Eldonioid

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. == Anatomy ==

(= Eldonia eumorpha''). A: Vertical cross-section B: Transverse section near the dorsal disc C: Transverse section near the ventral disc D: Underside of the ventral disc 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. 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 (or eldonids) 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). Eldoniidae is 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. Eldonioids, a broader definition than eldoniids, also include rotadiscids Most subsequent authors doubt a close relationship between Dinomischus and velumrellids, leaving Eldonioidea equivalent to Vellumbrellida. A 2018 assessment of Eldonioidea recognized the families Eldoniidae and Rotadiscidae alongside the informal group "paropsonemids." A 2026 overview of Moroccan eldonioid fossils proposed an order Eldonida directly under the cambroernids to replace Eldonioidea and Velumbrellida. This replacement was proposed to address inconsistent use of phylogenetic and functional aspects of the organisms in the classification, as well as problems arising from Eldonioidea, Velumbrellida, and Dinomischida being defined in terms of a transition from lophophorates to deuterostomes that has since been rejected. Taxonomy As of 2026, two taxonomies have been in recent use. As there has not yet been time for subsequent publications to ratify the 2026 proposal, it is unclear which will be utilized going forward. One classification is illustrated with a table in Schroeder, Paterson and Brock (2018), and is primarily based on Dzik (1991). It does not address Pseudodiscophyllum or the not-formally-published Praeclarus but is otherwise comprehensive at the genus level: • Class Eldonioidea (= Eldoniidae sensu lato in some publications) • Family Eldoniidae sensu stricto • Eldonia (=Yunnanomedusa ) • Stellostomites (?=Eldonia ) • Family Rotadiscidae • Rotadiscus (=Brzechowia ) • Pararotadiscus (=Rotadiscus pars) • ? Seputus • ? Velumbrella • An unnamed Pararotadiscus-like taxon from the early Cambrian Emu Bay Shale of Australia • "paropsonemids" • Discophyllum • Paropsonema The more recent classification is proposed by García-Bellido, Romero, and Gutiérrez-Marco (2026). This paper only mentions Rotadiscus as a cambroernid, but does not list it, nor its synonym Brzechowia, nor the family Rotadiscidae among the Eldonida. All other rotadiscids are left unassigned at the family level. The family Eldoniidae, the contents of which are not listed, is presumed to include only Eldonia as all other known genera are accounted for in the description of the order Eldonida: • Order Eldonida (=Velumbrellida pars) • Pararotadiscus • Velumbrella • Seputus • Family Eldoniidae • Eldonia (=Yunnanomedusa ; =Stellostomites ) • Family Paropsonemidae • Paropsonema • Discophyllum (?=Pseudodiscophyllum ) • "Praeclarus" == Occurrences ==

The following geostratigraphic ranges are known as of 2026: • Early Cambrian • Eldonia eumorpha (= Stellostomites eumorphus; Yunnanomedusa elegans): Yunnan, China • Rotadiscus grandis: Yunnan, China • Rotadiscus sp (=Brzechowia sp): Poland • Velumbrella czarnocki: Poland • "Emu Bay Shale taxon": Kangaroo Island, Australia • Middle Cambrian • Rotadiscus sp (=Brzechowia sp): Poland • Velumbrella czarnocki: Poland • Pararotadiscus guizhouensis (=Rotadiscus guizhouensis): Guizhou, China • Eldonia ludwigi (British Columbia, Canada; Utah, USA; Siberia, Russia) • Ordovician • Seputus pomeroii: Ireland • Discophyllum peltatum (=E. berbera): New York, USA; Morocco • Silurian • Pseudodiscophyllum windermerensis (?=Discophyllum peltatum): England • Paropsonema mirabile: Melbourne, Australia • Devonian • Paropsonema cryptophya: New York, USA ==References==