Unranked clade (stem group cnidarians)
Sphenothallus has been compared to some form of tubiculous annelid worm or the sessile polyp stage of a scyphozoan jellyfish that builds tapered, chitinous tubes fixed to the substrate by an attachment disc (Van Iten et al., 2002).
Van Iten et al.
Sphenothallus – from the Greek sphen, “wedge”, and thallos, “branch.”
Species name not determined.
Burgess Shale and vicinity: Many shared similarities suggest that other thecate Burgess Shale fossils such as Byronia annulata, Cambrorhytium major, C. fragilis and Tubulella flagellum, may be related to Sphenothallus sp.
Other deposits: Other species occur worldwide in rocks from the Cambrian to the Silurian periods. Sphenothallus is also known in the Kaili Formation (Zhu et al., 2000).
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Trilobite Beds on Mount Stephen.
Brief history of research:
Two specimens from the Trilobite Beds were illustrated in 2002 (Van Iten et al.). A third previously unrecognized specimen was identified in the Geological Survey of Canada collections in Ottawa (Billings collection) in the Spring of 2010. Owing to the relatively low degree of morphological variations among all known species, it is not currently possible to assign the Burgess Shale form to any particular species without better preserved specimens.
The chitinophosphatic tube (theca) of Sphenothallus consists of longitudinal thickenings which are particularly obvious towards the aperture area. The tube is gently curved and does not seem to branch. The maximum diameter of the largest specimen is about 4 mm for a length of about 75 mm. A thin wall is present between the longitudinal thickenings and terminates in a smooth margin near the aperture, a couple of millimeters beyond the longitudinal thickenings. The tube is roughly circular in the apical region and is very slender, with the two longitudinal thickenings less differentiated in this area. The surface of the entire tube including thickenings is smooth with no evidence of ridges or annulations. All three specimens lack the apical ends, so it is not evident that this species had a holdfast and there is no evidence of soft-tissue preservation.
Only three specimens known from the Trilobite Beds on Mount Stephen.
Carnivorous, suspension feeder
The theca of Sphenothallus was likely attached to the substrate via an apical disc as can be seen in other better known species. The absence of soft tissue preservation makes the assignment to a particular feeding strategy tentative. By comparison with possible related forms such as Cambrorhytium, a carnivorous or suspension feeding habit seems possible.