Tetrapod

A tetrapod is a vertebrate animal having four feet , legs or leglikeappendages. The technical term tetrapoda literally means "four-legged" (from the Greek ).

Devonian Tetrapods

The first tetrapods evolved in shallow and swampy freshwater habitats , towards the end of the Devonian period, a little more than360 million years ago. By the Late Devonian, land plants had stabilized freshwater habitats, wetland ecosystems had developed,with increasingly complex food webs that afforded new opportunities. The primitive tetrapods developed from osteolepiform lobe-fin fish , the Sarcopterygii . The "living fossil" coelacanth is a lobe-finned fish which had already developed some adaptations of finswith fleshy bases and bones , which they used as paddles in shallow water habitats choked with plants and detritus . The universal tetrapod characters of front limbs that bend backward at the elbow and hind limbs that bend forward at the knee can plausibly be traced to early tetrapods living in shallow water.

The evolution of the air-breathing lung from the primitive swim bladder of lobe-finned fishes has not yet been worked out in detail. Functioning internal gills were present in at least one Late Devonian tetrapod, Acanthostega .

Nine genera of Devonian tetrapods have been described, several known mainly or entirely from lower jaw material. All of themwere from the Euramerican supercontinent, which comprised Europe, North America and Greenland, except for a single Gondwanan genus, Metaxygnathus , from Australia. The first Devonian tetrapod identified from Asia, was recognized from afossil jawbone and reported in 2002, a finding that substantially extends the geographical range of these animals and raises newquestions about the worldwide distribution and great taxonomic diversity they achieved within a relatively short time. TheChinese tetrapod, named Sinostega pani, was discovered among fossilized tropical plants and lobe-finned fish in the redsandstone sediments of the Ningxia Hui Autonomous Region of northwestChina.

These earliest tetrapods were not terrestrial . The earliest confirmedterrestrial forms are known from the early Carboniferous deposits, some20 million years later. At last they would have used their legs to paw their way through the mud .

Carboniferous Tetrapods

Until the 1990s , there was a 30-million year gap in the fossil record between the late Devonian tetrapods andthe reappearance of tetrapod fossils, in recognizable amphibian lineages, in themid- Carboniferous . It was referred to as " Romer's Gap " for the palaeontologist who recognized it.

During the "gap", tetrapod backbones developed as did limbs with digits and other adaptations for terrestrial life and walkinggaits. Ears, skulls and vertebral columns all underwent changes too. The number of digits on manus ( hand ) and ( pes ) foot became standardized. The very few tetrapod fossils found in the"gap" are all the more precious.

By the Visean era of mid-Carboniferous times, recognizable basal-group Amphibia ( frogs , salamanders and caecilians ) are represented by the labyrinthodonts , which as comprisedof the temnospondyls (e.g. Eryops ), and similarly primitive Amniota (which now include mammals , turtles , crocodiles , birds , lizards and snakes ) are represented by the anthracosaurs . Living members of thetetrapod clan (that is, "crown-group") tetrapods represent the phylogenetic end-points of these two divergent lineages. A third Palaeozoic group, the baphetids left no modern survivors.

Permian Tetrapods

In the Permian period theterm "tetrapoda" becomes less useful, as the separate tetrapod lineages each developed in their own way. Each lineage, however,remained part of the tetrapoda, just as Homo sapiens could be considered a very highly-specialized kind oflobe-finned fish!

Most tetrapods today are land-dwelling, at least in their adult forms, but some species, such as the axolotl , remain aquatic. Whales and ichthyosaurs are tetrapods that have returned to the sea.

Classification of Tetrapods

There are four main categories of living ("crown group") tetrapods:

Amphibia  

frogs and toads , newts and salamanders

Anapsida  

only extant examples are turtles

Synapsida  

many extinct species and all mammals

Diapsida  

dinosaurs , most modern reptiles , birds

Note that snakes are considered tetrapods because they are descended from ancestorswho had a full complement of limbs. Similar considerations apply to aquatic mammals.

External links

• What is a tetrapod?
• UCMP Taxonomy page.
• Tetrapod cladograms. These are like genealogicalfamily trees.

Devonian tetrapods: external links

• Sinostegia discovery reported inNational Geographic.
• Devonian tetrapods.
• Devonian tetrapod trackways.

Carboniferous tetrapods: external links

• Detailed account of early tetrapod evolution. This website, geared to the layman, also describesJennifer Clack's role in untangling the early evolution of tetrapods.
• Abstract ofJennifer A. Clack, An early tetrapod from "Romer's Gap", Nature July 2002.
• Abstract of JenniferClack's identification of a primitive baphetid tetrapod.
• "Theevolution of tetrapods and the closing of Romer's Gap."