Long considered the most primitive of sea slugs and previously placed in the Cephalaspidea due to perceived similarities to those headshield slugs (the headshield and robust oval shell), the ringiculids have recently been shown to be a sister clade to the quite dissimilar Nudipleura – the pleurobranchs and nudibranchs. “This highly remarkable and counterintuitive relationship” is based on assessment of molecular evidence and microanatomy of the nervous system and fusion of head to mantle. Today’s ringiculid shells are so similar to those from ancient eras that they have been referred to as “living fossils”.
The ringiculids are infaunal living, burrowing through soft silt in the hunt for prey, thus far identified as foraminiferans and copepods. They are small with a robust glossy shell having a large aperture and a columella with from one to four strong folds.