Australia: The Land Where Time Began

A biography of the Australian continent 

Aspidella, from the Ediacaran Abut 560 Ma - Evidence for Cnidaria-Like Behaviour

According to the authors¹ debate continues on when the first animals arose based on evidence in the geological record, and how far back before the Cambrian Explosion this occurred. The deepest divergence of the Metazoa that has been estimated by molecular clock is estimated to have occurred by the Ediacaran Period, 635-541 Ma, though evidence of animal activity from well below the Ediacaran-Cambrian boundary has been rare and is often disputed on the grounds of this often being questionable evidence. The Ediacaran macrobiota remain enigmatic, as is emphasised by recent controversial claims that the Ediacaran forms from South Australia are actually terrestrial forms, and instead of being marine animals they may actually be terrestrially-based lichens and microbial colonies. In this paper the authors¹ report evidence of animal-like behaviour in a submerged setting in Aspidella terranovica, Billings 1872, a key form from the Ediacaran, a discoidal fossil from the Fermeuse Formation, Newfoundland, Canada, dated to about 560 Ma. They describe sedimentary fabrics that indicate a vertical movement of an organism through sediment as a response to an aggrading sediment-water interface. As observed in partially buried marine animals, such as tube anemones, at the present, such equilibrium traces are also commonly known from the Phanerozoic. There are also horizontal tracks that are closely comparable to tracks that have previously been described from Mistaken Point, Newfoundland, dating to about 565 Ma, that have now been linked to Aspidella. The findings of this study are believed to constitute evidence of movement, both horizontal and vertical, in a key Ediacaran taxon, that is consistent with an animal of cnidarian grade. Also, the evidence reported from this study conflicts with the proposed radical interpretation of Retallack of the Ediacaran fossil assemblage from the Rawnsley Quartzite of South Australia, as Aspidella is also reported from the Rawnsley Quartzite.

Sources & Further reading

1. Menon, Latha R., Duncan McIlroy, and Martin D. Brasier. "Evidence for Cnidaria-Like Behavior in Ca. 560 Ma Ediacaran Aspidella." Geology 41, no. 8 (August 1, 2013 2013): 895-98.