Chlorophyta (Green algae)
Bryopsidophyceae (Order: Bryopsidales)
Walcott (1919) considered this species to be a green alga, but also noted some similarities with alcyonarian corals (closely related to the sea pens). Studies of specimens from the Burgess Shale and Utah suggest affinities with the modern green alga Caulerpa (Satterthwait, 1976; Conway Morris and Robison, 1988).
Margaretia – unspecified; possibly from the Greek margarites, “pearl,” in reference to the rounded structures present along the tegument.
dorus – unspecified; possibly from the Greek dora, “skin,” in reference to the skin-like tegument of this algae.
Holotype –USNM83922 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: none.
Other deposits: M. chamblessi from the Lower Cambrian Latham Shale of southeastern California (Waggoner and Hagadorn, 2004).
Upper Lower Cambrian, Olenellus Zone to the upper Middle Cambrian, Ptychagnostus punctuosus Zone.
Burgess Shale and vicinity: The Walcott, Raymond and Collins Quarries on Fossil Ridge. The Trilobite Beds, Collins Quarry and smaller sites on Mount Stephen. Mount Odaray and Stanley Glacier.
Other deposits: M. dorus is known from several Cambrian deposits in particular from the Middle Cambrian Spence Shale and Marjum Formation in Utah (Conway Morris and Robison, 1988).
Brief history of research:
Walcott collected about 70 specimens of this new species from the Burgess Shale. It is only after his death that this material was formally published by his assistant Charles Resser in 1931. This species was compared to the modern green alga Caulerpa in an unpublished thesis (Satterthwait, 1976) an interpretation followed by Conway Morris and Robison (1988) based on the study of well preserved material from several Utah deposits. No revisions of this alga using specimens from the Burgess Shale have been published since its original description by Walcott (1931).
This alga is the largest known in the Burgess Shale. It is composed of single or dichotomous (divided into two) tubular axes which are erect and are connected perpendicularly to simple root-like elements called rhizomes. The tubular axes do not vary in width and can reach at least 40 cm in length. The distal ends were rounded. The surfaces are covered with small protuberances of similar size and shape called papillae. These papillae are organized in a spiral pattern. Papillae are visible along the margins of the fossils, but when preserved perpendicularly, they tend to split off at their base giving the impression of holes along the stems. Contrary to Walcott’s initial assessment, Margaretia is not a “thin membranous perforated sheet.” The rhizomes are usually smaller in diameter than the stems and tend to have irregular undulations but no papillae.
Margaretia is present in many sites but is usually rare. No specimens of this alga were found in the Walcott Quarry community out of 52,620 specimen observed (Caron and Jackson, 2008).
Margaretia lived attached to the sea floor via its rhizomes, with its fronds floating above it in the water. The presence of mostly fragmentary specimens in many fossil deposits suggests this alga could have been transported from nearby environments.