Acrotretida

Acrotretides (Acrotretida) are an extinct order of linguliform brachiopods in the class Lingulata.

In the upper Cambrian, linguliforms reached the apex of their diversity: acrotretides and their relatives the lingulides together comprised nearly 70% of brachiopod genera at this time.

[1][2] Though acrotretides continued to diversify during the Ordovician, their proportional dominance declined, as rhynchonelliforms took on a larger role in brachiopod faunas.

These replaced the stout, column-like central muscles, which are present in other linguliforms but apparently absent in acrotretides.

The adult shell is smooth, with a rounded outline and an organo-phosphatic composition, like other linguliforms.