Eldoniids are an extinct

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

of enigmatic disc-shaped animals which lived in the early to middle Paleozoic (

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

). They 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 eldoniids are soft-bodied and can only be preserved in lagerstätten, but a few species may have hosted mineralized deposits. Historically, the affinities of eldoniids was enigmatic; recently, they been assessed as

cambroernid The cambroernids are an informally-named clade of unusual Paleozoic animals with coiled bodies and filamentous tentacles. They include a number of early to middle Paleozoic ( Cambrian to Devonian) genera noted as 'bizarre" or "orphan" taxa, mean ...

deuterostomes. Their lifestyle is still an unresolved question; some authors reconstruct eldoniids as free-floating

planktonic Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...

predators similar to jellyfish, while others argue that they were passive detritivores, embedded within the seabed for much of their life.

Classification

Eldoniids have a frequently fluctuating

species composition Species richness is the number of different species represented in an community (ecology), ecological community, landscape or region. Species richness is simply a count of species, and it does not take into account the Abundance (ecology), abunda ...

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 The lophophore () is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Hyolitha, and Phoronida, which collectively constitute the protostome group Lophophorata.lophotrochozoans with ciliated tentacles known as lophophores). In 2010, eldoniids 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 Stem or STEM may refer to: Plant structures * Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang * Stipe (botany), a stalk to support some other structure * Stipe (mycology), the stem of a mushro ...

- or crown-group ambulacrarians related to

echinoderms An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea li ...

and hemichordates. Eldoniidae was a family first established in reference to ''

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 b ...

'', 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 Tafilalt or Tafilet (; ar, تافيلالت), historically Sijilmasa, is a region and the largest oasis in Morocco. Etymology The word "Tafilalt" is an Amazigh word and it means "Jug", which is specifically a pottery jar used to store water. H ...

of Morocco. ''Eldonia'' was initially mistaken for a

scyphozoan The Scyphozoa are an exclusively marine class of the phylum Cnidaria, referred to as the true jellyfish (or "true jellies"). The class name Scyphozoa comes from the Greek word ''skyphos'' (), denoting a kind of drinking cup and alluding to the ...

(jellyfish), and was not the only member of the group originally misidentified as a

cnidarian Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that th ...

. Others include '' Vellumbrella'' (mistaken for a jellyfish), '' Discophyllum'' (mistaken for a coral), and '' Paropsonema'' (mistaken for a jellyfish-like porpitoid hydrozoan). Eldoniids, in a broad definition, also include '' Rotadiscus'', '' Stellostomites'', ''

Pararotadiscus ''Pararotadiscus'' is an abundant Eldonioid fossil from the mid-Cambrian, and one of the most abundant taxa in the Kaili biota. References {{Taxonbar, from=Q60790429 Cambroernids ...

'', and '' Seputus.'' Apart from ''Eldonia'', the most abundant and well-preserved eldoniid species are from the Cambrian of

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

: ''Stellostomites eumorphus'' and ''Rotadiscus grandis'' are from the

Chengjiang biota The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales ...

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 ''Dinomischus ''is a rare fossil animal from the Cambrian period. It reached 20 mm in height, was attached to the sea floor by a stalk, and looked loosely like a flower. The cup-shaped body at the top of the stalk probably fed by filter feed ...

''. 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. As a result, "Eldonioid" and "Eldoniid" have been used interchangeably by many recent publications. Some authors classify the rotadiscids and post-Cambrian "paropsonemids" as informal groups within Eldoniidae, while others maintain rotadiscids and "paropsonemids" as separate eldonioid families.

Anatomy

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. Walcott Cambrian Geology and Paleontology II plate 09

Walcott Cambrian Geology and Paleontology II plate 09

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 eldoniid 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. 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 The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

. It sheaths the

alimentary canal The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans ...

(gut), which is thinner and more difficult to discern in most fossils. In the best-preserved eldoniids, the gut can be divided into three regions: the esophagus or pharynx (front), stomach (middle), and

intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans ...

(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. A small number of circumoral feeding tentacles project out of 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 The lophophore () is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Hyolitha, and Phoronida, which collectively constitute the protostome group Lophophorata.brachiopods and bryozoans. 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 ''

'' 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.

Taxonomy

* Eldonioidea (= Eldoniidae ''sensu lato'') ** Eldoniidae ''sensu stricto'' *** ''

'': middle Cambrian-Late Ordovician of Canada ( British Columbia),

USA The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

( Utah), Russia, and Morocco *** '' Stellostomites'' (sometimes considered a species of ''Eldonia''): early Cambrian of China (Yunnan) ** Rotadiscidae *** ''

Pararotadiscus ''Pararotadiscus'' is an abundant Eldonioid fossil from the mid-Cambrian, and one of the most abundant taxa in the Kaili biota. References {{Taxonbar, from=Q60790429 Cambroernids ...

'' (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 Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

** " Paropsonemids" *** '' Discophyllum'': Ordovician of USA (

New York New York most commonly refers to: * New York City, the most populous city in the United States, located in the state of New York * New York (state), a state in the northeastern United States New York may also refer to: Film and television * '' ...

) *** '' Paropsonema'': Silurian-Devonian of USA (New York) and Australia

References

{{Reflist Prehistoric deuterostomes Burgess Shale fossils