Synapsids are one of the two major groups ofvertebrates which evolved from basalamniotes.
'Synapsids' means 'fused arch', and refers to their skulls. The group includesmammals and earlier groups related to mammals. They were originally called "mammal-like reptiles", which is a bit confusing.
Synapsids are one of the two major groups ofamniotes. Both groups evolved from early amniotes about 345 million years ago during the early or midCarboniferous period. The other is thesauropsids, the group which includesreptiles andbirds.[2]
Synapsids have a primitive group and an advanced group. These are known, respectively, aspelycosaurs andtherapsids. Pelycosaurs are aparaphyletic group of six primitive families of synapsids.[4]120 They were all rather lizard-like with sprawling gait and possibly hornyscutes.
The therapsids are the more advanced synapsids. They have a more erect posture (way of standing) and maybe hair, at least in some forms.[5] They are the ancestors of mammals. So each of the two synapsid groups representevolutionary grades: the pelycosaurs have given rise to the therapsids, who in their turn have given rise to the mammals.
The therapsids ('beast face') used to be calledmammal-like reptiles. This was a mistake, because they were never reptiles. The split between Synapsids andSauropsids (the group which gave rise to reptiles) took place long before therapsids evolved.[6]
There are a number of characteristics which cannot be seen onfossils, but which are of great importance. Mammals are distinguished from reptiles by fundamental differences in the development of the blood system.[7][8] These differences are such that it would be almost impossible for mammals to be derived from reptiles.
"It is clearly quite impossible for the condition found in birds and modern reptiles to have arisen from that found in mammals or vice versa". Kermack.[8]6
This supports the division of amniotes into the Sauropsida and the Synapsida.
The synapsids have a single hole behind each eye, on the lower part of the skull.
Synapsids evolved a fenestra (hole) behind each eye orbit. It allows better attachment sites forjaw muscles than the originalanapsid condition.
A parallel development took place in thediapsida, who evolved two rather than one opening behind each eye.
Originally, the opening in the skull left the inner cranium only covered by the jaw muscles, but in highertherapsids andmammals thesphenoid bone has expanded to close the opening.
The trend towards differentiation is found in somelabyrinthodonts and earlyanapsid reptilians. They have enlarged first teeth on themaxilla, forming a form of proto-canines. This trait was subsequently lost in thesauropsid line, but developed further in the synapsids. Early synapsids could have 2 or even 3 enlarged 'canines', but in the therapsids, the pattern had settled to one canine in each upper jaw half. The lower canines developed later.
Note: in evolutionary terms, the mammals are entirely within the Synapsida. Their status as a separate class is traditionalLinnaean taxonomy: as aclade they are a sub-group of the Therapsida.
↑Steen, Margaret C. (1934). "The amphibian fauna from the South Joggins. Nova Scotia".Journal of Zoology.104 (3):465–504.doi:10.1111/j.1096-3642.1934.tb01644.x.
