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The earliest known synapsid Asaphestera coexisted with the earliest known sauropsid Hylonomus which lived during the Bashkirian age of the Late Carboniferous. [ 26 ] [ 14 ] It was one of many types of primitive synapsids that are now informally grouped together as stem mammals or sometimes as protomammals (previously known as pelycosaurs ).
The most popular definition states that Sauropsida is the sibling taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early synapsids have historically been referred to as "mammal-like reptiles", all synapsids are more closely related to mammals than to any modern reptile.
The synapsid lineage became distinct from the sauropsid lineage in the late Carboniferous period, between 320 and 315 million years ago. [2] The only living synapsids are mammals, [3] while the sauropsids gave rise to the dinosaurs, and today's reptiles and birds along with all the extinct amniotes more closely related to them than to mammals. [2]
Synapsids are a clade that includes mammals and their extinct ancestors. The latter group are often referred to as mammal-like reptiles, but should be termed protomammals, stem mammals, or basal synapsids, because they are not true reptiles by modern cladistic classification.
This single temporal fenestra is homologous to the infratemporal fenestra, as displayed most clearly by early synapsids. [2] In later synapsids, the cynodonts, the orbit fused with the fenestral opening after the latter had started expanding within the therapsids. Most mammals have this merged configuration.
These lists of synapsids collectively include every genus that has ever been included in the clade Synapsida- the mammals and their evolutionary precursors. The lists includes accepted genera along with those now considered invalid, doubtful (nomina dubia), not formally published (nomina nuda), junior synonyms of more established names, as well as genera that are no longer considered synapsids.
Today, the synapsids are often not considered true reptiles, while Euryapsida were found to be an unnatural assemblage of diapsids that had lost one of their skull openings. Genetic studies and the discovery of the Triassic Pappochelys have shown that this is also the case in turtles, which are actually heavily modified diapsids.
In anapsids, the ancestral condition, there are none; in synapsids (mammals and their extinct relatives) there is one; and in diapsids (including birds, crocodilians, squamates, and tuataras), there are two. Turtles have secondarily lost their fenestrae, and were traditionally classified as anapsids because of this.