Sous-embranchement (subphylum) des Rhynchonelliformea
Classe des Rhynchonellata
Ordre des Terebratulida
Sous-Ordre des Terebratellidina
Super-famille des Megathyridoidea


Genus Gyrothyris Thomson, 1918

[Type species= Gyrothyris mawsoni Thomson, 1918 (p. 28)]

Medium, transversely ovate to elongate oval; finely multicostate, anterior commissure weakly unisulcate; beak suberect to erect, foramen small, mesothyrid, attrite; deltidial plates usually conconjunct, concave, almost hidden. Cardinalia thickened; cardinal process transversely oval myophore, median septum uniting posteriorly with narrow septalium; adult loop typically trabecular but occasionally teloform.


Diagnosis from volume 5 of the
Treatise on Invertebrate Paleontology (2006)

Extant Species of Gyrothyris
  • G. mawsoni Thomson, 1918
  • Diagnosis

    Gyrothyris mawsoni   Thomson, 1918

    Type locality: “one mile off south end of Macquarie Island.”
    Depth range: 79 - 563 m

    Gyrothyris mawsoni antipodesensis Foster, 1974
    Gyrothyris mawsoni aucklandensis Foster, 1974

    Note: Tomson (1918) wrote: “Gyrothyris resembles Pachymagas more than Terebratella, but, differs from the former, in the absence of beak ridges, the shallowness of the hinge trough, and the transverse nature of the cardinal process. Further, Gyrothyris mawsoni possesses a radial ornament which has never been observed on any species of Pachymagas.”

    According to Bowen (1968), “the Macquarie Island specimens of G. mawsoni resemble Magasella sanguinea but lack its characteristic reddish colour. On the inside, the shell structures are more thickened than in M. sanguinea.”

    Diagnosis in preparation -


    Should be added:
  • Muscle disposition: see Fig. in front - dorsal pedicle adjustor muscle bases not located at the anterior edge of the hinge plate


    Muscle attachment (modified, from Foster, 1974 - with absence of separate posterior accessory diductor muscles base, the pair of ventral adductor muscle bases in a single unit)