Eldonia

Eldonia is an extinct soft-bodied cambroernid animal of unknown affinity, best known from the Fossil Ridge outcrops of the Burgess Shale, particularly in the 'Great Eldonia layer' in the Walcott Quarry. In addition to the 550 collected by Walcott, 224 specimens of Eldonia are known from the Greater Phyllopod bed, where they comprise 0.43% of the community. Species also occur in the Chengjiang biota, Siberia, and in Upper Ordovician strata of Morocco.

A specimen from the Lower Ordovician Madaoyu Formation in Hunan, South China, can be interpreted as the incomplete body of Eldonia or the similar animal. However, its annulation, the structure of the intestine and the shape of the body are more similar to those of Ottoia.

Description
It takes the form of a round, medusoid disk (which originally led to suggestions of a jellyfish affinity) with a C-shaped gut trace. The gut is recalcitrant and can be extracted using Hydrofluoric acid.

Classification
Walcott's original interpretation as a holothurian was rapidly disputed. Alternative affinities to be suggested, which did not stand the test of time, included the siphonophores  and a coelenterate medusa.

The eldoniids form a clade that also includes Paropsonema, Rotadiscus, and Stellostomites.

Paleoecology
The organism is frequently found in association with the lobopod Microdictyon, which is presumed to have fed on Eldonia.