38 Massive free-lying brachiopod

Brachiopoda, Terebratulida: Erihadrosia epsilon

One of the variety of life habits that brachiopods have evolved is known as ‘free-lying’. While juveniles of free-lying brachiopods are probably always attached to a hard substrate (ie a shell or pebble) by their pedicle, as these species get larger they allow their pedicle to wither and they lie on the sea floor on soft sediments. The foramen is tiny in adults.

An example of the brachiopod Erihadrosia epsilon in different views, including a reconstruction of how it would have sat on the sea floor (specimen OU 6957). Image credit: JH Robinson with illustration adapted from Richardson (1981).

Brachiopods must open their shells (at the ‘anterior’ end) in order to feed by filtering tiny food particles out of the sea water.  Free-lying brachiopods thicken the posterior end of the shell so it is much heavier and sinks a little into the soft sediment, raising up the anterior end.

Many different genera and species of brachiopod have adopted this free-lying habit over their 500 million year history. In New Zealand the most impressive species is Erihadrosia epsilon (originally called Stethothyris epsilon); single specimens can be up to 10 cm long.

Erihadrosia lived in shallow shelf environments around New Zealand during the Middle Miocene.

—Written by Jeffrey H Robinson