Frogs are often thought of as ‘lower forms’ of life, not only by the general public but also by past influential scientists. In the eighteenth century, the great Scandanavian biologist, Carolus Linnaeus writing about amphibians stated in his book The System of Nature that, "These foul and loathsome animals are abhorrent because of their cold body, pale colour, cartilaginous skeleton, filthy skin, fierce aspect, calculating eye, offensive smell, harsh voice, squalid habitation, and terrible venom....". This statement from Linnaeus was perhaps more a reflection of the Western World’s repulsion of amphibians as they attributed witchcraft and many common ailments such as warts to the common toad. Over the last 250 years many generations of biologists and people interested in nature have discovered that Linnaeus was sadly mistaken and the amphibians consist of a beautiful array of varied, colourful, harmless and often comical creatures.
Amphibians are terrestrial vertebrates and could be regarded as a link between the fishes and other land vertebrates. They were the first vertebrates to walk on land and probably evolved from the lobe-finned fishes. There are several hypotheses on the origin and evolution of amphibians and it is still a hotly debated issue. The well known Icthyostega was probably one of the earliest amphibian fossils and was discovered in Greenland in 1932 from late Devonian deposits (about 370 million years ago). Amphibians first became common during the Carboniferous Period when they were nearly all aquatic or semi-aquatic. By the mid-Permian there were over 40 families with more than 60% of these being completely terrestrial. This was the peak of the Age of Amphibians, but during the late Permian reptiles became more common and the amphibians started their decline.
The amphibians represent a strange group of animals with almost all of their members having been extinct for the last 150 million years. During the Triassic they became virtually extinct on land with only a few aquatic families left including the first frog - only two groups survived from the Jurassic period (the frogs and the salamanders). The number of amphibian species present today has recently overtaken the number of mammals and they represent key components in terrestrial food chains. There are more than 6200 described species of amphibians (for the latest figures click here) and there is a rapid rate of discovery of new species.
The picture to the right is a species of frog (Arthroleptella ngongoniensis Bishop and Passmore 1993) discovered by Dr Phil Bishop in a relatively well-studied area of Africa, so they could be right under your nose! It has been suggested that there may be several more cryptic species of Leiopelma in New Zealand waiting to be discovered.
As you can see frogs have been around for a long time – they were the first vertebrates to communicate using air-borne sound, they ‘invented’ the lung and the webbed foot. They saw the dinosaurs and mammoths come and go, they survived the great heat waves of the Tertiary Period and the ice ages of the Quarternary Period but the $6 million dollar question is “Can they survive 21st century man?”.
Friends in low places
Herpetology (from the Greek herpeton, meaning "crawling things") is the name given to the study of amphibians and reptiles, and therefore a herpetologist is somebody who studies amphibians and/or reptiles. The two groups that go together to make up the herpetofauna (reptiles and amphibians, sometimes referred to as ‘herps’) have only been lumped together as they live in similar habitats and people who are interested in one group are invariably keen on the other group as well. Because reptiles and amphibians can often be found under rocks and logs, or on the ground, and are important environmental sentinels (as we’ll see later on) they can be considered as our ‘friends in low places’.
Apart from both groups being ectothermic (getting their body heat from the external environment), and a few other similarities such the structure of the heart, reptiles and amphibians differ markedly. Amphibians have a moist, smooth skin that is permeable to water whilst reptiles are covered in a waterproof dry scaly skin. The eggs of amphibians lack a waterproof shell and are always laid in water or in moist soil, however the reptilian egg has a thick, hard or leathery shell that holds moisture in, enabling the young to develop within it even on dry land.
While the reptiles consist of lizards, snakes, turtles, crocodilians and tuatara, amphibians can only be divided into three distinct groups. These three groups (or Orders) of amphibians are known as:
The Caudata (or Urodela) – salamanders, newts and mudpuppies (564 species)
The Gymnophiona – caecilians (173 species)
The Anura – frogs and toads (5505 species)
Only members of the Anura occur in Australia and New Zealand.