{{short description|Palaeozoic lophophorates with small conical shells}} {{Use dmy dates|date=January 2017}} {{automatic taxobox | fossil_range = Fortunian to end Permian, {{fossilrange|536|{{Rnd|{{Period end|Permian}}|1}}|ref=<ref name="kouchinsky2012">{{cite journal |doi=10.1017/S0016756811000720|title=Chronology of early Cambrian biomineralization |journal=Geological Magazine |volume=149 |issue=2 |pages=221–251 |date=March 2012 |last1=Kouchinsky |first1=A. |last2=Bengtson |first2=S. |last3=Runnegar |first3=B. |last4=Skovsted |first4=C. |last5=Steiner |first5=M. |last6=Vendrasco |first6=M.|bibcode=2012GeoM..149..221K |doi-access=free }}</ref><ref name="ref_" /><ref name="Runnegar1975">{{cite journal |doi=10.1111/j.1502-3931.1975.tb01311.x|title=Biology of the Hyolitha|journal=Lethaia|volume=8|issue=2|page=181|year=1975|last1=Runnegar|first1=Bruce|last2=Pojeta|first2=John|last3=Morris|first3=Noel J.|last4=Taylor|first4=John D.|last5=Taylor|first5=Michael E.|last6=McClung|first6=Graham|bibcode=1975Letha...8..181R }}</ref>}} | image = Hyoliths01.JPG | image_caption = ''Hyolithes cerops'', Spence Shale, Idaho (Middle Cambrian) | taxon = Hyolitha | authority = Marek, 1963 | subdivision_ranks = Orders | subdivision = See text. }}
'''Hyoliths''' are an extinct group of invertebrates with small conical shells, known from fossils from the Palaeozoic era. They are at least considered lophotrochozoans, possibly being lophophorates, a group which includes the brachiopods (hyoliths may even be brachiopods themselves<ref name=brachiopodsatlast>{{cite journal |last1=Smith |first1=Martin R |title=Finding a home for hyoliths |journal=National Science Review |date=1 February 2020 |volume=7 |issue=2 |pages=470–471 |doi=10.1093/nsr/nwz194|pmid=34692061 |pmc=8288929 }}</ref>), while others consider them as being basal lophotrochozoans, or even molluscs.<ref name="ReferenceA" /><ref name=":0">{{Cite journal |last1=Liu |first1=Fan |last2=Skovsted |first2=Christian B |last3=Topper |first3=Timothy P |last4=Zhang |first4=Zhifei |last5=Shu |first5=Degan |date=2020-02-01 |title=Are hyoliths Palaeozoic lophophorates? |journal=National Science Review |volume=7 |issue=2 |pages=453–469 |doi=10.1093/nsr/nwz161 |pmid=34692060 |pmc=8289160 |issn=2095-5138}}</ref><ref name="LiSkovstedTopper2022">{{cite journal |last1=Li |first1=Luoyang |last2=Skovsted |first2=Christian B. |last3=Topper |first3=Timothy |date=August 2022 |title=Deep origin of the crossed-lamellar microstructure in early Cambrian molluscs |url=https://www.researchgate.net/publication/362983334 |journal=Palaeontology |volume=65 |issue=4 |article-number=12620 |doi=10.1111/pala.12620 |bibcode=2022Palgy..6512620L |s2cid=251866827 |access-date=25 November 2022}}</ref>
==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''<ref name="ReferenceA"/> They are calcareous – probably aragonitic<ref name="Mus2007">{{Cite journal|pages= 1231–1243|doi=10.1111/j.1475-4983.2007.00700.x|title=Skeletal Microstructure of Helens, Lateral Spines of Hyolithids|journal=Palaeontology|volume=50|issue=5|date=September 2007|last1=Mus|first1=M. Martí|last2=Bergström|first2=J.|doi-access=free|bibcode=2007Palgy..50.1231M }}</ref> 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.<ref name="Mus2007" />
=== Helens === Some hyoliths had helens, long structures that taper as they coil gently in a logarithmic spiral in a ventral direction.<ref name="Mus2007" /><ref name="ReferenceA"/> 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.<ref name="Mus2007" />
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 adopted the name helen when they were recognized as part of the hyolith organism.<ref name="Mus2007" /> 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, above the sea bed.<ref name="ReferenceA">{{cite journal|last1=Moysiuk|first1=Joseph|last2=Smith|first2=Martin R.|last3=Caron|first3=Jean-Bernard|title=Hyoliths are Palaeozoic lophophorates|journal=Nature|date=11 January 2017|volume=541|issue=7637|pages=394–397|doi=10.1038/nature20804|pmid=28077871|bibcode=2017Natur.541..394M |s2cid=4409157 |url=http://dro.dur.ac.uk/20195/1/20195.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://dro.dur.ac.uk/20195/1/20195.pdf |archive-date=2022-10-09 |url-status=live}}</ref>
