Synapsida (Extinct Mammal Relatives)

Abstract In the phylogenetic system, the Synapsida include Mammalia and all other amniotes more closely related to Mammalia than to Reptilia. The most characteristic feature is the presence of a single large opening behind the orbit on either side of the skull. This opening is small in basal synapsids but increases in size in later, more derived forms including mammals. Nonmammalian synapsids form an extensive series of taxa that are successively more closely related to mammals and thus are now often referred to as stem‐mammals or ancient mammal relatives. They range from the Late Carboniferous to the Early Cretaceous. Crown‐group mammals (which include all present‐day mammals) are first known from the Middle Jurassic. Synapsida and Reptilia are now considered two entirely distinct evolutionary lineages of Amniota with a common ancestor in the Carboniferous. Key Concepts Synapsida is one of the two principal groups of Amniota. Synapsida comprises mammals and all extinct taxa more closely related to mammals than to reptiles including birds. Synapsids form a distinct evolutionary lineage from reptiles. There was a gradual evolutionary acquisition of mammal‐like features during synapsid evolution. The ear bones of mammals developed from the bones at the back of the lower jaw in their antecedents.