{{Short description|Extinct clade of disc-shaped animals}} {{Automatic taxobox | name = Eldonioidea | fossil_range = {{fossilrange|Cambrian|Devonian}} | image = Eldonia berbera 1.JPG | image_caption = Fossils of ''Discophyllum peltatum'', from the Late Ordovician of Morocco | taxon = Eldonioidea | authority = Dzik, 1991 | display_parents = 3 | subdivision_ranks = Families | subdivision = *Eldoniidae *Rotadiscidae *Paropsonemidae }}
'''Eldonioids''' or '''velumbrellids''' are an extinct clade of disc-shaped cambroernids which lived in the early to middle Paleozoic (Cambrian to Devonian). Depending on the researcher, the clade is either considered to either be the class '''Eldonioidea''' (containing only the order '''Velumbrellida''') or be the order '''Eldonida'''.<ref name="G-B2026">{{harvnb|García-Bellido|Romero|Gutiérrez-Marco|2026}}</ref> The lifestyle of eldonioids is still an unresolved question; some authors reconstruct eldonioids as free-floating planktonic predators similar to jellyfish,<ref name="Zhu2002">{{harvnb|Zhu|Zhao|Chen|2002}}</ref> while others argue that they were passive detritivores, embedded within the seabed for much of their life.<ref name=Dzik1997 />
== Names == 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.<ref name=Schroeder2018 /> When using the order Eldonida, the adjectival form is "eldonid".<ref>{{harvnb|Lieberman|Kurkewicz|Shinogle|Kimmig||MacGabhann|2017}}</ref>
== Anatomy == [[File:Stellostomites diagrams.png|thumb|264x264px|Cross-sectional diagrams of ''Stellostomites eumorphus'' (= ''Eldonia eumorpha'').<br /> A: Vertical cross-section<br /> B: Transverse section near the dorsal disc<br /> C: Transverse section near the ventral disc<br /> 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.<ref name=Zhu2002 /> 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.<ref name=Chen1995>{{harvnb|Chen|Zhu|Zhou|1995}}</ref><ref name=Dzik1997 /> Historically, the affinities of eldonioids was enigmatic; recently, they been assessed as cambroernid deuterostomes.<ref name="Caron2010" /><ref name=Li2023 />
The basic eldonioid body plan is oriented around two circular discs sealed together with the viscera in between.<ref name=Zhu2002 /> 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.<ref name=Zhu2002 />
=== Disc structures === [[File:Walcott Cambrian Geology and Paleontology II plate 09.jpg|left|thumb|Fossils of ''Eldonia ludwigii'', showing the prominent coiled sac, radial lobes, and central ring canal]] The ventral disc acts as a platform for numerous elongated internal structures arranged in a radiating pattern.<ref name=Zhu2002 /><ref name="Caron2010" /><ref name=Hagadorn2018 /> These narrow structures, known as "radial sacs"<ref name=Chen1995 /><ref name=Zhu2002 /> (also "radial lobes"<ref name=Schroeder2018 /> or "internal lobes"<ref name=Schroeder2018 />), 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".<ref name=Zhu2002 /> In most eldonioid species, each radial sac bifurcates (divides into two parts) near the rim of the disc.<ref name=Zhu2002 /> 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.<ref name=Zhu2002 /> The radial sacs and ring cavity were likely filled with fluids, akin to a hydrostatic skeleton<ref name="Caron2010" /> or (less likely)<ref name=Zhu2002 /><ref name="Caron2010" /> an echinoderm-like water vascular system.<ref name=Chen1995 />
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.<ref name=Chen1995 /><ref name=Dzik1997 /><ref name=Zhu2002 /><ref name="Caron2010" /><ref name="MacGabhann2010">{{harvnb|MacGabhann|Murray|2010}}</ref><ref name=Schroeder2018 /> According to some interpretations, radial ridges, wrinkles, or grooves also ornament part of the dorsal disc in some rotadiscids.<ref name=Dzik1997 />''<ref name=MacGabhann2010 /><ref name=Schroeder2018 />'' Paropsonemids have even more complex dorsal discs, combining radial and concentric ornamentation at a fine level of detail.''<ref name=MacGabhann2010 />''<ref name=Schroeder2018 /> 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.<ref name=Zhu2002 />
=== 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.<ref name=Chen1995 /><ref name=Dzik1997 /><ref name=Zhu2002 /> 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.<ref name=Chen1995 /><ref name=Dzik1997 /><ref name=Zhu2002 /><ref name="Caron2010" /><ref name=Hagadorn2018 /> 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).<ref name=Chen1995 /><ref name=Zhu2002 /><ref name="Caron2010" /> The stomach is usually the broadest part of the gut, and the portion of the coiled sac surrounding it is stained a dark color.<ref name=Chen1995 /> The mouth and anus are positioned close to each other and open through the ventral disc.<ref name=Zhu2002 />
