What Is an Edrioasteroid? Starfish Relatives That Lived Glued Down

The body is a flattened sac covered in small calcite plates, attached at its base to a shell, a rock or another animal, and carrying five food grooves radiating across the upper surface from a central mouth. Those grooves were lined with tiny tube feet that collected particles from the water and moved them towards the centre, which makes the animal a suspension feeder holding still rather than a predator moving about. The grooves curve, frequently all in the same rotational direction, which gives the fossils a distinctive pinwheel appearance. An anal opening sits to one side, covered by a small pyramid of plates. Some forms could partly contract, pulling the plates together to close up, which was presumably a response to disturbance. The whole animal is typically a few centimetres across.

The group belongs to a much larger phylum whose living members are familiar:

• •Echinoderms include starfish, sea urchins, brittle stars, sea cucumbers and sea lilies, all built on a five-part radial plan
• •That plan is secondary, since the larvae are bilaterally symmetrical and the adult symmetry develops later
• •The phylum has a far larger fossil record than its living members suggest, with numerous extinct classes
• •Several of those extinct groups had three-part or irregular symmetry rather than five, which complicates the story
• •Edrioasteroids are one of the earliest classes to appear and one of the longest lived, spanning the whole Palaeozoic
• •They are not ancestral to starfish despite the resemblance, which is a case of similar appearance from a shared body plan rather than close kinship

Because they attached to hard surfaces, these animals are frequently found still cemented to whatever they settled on, which preserves an ecological relationship directly. Brachiopod shells are a common substrate, and specimens are known with many individuals crowded onto a single shell, oriented consistently in relation to it. That arrangement supplies information nothing else does, including whether the host was alive when the settlement happened, which can be inferred from where on the shell the animals sat and whether they avoided the opening. Some assemblages record a sequence, with successive generations settling on the same surface. Where a hard substrate was scarce, competition for it was evidently intense, and shells are found crowded with several different kinds of encrusting organism. These are among the best available records of Palaeozoic hard-substrate communities, which are otherwise difficult to reconstruct.

Living permanently fixed to one spot imposes constraints that shape everything about such an animal. Food must come to it, which requires water movement and rules out still settings. Waste must be carried away rather than left, which is why the anal opening sits off to one side away from the incoming food. Reproduction cannot involve finding a partner, so gametes are released into the water and fertilisation happens there, which requires neighbours and synchrony. Dispersal has to happen at the larval stage, since the adult never moves again, and the settlement decision made by a larva is irreversible and therefore critical. Defence cannot involve fleeing, so it depends on armour, on closing up or on being unrewarding to eat. Those constraints apply equally to barnacles, corals, oysters and sea lilies, which is why animals from unrelated groups living this way resemble each other in so many respects.

Their interest exceeds their fame for a few reasons. They document an early stage in echinoderm evolution and preserve features that clarify how the group's distinctive anatomy assembled, including the water vascular system that operates the tube feet. Their attachment makes them useful indicators of the environment, since they require a hard surface, moving water to bring food and conditions that permit settlement. They occur in mass mortality assemblages where a sudden sediment influx buried an entire community in place, and those provide snapshots of a seafloor community as it actually stood, which is rare. And they disappeared in the end-Permian extinction along with a great deal else, so their record ends where the Palaeozoic does, making them a useful marker as well as a subject.

A flattened plated sac cemented to a hard surface, with five curving grooves lined with tube feet collecting particles, a few centimetres across. They are echinoderms, sharing the five-part plan with starfish and urchins without being closely related to starfish. Found still attached to the shells they settled on, they record hard-substrate communities that are otherwise hard to reconstruct.