Fish to Land Animal

Australia: The Land Where Time Began

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Fish to Land Animal

Land Invasion

Panderichthyids-Fish or Finned Amphibian

The Earliest Tetrapods

Place of Origin of the First Tetrapods

Possible Reasons the Evolution of the Amphibians

The osteolepiforms had already evolved most of the requirements for life on land. They already had the strong anatomical structures such as limbs, shoulder and hip girdles, that were pre-adapted for walking on land. Their pectoral and pelvic fins already had the same set of bones that the first amphibians, and all subsequent tetrapods, would retain. They had a swim bladder that was easily converted to use as a lung, as had already occurred in at least 1 or 2 fish types. The cheek and skull bone pattern didn't change during the transition to amphibian.

The first known amphibians would have appeared more like walking fish than amphibian, mostly retaining the appearance of a fish, with a fish tail and gills, and had many fingers and toes, as in the fin rays, and most of the body covered by scales. By the Early Carboniferous the first known proto-reptiles begin to appear in the fossil record.

Land Invasion

The earliest evidence of life on land were plant spores from the Late Ordovician, about 470 Ma. These spores resemble the spores of very primitive living plants like liverworts and mosses. By about 420 Ma the first plant fossils begin to appear in the Late Silurian. These plants had thick stems and spiny leaf-like structures, appearing sedge-like. Baragwanathia, from Australia was among the early plants. Its fossil remains are often found in marine deposits, so it probably grew near the seashore.

The first good evidence of a terrestrial ecosystem inhabited by animals and plants comes from about 400 Ma in the Rhynia Chert of Aberdeenshire, Scotland. Among the animals inhabiting this ecosystem are trigonotarbids, mites, springtails, and some shrimp-like animals. The plants include simple vascular plants such as Rhynia and Algaophyton. The environment of Rhynia at the time is believed to have been around a series of hot springs in a volcanic region. The land would have had a sparse plant cover on poorly developed soils. The animals of this habitat were a variety of small invertebrates a few cm long.

The earliest known evidence of animals life on land comes for a the slightly earlier Tumblagooda Sandstone, near Kalbarri, Western Australia, in the form of an assemblage of trace fossils and rare body fossils. Evidence from this site indicates that in the Late Silurian animals such as large sea-scorpions, eurypterids, up to 1 m long, had been moving around on the shore, and euthycarcinpoids, primitive, multi-segmented arthropods, believed to be a possible ancestor of first insects inhabited these river plains. There were a great diversity of animals known only from their tracks, feeding traces and burrows. There were at least 15 different animals living at the water's edge by the Late Silurian, so their ancestors must have moved to the land well before this time.

There is a large increase in the diversity of land plants and invertebrates by the start of the Late Devonian. Horse-tails (lycopods) and other large plants reached up to 10s of m high, forming the first real forests. There were also simple ferns, psilophytes, as well as lower plants like worts, mosses and sedges. Fungi thrived in these primitive environments. Among the animals were large scorpions, spiders, many different mites, centipede-like invertebrates and millipede, and worms had all made the move by the Middle Devonian. This was the time when fish were developing the ability to breathe air. The multitude of prey in the terrestrial ecosystems around the waterways may be what enticed the first air-breathing fish on to land.

All the known terrestrial invertebrate communities were fully carnivorous, feeding on each other. The first evidence of herbivory comes form the Carboniferous, fossil leaves showing the damage done by herbivores, insects. It has been postulated that prior to the Carboniferous insects had not evolved the ability to digest plant lignin.

By the Middle Devonian the land had been thoroughly colonised by plants and invertebrates, providing a food source for the first fish-amphibians.

As well as the obvious adaptations that needed to have taken place before the move to land, there are a number of other characteristics of fish that needed to be adapted to life out of water. Sensing, eating, excreting, feeding, breeding, and predator evasion are some of the them.

The ability to breathe air, at least to a limited extent, has been achieved in many bony fish. Mud skippers extend the length of time they can remain out of water by pushing air into the swim bladder. Though it is much less efficient than a true lung at exchanging gases, it allows the mudskipper to remain completely out of water for short periods. Another method has been developed by fish living in water that can become stagnant with low oxygen levels, such as gouramis and similar fish. They have a labyrinth of tubes in the head that gulped air goes into and the gas exchange takes place across the membranes lining the tubes. Again, it is no lung, but it gets enough oxygen into the fish to allow it to survive at water oxygen levels that are too low to sustain its life if it depended on gills alone. The lineage leading to amphibians would have had some level of pre-adaptation to the swim bladder that could be worked on by selection pressures to evolve into true lungs.

The first amphibians were not shaped for rapid movement on land, so it is unlikely they could successfully hunt insects on land from the start, it would have been a gradual process to reach the level of mobility on land that would be required to leave the water to hunt. Maybe they first left the water to escape ever-present predators. To hunt on land they would have had to be capable of moving rapidly, at least for short periods, or they could have used a long elastic tongue as frogs do. There is no evidence that the earliest amphibians had long tongues, so they probably hunted in the water until they evolved the body structure necessary for land hunting.

