Tribrachidium is a fossil animal with a three-fold symmetrical shape that lived in the late Ediacaran (Vendian) period in ancient seas. When it was alive, it had a shape similar to half of a sphere. T. heraldicum is the most well-known species in the extinct group called Trilobozoa.

Etymology

The scientific name Tribrachidium comes from the combination of the Ancient Greek word "tria," meaning "three," the Latin word "brachium," meaning "arm," and the suffix "-idium," which is used to form scientific names. The specific name T. heraldicum is named after the similarity between the pattern of this fossil and a well-known symbol called a heraldic triskelion, such as the one found in the coat of arms of the Isle of Man.

Occurrence

Tribrachidium fossils were first found in the Ediacara Member of the Rawnslay Quartzite in the Flinders Ranges, South Australia. These fossils have also been discovered in the Mogilev Formation of the Dniester River Basin in Podolia, Ukraine, and in the Verkhovka, Zimnegory, and Yorga formations of the White Sea area in the Arkhangelsk Region, Russia. Additionally, Tribrachidium fossils have been identified in the Sonia Formation of the Marwar Supergroup near Jodhpur, India.

Description

Tribrachidium heraldicum is preserved as imprints on the bottom of sandstone layers. These fossils have a round, three-lobed shape with straight or three-leaf clover-like edges. They are often covered with many branching furrows that spread out from the center. The middle part of the fossil has three curved ridges ("arms"). The lobes are slightly twisted into gentle spirals.

The size of these fossils ranges from 3 to 40 millimeters (0.3 to 4.0 centimeters). They are found in areas ranging from a few meters to larger scales, and they can be found either alone or in groups.

In a 2015 study, Rahman et al. suggested that Tribrachidium heraldicum used a special feeding method called "gravity settling." Computer models showed that water flow was guided by the arms toward three small pits ("apical pits"), where the water slowed down, allowing food particles to settle out of the water.

Reconstruction and affinity

Tribrachidium was first described by Martin Glaessner as a difficult-to-classify organism because its tri-radial symmetry did not match any major groups of animals. Researchers noted that Tribrachidium looked similar to edrioasteroid echinoderms, but Glaessner later said it did not belong to any known animal groups, leaving its classification unclear. Early studies of poorly preserved Australian fossils suggested features like tentacles, unusual arms, and a mouth, but these ideas were later dismissed. Scientists still do not know how Tribrachidium moved in life.

When closely related fossils, such as Albumares and Anfesta, were discovered along with better-preserved Russian specimens, Mikhail Fedonkin proposed a new group called Trilobozoa. This group included Tribrachidium and other similar organisms. Initially, Trilobozoa was classified as a class within the phylum Coelenterata. However, when Coelenterata was split into separate phyla, Cnidaria and Ctenophora, Trilobozoa was reclassified as its own phylum.

Fedonkin showed that Tribrachidium fossils are impressions of the animal’s upper body surface, showing parts of its external and internal structure. The radial furrows on the fossil represent grooves on the living animal’s surface, while the three hooked ridges in the center are impressions of body cavities. Tribrachidium was a soft-bodied organism that lived on the ocean floor and temporarily attached itself to microbial mats without permanently sticking to them.