Evolutionary History of Amphibians and Reptiles
Phylogeny of extant vertebrates
Reptiles are not a natural group because birds fall within them. This is a problem for evolutionary
biologists because it means some reptiles (crocodiles) are more closely related to nonreptiles than
to other reptiles, so we would expect them to show similarities
Monophyly: taxon (evolutionary group) that includes ancestor and all of its descendants. So,
mammals are monophyletic – ancestor was mammal, and all descendants are mammals.
Amphibians are also monophyletic.
Paraphyly – taxon that includes ancestor but not all descendents, like Reptilia (birds are not
Polyphyly – taxon that does not include the ancestor of all taxa – ie, Haemothermia (“warmblooded
animals”) – such as mammals and birds. But ancestor of these is not warmblooded, we’re pretty
sure. Usually relates to a trait that has evolved multiple times independently
Clade – monophyletic taxon. Clades are identified using synapomorphies (shared, derived
characters). But, all members of a group don’t necessarily have to have that trait – it can be
subsequently lost. Digits are a synapomorphy of tetrapods, but some have lost their digits, like
snakes. It’s lost secondarily.
Devonian period – first amphibians appear
The conquest of land
Why crawl out of water onto land?
Get out of drying pools
New things to drink
What are challenges of colonizing land?
Respiration – had already been solved, a number of fish had evolved lungs such as lungfish
Support – variety of structural changes, backbone, limbs, etc. Tiktaalik started to adapt ribs,
backbones, etc. Evolved in shallow water, enhanced on land.
Feeding – most fish use suction, doesn’t work well in air. Freeing of neck enhanced ability to move
from suction feeding to apprehension (grabbing)
Reproduction – fish eggs dry up, need water – likely that early amphibians continued to reproduce
in water like most amphibians today. We have a few juvenile fossils that suggest this.
Water balance – how to keep from drying out – probably just stayed near water, moved in and out
Eusthenopteron – Devonian period – 385 million years ago – paired limbs are true fins, but
structurally similar to tetrapod limbs, with articulated joints and muscles Tiktaalik – 375 million years ago – intermediate between fish and primitive landliving animal.
Hands hadn’t changed too much, but analysis said it could support itself much better on this finned
limb, not yet a foot. Biggest changes in neck – head no longer attached to body – more mobility
and freedom to move. Had also evolved massive ribs and musculature to support body on land.
[but, fossil has no back of the body to talk about. But, three weeks ago, found a more complete
specimen and now know what hind legs were like! Much more robust than they had imagined]
Acanthostega – 365 million years ago – clearly has evolved real limbs with digits, but still aquatic –
forelimb couldn’t move forward and bend at elbow. Suggested that they evolved for walking around
in cluttered, shallow water, not on land. So, digits evolved before coming out of water.
Ichthyostega – got out and walked around. But, has a tail fin – adaptation for swimming, so
presumably lived at interface of land and water.
But, 2 years ago, paper published showing footprint from 390 million years ago! With digits! Much
further back than we thought.
Diversity of early amphibians
Tetrapod – four limbs
Amphibians – double life. Live on land, but also go back to water for reproduction and sometimes
spending considerable amount of time in water
Earliest tetrapods were amphibians
Amphibians diversified in Carboniferous period (300350 million years ago) – great diversity
Frogs appear in Triassic period (225 million years ago)
Salamanders appear from Jurassic (middle of age of dinosaurs)
Caecilians – from early Jurassic – had little legs. We have a transitional fossil, hasn’t lost legs yet.
Amniots – stuck right in the middle of amphibians (egg is amniotic – reptiles, birds, mammals). This
has a problem – means that amphibians are paraphyletic with respect to amniots, creates some
Many amphibians were extinct by Permian age.
Cleidoic Egg – “closed egg” – shell