I've been reading quite a bit recently about the stage in vertebrate evolution when early amphibians first colonized land. It's an intriguing time in Earth's history. Land habitats were frontiers to vertebrates. A vast contrast to the aquatic habitats of that time, which for many species were wrought with predators and competitors.

The first thing I realized after digging into this topic, is the ambiguity of the term 'amphibians'. It's a term that is generally accepted to mean the Class Amphibia (which includes frogs and toads, salamanders and newts, and caecilians). That's all well and good for present day purposes, but when you start to muddle about in the past and try to talk about early amphibians, it starts to get a bit foggy.

You see, the traditional usage of the term amphibians also includes a number of species that are now extinct (such as Ichtyostega and Nectridea). And among those extinct amphibians is the ancestor of the amniotes (a group that gave rise to present day reptiles, mammals, and birds). So using the term amphibians in this way to describe these early vertebrates means that reptiles mammals and birds would, technically, be subgroup of the amphibians. But the Class Amphibia (present day toads, frogs, salamanders, newts and caecilians) does not include amniotes (reptiles, birds, and mammals). So it is helpful to identify another term to accurately describe early 'amphibians', something that reflects the fact that the ancestors of present day amphibians branched off from the ancestors of present day amniotes and both lineages evolved separately from that point on.

Let's take a look at the current understanding of evolution of the first land vertebrates (Tudge, 2000):

Sarcopterygii (Lobed-finned fishes)
|
`--Tetrapoda (Four-legged vertebrates)
    |--Batrachomorpha
    |    `--Lissamphibia
    |         |--+--Anura (Frogs and Toads)
    |         |   `--Urodela (Newts and Salamanders)
    |         `--Gymnophiona (Caecilians)
    `--Reptilomorpha
         `--Amniota (Reptiles, Birds, Mammals)

The first vertebrates to venture onto dry land for at least part of their life cycle are believed to have evolved from the lobed-finned fishes (Sarcopterygii). Those early land vertebrates developed four limbs and are therefore referred to as tetrapods (Tetrapoda). Tetrapods then evolved and eventually diverged into two groups, the batrachomorphs (retained amphibian-like characteristics) and the reptilomorphs (assumed reptilian-like characteristics).

The batrachomorphs later gave rise to a group of vertebrates known as the Lissamphibia. It is this group, the Lissamphibians, that includes present day frogs, toads, salamanders, newts, and caecilians. So rather than referring to modern day frogs, toads, salamanders, newts, and caecilians as amphibians, it is more accurate to refer to them as Lissamphibians.

The other group of early tetrapods, the reptilomorphs, eventually gave rise to reptiles. The following diagram illustrates how reptiles and lisamphibians are related. Notice that their common ancestor is a tetrapod and their evolutionary history stretches back through different tetrapod clades, batrahomorpha and reptilomorpha (Tudge, 2000):

Tetrapoda
|
|--Batrachomorpha--Lisamphibia
`--Reptilomorpha--Amniota--Reptilia

Find out more:

• Tudge, C. 2000. The Variety of Life. Oxford: Oxford University Press.
• Halliday T and K Adler. 2002. Reptiles and Amphibians. Toronto: Firefly Books.

Photo © RJ Lerich / Shutterstock.