linguistic:paraphrasing, linguistic:voice_change
Linnaean and cladistic classifications

At the turn of the 20th century synapsids were thought to be one of the four main subclasses of reptiles. However, this notion was disproved upon closer inspection of skeletal remains, as synapsids are differentiated from reptiles by their distinctive temporal openings. These openings in the skull bones allowed the attachment of larger jaw muscles, hence a more efficient bite.

Synapsids were subsequently considered to be a later reptilian lineage that became mammals by gradually evolving increasingly mammalian features, hence the name "mammal-like reptiles" (also known as pelycosaurs). These became the traditional terms for all Paleozoic (early) synapsids. More recent studies have debunked this notion as well, and reptiles are now classified within Sauropsida (sauropsids), the sister group to synapsids, thus making synapsids their own taxonomic group.

As a result, the paraphyletic terms "mammal-like reptile" and "pelycosaur" are seen as outdated and disfavored in technical literature, and the term stem mammal (or sometimes protomammal or paramammal) is used instead. Phylogenetically, it is now understood that synapsids comprise an independent branch of the tree of life. The monophyly of Synapsida is not in doubt, and the expressions such as "Synapsida contains the mammals" and "synapsids gave rise to the mammals" both express the same phylogenetic hypothesis. This terminology reflects the modern cladistic approach to animal relationships, according to which the only valid groups are those that include all of the descendants of a common ancestor: these are known as monophyletic groups, or clades.

Additionally, Reptilia (reptiles) has been revised into a monophyletic group and is considered entirely distinct from Synapsida, falling within Sauropsida, the sister group of Synapsida within Amniota.