The 555-Million-Year-Old Cones That Fed on the Seafloor

In a slab of grey sandstone from the Flinders Ranges, there is a cone the size of a thimble. It is 555 million years old, and it once held a mouth.

The fossil is Conomedusites. For decades, palaeontologists classified it as a jellyfish — a drifting medusa, like those that pulse through modern seas. But the rock tells a different story. The cone is not a bell. It is a holdfast, and it was anchored to the seafloor for its entire life.

The Cone That Would Not Drift

Conomedusites belongs to the Ediacara biota, the strange community of soft-bodied organisms that lived in the shallow seas of the late Precambrian. Most were fronds, discs, or quilted mats. But Conomedusites built a four-part radial body — a cone with a central cavity, surrounded by concentric rings preserved as faint ridges in the sandstone.

The fossils are found in the Ediacara Member of the Rawnsley Quartzite, a sequence of sandstones deposited by storm events in the ancient sea. They are preserved as casts and moulds, the sediment having filled the cavity of the organism before it decayed. What remains is not the animal itself, but the shape of its living space.

Recent re-examination of these specimens has changed how scientists interpret the cone. The concentric rings are not the bell of a jellyfish. They are the margin of a polyp — a sessile predator that sat on the seafloor and waited.

A Mouth That Faced Up

The anatomy is subtle but decisive. The central cavity of Conomedusites is surrounded by a ring of what appear to be tentacle bases, arranged in four-fold symmetry. This is the body plan of a cnidarian polyp — the same architecture used by modern sea anemones and corals. The cone was a column, the rings a crown of tentacles, and the cavity a gut.

Unlike the frondose Ediacarans that absorbed nutrients from the water, Conomedusites was a true predator. It captured small organic particles — perhaps even tiny animals — with its tentacles and drew them into its mouth. It could also retract its tentacles into the cone for protection, a behaviour preserved in the layered sediment that filled the contracted cavity.

This makes Conomedusites one of the earliest known animals with a functional gut. It did not graze microbial mats like Dickinsonia. It did not filter seawater like a sponge. It fed like a modern sea anemone, 555 million years before the first reef.

The first mouth was also the first stomach. Both fit inside a cone no larger than a fingernail.

The Seafloor That Waited

The Ediacaran seafloor in the Flinders Ranges was not a quiet garden. It was a competitive landscape. Microbial mats covered the sediment, binding it into a firm surface. Fronds like Rangea stood upright, capturing nutrients from the water column. Dickinsonia grazed the mats in slow, deliberate paths. And Conomedusites sat among them, its tentacles extended into the current.

The fossils are found in storm beds — layers of sand that smothered the community in a single event. What was preserved is a snapshot of a living seafloor, frozen in the instant of its burial. The cones are oriented upright, caught in the posture of feeding.

There are no relatives of Conomedusites alive today. The Ediacaran polyp lineage did not survive into the Cambrian. But its body plan — the cone, the mouth, the radial symmetry — became the template for an entire branch of animal life. Jellyfish, corals, and anemones all descend from this basic design.

The cone that was mistaken for a jellyfish turns out to be the ancestor of the jellyfish. It just never learned to swim.