=== 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.<ref name=Dzik1997 /><ref name=Zhu2002 /><ref name="Caron2010" /> 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.<ref name=Chen1995 /><ref name=Dzik1997 /><ref name=Zhu2002 /> 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.<ref name=Li2023 />
== 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).<ref name=Dzik1997>{{harvnb|Dzik|Zhao|Zhu|1997}}</ref><ref name=Zhu2002 /> 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.<ref name="Caron2010">{{harvnb|Caron|Conway Morris|Shu|Soares|2010}}</ref><ref name=Li2023>{{harvnb|Li|Dunn|Murdock|Guo|2023}}</ref>
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.<ref name=Chen1995 /> ''Eldonia'' was initially mistaken for a scyphozoan (jellyfish), and was not the only member of the group originally misidentified as a cnidarian. Others include ''Velumbrella'' (mistaken for a jellyfish), ''Discophyllum'' (mistaken for a coral), and ''Paropsonema'' (mistaken for a jellyfish-like porpitoid hydrozoan).<ref name="Hagadorn2018">{{harvnb|Hagadorn|Allmon|2019}}</ref>
Eldonioids, a broader definition than eldoniids, also include rotadiscids<ref name="Dzik1997" /> and paropsonimeids<ref name=Schroeder2018 /> 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.<ref name="Schroeder2018" /> Beyond the Cambrian, ''Discophyllum peltatum'' (formerly known as ''"Eldonia berbera"'') is an abundant fossil in the Late Ordovician Tafilalt biota of Morocco.<ref name="G-B2026"/>
Dzik (1991) named the class Eldonioidea to encompass "velumbrellids" (i.e. eldoniids and rotadiscids) as the order Vellumbrellida and the goblet-shaped Cambrian animal ''Dinomischus'' as the order Dinomischida. The Vellumbrellida were divided into the families Eldoniidae and Rotadiscidae.<ref>{{harvnb|Dzik|1991}}</ref> 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."<ref name=Schroeder2018>{{harvnb|Schroeder|Paterson|Brock|2018}}</ref><ref name=Li2023 />
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.<ref name="G-B2026" />
=== 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:<ref name=Schroeder2018 />
* Class Eldonioidea {{small|Dzik 1991}} (= Eldoniidae ''sensu lato'' in some publications)<ref name=Caron2010 /> ** Family Eldoniidae {{small|Walcott 1911}} ''sensu stricto'' *** ''Eldonia'' {{small|Walcott 1911}} (=''Yunnanomedusa'' {{small|Sun & Hou 1987}}) *** ''Stellostomites'' {{small|Sun & Hou 1987}} (?=''Eldonia'' {{small|Walcott 1911}}) ** Family Rotadiscidae {{small|Dzik 1991}} *** ''Rotadiscus'' {{small|Sun & Hou 1987}} (=''Brzechowia'' {{small|Dzik 1991}}) *** ''Pararotadiscus'' {{small|Zhao & Zhu 1994}} (=''Rotadiscus'' {{small|Sun & Hou 1987}} ''pars'') *** ? ''Seputus'' {{small|MacGabhann & Murray 2010}} *** ? ''Velumbrella'' {{small|Stasińska 1960}} *** An unnamed ''Pararotadiscus''-like taxon from the early Cambrian Emu Bay Shale of Australia ** "paropsonemids" *** ''Discophyllum'' {{small|Hall 1847}} *** ''Paropsonema'' {{small|Clarke 1900}}
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:<ref name=G-B2026 />
* Order Eldonida {{small|García-Bellido, Romero & Gutiérrez-Marco 2026 ''pro'' MacGabhann 2012}} (=Velumbrellida {{small|Dzik 1991}} ''pars'') ** ''Pararotadiscus'' {{small|Zhao & Zhu 1994}} ** ''Velumbrella'' {{small|Stasińska 1960}} ** ''Seputus'' {{small|MacGabhann & Murray 2010}} ** Family Eldoniidae {{small|Walcott 1911}} *** ''Eldonia'' {{small|Walcott 1911}} (=Yunnanomedusa {{small|Sun & Hou 1987}}; =Stellostomites {{small|Sun & Hou 1987}}) ** Family Paropsonemidae {{small|García-Bellido, Romero & Gutiérrez-Marco 2026 ''pro'' MacGabhann 2012}} *** ''Paropsonema'' {{small|Clarke 1900}} *** ''Discophyllum'' {{small|Hall 1847}} (?=Pseudodiscophyllum {{small|Fryer & Stanley 2004}}) *** "''Praeclarus''" {{small|''nomen nudum'', named in unpublished thesis MacGabhann 2012}}
== Occurrences ==
The following geostratigraphic ranges are known as of 2026:<ref name=Schroeder2018 /><ref name=G-B2026 />
* 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== {{Reflist}}
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Category:Cambroernids Category:Burgess Shale fossils