''Tribrachidium heraldicum'' is a tri-radially symmetric fossil animal that lived in the late

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 ...

(Vendian) seas. In life, it was hemispherical in form. ''T. heraldicum'' is the best known member of the extinct group

Trilobozoa Trilobozoa, from Ancient Greek τρεῖς (''treîs''), meaning "three", λοβός (''lobós''), meaning "lobe", and ζῷον (''zôion''), meaning "animal", is a phylum of extinct, sessile animals that were originally classified into the Cnid ...

Etymology

The generic name ''Tribrachidium'' is derived from combination of the (, "three") + ("arm") + diminutive suffix . The specific name ''T. heraldicum'' references the similarity of the pattern of this fossil with the well-known heraldic triskelion design, such as the coat of arms of the Isle of Man.

Occurrence

''Tribrachidium'' fossils were first discovered in the Ediacara Member of the Rawnslay Quartzite, Flinders Ranges in

South Australia South Australia (commonly abbreviated as SA) is a States and territories of Australia, state in the southern central part of Australia. With a total land area of , it is the fourth-largest of Australia's states and territories by area, which in ...

. This fossil is also known from the Mogilev Formation in the

Dniester River The Dniester ( ) is a transboundary river in Eastern Europe. It runs first through Ukraine and then through Moldova (from which it more or less separates the breakaway territory of Transnistria), finally discharging into the Black Sea on Uk ...

Basin, Podolia,

Ukraine Ukraine is a country in Eastern Europe. It is the List of European countries by area, second-largest country in Europe after Russia, which Russia–Ukraine border, borders it to the east and northeast. Ukraine also borders Belarus to the nor ...

and from the Verkhovka, Zimnegory and Yorga formations in the

White Sea The White Sea (; Karelian language, Karelian and ; ) is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is surrounded by Karelia to the west, the Kola Peninsula to the north, and the Kanin Peninsula to the nort ...

area of the Arkhangelsk Region,

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

. This fossil is also known from the Sonia Formation of Marwar Supergroup near

Jodhpur Jodhpur () is the second-largest city of the north-western Indian state of Rajasthan, after its capital Jaipur. As of 2023, the city has a population of 1.83 million. It serves as the administrative headquarters of the Jodhpur district and ...

, India.

Description

''T. heraldicum'' is preserved as negative impressions on the base of sandstone beds. These fossils have a circular, three-lobe form, with straight or

trefoil A trefoil () is a graphic form composed of the outline of three overlapping rings, used in architecture, Pagan and Christian symbolism, among other areas. The term is also applied to other symbols with a threefold shape. A similar shape with f ...

-like edges; they are usually covered by numerous radial branched furrows. The central part of the fossil has three hooked ridges ("arms"). The lobes are twisted into weak spirals. The diameter of specimens ranges from . Its meter-scale distribution was found to be variable occurring both as solitary individuals and in groups.

Feeding Method

In a 2015 study, Rahman et al. proposed that ''Tribrachidium heraldicum'' used a rare 'gravity settling' mode of suspension feeding based on computational fluid dynamics simulations, which showed that water flow was directed passively by the arms, funneling it towards three depressions (‘apical pits’) where water flow slowed down so that food particles would fall out of suspension.

Reconstruction and affinity

''Tribrachidium'' was originally described by Martin Glaessner as a problematic organism, one that is excluded from all known major groups of animals by its tri-radial symmetry. However, ''Tribrachidiums superficial resemblance to edrioasteroid

echinoderms An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larv ...

was well known to researchers and discussed. Later, Glaessner rejected any putative affinities of this animal with any known phyla, leaving the status of its taxonomy uncertain. Originally, the various structures on the poorly preserved Australian specimens were interpreted as tentacles, peculiar arms and mouth, but later this interpretation was rejected. Its mode of locomotion in life also remains unknown. Pomoria_corolliformis

With the discovery of the closely related '' Albumares'' and '' Anfesta'', along with the discoveries of much better-preserved Russian specimens, Mikhail Fedonkin proposed for these animals the new taxon,

– an extinct group of tri-radially symmetrical coelenterate-grade animals. Originally, Trilobozoa was erected as a separate

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

in the phylum Coelenterata, but after Coelenterata was divided into separate phyla

Cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...

and

Ctenophora Ctenophora (; : ctenophore ) is a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and they are ...

, the Trilobozoa was transferred to the rank of

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

. M. Fedonkin has shown that the fossil of ''Tribrachidium'' is an imprint of the upper side of the animal's body, with some elements of its external and internal anatomy. The radial furrows on the fossil are radial grooves on the surface of the living animal, while the three hooked ridges in central part of the fossil are imprints of cavities within the body. ''Tribrachidium'' was a soft-bodied

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

organism that temporarily attached (but did not accrete) to the substrate of its habitat ( microbial mats).

References

External links

Palaeos dendrogram
University of Bristol
Anatomical Information Content in the Ediacaran Fossils and Their Possible Zoological Affinities
Jerzy Dzik, Instytut Paleobiologii PAN, Twarda 51/55, 00-818 Warszawa, Poland {{Taxonbar, from1=Q2210059, from2=Q20719482 Trilobozoa Monotypic prehistoric animal genera Ediacaran life Fossils of Australia Fossils of Russia White Sea fossils