Within tetrapods, excluding mammals, the main sister groups that can be defined are:
- Amphibians (class Amphibia)
- Sauropsids, which include reptiles and birds (class Reptilia and Aves)
These two groups represent the major non-mammalian branches of tetrapods. Tetrapods broadly split into two major clades: synapsids (which include mammals and their extinct relatives) and sauropsids (which include reptiles and birds). If mammals (synapsids) are excluded, the remaining tetrapods primarily form two sister groups: amphibians and sauropsids
. Thus, excluding mammals, there are essentially two major sister groups within tetrapods: amphibians and sauropsids.