=== Operculum === The operculum closes over the aperture of the shell, leaving (in hyolithids) two gaps through which the helens can protrude.<ref name="Mus2007" /> 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.<ref name="Mus2007" /> The inside of the shell{{fix|text=the inside of the operculum?}} bears a number of protrusions, notably the dorsal cardinal processes and the radially-arranged clavicles.<ref name="Mus2007" /> [[File:Haplophrentis.png|thumb|Reconstruction of ''Haplophrentis'', hyolith with known soft tissue]]
===Soft tissues=== The soft tissues of the mid-Cambrian hyolith ''Haplophrentis'', from the Burgess Shale 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.<ref name="ReferenceA"/> Preserved intestines have been described from the Ordovician hyolith ''Girvanolithes thraivensis''.<ref name="Malinky2003">{{cite journal |last1=Malinky |first1=John M. |date=July 2003 |title=Ordovician and Silurian hyoliths and gastropods reassigned from the Hyolitha from the Girvan district, Scotland |url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/abs/ordovician-and-silurian-hyoliths-and-gastropods-reassigned-from-the-hyolitha-from-the-girvan-district-scotland/3B5962257238EC74BB2CA143B69AB2C0 |journal=Journal of Paleontology |volume=77 |issue=4 |pages=625–645 |doi=10.1666/0022-3360(2003)077<0625:oashag>2.0.co;2 |access-date=26 November 2022|url-access=subscription }}</ref>
== Taxonomy == {{See also|List of bilaterial animal orders}} [[Image:Hyoliths02.JPG|thumb|250px|Hyoliths from the Middle Ordovician of northern Estonia; these are internal molds.]] The hyoliths are divided into two orders, the Hyolithida and the Orthothecida.
Hyolitha have dorso-ventrally differentiated opercula, with the ventral surface of the shell extending forwards to form a shelf termed the ligula.<ref name="Mus2007" />
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.<ref name="Mus2007" /> 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.<ref name=Malinky2004/>
''Hyptiotheca'' is an unusual hyolithid, in that it lacks clavicles.<ref name="Malinky2004">{{Cite journal | author1=Malinky, J.M. | author2=Skovsted, C.B. | title = Hyoliths and small shelly fossils from the Lower Cambrian of North−East Greenland | journal = Acta Palaeontologica Polonica |volume=49 |issue=4 |pages=551–578 | year=2004|url=http://app.pan.pl/article/item/app49-551.html}}</ref>
Orthothecids fall into two groups: one, the orthothecida ''sensu stricto'',<ref name="ref_a">{{Cite journal| last1 = Malinky | first1 = J. M.| title = First Occurrence of ''Orthotheca'' Novák, 1886 (Hyolitha, Early Devonian) in North America| journal = Journal of Paleontology| volume = 83| issue = 4| pages = 588–596| year = 2009| doi = 10.1666/08-164R.1| bibcode = 2009JPal...83..588M| s2cid = 130227630}}</ref> 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.<ref name=Malinky2004/> All were sessile and benthic; some may have been filter feeders.<ref name="ref_a" />
== Phylogenetic position == [[File:Haplophrentis Burgess Shale.jpg|thumb|''Haplophrentis carinatus'' from the Stephen Formation, Burgess Shale (Middle Cambrian), Burgess Pass, British Columbia, Canada.]] Because hyoliths are extinct and do not obviously resemble any extant 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 phylogenetic tree.<ref name=Sumrall2009>{{cite journal |pages=147–152 |doi=10.1666/08-094R.1 |jstor=29739075 |title=Permian Hyolithida from Australia: The Last of the Hyoliths? |journal=Journal of Paleontology |volume=83 |issue=1 |year=2009 |last1=Malinky |first1=John M.|bibcode=2009JPal...83..147M }}</ref> Their grade of organization was historically considered to be of the 'mollusc-annelid-sipunculid' level,<ref name="Runnegar1980">{{cite journal |last1=Runnegar |first1=B. |doi=10.1111/j.1502-3931.1980.tb01025.x |title=Hyolitha: Status of the phylum |journal=Lethaia |volume=13 |pages=21–25 |date=January 1980 |issue=1 |bibcode=1980Letha..13...21R }}</ref> consistent with a Lophotrochozoan affinity, and comparison was primarily drawn with the molluscs or sipunculids.<ref name="Runnegar1980" /><ref name="Kouchinsky2000">{{cite journal |last1=Kouchinsky |first1=A. V. |title=Skeletal microstructures of hyoliths from the Early Cambrian of Siberia |doi=10.1080/03115510008619525 |journal=Alcheringa: An Australasian Journal of Palaeontology |volume=24 |issue=2 |pages=65–81 |year=2000 |bibcode=2000Alch...24...65K |s2cid=140660142 }}</ref> Older studies (predating the Lophotrochozoan concept) consider hyoliths to represent a stem lineage of the clade containing (Mollusca + Annelida + Arthropoda).<ref name="Runnegar1975" />
A secure classification at last became possible in 2017, on the basis of Burgess Shale specimens that preserve lophophores. This diagnostic characteristic demonstrates an affinity with the Lophophorata, a group that contains Brachiopoda, Bryozoa (perhaps), and Phoronida.<ref name="ReferenceA"/>
