The very earliest discovered amphibian is Elginerpeton,
which was found in Scotland and dates back 368 million years.The
earliest well known amphibians come from the Late Devonian, some
360 million years ago, and are Acanthostega and Ichthyostega.
- Acanthostega was redescribed in the 1990s by
Jenny Clack and Mike Coates, and represents the most primitive
tetrapod that has hands and feet for which we have a full skeleton.
It had toes (eight per limb), no fin rays, a large load-bearing
pelvis and is thought to have retained gills into adulthood.
- Ichthyostega was a large carnivore, ranging
in size from 0.5 - 1.2 m. The earliest known Ichthyostega comes
from 363 million year old deposits in Greenland. It was largely
aquatic but had massive broad ribs that could have been used
for support of internal organs while on land.
Reconstructions of (a) Acanthostega and (b) Ichthyostega,
from Benton, 1997.
Artistic reconstruction of Ichthyostega, from www.myherp.com
- Order Anthracosauria, also known as coal lizards,
are early stem-amniotes (not true amniotes though, as they still
had a larval from) known from the Carboniferous to the Triassic.
They were fish-eating carnivores that seem to have been terrestrial
animals that became secondarily adapted for life in water. An
example is Proterogyrinus from the Early Carboniferous
of the USA and Scotland. This was 1 m long with an elongated
skull, well developed limbs, a tail for swimming and may have
had an eardrum.
- Order Temnospondyli. The main Carboniferous tetrapod
group with some 170 genera. They were abundant until the end
of the Triassic, even reaching the Cretaceous. There were three
main body shapes, crocodile-like, neotenous (larval characters in the adult, and
salamander-like. Eryops was a
temnospondyl from the Early Permian of North America. It had
a massive skeleton and heavy limbs. Eryops was about 2
m long and was the top carnivore, eating fish and small tetrapods.
Artistic reconstruction of Eryops, from www.myherp.com
Reconstruction of Sauropleura, from Benton, 1997.
- Order Nectridea. A group of aquatic early
amphibians from the Late Carboniferous to the Early Permian.
Many were newt-like in appearance with long flat tails for swimming,
such as Sauropleura from Late Carboniferous Europe and
North America. Interesting members of this order are Diplocaulus
and Diploceraspis from the Late Carboniferous and Early Permian of the USA, which have
expanded skulls with large horns growing out of the sides. The
juvenile forms have no horns at all and they enlarge throughout
life. The function of the horns is still not understood.
- Order Microsauria. A large group (at least 30 genera)
from Carboniferous and Early Permian North America and Europe
of mainly terrestrial amphibians, that are characterized by the reduced
number of bones in the skull and the occiput-atlas complex. They
show some evolution towards a burrowing life style. Two examples
of microsaurs are Tuditanus from Ohio, USA, which had
lizard-like proportions and Microbrachis from the Czech
republic, which is thought to have become secondarily aquatic.
Reconstruction of (a) Tuditanus and (b) Microbrachis,
from Benton, 1997.
Artistic reconstruction of Seymouria, from www.myherp.com
- Order Seymouriamorpha. A small group of
reptilomorphs that are close to the intermediary link between
reptiles and amphibians, from the Early Permian. They were abundant
in the USA and are thought to have had amphibian-like skulls
but more reptile-like bodies with powerful limbs that held the
body high off the ground.
- Order Diadectomorpha. A Late Carboniferous to Early
Permian group that were close to amniotes. Most were herbivorous
leaf strippers, although some were carnivorous. Diadectes,
from the Western USA, Was heavily built with massive limb girdles,
but short limbs.
Reconstruction of Diadectes, from Benton, 1997.
- Lissamphibia. Modern
amphibians are known first from the Triassic, but they might have arisen
earlier. Triassic fossils are rare, but the fossil record for
frogs and urodelans after the Jurassic is quite extensive, but it is not for caecilians,
probably because of their reduced skeleton.
- Early frogs are from the Lower Jurassic of Argentina
and Arizona, but there are some 'proto-frogs' from the Lower
Triassic of Madagascar (Triadobatracus) and Poland (Czatkobatrachus).
- The earliest salamanders are from Middle Jurassic
Europe (Marmorerpeton) and Asia (Karaurus), while
most later fossils are from all over the Northern Hemisphere.
- The earliest fossil caecilian is Eocaecilia
from the Lower Jurassic of Arizona, a form that still has reduced limbs,
although in modern caecilians the limbs are absent.