Classification Construct : The Second Lineage Of Terrestrial Vertebrates ( Amniote Tetrapods )
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The Synapsid clade (classification construct), are known as the second lineage of terrestrial vertebrates (amniote tetrapods), that originated at the end of the Paleozoic era (Pough, Janis, and Heiser 448). According to Pough, Janis, and Heiser, “Synapsids include mammals and their extinct pre-decessors, commonly called “mammal-like reptiles”, (can also be referred to as “non-mammalian”)”. The key distinction in distinguishing synapsids from other amniotes is their synapsid skull; the synapsid skull is a skull with the presence of a lower temporal (synapsid) fenestra, plus a few other skull features (Fig. 1) (Pough, Janis, and Heiser 448).
Fig. 1- Diagram of the Synapsid Skull. The diagram depicts a generalized image of the Synapsid skull. The key in differentiating the Synapsid skull from other skulls is that temporal fenestra, which is located behind the orbit. Dilkes DW & RR Reisz (1996), First record of a basal syanapsid ("mammal-like reptile") in Gondwana. Proc. Royal. Soc. Lond. B 263: 1165-1170. Aerosaurus, Elliotsmithia, SYNAPSIDA, Varanops, Varanopseidae. < http://palaeos.com/vertebrates/synapsida/synapsida.html>
When defining Synapsid, this term is often misused to only refer to the only extinct non-mammalian forms, when in actuality this group includes all the amniotes that descended from a common ancestor who had a synapsid type of temporal fenestration (Pough, Janis and Heiser, 448). A term when defining the Synapsid lineage is “mammal-like reptile” this term