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 The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

(

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 Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...

-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 Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some exampl ...

. 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 Detritivores (also known as detrivores, detritophages, detritus feeders, or detritus eaters) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). There are many kinds of invertebrates, ...

, 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 Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuria ...

), 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 ''Herpetogaster'' is an extinct cambroernid genus of animal from the Early Cambrian Chengjiang biota of China, Pioche Formation of Nevada and Middle Cambrian Burgess Shale of Canada containing the species ''Herpetogaster collinsi'' and ''Herpetog ...

'' 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 In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

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 Hemichordata is a phylum which consists of triploblastic, enterocoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include ...

. Eldoniidae was a

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

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 The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest foss ...

of Canada. It is also a major component of the

Late Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. The ...

Tafilalt biota of

Morocco Morocco (),, ) officially the Kingdom of Morocco, is the westernmost country in the Maghreb region of North Africa. It overlooks the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to ...

. ''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 Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...

), and ''

Paropsonema ''Paropsonema'' is a discoidal animal known from the Cambrian 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 ...

'' (mistaken for a jellyfish-like porpitoid

hydrozoan Hydrozoa (hydrozoans; ) are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specialized ...

). Eldoniids, in a broad definition, also include ''

Rotadiscus ''Rotadiscus'' is a genus of discoidal animal known from the Cambrian Chengjiang biota and classified with the eldoniid Eldoniids are an extinct clade of enigmatic disc-shaped animals which lived in the early to middle Paleozoic (Cambrian to D ...

'', ''

Stellostomites ''Stellostomites'' is a discoidal animal known from the Cambrian Chengjiang biota and classified with the eldoniid Eldoniids are an extinct clade of enigmatic disc-shaped animals which lived in the early to middle Paleozoic (Cambrian to Devonian ...

'', '' Pararotadiscus'', 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 ...

Yunnan Yunnan , () is a landlocked Provinces of China, province in Southwest China, the southwest of the People's Republic of China. The province spans approximately and has a population of 48.3 million (as of 2018). The capital of the province is ...

, and ''Pararotadiscus guizhouensis'' is from the Kaili Formation of

Guizhou Guizhou (; formerly Kweichow) is a landlocked province in the southwest region of the People's Republic of China. Its capital and largest city is Guiyang, in the center of the province. Guizhou borders the autonomous region of Guangxi to t ...

. 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 biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a ...

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 The Southeastern Pennsylvania Transportation Authority (SEPTA) is a regional public transportation authority that operates bus, rapid transit, commuter rail, light rail, and electric trolleybus services for nearly 4 million people in five coun ...

” or “

mesenteries In zoology, a mesentery is a membrane inside the body cavity of an animal. The term identifies different structures in different phyla: in vertebrates it is a double fold of the peritoneum enclosing the intestines; in other organisms it forms co ...

”. 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 A hydrostatic skeleton, or hydroskeleton, is a flexible skeleton supported by fluid pressure. Hydrostatic skeletons are common among simple invertebrate organisms. While more advanced organisms can be considered hydrostatic, they are sometimes refe ...

or (less likely) an echinoderm-like

water vascular system The water vascular system is a hydraulic system used by echinoderms, such as sea stars and sea urchins, for locomotion, food and waste transportation, and respiration. The system is composed of canals connecting numerous tube feet. Echinoderms mov ...

. 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 The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...

(front),

stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...

(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 In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

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 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.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 Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, whi ...

and

bryozoans Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a l ...

. 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 ''

'', 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 British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...

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 Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...

Russia Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...

, and Morocco *** ''

'' (sometimes considered a species of ''Eldonia''): early Cambrian of China (Yunnan) ** Rotadiscidae *** '' Pararotadiscus'' (previously considered a species of ''Rotadiscus''): middle Cambrian of China (Guizhou) *** ''

'': early to middle Cambrian of China (Yunnan) and

Poland Poland, officially the Republic of Poland, is a country in Central Europe. It is divided into 16 administrative provinces called voivodeships, covering an area of . Poland has a population of over 38 million and is the fifth-most populous ...

*** '' Seputus?'': Ordovician of

Ireland Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic Ocean, in Northwestern Europe, north-western Europe. It is separated from Great Britain to its east by the North Channel (Grea ...

*** '' Vellumbrella?'': early to middle Cambrian of Poland *** An unnamed ''Pararotadiscus''-like taxon from the early Cambrian

Emu Bay Shale The Emu Bay Shale is a geological formation in Emu Bay, South Australia, containing a major Konservat-Lagerstätte (fossil beds with soft tissue preservation). It is one of two in the world containing Redlichiidan trilobites. The Emu Bay Shale is ...

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 * '' ...

) *** ''

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

References

{{Reflist Prehistoric deuterostomes Burgess Shale fossils