within the tetrapods, if mammals are excluded, how many different sister groups can be defined?

Within tetrapods, if you exclude mammals, you can define two main sister groups: amphibians and sauropsids (the clade that includes reptiles plus birds).

What this means in practice

• Tetrapods = four-limbed vertebrates.
• The major living tetrapod lineages are:
  • Amphibians
  • Mammals
  • Sauropsids (reptiles including birds).

If you remove mammals from the picture, you are left with:

1. Amphibians
2. Sauropsids (turtles, lizards and snakes, crocodilians, birds).

These two clades share a more recent common ancestor with each other than with any excluded group (mammals), so they form two sister groups within tetrapods once mammals are left out.

So the answer to:

“Within the tetrapods, if mammals are excluded, how many different sister groups can be defined?”

is: Two sister groups – amphibians and sauropsids.

Information gathered from public forums or data available on the internet and portrayed here.