In one of the earliest phylogenetic analyses of the group , Gardiner ( 1983 ) recognized five characteristics that made Temnospondyli a clade : a bone at the back of the skull , the parasphenoid , is connected to another bone on the underside of the skull , the pterygoid ; large openings called interpterygoid vacuities are present between the pterygoids ; the stapes ( a bone involved in hearing ) is connected to the parasphenoid and projects upward ; the cleithrum , a bone in the pectoral girdle , is thin ; and part of the vertebra called the interdorsal attaches to the neural arch . Additional features were given by Godfrey et al . ( 1987 ) , including the contact between the postparietal and exoccipital at the back of the skull , small projections ( uncinate processes ) on the ribs , and a pelvic girdle with each side having a single iliac blade . These shared characteristics are called synapomorphies .