Mammalian evolution began with the evolution of the first synapsids. What is the defining synapomorphy of synapsis?

a. the possession of the amniotic egg

b. a single temporal fenestra

c. the dentary squamosal joint

d. respiration with a diaphragm

e. endothermic metabolism

b. a single temporal fenestra

Explanation:

Synapsids have a temporary fossa in the lower part of the temporal bone. These reptiles mamiferoides only have a single inferior temporal fenestra.

The synapsids were the first amniots to diversify and appeared in the middle of the Carboniferous. These first synapses were characterized by the presence of a single temporary fenestra behind each orbit, through which the mandibular muscles pass.