Thectardis avalonensis is a triangular-shaped member of the Ediacaran biota, dating from 574 to 565 million years ago.

[2] The fossils take the form of a triangle with a central depression, suggesting that the original organism was conical.

[2] A total of 205 specimens of Thectardis are known, from two bedding surfaces, separated by 2 km and 10 million years at Mistaken Point, Newfoundland.

[2] Twelve specimens of Thectardis also known from a new location; these fossilised animal remains were discovered on the bedding planes of the Sonia Formation of Marwar Super Group, which is exposed in the Sursagar region of Jodhpur, India.

As there is no evidence for a holdfast anchoring them to the sea floor, it remains a matter of speculation how they were attached.