bird-mammal common ancestor

Ken Brown rkjb at
Wed Sep 7 18:12:28 EST 1994

mwspitze at (matt spitzer) wrote:
> Subject: bird-mammal common ancestor
> What is the current thinking on the common ancestor of birds and mammals?
> Was it a primitive turtle-like reptile?  What would be the closest living
> species?

I don't know about current thinking, but when I studied (there was a "7" in
the decade) the common ancestor of birds & mammals was thought to be way back
in the Permian if not before, possibly even an amphibian rather than a
reptile. Almost certainly not "turtle-like" either - imagine a  slow-moving,
land-dwelling carnivorous beast, unable to lift it's belly off the ground.
Perhaps something like the famous fossil "Seymouria".

The following account is half from memory & probably very out of date:

These amphibian-like reptiles would have evolved into the both the  synapsids
and the cotylosaurs.

Synapsids evolved into therapsids & a number of large Permian animals (such
as the Pelycosaurs - the ones with "sails" on their backs, often wrongly
called dinosaurs) and therapsids became mammals in the Triassic.

The cotylosaurs gave rise to the turtles, lizards, parapsids (marine reptiles
like icthyosaurs & pleiosaurs) and the archosaurs. Archosaurs split into
crocodiles, pterosaurs & the "dinosaur" groups: saurischians &
ornithischians. The ornithischians include things like stegosaurs and
ceratopsians and birds.

As for "closest living species", well we're all as close as each other. I
suppose  the modern species with the nearest niche might be something like a
monitor lizard, but you & me and the sparrow in the street are just as
closely related to each other and to the common ancestor as a lizard is.
That's what we mean by a "common ancestor" :-)

Of the other living groups crocodiles are more closely related to birds than
mammals are.

I also remember an idea that pterosaurs (flying reptiles) were closer to
mammals than to dinosaurs, but I don't know what the thinking behind this is.
I do remember that it wasn't until I got to University that I realised that
mammals are an *older* group than dinosaurs - back in the Triassic the small,
fast-moving dinosaurs replaced the large, lumbering mammals :-)

More information about the Bioforum mailing list