From Tetrapod to Amphibian

The Basics

In my previous article I discussed the evolution of tetrapods – 4 limbed creatures. This transition from aquatic to terrestrial environments let to the evolution of amphibians. Amphibian means ‘double life’ meaning that they can be either semi/aquatic or completely aquatic. They emerged ~300-310 million years ago. There are now ~6500 amphibian species which are split into 3 subgroups (most of which are tetrapods):

Anuraus – Frogs and Toads

Candates – Salamanders and Newts

Gymnophions – Caecilians

 FrogAmphibianslARVAL DEVE

Fig 1: An illustrations of the 3 subgroups in amphibians. Frog(left), Caecilians (middle), and salamanders (bottom).

All amphibians apart from Caecilians are tetrapods as they have no limbs. Amphibians start off their life in an aquatic environment. In early stage larval development they contain gills to facilitate breathing. During development the larvae undergo a metamorphosis event in which they obtain lungs, a 3 chambered cardiovascular system (2 atria, 1 ventricle) and limbs.

Some will remain in this aquatic environment, but others will venture out onto land for the remainder of their lives. A key characteristic of amphibians is that at least some of their life is spent in water. Although they may appear to look like reptiles there is one significant difference amphibians have moist scale-free skin but reptiles have scaley skin. It’s hard for us to imagine but around 300 million years ago, amphibians would have been the predominant land species until larger reptilian species took over, and the amphibians retreated back to swamplands.

Characteristics of Amphibians

Many decades ago it was thought that early tetrapods were amphibians, it was later found that they did not contain all the characteristics of amphibians therefore segregating the two species. Amphibian body plans are extremely variable, this is illustrated adequately in fig 1. They have variable numbers of vertebrae, ribs can wither be absent of present and they have no exoskeleton (e.g a protective shell). As aforementioned, most are tetrapods as they contain 4 limbs but others such as Caecilians are not tetrapods. Frogs and salamanders contain 4 limbs, with webbed feet.

 FrogFeet

Fig 2: Frog webbed feet.

Primitive amphibians contained scales in their early development, throughout development they evolved into large protective plates on the stomach of species. These plates overlapped to ensure adequate flexibility.

SnakeScaleFrogMoist

Fig 3: An illustration of the difference between reptilian scale-y skin (left) and the frogs moist skin (right).

They contain teeth in upper jaws and nostrils open to the inside of the mouth. As mentioned in the previous article they keep all of these systems moist using a choana. Respiration usually occurs through either lungs or through their moist skin. Some will have gills throughout their entire life. Salamanders respire by expansion and contraction of ribs and costal muscles, frogs however implement buccal pumping whereby they use their mouth and throat to force air into their lungs. Their kidneys excrete urea. These species are cold-blooded, regulating temperature by exchanging heat with it’s surrounding environment.

Ancient Amphibians and Modern Amphibians

It is not yet known the ancestor which linked ancient amphibians to modern amphibians but it is known that they are very different. Currently there is thought to be 2 species of ancient amphibian, the lepospondyls which were weird looking, aquatic/semi aquatic species with slimey skin and characteristics more similar to modern day amphibians. An example is Diplocaulus which is a 3ft long amphibian. The second type of prehistoric amphibian is Temnospondyls which are more reptile like and resemble crocodiles. They had long bodies with flat skulls and short legs with 4 digits on fore-limbs and 5 digits on hind-limbs. An example is Mastodonsaurus and amphibamus. Amphibamus is the species in which gave this family it’s name.

 Amphibamus

Fig 4: An reconstruction of amphibamus from fossils.

1. Caecilians

As previously mentioned there are 3 groups of modern amphibian: frogs/toads, salamanders and newts and caecilians. Caelcilians have an appearance similar to worms and snakes, however on closer inspection they lack scales (ruling them out as a retile) and contain a mouth and eyes ( ruling them out as a worm). They are mainly found burrowing underground or in more aquatic environments. There are currently 160 extant (still alive today) species and they feed off insects and worms.

2. Salamanders and Newts

There are 470 species of salamanders and newts extant today. Unlike other amphibians which loose their tail during metamorphosis, these species retain their tail throughout adulthood. They live in cool, shadey environments and conduct much of their activities at night. Unlike frogs and toads, salamanders and newts are silent creatures which feed on insects and worms. Many species have poison glands which protect them against predation.

3. Frogs and Toads

The earliest frogs were found ~200-250 million years ago during the triassic period.Most are surprised to hear that all toads are in fact frogs. Frogs have eyes, strong webbed feet, smooth skin, and they lay eggs in clusters. Toads have stubby bodies which short hind legs, dry skin, poison glands, and lay eggs in long chains. A strange fact about frogs is that they shed their skin periodically, after which… they eat it! The “ribbit” sound that is associated with frogs is as a result of air being pushed over their vocal sac.

Amphibians are now under extreme threat of extinction due to deforestation, disease, pollution and global warming. If they go extinct they will be the first vertebrate family to undergo complete extinction. Stay tuned for my next article all about how amniotes (reptiles, birds and mammals) diverged from amphibians.

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