''Etacystis communis'', colloquially known as the H-animal or aitch, was a soft-bodied

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

that lived in shallow tropical coastal waters of muddy

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environment ...

during the Pennsylvanian

geological period The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronol ...

, about 300 million years ago. The classification is uncertain: the animal had a unique H-shaped body ranging from 2 to 11 cm long, and researchers have suggested a

hemichordate 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 ...

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 ...

affinity. Examples of ''Etacystis'' have been found only in the

Mazon Creek fossil beds The Mazon Creek fossil beds are a conservation ' found near Morris, in Grundy County, Illinois. The fossils are preserved in ironstone concretions, formed approximately in the mid- Pennsylvanian epoch of the Carboniferous period. These concretio ...

of the Midwestern United States.

Etymology

''Etacystis communis'' takes its genus name from its shape, being roughly similar in shape to the letter "H". ''Eta'' is the Greek letter "H" while ''cystis'', from the Greek ''kystis'', means sac or bladder, referring to the organism's sac-like body plan. The specific name refers to the relatively common occurrence of the animal in the Mazon Creek fossil beds. Only a single species has been described.

Description

''Etacystis'' is an unusual animal of uncertain affinities, found among the Essex fauna of the Mazon Creek beds. It displays, as described above, a roughly H-shaped body plan. It has a

stolon In biology, stolons (from Latin '' stolō'', genitive ''stolōnis'' – "branch"), also known as runners, are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external s ...

-like structure from which a peduncle arises at approximate right angles. The distal end of the peduncle has two arm-like extensions of unequal length, and a sac-like structure is attached to the peduncle by a short stalk, in the direction of the shorter of the two arms. Relatively large for the animals of its time, ''Etacystis'' fossils of up to 11 cm have been known, with incomplete specimens considered to have been up to 15 cm in life. The holotype is 5 cm across from tip of sac to tip of arm, and the smallest specimens have measured as little as 2 cm. The H-animal is most likely a colonial organism that continued to grow throughout its lifetime, explaining the wide variation in specimen sizes found. The exact shape of the main sac varies significantly from specimen to specimen, from lobe-shaped to heart-shaped and others.

Affinity

There is insufficient evidence to align ''Etacystis'' to any

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

with certainty. However, it is considered by Nitecki and Schram that there are similarities with the Hemichordata which includes the acorn worms, and an affinity to this group is considered likely. The organism may have a close affinity to the

Pterobranchia Pterobranchia is a class of small worm-shaped animals. They belong to the Hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water with the help of cilia attached to tentacles. The ...

class, which likely includes the

graptolite Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and ...

organisms, however it is not certain if it belongs within the class or is a sister clade. The modern pterobranchs usually only grow to about a centimetre, unlike the relatively large H-animals. An alternate opinion considers ''Etacystis'' to be a

hydrozoa 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 specializ ...

n, provisionally in the

Siphonophorae Siphonophorae (from Greek ''siphōn'' 'tube' + ''pherein'' 'to bear') is an order within Hydrozoa, which is a class of marine organisms within the phylum Cnidaria. According to the World Register of Marine Species, the order contains 175 specie ...

References

{{Taxonbar, from=Q22285659 Carboniferous animals of North America Enigmatic deuterostome taxa Fossil taxa described in 1976

Etymology

Description

Affinity

See also

References