Eldonioids are an extinct clade of disc-shaped cambroernids, the Eldonioidea, which lived in the early to middle Paleozoic (Cambrian to Devonian).

The terms "eldonioid" and "eldoniid" have been used somewhat informally and interchangeably, but technically refer to members of the class Eldonioidea and the family Eldoniidae, respectively.

[2] Eldonioids are characterized by their "medusoid" (jellyfish-shaped) bodies, with the form of a shallow dome opening below to an offset mouth supplemented by filamentous tentacles.

[1] Internally, they have a distinctive C-shaped cavity encompassing the gut, as well as hollow radial (radiating) structures arranged around a central ring canal.

Sediment infillings or differences in preservation allow for hollow internal structures to be differentiated from solid sheets of tissue within the body.

[1] The radial sacs and ring cavity were likely filled with fluids, akin to a hydrostatic skeleton[4] or (less likely)[1][4] an echinoderm-like water vascular system.

In several taxa, the dorsal disk is ornamented by one or more irregular concentric (ring-shaped) wrinkles, which may be growth lines or artefacts of compression.

[1] In most eldonioids, the center of the body is solid, but in other species, there is a rigid central cavity which tapers towards the ventral or dorsal disc.

In the best-preserved eldonioids, the gut can be divided into three regions: the esophagus or pharynx (front), stomach (middle), and intestine (rear).

Rotadiscus helps to establish homology between the coelomopore and additional pharyngeal openings present in tunicates and cephalochordates, as well as part of the vertebrate pituitary gland.

[8] Apart from Eldonia, the most abundant and well-preserved eldonioid species are from the Cambrian of China: Stellostomites eumorphus and Rotadiscus grandis are from the Chengjiang biota of Yunnan, and Pararotadiscus guizhouensis is from the Kaili Formation of Guizhou.