Edrioasteroidea (Order: Edrioasteroida, stem group echinoderms)
Walcottidiscus is a poorly known edrioasteroid, an extinct group of echinoderms (Smith, 1985).
Walcottidiscus – from Charles Walcott, the discoverer of the Burgess Shale, and the Greek diskos, “disc.” The name refers to the flattened appearance of the fossils.
typicalis – from the Greek typikos, “type,” perhaps in reference to the single specimen originally described.
Holotype –USNM90754 (W. typicalis),USNM90755 (W. magister) in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: W. magister Bassler, 1935 from the Walcott Quarry on Fossil Ridge (but see below paragraph brief history of research).
Other deposits: none.
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott Quarry on Fossil Ridge.
Brief history of research:
Two species known from a single specimen each were originally described by Bassler in 1935 and 1936: a small form W. typicalis, and a larger form W. magister respectively. However, W. magister is now thought to belong to W. typicalis (Smith, 1985) but additional fossil material would be required to confirm this hypothesis. Walcottidiscus resembles Kailidiscus chinensis, a chinese form from the Middle Cambrian Kaili deposit, but remains too poorly known to draw more detailed comparisons between the two genera (Zhao et al., 2010).
The body (theca) is ovoid in outline and has a relatively small dorsal surface compared to the ventral one. The upper central part of the theca is not calcified, but the outer zone is composed of small calcified plates. A five star-shaped food groove lined with small plates (the ambulacra) is present on the upper surface. The five arms of the ambulacra are arranged in a 2:1:2 fashion around the mouth, and they are at first straight and then turn to the left near the edge of the theca. Differences between the two species are the size and degree of ambulacral curvature, but those differences could simply be a factor of growth.
Walcottidiscus is very rare only two specimens were originally described. A few additional specimens are known in the Burgess Shale collections of the Geological Survey of Canada and the Royal Ontario Museum.
Walcottidiscus was most likely resting on the seafloor. Food particles were transported by food grooves (ambulacrum) into a central mouth at the top of the theca.