Eldonioids or velumbrellids are an extinct

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

of disc-shaped

cambroernid The Cambroernida are a clade of 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, meaning that their affinities wi ...

s, the Eldonioidea (or Velumbrellida), which lived in the early to middle

Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...

(

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

). 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 Plankton are the diverse collection of organisms that drift in water (or air) but are unable to actively propel themselves against currents (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they pro ...

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.

Anatomy

Eldonioids are characterized by their "medusoid" (

jellyfish Jellyfish, also known as sea jellies or simply jellies, are the #Life cycle, medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animal ...

-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

deuterostomes Deuterostomes (from Greek: ) are bilaterian animals of the superphylum Deuterostomia (), typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia comprises three phyla: Chordata, Echinodermata, ...

. The basic eldonioid body plan is oriented around two circular discs sealed together with the

viscera In a multicellular organism, 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 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.

Disc structures

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 SEPTA, the Southeastern Pennsylvania Transportation Authority, is a regional public transportation authority that operates bus, rapid transit, commuter rail, light rail, and electric trolleybus services for nearly four million people througho ...

” or “

mesenteries In human anatomy, the mesentery is an organ that attaches the intestines to the posterior abdominal wall, consisting of a double fold of the peritoneum. It helps (among other functions) in storing fat and allowing blood vessels, lymphatics, a ...

”. 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 A hydrostatic skeleton or hydroskeleton is a type of skeleton supported by hydrostatic fluid pressure or liquid, common among soft-bodied organism, soft-bodied invertebrate animals colloquially referred to as "worms". While more advanced organisms ...

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 In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

. 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 tract is the largest of the body's systems, after the cardiovascular system. ...

(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 The esophagus (American English), oesophagus (British English), or œsophagus (Œ, archaic spelling) (American and British English spelling differences#ae and oe, see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), c ...

pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

(front),

stomach The stomach is a muscular, hollow organ in the upper gastrointestinal tract of Human, humans and many other animals, including several invertebrates. The Ancient Greek name for the stomach is ''gaster'' which is used as ''gastric'' in medical t ...

(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 tract is the largest of the body's systems, after the cardiovascular system. T ...

(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 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 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 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 such as

brachiopods Brachiopods (), phylum Brachiopoda, are a phylum of 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, while the fron ...

and

bryozoans Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...

. 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 The pituitary gland or hypophysis is an endocrine gland in vertebrates. In humans, the pituitary gland is located at the base of the human brain, brain, protruding off the bottom of the hypothalamus. The pituitary gland and the hypothalamus contr ...

. 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 Relative species abundance is a component of biodiversity and is a measure of how common or rare a species is relative to other species in a defined location or community.Hubbell, S. P. 2001. ''The unified neutral theory of biodiversity and biogeog ...

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 ''Phlogites'' is a member of the extinct ambulacrarian stem group Cambroernida, occupying an intermediate position between the basal ''Herpetogaster'' and the more derived Eldonioidea. It is known from the Cambrian, Lower Cambrian Haikou Chengji ...

''. Together, they comprise the cambroernids, an informal clade. Cambroernids are probably deuterostomes, specifically

stem Stem or STEM most commonly refers to: * Plant stem, a structural axis of a vascular plant * Stem group * Science, technology, engineering, and mathematics Stem or STEM can also refer to: Language and writing * Word stem, part of a word respon ...

- 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 animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larv ...

and hemichordates. Eldoniidae was a

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

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 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 fos ...

of Canada. It is also a major component of the Late Ordovician Tafilalt biota of

Morocco Morocco, officially the Kingdom of Morocco, is a country in the Maghreb region of North Africa. It has coastlines on the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to Algeria–Morocc ...

. ''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 language, Greek word ''skyphos'' (), denoting a kind of drinking cup and ...

(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 invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...

. Others include '' Velumbrella'' (mistaken for a jellyfish), '' Discophyllum'' (mistaken for a

coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...

), and '' Paropsonema'' (mistaken for a jellyfish-like porpitoid

hydrozoan Hydrozoa (hydrozoans; from Ancient Greek ('; "water") and ('; "animals")) is 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 sp ...

). Eldonioids, a broader definition than Eldoniids, also include ''

Rotadiscus ''Rotadiscus'' is a genus of discoidal animal known from the Cambrian Chengjiang biota and classified with the eldonioids. As with other eldonioids, it was originally thought to have been pelagic, but is now thought to be benthic. In addition ...

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

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

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

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

Yunnan Yunnan; is an inland Provinces of China, province in Southwestern China. The province spans approximately and has a population of 47.2 million (as of 2020). The capital of the province is Kunming. The province borders the Chinese provinces ...

, and ''Pararotadiscus guizhouensis'' is from the

Kaili Formation The Kaili Formation () is a Formation (stratigraphy), stratigraphic formation which was deposited during the Lower and Middle Cambrian (~513 to 506 million years ago). The formation is approximately thick and was named after the city Kaili City, ...

Guizhou ) , image_skyline = , image_caption = , image_map = Guizhou in China (+all claims hatched).svg , mapsize = 275px , map_alt = Map showing the location of Guizhou Province , map_caption = Map s ...

. Dzik (1989) named the class Eldonioidea to encompass "velumbrellids" (i.e. eldoniids) and the goblet-shaped Cambrian animal ''

Dinomischus ''Dinomischus ''is an extinct genus of stalked filter-feeding animals within the Cambrian period, with specimens known from the Burgess Shale and the Maotianshan Shales. While long of uncertain affinities, recent studies have suggested it to be a ...

''. Eldonioidea was divided further into the orders Dinomischida (containing solely ''Dinomischus'') and Velumbrellida. 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 British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...

), USA (

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

, 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) *** ''

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

Poland Poland, officially the Republic of Poland, is a country in Central Europe. It extends from the Baltic Sea in the north to the Sudetes and Carpathian Mountains in the south, bordered by Lithuania and Russia to the northeast, Belarus and Ukrai ...

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

Ireland Ireland (, ; ; Ulster Scots dialect, Ulster-Scots: ) is an island in the North Atlantic Ocean, in Northwestern Europe. Geopolitically, the island is divided between the Republic of Ireland (officially Names of the Irish state, named Irelan ...

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

Emu Bay Shale The Emu Bay Shale is a Formation (stratigraphy), 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 trilob ...

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

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

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

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

References

Works cited

* * * * * * * * * {{Taxobar, from=Q131521311 Cambroernids Burgess Shale fossils