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DrDirtPhD t1_j551hg3 wrote

We're all just fish anyway.

Seriously though, synapsids and sauropsids share a common ancestor but are two distinct monophyletic sister lineages. Synapsids gave rise to mammals and sauropsids to reptiles (including birds which are just highly derived reptiles).

Your question just seems to be one of nomenclature rather than taxonomy/systematics though. Reptiles are reptiles from cultural carryover, even though classically the definition is paraphyletic (by excluding birds it doesn't include all descendant species of a common ancestor); in modern systematics it includes birds and makes a single clade. Mammals are a distinct monophyletic grouping and so remain a valid clade.

Changing amniote to reptile and synapsids to "mammal like reptiles" and sauropsids to "lizard like reptiles" doesn't add any clarity to things, and because lizards/birds/snakes/turtles are all fairly distinct groupings on their own it actually muddies the definition some.

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Ameisen t1_j585zeq wrote

The diapsid infratemporal fenestra is homologous to the synapsid single temporal fenestra, so this asserts that the amniote line first developed a single fenestra (the [infra]temporal fenestra) and then later one lineage gained another (the supratemporal fenestra), with this lineage becoming the diapsids while the other being synapsids.

With this in mind, why are the early amniotes who first developed a fenestra not considered synapsids? Is it to maintain synapsida and diapsida as monophyletic that we only consider them synapsids a while after their defining trait developed?

Or, rather, why are diapsida not considered a clade of synapsida given that the common ancestor of both lines already possessed the infratemporal fenestra? It would seem sensible to me to put them both in a clade specifying a single fenestra ("monapsid"?) with the synapsids just being those more closely related to [insert synapsid here] than to diapsids.

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aphilsphan t1_j57n7o0 wrote

Are we sure Synapsids and Sauropsids are distinct clades? So their common ancestor is neither?

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