Hyoliths are

animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...

s with small

conical A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex. A cone is formed by a set of line segments, half-lines, or lines c ...

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses ** Thin-shell structure Science Biology * Seashell, a hard o ...

s, known as fossils from the

Palaeozoic 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 '' ...

era. They are at least considered as

lophotrochozoa Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, bra ...

n, and possibly being

lophophorate The Lophophorata are a Lophotrochozoan clade consisting of the Brachiozoa and the Bryozoa. They have a lophophore. Molecular phylogenetic analyses suggest that lophophorates are protostomes Protostomia () is the clade of animals once though ...

s, a group which includes the

brachiopod 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, w ...

s, while others consider them as being basal lophotrochozoans, or even

molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estim ...

Morphology

The shell of a hyolith is typically one to four centimeters in length, triangular or elliptical in cross section. Some species have rings or stripes. It comprises two parts: the main conical shell (previously referred to as a ‘conch’) and a cap-like operculum. Some also had two curved supports known as ''helens'' They are

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an ad ...

– probably aragonitic All of these structures grew by marginal accretion.

Shell microstructure

The orthothecid shell has an internal layer with a microstructure of transverse bundles, and an external layer comprising longitudinal bundles.

Helens

Some hyoliths had helens, long structures that taper as they coil gently in a

logarithmic spiral A logarithmic spiral, equiangular spiral, or growth spiral is a self-similar spiral curve that often appears in nature. The first to describe a logarithmic spiral was Albrecht Dürer (1525) who called it an "eternal line" ("ewige Linie"). More ...

in a ventral direction. The helens had an organic-rich central core surrounded by concentric laminae of calcite. They grew by the addition of new material at their base, on the cavity side, leaving growth lines. They were originally described by Walcott as separate fossils under the genus name ''Helenia'', (Walcott's wife was named Helena and his daughter Helen);

Bruce Runnegar Bruce Norman Runnegar (born 2 February 1941, Brisbane) is an Australian-born paleontologist and professor at UCLA. His research centers on using the fossil record to determine how, where, and when life originated and evolved. He has published on ...

adopted the name helen when they were recognized as part of the hyolith organism. Encrusting organisms have been found on helens, and also on both sides of the main shell, all of which are therefore supposed to have been raised above the sea bed. The helens have been interpreted as props that supported the feeding organ, the

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.

Operculum

The operculum closes over the aperture of the shell, leaving (in hyolithids) two gaps through which the helens can protrude. It comprises two parts: the cardinal shield, a flat region at the top of the shell; and the conical shield, the bottom part, which is more conical. The inside of the shell bears a number of protrusions, notably the dorsal cardinal processes and the radially-arranged clavicles.

Soft tissues

The soft tissues of the mid-Cambrian hyolith ''Haplophrentis'', from the

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 fo ...

and Spence Shale Lagerstätten include a gullwing-shaped band below the operculum. This band is interpreted as a lophophore, a feeding organ with a central mouth; it bears 12 to 16 tentacles. From the mouth a muscular pharynx leads to a gut, which loops back and exits beyond the crown of tentacles. Next to the gut are a pair of large kidney-shaped organs of uncertain nature. Under the operculum are muscles. The thin body wall circumscribes the interior of the shell, except the apex. Preserved intestines have been described from the Ordovician hyolith ''Girvanolithes thraivensis''.

Taxonomy

The hyoliths are divided into two orders, the

Hyolithida The Hyolithida are lophophorate The Lophophorata are a Lophotrochozoan clade consisting of the Brachiozoa and the Bryozoa. They have a lophophore. Molecular phylogenetic analyses suggest that lophophorates are protostomes Protostomia () i ...

and the

Orthothecida The orthothecids are one of the two hyolith Hyoliths are animals with small conical shells, known as fossils from the Palaeozoic era. They are at least considered as lophotrochozoan, and possibly being lophophorates, a group which includes t ...

