The linguliformean subphylum is regarded as the most primitive of the brachiopods. They lack any form of shell articulation and rely on a complex internal musculature to keep the valves in place and to move them. The pedicle in these forms appears from between the two valves rather than a dedicated foramen in one of the valves as seen in the more derived brachiopod subphyla.
Also, unlike the other two brachiopod subphyla which have calcium carbonate shells, the ligulids have an organophosphatic shell. Again this hints at a primitive condition.
Well-preserved, 500 million year old specimens found in China have indicated that Lingula anatina has undergone significant soft tissue morphology and anatomy changes. Find out more about the evolution of this brachiopod.