They are defined as all eryopoids with interpterygoid vacuities (spaces in the interpterygoid bone) that are rounded at the front; and large external nares (Laurin and Steyer 2000).
They are characterized by specific anatomical features, including empty spaces within the interpterygoid bone, rounded foreparts, and large external nostrils, as noted by Laurin and Steyer in their 2000 study.