# Eldonioid

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. The lifestyle of eldonioids is still an unresolved question; some authors reconstruct eldonioids as free-floating planktonic predators similar to jellyfish, while others argue that they were passive detritivores, embedded within the seabed for much of their life.

## Anatomy

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. Internally, they have a distinctive C-shaped cavity encompassing the gut, as well as hollow radial (radiating) structures arranged around a central ring canal. Most eldonioids are soft-bodied and can only be preserved in lagerstätten, but a few species may have hosted mineralized deposits. Historically, the affinities of eldonioids was enigmatic; recently, they been assessed as cambroernid deuterostomes.
The basic eldonioid body plan is oriented around two circular discs sealed together with the viscera in between. The ventral (lower) disc is concave from below while the dorsal (upper) is convex from above, providing a dome-like profile to the body. The rim of the body is smooth, though the margin may furl slightly inwards under the ventral disc. Sediment infillings or differences in preservation allow for hollow internal structures to be differentiated from solid sheets of tissue within the body.

### Disc structures

The ventral disc acts as a platform for numerous elongated internal structures arranged in a radiating pattern. These narrow structures, known as “radial sacs” (also “radial lobes” or "internal lobes"), were most likely hollow cavities. They are among the most visible features in most fossils. The sacs are divided from each other by thin ligamentous walls, described as “septa” or “mesenteries”. In most eldonioid species, each radial sac bifurcates (divides into two parts) near the rim of the disc. The radial sacs thin towards the center of the disc, but do not converge at a single point. Instead, they intersect with a small ring-shaped cavity or canal in the central part of the ventral disc. The radial sacs and ring cavity were likely filled with fluids, akin to a hydrostatic skeleton or (less likely) an echinoderm-like water vascular system.
The dorsal disc is usually more rigid and resistant to distortion than the ventral disc. 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. According to some interpretations, radial ridges, wrinkles, or grooves also ornament part of the dorsal disc in some rotadiscids. “Paropsonemids” have even more complex dorsal discs, combining radial and concentric ornamentation at a fine level of detail. In Eldonia and Stellostomites, the dorsal disc appears to bear radial internal structures, mimicking the radial sacs above the ventral disc. The dorsal disc has been interpreted as almost completely solid, with the radiating structures identified as very thin tubular canals. This is opposite to what is seen in the ventral disc, which has proportionally larger radiating cavities and slender solid septa.

### Body cavity

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. All eldonioids bear a thick, easily-recognizable coiled sac which rings around the middle of the body. The coiled sac is a horseshoe-shaped cavity that curves dextrally (clockwise) from the mouth to the anus. It sheaths the alimentary canal (gut), which is thinner and more difficult to discern in most fossils. In the best-preserved eldonioids, the gut can be divided into three regions: the esophagus or pharynx (front), stomach (middle), and intestine (rear). The stomach is usually the broadest part of the gut, and the portion of the coiled sac surrounding it is stained a dark color. The mouth and anus are positioned close to each other and open through the ventral disc.

### Feeding tentacles

A small number of circumoral feeding tentacles project out from near the mouth. Their basic form is shrub-shaped, with a pair of main shafts that split away from each other and divide further into smaller filaments. Some authors referred to the circumoral tentacles as a “lophophore”, a term used for the hollow filamentous tentacles of lophophorates such as brachiopods and bryozoans. In Rotadiscus, a pair of tightly coiled spiral structures connect to the base of the tentacles. These structures covered a hole called the coelomopore, seen in living ambulacrarians. 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. Moreover, it suggests that the tentacles develop from outgrowths of the coelom (body cavity), more akin to ambulacrarians rather than lophophorates. Further similarities include detachment of the tentacles from the mouth, and their tapering structure.

## Classification

Eldonioids have a frequently fluctuating species composition and their relationship to other animals has been controversial. Some authors argued that they represented early holothurians (sea cucumbers), while others stressed their similarity to lophophorates (a subset of lophotrochozoans with ciliated tentacles known as lophophores). In 2010, eldonioids were allied with two other unusual Cambrian genera: Herpetogaster and Phlogites. Together, they comprise the cambroernids, an informal clade. Cambroernids are probably deuterostomes, specifically stem- or crown-group ambulacrarians related to echinoderms and hemichordates.
Eldoniidae was a family first established in reference to Eldonia, a disc-shaped fossil named in 1911. Eldonia was one of many mysterious soft-bodied animals discovered in the Cambrian-age Burgess Shale of Canada. It is also a major component of the Late Ordovician Tafilalt biota of Morocco. Eldonia was initially mistaken for a scyphozoan (jellyfish), and was not the only member of the group originally misidentified as a cnidarian. Others include Vellumbrella (mistaken for a jellyfish), Discophyllum (mistaken for a coral), and Paropsonema (mistaken for a jellyfish-like porpitoid hydrozoan). Eldonioids, a broader definition than Eldoniids, also include Rotadiscus, Stellostomites, Pararotadiscus, and Seputus. 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.
Dzik (1989) named the class Eldonioidea to encompass "velumbrellids" (i.e. eldoniids) and the goblet-shaped Cambrian animal Dinomischus. Eldonioidea was divided further into the orders Dinomischida (containing solely Dinomischus) and Vellumbrellida. The latter order was divided into Eldoniidae and Rotadiscidae. Most subsequent authors doubt a close relationship between Dinomischus and eldoniids. A 2018 assessment of Eldonioidea recognized the families Eldoniidae and Rotadiscidae alongside the informal group "paropsonemids."

### Taxonomy

The following classification is based on Schroeder et al. (2018):
* Eldonioidea (= Eldoniidae sensu lato in some publications)
** Eldoniidae sensu stricto
*** Eldonia: middle Cambrian-Late Ordovician of Canada (British Columbia), USA (Utah), Russia, and Morocco
*** Stellostomites (sometimes considered a species of Eldonia): early Cambrian of China (Yunnan)
** Rotadiscidae
*** Pararotadiscus (previously considered a species of Rotadiscus): middle Cambrian of China (Guizhou)
*** Rotadiscus: early to middle Cambrian of China (Yunnan) and Poland
*** Seputus?: Ordovician of Ireland
*** Vellumbrella?: early to middle Cambrian of Poland
*** An unnamed Pararotadiscus-like taxon from the early Cambrian Emu Bay Shale of Australia
** "Paropsonemids"
*** Discophyllum: Ordovician of USA (New York)
*** Paropsonema: Silurian-Devonian of USA (New York) and Australia