Thectardis avalonensis is a triangular member of the Ediacara biota

Ediacara biota

The Ediacara biota consisted of enigmatic tubular and frond-shaped, mostly sessile organisms which lived during the Ediacaran Period . Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.Simple multicellular organisms such as...

, dating from . The organism took the form of an elongated cone

Cone

A cone is a basic geometrical shape; see cone .Cone may also refer to:-Mathematics:*Cone , a family of morphisms resembling a geometric cone...

 with a central depression: its apex sticking into the substrate.

Morphology

The fossils take the form of a triangle

Triangle

A triangle is one of the basic shapes of geometry: a polygon with three corners or vertices and three sides or edges which are line segments. A triangle with vertices A, B, and C is denoted ....

 with a central depression, suggesting that the original organism was conical.
The diameter to height ratio of the organism

Organism

In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

 is roughly constant in each location at 1 to 3 in the younger beds, and from 1 to 2.5 in the older beds. The constant ratio suggests that it grew by adding to its body at the base of the cone. The triangle has a raised margin about a quarter of the width of the triangle. The interior either is blank, depressed, or has some vague transverse markings. The impression occurs in the upper bed rather than the lower surface.

Occurrence

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

Thectardis bearing bedding surfaces also contain Charnia

Charnia

Charnia is the genus name given to a frond-like Ediacaran lifeform with segmented ridges branching alternately to the right and left from a zig-zag medial suture. The genus Charnia was named after Charnwood Forest in Leicestershire, England, where the first fossilised specimen was found.- Diversity...

and Ivesheadia.

Ecology

Water currents knocked down the triangles in the same direction, and where they fall on top of other objects they flex over the top.
When alive, the organism probably stuck to the microbial mats that bound the Ediacaran

Ediacaran

The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...

 sea floor, standing on their tips like a pin in a pin cushion, so that the organism would have resembled an inverted cone. They probably fed on suspended particles. As there is no evidence for a holdfast

Holdfast

A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic cnidarians, and sponges, to the substrate. ...

 anchoring them to the sea floor, it remains a matter of speculation how they were attached.

Etymology

The generic name Thectardis is derived from the Greek thektos, sharp-pointed, and ardis, arrow-point. The specific name derives from the Avalon Peninsula

Avalon Peninsula

The Avalon Peninsula is a large peninsula that makes up the southeast portion of the island of Newfoundland.The peninsula is home to 257,223 people, which is approximately 51% of Newfoundland's population in 2009, and is the location of the provincial capital, St. John's. It is connected to the...

, where it was found. Thus, Thectardis avalonensis translates as "sharp arrow-point of Avalon (Peninsula)."