Sensing what is happening around them is an important part of survival, whether for feeding or for avoiding danger. Fish use their eyes and hearing, but depend much more heavily on their lateral line for these functions. This is not possible on land, so vision and hearing became more important. One of the big differences between fish and fossil tetrapods is that in fish the canals of the lateral lines are enclosed and has rows of pores open to the environment. The early tetrapods had wide open grooves in the dermal bones for their sensory lines. As they gradually evolved for life on land their dependence on the lateral line diminished until it was completely replaced by the vision and hearing.

The development of the ear drum in early amphibians was a major step towards permanent life on land. The crossopterygian jaw joint is braced by the hyomandibular bone, in the early tetrapods the same bone was modified to brace the otic membrane, or skin above the inner ear. The stapes bone evolved to transmit the vibrations from the air to the inner ear. The incus and malleus didn't evolve until the arrival of mammals and reptiles. There was virtually no change in the nostrils and eyes of the fish in the transition to amphibians.

The mechanism of excretion is much different in land animals than it was in the water. Once they left the water dehydration became an enormous problem, requiring the early amphibians to stay close enough to water to be able to return to it when necessary. Even when an animal doesn't sweat, it is continuously losing water to the environment. Urine and moist fasces, as well as water lost to the air in the process of breathing, all lower the water content of the body that must be replenished by drinking. The first amphibians to emerge still had the scales of fish, which would reduce water loss to some extent. The reptiles were the first tetrapods to become waterproofed enough to move further from water.

A great innovation was the development of hard-shelled eggs. This allowed the reptiles to move away from the water that the amphibians were never able to do completely.

Place of Origin of the First Tetrapods

The Late Devonian has been divided into 2 sections, the Frasnian, the oldest, and the Famennian, the youngest. Some forms come from the late Famennian of East Greenland. The skeletons of Devonian tetrapods have been found in Russia and the Baltic from the late Famennian - Tulerpeton, Ventustega, late Frasnian of Scat Craig, Scotland, and from middle Famennian of Australia - Metaxygnathus. Footprints of Devonian amphibians have been found in the Famennian red sandstone outcrops on the Genoa River in eastern Victoria and also from similar rocks possibly from the Early Devonian of the Grampian Ranges of western Victoria. The footprints are the oldest known evidence of amphibians anywhere in the world. The 2nd oldest evidence is the Metaxygnathus jaw from Australia.

The distribution of the early Devonian tetrapods doesn't fit with the known distribution of the panderichthyids, which are known only from Euramerica. Based on the panderichthyid distribution it would seem most logical to place the origin of the tetrapods also in Euramerica. But much of the crossopterygian evolution seems to have taken place in Gondwana. Among the unique Gondwanan faunas are the stem group osteolepiforms, canowindrids, earliest rhizodontiforms and primitive known eusthenopterids. This suggests that the evolution of higher crossopterygians probably occurred in Gondwana.

When the evidence for the evolution of higher crossopterygians occurring in Gondwana is considered in conjunction with the evidence for the oldest tetrapods also being in Gondwana, it seems likely that the transition to amphibians took place in Gondwana.

Possible Reasons the Evolution of the Amphibians

Study of the Greenland amphibians from the Late Devonian shows that they are only slightly modified fish. The adaptations needed for life on land were already present, having been evolving to assist them in their role as ambush predators. It is believed they were already air breathing because they had a choana, though they no doubt still lived in water. The digits in their hands and feet are very similar to the fins in their closest fish ancestors.

It is thought many of the lobe-finned fish were capable of short excursions onto land along river banks, one suggestion is that they may have sometimes done this to find safer pools for their eggs.

Another possible reason for the move is changing rates of development. Eusthenopteron is a well-known example of this mechanism. The juvenile stage of this osteolepiform fish has more features in common with early tetrapods than with the adults of the same species. This appears to be a common mechanism of evolution among amphibians, as well as many other higher animals. In heterochrony (neoteny), the lineage evolves by retaining juvenile stages of the ancestor, sexual maturity being reached at a stage where the ancestral species would be immature. So the juvenile osteolepiform skulls look more like those of adult early amphibians.

In the heads of amphibians there are characters of the juvenile stage of the ancestral fish, this is paedomorphosis. In the limb bones the process is different, the front and hind limbs are not the juvenile condition retained by the amphibian, instead, the development of the limbs has accelerated during the transformation from fish. So additional growth stages have been added, the limbs being more elongate. This process is paramorphosis. Major morphological changes can be achieved by combinations of these 2 processes without great genetic changes. The process involved in the evolution by the combined effects of paedomorphosis and paramorphosis is disassociated heterochrony. The situation in the amphibian case is similar to that in humans were we evolved from apes by retaining the large head with a flat face of a juvenile ape, paedomorphosis, while accelerating the development of the hind limbs, paramorphosis of the legs.

Sources & Further reading

Michael J. Benton Ph D, The Reign of the Reptiles, Quarto Publishing, 1990
John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
Development and Evolution of the Muscles of the Pelvic Fin

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