Acrothyra belongs within the Family Acrotretidae.
Acrothyra – from the Greek akros, “at the extremity,” and thyra, “door.”
gregaria – from the Latin gregaria, “in clusters,” and thyra, “door.”
Burgess Shale and vicinity: None to date. At least one possible related form is known from the Trilobite Beds – originally described as Acrotreta gemma var. depressa (Walcott, 1889) and later renamed Acrotreta depressa (Matthew, 1902; Walcott, 1908, 1912). The Burgess Shale brachiopods in particular from the Trilobite Beds on Mount Stephen need to be re-examined (see also Brief history of research).
Other deposits: Many species of Acrothyra are known in the Middle Cambrian of North America, Russia and possibly Europe.
The Walcott Quarry on Fossil Ridge.
Acrothyra gregaria has not been studied since its original description by Walcott in 1924. Walcott’s description is cursory, inadequately diagnosing the specimen, and no primary types were designated. The species needs to be redescribed.
Dorsal and ventral valves measuring up to 2 mm in length, with a long medium ridge (or septum) on the dorsal valve. The shell was originally mineralized.
Acrothyra gregaria is known from several hundred specimens in the Walcott Quarry but overall represents a small fraction of the fauna (<0.5%) (Caron and Jackson, 2008).
Acrothyra most likely had a short pedicle by which it was attached to the substrate near the sediment-water interface, or raised on small organic material. As the species name indicates, many specimens would have lived together, filtering small food particles from the water with their filter-feeding apparatus (located between the valves) called a lophophore.
MATTHEW, G. F. 1902. Notes on Cambrian Faunas: Cambrian Brachiopoda and Mollusca of Mt. Stephen, B.C. with the description of a new species of Metoptoma. Transactions of the Royal Society of Canada, 4: 93-112.
WALCOTT, C. 1889. Description of new genera and species of fossils from the Middle Cambrian. United States National Museum, Proceedings for 1888: 441-446.
WALCOTT, C. 1908. Mount Stephen rocks and fossils. Canadian Alpine Journal, 1: 232-248.
WALCOTT, C. D. 1912. Cambrian Brachiopoda. United States Geological Survey, Monograph, 51: part I, 812 p; part II, 363 p.
WALCOTT, C. D. 1924. Cambrian and Ozarkian Brachiopoda. Cambrian Geology and Paleontology IV. Smithsonian Miscellaneous Publications, 67: 477-554.