A study in 2019 estimated that hyoliths are more likely to be basal members of the lophotrochozoans rather than lophophorates.<ref name=":0" /> Meanwhile, a study in 2020 instead concluded that hyoliths belong to Mollusca,<ref name=":1">{{Cite journal |last1=Li |first1=Luoyang |last2=Skovsted |first2=Christian B. |last3=Yun |first3=Hao |last4=Betts |first4=Marissa J. |last5=Zhang |first5=Xingliang |date=2020-08-26 |title=New insight into the soft anatomy and shell microstructures of early Cambrian orthothecids (Hyolitha) |journal=Proceedings of the Royal Society B: Biological Sciences |volume=287 |issue=1933 |article-number=20201467 |doi=10.1098/rspb.2020.1467 |pmc=7482263 |pmid=32811320}}</ref> as did a different study in 2022.<ref name="LiSkovstedTopper2022" />
== Ecology == Hyolithids were benthic (bottom-dwellers), using their helens as stilts to hold the opening of their shells above the sea floor.<ref name="ReferenceA"/> 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.<ref name=Malinky2004/>
Some orthothecids are preserved in vertical [life] orientation, 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.<ref>{{cite book |last=Kouchinsky |first=A.V. |year=2000 |chapter=Mollusks, hyoliths, stenothecoids and coeloscleritophorans |editor1=A. Yu. Zhuravlev |editor2=R. Riding |title=The Ecology of the Cambrian Radiation |publisher=Columbia University Press |pages=326–349|doi=10.7312/zhur10612-015 |series=The Critical Moments and Perspectives in Earth History and Paleobiology|isbn=978-0-231-50516-1}}</ref>
==Occurrence== The first hyolith fossils appeared about {{ma|540}} in the ''Purella antiqua'' Zone of the Nemakit-Daldynian Stage of Siberia and in its analogue the ''Paragloborilus subglobosus–Purella squamulosa'' Zone of the Meishucunian Stage of China. Hyolith abundance and diversity attain a maximum in the Cambrian, followed by a progressive decline up to their Permian extinction.<ref name="ref_">{{Cite journal | last1 = Malinky | first1 = J. M. | doi = 10.1666/08-094R.1 | title = Permian Hyolithida from Australia: The Last of the Hyoliths? | journal = Journal of Paleontology | volume = 83 | pages = 147–152 | year = 2009 | bibcode = 2009JPal...83..147M }}</ref><ref name="Steiner2007">{{Cite journal | doi = 10.1016/j.palaeo.2007.03.046| last1 = Steiner | first1 = M. | last2 = Li | first2 = G. | last3 = Qian | first3 = Y. | last4 = Zhu | first4 = M. | last5 = Erdtmann | first5 = B. D. | title = Neoproterozoic to Early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China) | journal = Palaeogeography, Palaeoclimatology, Palaeoecology | volume = 254 | issue = 1–2 | page = 67 | year = 2007| bibcode = 2007PPP...254...67S }}</ref>
== Similar organisms == Due to the simple shape of their shell, hyoliths have been something of a wastebasket taxon, and organisms originally interpreted as hyoliths have sometimes later been recognized as something else – as for example in the case of the cnidarian-like ''Glossolites''<ref>{{cite journal | doi = 10.1002/spp2.1473 | title = Anatomy, palaeoautecology and phylogenetic affinity of tubular Glossolites magnus from the early Cambrian Chengjiang biota, South China | year = 2022 | last1 = Sun | first1 = Haijing | last2 = Zhao | first2 = Fangchen | last3 = Zhu | first3 = Maoyan | journal = Papers in Palaeontology | volume = 8 | issue = 6 | bibcode = 2022PPal....8E1473S | s2cid = 255125977 }}</ref> and ''Palaeoconotuba''.<ref>{{Cite journal | doi = 10.1111/pala.12636 | title = Thecate stem medusozoans (Cnidaria) from the early Cambrian Chengjiang biota | year = 2023 | last1 = Qu | first1 = Hanzhi | last2 = Li | first2 = Kexin | last3 = Ou | first3 = Qiang | journal = Palaeontology | volume = 66 | issue = 1 | article-number = 12636 | s2cid = 256562444 | doi-access = free | bibcode = 2023Palgy..6612636Q }}</ref> ==See also== * List of hyolith genera
==References== {{Reflist}}
{{Portal bar|Evolutionary biology|Paleontology}} {{Taxonbar|from=Q921087}}
Category:Hyolitha Category:Enigmatic protostome taxa Category:Paleozoic invertebrates Category:Protostome phyla Category:Cambrian first appearances Category:Paleozoic life of Ontario Category:Paleozoic life of British Columbia Category:Paleozoic life of New Brunswick Category:Paleozoic life of Nova Scotia Category:Paleozoic life of Nunavut Category:Paleozoic life of Quebec Category:Paleozoic life of Yukon