Chapter Eleven

The Evolution of Fish

11.1   The evolution of fish began about 530 million years ago during the Cambrian explosion. Early fish from the fossil record are represented by a group of small, jawless, armoured fish known as ostracoderms. Jawless fish lineages are by now mostly extinct. An extant clade, the lampreys seen in the picture above may approximate ancient pre-jawed fish.

11.2   Ray-finned fishes, class Actinopterygii, differ from lobe-finned fishes in that their fins consist of webs of skin supported by spines ("rays") made of bone or horn. There are other differences in respiratory and circulatory structures. Ray-finned fishes normally have skeletons made from true bone.

11.3   The first ray-finned and lobe-finned bony fish appeared in the Devonian period beginning around 420 million years ago.  Meanwhile, another class, the Sarcopterygii, including lobe-finned fishes, coelacanths, lungfish and tetrapods, was the most diverse group of bony fishes in the Devonian.

11.4   During the Late Devonian around 360 million years ago the first forests were taking shape on land. The first tetrapods appear in the fossil record over a period, the beginning and end of which are marked with extinction events. The ancestors of all tetrapods began adapting to walking on land, their strong pectoral and pelvic fins gradually evolved into legs.

11.5   The group of lobe-finned fishes that were the ancestors of the tetrapods are grouped together as the Rhipidistia, and the first tetrapods evolved from these fish over a relatively short timespan between 385–360 million years ago. The early tetrapod groups themselves are grouped as Labyrinthodontia. They retained aquatic, fry-like tadpoles, a system still seen in modern amphibians.  Our understanding of how land animals slowly evolved during the forty million or so years until the end of the Devonian, 359 million years ago, is itself evolving rapidly, almost year by year.

11.6   From the 1950s to the early 1980s it was thought that tetrapods evolved from fish that had already acquired the ability to crawl on land, possibly so they could go from a pool that was drying out to one that was deeper. However, in 1987, nearly complete fossils of Acanthostega from about 363 million years ago showed that this Late Devonian transitional animal had legs and both lungs and gills, but could never have survived on land: its limbs and its wrist and ankle joints were too weak to bear its weight; its ribs were too short to prevent its lungs from being squeezed flat by its weight; its fish-like tail fin would have been damaged by dragging on the ground.

11.7   The current hypothesis is that Acanthostega, which was about 1 metre long, was a wholly aquatic predator that hunted in shallow water. Its skeleton differed from that of most fish, in ways that enabled it to raise its head to breathe air while its body remained submerged, including: its jaws show modifications that would have enabled it to gulp air; the bones at the back of its skull are locked together, providing strong attachment points for muscles that raised its head; the head is not joined to the shoulder girdle and it has a distinct neck.

11.8   The Devonian proliferation of land plants may help to explain why air-breathing would have been an advantage: leaves falling into streams and rivers would have encouraged the growth of aquatic vegetation.  This would have attracted grazing invertebrates and small fish that preyed on them.  They would have been attractive prey but the environment was unsuitable for the big marine predatory fish.  Air-breathing would have been necessary because these waters would have been short of oxygen, since warm water holds less dissolved oxygen than cooler marine water and since the decomposition of vegetation would have used some of the oxygen.

11.9   There are three major hypotheses as to how tetrapods evolved their stubby fins to become proto-limbs. The traditional explanation is the "shrinking waterhole hypothesis" or "desert hypothesis" posited by the American paleontologist Alfred Romer. He believed limbs and lungs may have evolved from the necessity of having to find new bodies of water as old waterholes dried up.

11.10  The second hypothesis is the "inter-tidal hypothesis" put forward in 2010 by a team of Polish paleontologists led by Grzegorz Niedźwiedzki. They argued that sarcopterygians may have first emerged unto land from intertidal zones rather than inland bodies of water. Their hypothesis is based on the discovery of the 395 million-year-old Zachełmie tracks in Zachełmie, Poland, the oldest ever discovered fossil evidence of tetrapods.

11.11   The third hypothesis, the "woodland hypothesis", was proposed by the American paleontologist Gregory J. Retallack in 2011. He argues that limbs may have developed in shallow bodies of water in woodlands as a means of navigating in environments filled with roots and vegetation. He based his conclusions on the evidence that transitional tetrapod fossils are consistently found in habitats that were formerly humid and wooded floodplains.

11.12   By the late Devonian, land plants had stabilized freshwater habitats, allowing the first wetland ecosystems to develop, with increasingly complex food webs that afforded new opportunities. Freshwater habitats were not the only places to find water filled with organic matter and choked with plants with dense vegetation near the water's edge. Swampy habitats like shallow wetlands, coastal lagoons and large brackish river deltas also existed at this time, and there is much to suggest that this is the kind of environment in which the tetrapods evolved. Early fossil tetrapods have been found in marine sediments, and because fossils of primitive tetrapods in general are found scattered all around the world, they must have spread by following the coastal lines — they could not have lived in freshwater only.

11.13   Ray-finned fishes are by now the dominant vertebrate group, containing half of all known vertebrate species. They inhabit abyssal depths in the sea, coastal inlets and freshwater rivers and lakes, and are a major source of food for humans.

11.14   Although the evolutionary origin of ray finned fish dates back to the Devonian period, the major species of fish in Lake Annecy evolved much later.  See above helpful evolutionary chart  by Micheal Bention.  The Salmonidae first appear in the fossil record in the middle Eocene around 45 million years ago,  with the fossil Eosalmo driftwoodensis, which was first described from fossils found at Driftwood Creek, central British Columbia. This genus shares traits found in the Salmoninae, whitefish, and grayling lineages.

11.15   The three main species of fish in Lake Annecy are all subfamilies of the Salmonid family: whitefish (Coregoninae) known locally as the Fera, Artic Char known as the Omble chevalier, and Trout.

11.16   The fish themselves of course appeared only after the Lake was formed around 18,000 years ago, but we know from evidence remarkably well preserved in the mud at the bottom of the lake, that home sapiens had developed a fishing community there as long as 6000 years ago.  In fact one of the settlements whose remains have been particularly well preserved is on the shoreline at Saint Jorioz and is now included as a world heritage site.

11.17   These fish eat up zooplankton and so control its population, but also provide food for fish eating birds such as grebes.

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