#edrioasteroid

Early echinoderms seem to have gone through an asymmetrical phase before starting to evolving their characteristic radial symmetry. The first truly radial forms had three-way symmetry, but soon a group called the edrioasteroids upped that count to five.

First appearing in the fossil record around 525 million years ago in the early Cambrian, edrioasteroids were mostly shaped like discs or domes, and were immobile filter-feeders that lived permanently attached onto surfaces like the seafloor or the shells of other animals. Unlike most modern echinoderms their pentaradial symmetry was actually created by taking a tri-radial body plan and forking two of their arms near the bases to create a total of five.

Thresherodiscus ramosus was an unusal edrioasteroid that lived in the shallow seas of what is now central Canada during the late Ordovician, around 460-450 million years ago. Up to about 4cm in diameter (~1.6"), its arms split additional times at irregular intervals, creating a complex asymmetrical branching pattern across its upper surface.

The tips of its arms protruded slightly over the rim of its body, and along with the erratic extra branching this may have been an adaptation to increase its food-gathering surface area.

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Another group of early pentaradial echinoderms known as the blastozoans were characterized by erect feeding appendages called brachioles. But some blastozoans abandoned their five-way symmetry in favor of much stranger arrangements, sometimes having as few as two arms – and, in some cases, two mouths.

Known from the same general area and time period as Thresherodiscus, Amygdalocystites radiatus was part of an Ordovician-to-early-Silurian lineage known as paracrinoids, which attached their irregularly-shaped bodies to the seafloor via a stem.

About 5cm long (~2") Amygdalocystites had just two asymmetric arms forming "food grooves" along its upper edge, each lined with numerous long brachioles along just one of their sides. It probably orientated itself so its body was facing down-current, which would have created eddies that brought suspended food particles within easier reach of its brachioles.

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Despite looking more like some sort of scaly tubeworm, Rhenopyrgus viviani here was actually an echinoderm, distantly related to modern starfish, brittle stars, sea urchins, crinoids, and sea cucumbers.

It was part of an extinct Paleozoic echinoderm lineage known as edrioasteroids, which lived attached to the seabed or on hard surfaces like the shells of other marine animals, using the tube feet on their five arms to catch food particles from the water around them.

Living during the Silurian, about 435 million years ago, in what is now Quebec, Canada, it stood around 3-4cm tall (1.2-1.6"), firmly anchored into the seafloor sediment by a bulbous sac-shaped base. Its long stalked body was somewhat flexible, and it was able to partially contract the top feeding region down under a "collar" of large scale-like armor plates.

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Cambrian Explosion Month #13: Phylum Echinodermata – Sticking Around

It seems like echinoderms became five-way symmetric incredibly quickly following the group's first appearances in the early Cambrian. We don't really know why this secondary radial symmetry evolved in the group – but we do know that the common ancestors of all modern pentaradial echinoderms were suspension-feeding animals that lived attached to the sea floor.

And those ancestors were probably a group called the edrioasteroids.

Edrioasteroids looked like discs or pillows with a starfish stuck on the top (although a few later forms resembled turrets), and they lived permanently attached to either the seafloor sediment or to hard surfaces like rocks and shells. Their five arms could grow straight, curved, or spiralling over the top surface of their bodies, and they captured food particles from the water using tube feet.

They first appeared in the fossil record at around the same time as helicoplacoids, about 525 million years ago, and were either ancestral to all modern echinoderms or to everything except crinoids (depending on whether eocrinoids and blastozoans were related to true crinoids or not).

Camptostroma roddyi was an unusual early edrioasteroid that resembled a cupcake, known from Pennsylvania, USA, around 516-513 million years ago. Growing up to about 6cm in diameter (2.4"), its wide thick base helped it suction onto soft sediment and algal mats.

Most other early edrioasteroids were also soft-surface suction-stickers, but later in the Cambrian microbial mats became less common and members of the group began to specialize for attaching to hard surfaces instead.

Totiglobus spencensis was one of the first species to use hard surfaces, living in Idaho, USA, about 509-504 million years ago. The only known specimen was about 2cm in diameter (0.8"), and was found attached onto a hyolith shell – possibly with the aid of a bioglue – with its position and the completeness of the hyolith suggesting it had been growing on a living host until they both got buried in sediment and preserved.

A very closely related species, Totiglobus nimius, was still a soft-sediment-sticker, and the two had differently shaped basal regions of their body – T. nimius had a wide flat attachment surface, while T. spencensis had a conical tapering base that ended in a smaller attachment disc.

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While edrioasteroids were never particularly common echinoderms, they were still widespread around the world and surprisingly long-lived as a group – they actually continued on through most of the Paleozoic, with their last known fossils coming from the Mid-Permian about 270 million years ago.

And, of course, if early edrioasteroids were the ancestors of modern echinoderms then their descendants are still going strong today. Even now, half a billion years later, some modern echinoderm larvae still may show some traces of that ancestry, attaching themselves onto the seafloor before metamorphosing.

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