Eldonia is an odd genus of animal that lived from the Cambrian to the Ordovician. When Charles Doolittle Walcott first found Eldonia in the Burgess Shale, he classified it as a holothurian (a sea cucumber), but other scientists didn't agree. They classified it as a medusoid (a jellyfish). Others classified it as a siphonophore. But no one knows for sure what it was.
There are three species of Eldonia: Eldonia ludwigii, Eldonia eumorpha, and Eldonia berbera. Although many people believe that Eldonia was a pelagic animal, some now believe that it was benthic and lived on the bottom of the ocean.
Some fossils of Eldonia show the lobopod Microdictyon feeding on them. Eldonia could grow up to 4 inches, but most specimens of Eldonia are smaller than this.
The part of the Burgess Shale where Walcott found so many Eldonia is called "The Great Eldonia Layer." Eldonia has also been found in China and in Morocco.