''Thectardis avalonensis'' is a triangular-shaped member of the

Ediacaran biota The Ediacaran (; formerly Vendian) biota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period (). These were enigmatic tubular and frond-shaped, mostly sessile, organis ...

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

cone In geometry, a cone is a three-dimensional figure that tapers smoothly from a flat base (typically a circle) to a point not contained in the base, called the '' apex'' or '' vertex''. A cone is formed by a set of line segments, half-lines ...

with a central depression, and its apex was anchored to the substrate. Sperling et al. (2011) suggest that ''Thectardis'' was a

sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

, while Antcliffe et al. (2014, 2015) instead suggest that it is the decayed remains of

rangeomorph The rangeomorphs are a group of Ediacaran Ediacaran biota, fossils. Ediacarans are the oldest large fossil organisms on earth, and many are not self-evidently related to anything else that has ever lived. However, some Ediacarans clearly resemble ...

Morphology

The

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

take the form of a

triangle A triangle is a polygon with three corners and three sides, one of the basic shapes in geometry. The corners, also called ''vertices'', are zero-dimensional points while the sides connecting them, also called ''edges'', are one-dimension ...

with a central depression, suggesting that the original

organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...

was conical. They range from 26 to 165mm in length and from 24 to 96mm in width at the triangular base. The diameter to height ratio of the organism 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

A total of 205 specimens of ''Thectardis'' are known, from two bedding surfaces, separated by 2 km and 10 million years at

Mistaken Point Mistaken Point Ecological Reserve is a wilderness area and a UNESCO World Heritage Site located at the southeastern tip of Newfoundland's Avalon Peninsula in the Canadian province of Newfoundland and Labrador. The reserve is home to the namesake ...

Newfoundland Newfoundland and Labrador is the easternmost province of Canada, in the country's Atlantic region. The province comprises the island of Newfoundland and the continental region of Labrador, having a total size of . As of 2025 the population ...

. ''Thectardis'' bearing bedding surfaces also contain ''

Charnia ''Charnia'' is an extinct genus of frond-like lifeforms belonging to the Ediacaran biota with segmented, leaf-like ridges branching alternately to the right and left from a zig-zag medial suture (thus exhibiting glide reflection, or opposite is ...

'' and ''

Ivesheadia The "ivesheadiomorphs" are a group of fossilised structures known from Ediacaran localities in England and Newfoundland. They are considered to be taphomorphs, representing the poorly preserved biological remains of various contemporary taxa such ...

''. 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. Thectardis avalonensis - MUSE

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 The Ediacaran ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...

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 most often refers to: *Holdfast (biology), a root-like structure that anchors aquatic sessile organisms to their substrate *Holdfast (tool), a tool used to secure a workpiece to a workbench or anvil Holdfast or hold fast may also refer t ...

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 The Avalon Peninsula () is a large peninsula that makes up the southeast portion of the island of Newfoundland in Canada. It is in size. The peninsula is home to 270,348 people, about 52% of the province's population, according to the 2016 Ca ...

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

References

{{Taxonbar, from1=Q15103985, from2=Q7777663 Ediacaran life Ediacaran Canada Enigmatic prehistoric animal genera Fossil taxa described in 2004

Morphology

Occurrence

Ecology

Etymology

See also

References