. Hyolitha have dorso-ventrally differentiated opercula, with the ventral surface of the shell extending forwards to form a shelf termed the ligula. The Orthothecida are somewhat more problematic, and probably contain a number of non-hyoliths simply because they are so difficult to identify with confidence, especially if their operculum is absent. They have a straight (planar) opening, sometimes with a notch on the bottom side, and sealed with an operculum that has no ligula, clavicles, furrow or rooflets. ''Hyptiotheca'' is an unusual hyolithid, in that it lacks clavicles. Orthothecids fall into two groups: one, the orthothecida ''sensu stricto'', is kidney or heart shaped in cross-section due to a longitudinal groove on its ventral surface, and its opercula bear cardinal processes; the other has a rounded cross-section and often lacks cardinal processes, making them difficult to distinguish from other cornet-shaped calcareous organisms. All were sessile and benthic; some may have been filter feeders.

Phylogenetic position

Because hyoliths are extinct and do not obviously resemble any

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

group, it has long been unclear which living group they are most closely related to. They have been supposed to be molluscs; or to belong to their own phylum in an unspecified part of the tree of life. Their grade of organization was historically considered to be of the 'mollusc-annelid-sipunculid' level, consistent with a Lophotrochozoan affinity, and comparison was primarily drawn with the molluscs or

sipunculid The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a class containing about 162 species of unsegmented marine annelid worms. The name ''Sipuncula'' is from the genus name '' Sipunculus'', and comes from the Latin ...

s. Older studies (predating the Lophotrochozoan concept) consider hyoliths to represent a stem lineage of the clade containing (Mollusca + Annelida + Arthropoda). A secure classification at last became possible in 2017, on the basis of

specimens that preserve lophophores. This diagnostic characteristic demonstrates an affinity with the

Lophophorata The Lophophorata are a Lophotrochozoan clade consisting of the Brachiozoa and the Bryozoa. They have a lophophore. Molecular phylogenetic analyses suggest that lophophorates are protostomes, but on morphological grounds they have been assessed ...

, a group that contains Brachiopoda,

Bryozoa 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 ...

(perhaps), and Phoronida. A study in 2019 estimated that hyoliths are more likely to be basal members of the

ns rather than lophophorates. Meanwhile, a study in 2020 instead concluded that hyoliths belong to

Mollusca Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is es ...

, as did a different study in 2022.

Ecology

Hyolithids were benthic (bottom-dwellers), using their helens as stilts to hold the opening of their shells above the sea floor. Orthothecids did not have helens, but are presumed to have been sessile and benthic. In the Cambrian, their global distribution shows no sign of provinciality, suggesting a long-lived planktonic larval life stage (reflected by their protoconchs); but by the Ordovician distinct assemblages were becoming evident. Some orthothecids are preserved in vertical ifeorientation, suggesting a sessile suspension-feeding habit; hyolithids tend to be flat on the bottom, and their shape and the occurrence of epibionts are consistent with a sessile suspension feeding habit via orientation relative to passive currents.

Occurrence

The first hyolith fossils appeared about in the ''Purella antiqua'' Zone of the Nemakit-Daldynian Stage of

Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part ...

and in its analogue the ''Paragloborilus subglobosus–Purella squamulosa'' Zone of the

Meishucunian The Cambrian Period ( ; sometimes symbolized 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 years ago ( ...

Stage of

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...

. Hyolith abundance and diversity attain a maximum in the

Cambrian The Cambrian Period ( ; sometimes symbolized 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 years ago ...

, followed by a progressive decline up to their

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...

extinction.

References

{{Taxonbar, from=Q921087 Protostome enigmatic taxa Paleozoic invertebrates Animal phyla Cambrian first appearances Paleozoic life of Ontario Paleozoic life of British Columbia Paleozoic life of New Brunswick Paleozoic life of Nova Scotia Paleozoic life of Nunavut Paleozoic life of Quebec Paleozoic life of Yukon