story under development... Lingula is often considered a "living-fossil" based on its
supposed lengthy morphological conservatism owing to its absence of
evolution, and its remarkable survival for more than 550 M.Y. This
conclusion is based on the typical apparently unchanged "linguliform"
shape of the shell.
Lingula or forms very close in appearance have existed possibly since the
The linguliformean subphylum is regarded as the most primitive of the
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
This hints at a primitive condition.