During Devonian (named after Devon) the armour-plated fish placoderms (‘plated-skins’) became dominant. Instead of teeth they had self-sharpening bony plates in their jaws that performed the function of teeth. The two important groups of placoderms, the arthrodires (‘jointed neck’) and the antiarchs (‘opposite anus’), included the most powerful predators of the time. The Late Devonian arthrodire Dunkleosteus (named after David Dunkle) grew to six metres (20 ft) long. The chondrichthyans, e.g. sharks, thrived through the Devonian.
Also in the Late Devonian the first osteichthyans (‘bony fish’) appeared: sarcopterygians (‘fleshy lobe-finned fish’) and actinopterygians (ray-finned fish). Most primitive bony fishes seem to have possessed a pair of air-filled pouches that served as crude lungs, or buoyancy tanks, or both.
One group of lobe-finned fish was the dipnoans (‘double breathers’) or lungfish. Some species have survived and are seen to use their lungs to complement their gills when the ponds in which they live become stagnant and breathing becomes difficult. The African lungfish is able to burrow into the mud at the bottom of ponds that are drying out and produce a cocoon in which to breathe air until the waters return. Lungfish burrows have been found in the Devonian.
Another Devonian lobe-fin group is the coelacanths (‘hollow spine’). A large marine fish a metre (3 ft) or more in length was thought to have become extinct about 70 mya but a modern coelacanth was fished up in 1938. It was named Latimeria, after its discoverer Marjorie Courtenay-Latimer.
During the Devonian the lobefins diverged in shape, structure and ecology. Rhipidistians and the other Devonian lobe-fins were most successful in coastal and inland waters. When the rayfins with their lightness and manoeuvrability became the most successful group it was perhaps the competition that caused the rhipidistians to evolve the adaptations that enabled them to expand towards the land. It is thought that the rhipidistians include all the ancestors of extant lungfishes and tetrapods (‘four-footed’). The tetrapods include any vertebrate that has evolved from four-legged ancestors: amphibians, birds, mammals and reptiles. In some cases one pair of limbs has been lost or modified, for example into wings or flippers.
In moving to life on land, the first tetrapods had to overcome the problem of supporting their own weight. In water the weight of a fish is largely irrelevant; water provides buoyancy and flotation devices can provide compensation when required. On land the body has to be held up from the ground and the internal organs supported within a ribcage to prevent them from collapsing. In addition, reproductive, osmotic (water balance) and sensory systems had to adapt over time.
Although they have retained many water-living features the amphibians represent a halfway stage towards landlife. Like modern amphibians, the earliest tetrapods must have laid their eggs in water where they hatched into the tadpoles that later became the land-living form. The most completely known of the early amphibians is Ichthyostega (‘fish roof’), which lived during the Late Devonian in what is now Greenland. It was a one-metre-long animal with four limbs and a clear separation between the head and the rest of its body. It had a strong backbone and ribcage that supported its weight on land but it still had a fishlike skull and a tailfin.
At Rhynie in Scotland a Middle Devonian peat bog was occasionally flooded by silica-rich water, probably from nearby hot springs. As the silica precipitated to form chert, the plants in the peat were fossilised in great detail, along with a set of small arthropods that lived on and around them. The plants are frequently found as three-dimensional structures, rather than as squashed films. The arthropods include scorpions, pseudo-scorpions and mites.
Rhynie chert has enabled the reconstruction of the dominant Devonian plants. Aglaophyton grew to a height of about 20 cm (8 inches). It had most of the adaptations needed in land plants but it did not have xylem (‘wood’), only a simple conducting strand. Rhynia is a genuine vascular plant with true xylem, which allows it to grow taller than Aglaophyton. Rhynia has given its name to the rhyniophytes, the group that includes all the earliest vascular plants. There followed a rapid and complete successive replacement of these floras: trimerophytes dominated until the Middle Devonian; progymnosperms dominated the Late Devonian; and pteridosperms dominated the Early Carboniferous.
The trimerophyte Psilophyton had more of its stem devoted to xylem than did Rhynia and was more strongly constructed. Psilophyton was thus able to grow up to two metres high. Towards the end of the Devonian the progymnosperms produced a secondary xylem system, which gave greater strength and allowed higher growth and produced the first trees. They also probably evolved the first seeds. All previous plants needed a film of water in which sperm could swim to fertilise the ovum. Seed plants can produce away from water. Living alongside these branched plants were others known as lycopods (Lycopodiophyta) because they bear a strong resemblance to living clubmosses (Lycopodiopsida). Lycopods would probably have formed dense bushy ground cover from the Devonian onwards. In the Late Devonian the lycopods underwent a dramatic radiation, producing giant forms such as Lepidodendron and Sigillaria, the first genuinely tree-sized plants. The Devonian ended with another major extinction.
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