39 Free-living, unattached brachiopod

Brachiopoda, Terebratulida: Rhizothyris spp.

Rhizothyris was a genus of brachiopods with a life habit known as ‘free-living’. These brachiopods are not attached to anything (see: Large brachiopod that easily forms attachments) or are passively free-lying on the sea floor (see: Massive free-lying brachiopod), but have a ‘motile’ pedicle that they can move in and out to reorient themselves.

An example of the brachiopod Rhizothyris in different views, including a reconstruction of how it would have sat on the sea floor. Image credit: JH Robinson with illustration of the motile pedicle, adapted from Richardson (1981).

In brachiopods that attach to the substrate the umbo curves, the end of the pedicle has ‘roots’ that attach to the substrate (see Brachiopods) and the pedicle muscles allow the shell to be rotated.

Having a motile pedicle has involved modification of the umbo and foramen and the muscles controlling the pedicle. The umbo is straight and the foramen points posteriorly. The tip of the pedicle has small branches that can be pushed against exterior objects.

The free-living Rhizothyris lived on sandy and shelly sediments of the wide, shallow marine shelf around New Zealand from the latest Eocene to the Middle Miocene.

—Written by Jeffrey H Robinson