a somewhat enigmatic creature from the Cambrian-era Burgess Shale. When originally discovered, Eldonia was interpreted as a sea cucumber with an odd, circular mantle around its body. Later, it was thought to be some sort of jellyfish or maybe a lophophore. Now it's believed to be a primitive deuterostome-- the group of animals that includes echinoderms, chordates and hemichordates. Its closest relative is believed to be another Cambrian animal called Herpetogaster-- nicknamed the Parsley-Penis Worm (google Herpetogaster, you'll understand).
In the past Eldonia was thought to be a pelagic swimmer, due to its jellyfish-like shape. More recent reconstructions portray it as a benthic animal like a bowl with a plume of tentacles sticking out of the top-- even the model in the Field Museum looks that way. But.... I don't know, something just never looked right about that to me.
Here I've split the difference and reconstructed Eldonia as a kind of prehistoric Enypniastes, spending most of its time on the ground grubbing around in the mud, but with the ability to make short, jerky swims if it needs to. It swims by scrunching up its C-shaped body, which contracts the flexible gelatinous mantle.
Also featured here is a small school of Odaraia and a couple of Hallucigenia poking around for food that the Eldonia are stiring up.