrates, they have two, four or even eight copies of many individual invertebrate genes. At one time, molecular biologists thought that humans had as many as 100,000 genes, but the reduced figure was established in 2004 after the intense gene sequencing efforts of the Human Genome Project. What does genome size mean?

Some have suggested that genome size maps on to the complexity of an organism. Surely, a single-celled bacterium does not need many genes because it does not do much, and vertebrates, as much more complex organisms, would need more genes. Humans ought to have the largest genomes since we are somehow very complex and important. In fact, genome size is only loosely related to bodily complexity: the largest genome reported so far comes from a lungfish! Much of the genome is so-called junk DNA, or at least duplicate genes and non-coding sections, so the functional genome size might be a better correlate of function or bodily complexity.

Whether functional or not, molecular biologists have proposed that there were at least three genome duplication events (GDEs) in the history of vertebrates - times when evolutionary change was dramatic and large sectors of the genome duplicated. GDEs are identified at the origin of vertebrates, the origin of gnathostomes and the origin of teleosts, the hugely diverse modern bony fishes (Furlong & Holland 2004). Could the evolutionary jump have caused the GDE, or perhaps the GDE stimulated rapid and fundamental reorganization of the fishes at these three points?

Donoghue and Purnell (2005) suggest that molecular biologists have been misled. By omitting fossils, they see artificial morphological jumps in their cladograms, and then link this to the postulated GDE. In fact, when fossils are inserted, the "jumps" seem less clear. For the origin of gnathostomes, biologists have compared lampreys with sharks, and there is a wide gulf between these two groups, so suggesting quite a leap in terms of anatomic change and in terms of genome duplication. However, when fossils are inserted (Fig. 16.7), seven major ostracoderm and placoderm clades fall between the living groups, and the evolutionary transition is stretched. Some of the fossil groups (especially pteraspidimorphs, conodonts and placoderms) were diverse, and it is not clear that the GDE drove, or permitted, a single dramatic burst of speciation, as had been proposed. Further, it is not clear that there was a single reorganization of anatomy associated with the origin of jaws and the GDE: the fossils show step-by-step character changes over a long interval.

This is a developing field of study. The claim that genome duplication can drive major bursts of evolution is dramatic, and perhaps overstated. Paleontologists can make profound contributions in new areas of science by working hand-in-hand with molecular and developmental biologists.

Read more through http://www.blackwellpublishing.com/paleobiology/.


Figure 16.7 Phylogeny of the basal fishes. One major genome duplication event was apparently associated with the origin of jaws. When the fossil groups (open lines) are omitted, there is a large morphological and genomic leap from jawless lampreys and hagfishes; when the fossil groups are included, as here, the transition appear much more gradual. The timing of the genome duplication events is uncertain, and falls within the area of the gray box. The number of families within each living and fossil group is shown by the shaded vertical bars. (Courtesy of Phil Donoghue.)

spaced rows on their undersides (Fig. 16.8b). Acanthodians are often found preserved in vast numbers in the rock layers, so they probably swam in huge shoals in open water, perhaps feeding on small arthropods and plankton. They escaped predators by rapid darting from side to side in their shoals, and perhaps their exceptional spininess made them difficult to swallow.

Bony fishes: ray fins and lobefins

The osteichthyans, or bony fishes, also appeared in the Devonian. There are two groups: (i) those with ray-like fins, the actinopteryg-ians, ancestors of most fishes today from carp to salmon, and seahorse to tuna; and (ii) the lobefins, the sarcopterygians, that had thick, muscular, limb-like fins. Today, the lobefins are rare, being represented by only three species of lungfishes and the rare coelacanth. The coelacanth Latimeria is a famous "living fossil". Until 1938, coelacanths were only known as Devonian to Cretaceous fossils, but in 1938 the world was astounded to hear that a living coelacanth had been fished out of deep waters off East Africa, and more have been caught since then.

The ray fins of the Devonian include Chei-rolepis (Fig. 16.8c), which had a flexible body covered with small scales and a plated head. This was an active predator that may have fed on acanthodians. The Devonian lobefins include both lungfishes and "rhipidistians".

trunk shield head shield trunk shield head shield

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anal fin pelvic fin dorsal fin spines pectoral fin

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anal pelvic intermediate pectoral fin fin spine spines spine spine

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Figure 16.8 Jawed fishes of the Devonian: (a) the placoderm Coccosteus; (b) the acanthodian Climatius; (c) the actinopterygian bony fish Cheirolepis; (d) the lungfish Dipterus; and (e) the lobefin Osteolepis. (Based on Moy-Thomas & Miles 1971.)

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anal fin pelvic fin 10 mm pectoral fin

Figure 16.8 Jawed fishes of the Devonian: (a) the placoderm Coccosteus; (b) the acanthodian Climatius; (c) the actinopterygian bony fish Cheirolepis; (d) the lungfish Dipterus; and (e) the lobefin Osteolepis. (Based on Moy-Thomas & Miles 1971.)

The lungfish Dipterus (Fig. 16.8d) was a long, slender fish that hunted invertebrates and fishes, and crushed them with broad grinding tooth plates. The "rhipidistian" Osteolepis (Fig. 16.8e) was also long and slender, and was an active predator. These lobefins had muscular front fins, and could have used these to haul themselves over mud from pond to pond. Specimens of these fishes are known from the Devonian of many parts of the world (Box 16.5).

After the Devonian, the actinopterygians seem to have radiated three times. The first radiation (Devonian-Permian) consisted of the palaeonisciforms (Fig. 16.10a), a para-phyletic group of bony fishes with large bony scales and heavy skull bones. The second radiation of bony fishes, an assemblage termed the "holosteans", occurred in the Late Trias-sic and Jurassic. Semionotus (Fig. 16.10b), a small form that has been found in vast shoals, had more delicate scales than the palaeonisci-forms, and a jaw apparatus that could be partly protruded, hence providing a wider gape.

The third and largest radiation of actinop-terygian fishes, occurred in the Late Jurassic and Cretaceous (Fig. 16.10c), with the diversification of the teleosts. Teleosts are the most diverse and abundant fishes today, including 23,000 living species, such as eel, herring, salmon, carp, cod, anglerfish, flying fish, flatfish, seahorse and tuna. The huge success of this radiation may be the result of their remarkable jaws. Palaeonisciforms opened their jaws like a simple trapdoor, holosteans could enlarge their gape a little, but teleosts can project the whole jaw apparatus like an extendable tube (Fig. 16.10d). This came about because of great loosening of the elements of the skull: as the lower jaw drops, the tooth-bearing bones of the upper jaw (the maxilla and premaxilla) move up and forwards. Rapid projection of a tube-like mouth allows many teleosts to suck in their prey, while others use the system to vacuum up food particles from the seafloor, or to snip precisely at flesh or coral.

The evolution of sharks:

an arms race with their prey?_

During the Carboniferous, numerous extraordinary shark-like fishes arose, and these were clearly important marine predators. A second shark radiation took place in the Triassic and Jurassic. Hybodus (Fig. 16.11a) was a fast-swimming fish, capable of accurate steering using its large pectoral (front) fins. The hybodontiforms had a range of tooth types, from triangular pointed flesh-tearing teeth to broad button-shaped crushers, adapted for dealing with mollusks. It is rare to find whole

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