Editing Microdictyon

{{Short description|Extinct genus of lobopodian worm}}

{{Automatic taxobox
| fossil_range = {{fossilrange|Cambrian Stage 3|Middle Cambrian}}<!-- only include exact numbers if these can be supported by a reference.-->
| image = 20210915 Microdictyon sinicum diagrammatic reconstruction.png
| image_caption = Diagrammatic reconstruction of ''Microdictyon sinicum''
| image2 = Molting in Microdictyon sinicum from the early Cambrian Chengjiang Lagerstätte.png
| image2_caption = ''Microdictyon sinicum'' fossil<ref name="chen2024" />
| taxon = Microdictyon
| authority = Bengtson, Matthews & Missarzhevsky, 1986
| type_species = '''''Microdictyon effusum'''''
| type_species_authority = Bengtson, Matthews & Missarzhevsky, 1986
| subdivision_ranks = Species
| subdivision = *See text
}}

{{langnf|el|'''Microdictyon'''|small net|links=no}} is an extinct genus of [[Lobopodia|lobopodian]] worm characterized by its net-like [[sclerite]] armour plates, known from [[Cambrian]] deposits around the world. Soft-bodied fossils which preserve more than the sclerites are only known from the [[Maotianshan Shales|Chengjiang Lagerstätte]] of [[Yunnan]], [[China]].

== History ==

''Microdictyon'' [[sclerite]] plates have been recovered from around the globe, recovered from rock via acid dissolution which eats away at the rock but leaves behind compositionally distinct [[Microfossil|microfossils]]. The first of them were found in the ''[[Strenuella]]'' Limestone of [[Comley]], [[England]], in 1975.<ref name="pan2018">{{cite journal |last1=Pan |first1=Bing |last2=Topper |first2=Timothy P. |last3=Skovsted |first3=Christian B. |last4=Miao |first4=Lanyun |last5=Li |first5=Guoxiang |title=Occurrence of Microdictyon from the lower Cambrian Xinji Formation along the southern margin of the North China Platform |journal=Journal of Paleontology |date=January 2018 |volume=92 |issue=1 |pages=59–70 |doi=10.1017/jpa.2017.47 |bibcode=2018JPal...92...59P |url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/abs/occurrence-of-microdictyon-from-the-lower-cambrian-xinji-formation-along-the-southern-margin-of-the-north-china-platform/A6B2623E8BCB41BC8F50387DE708607A |language=en |issn=0022-3360}}</ref> The genus ''Microdictyon'' was erected by Stefan Bengston, Vladimir Missarzhevsky, and S. C. Matthews in 1981, as an enigmatic net-like microfossil, based on a few isolated plates from [[Turkistan Region|South Kazakhstan]], although this description lacked a type species and proper description, so a following publication by the same authors in 1986 corrected this.<ref name="pan2018" /><ref name="demidenko2006">{{cite journal |last1=Demidenko |first1=Yu. E. |title=New Cambrian lobopods and chaetognaths of the Siberian Platform |journal=Paleontological Journal |date=1 May 2006 |volume=40 |issue=3 |pages=234–243 |doi=10.1134/S0031030106030026 |bibcode=2006PalJ...40..234D |url=https://link.springer.com/article/10.1134/S0031030106030026 |language=en |issn=1555-6174|url-access=subscription }}</ref> It was unknown at the time what animal could have produced it - suggestions for its producer included [[Echinoderm|echinoderms]], [[Demosponge|sponges]],<ref name="mcmenamin1984">{{cite thesis |last=McMenamin|first=Mark |title=Paleontology and stratigraphy of Lower Cambrian and Upper Proterozoic sediments, Caborca region, northwestern Sonora, Mexico |date=1984 |degree=PhD |publisher=University of California}}</ref> and [[Radiolaria|radiolarians]].<ref name="junyuan1995" /> A number of ''Microdictyon'' species were named in the following years. The genus ''Eoconcharium'' was named in 1987 based on fossils from China, and a family was erected to contain it, the [[Eoconchariidae]]. Although it was later recognized as a junior synonym of the earlier named ''Microdictyon'', the name of the family group Eoconchariidae, which today contains ''Microdictyon'', ''[[Fusuconcharium]]'', and ''[[Quadratapora]]'', kept priority. The first complete specimens of Microdictyon were found in the [[Maotianshan Shales|Chengjiang Biota]] of Yunnan, China. The two soft bodied fossils, which showed the characteristic Microdictyon plates armouring the body of a caterpillar-like worm, were discovered in 1989 and given the name ''Microdictyon sinicum''. A more complete description based on over 70 newly discovered fossils, also from Chengjiang, was published in 1995.<ref name="junyuan1995">{{cite journal |last1=Jun-Yuan |first1=Chen |last2=Gui-Qing |first2=Zhou |last3=Ramsköld |first3=Lars |title=The Cambrian lobopodian Microdictyon sinicum |journal=Bulletin of National Museum of Natural Science |date=1995 |volume=5 |pages=1–93}}</ref> Additional soft-bodied fossils of ''Microdictyon'' were reported from the [[Kaili Biota]] in 1999, which have not been assigned to a species.<ref name="zhang1999">{{cite journal |last1=Yuanlong |first1=Zhao |last2=Maoyan |first2=Zhu |last3=Babcock |first3=Loren E. |last4=Jinliang |first4=Yuan |last5=Parsley |first5=Ronald L. |last6=Jin |first6=Peng |last7=Xinglian |first7=Yang |last8=Yue |first8=Wang |title=Kaili Biota: A Taphonomic Window on Diversification of Metazoans from the Basal Middle Cambrian: Guizhou, China |journal=Acta Geologica Sinica - English Edition |date=2005 |volume=79 |issue=6 |pages=751–765 |doi=10.1111/j.1755-6724.2005.tb00928.x |bibcode=2005AcGlS..79..751Z |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1755-6724.2005.tb00928.x |language=en |issn=1755-6724|url-access=subscription }}</ref>

== Description ==
[[File:Microdictyon sinicum from the early Cambrian Chengjiang Lagerstätte.png|thumb|left|Moulting specimens of ''Microdictyon sinicum''.<ref name="chen2024" />]]
[[File:Assumed molting process in Microdictyon.png|thumb|left|Assumed moulting process of ''Microdictyon''<ref name="chen2024" />]]

Excluding their sclerites, ''Microdictyon'' had a relatively simple, conserved body plan among lobopodians. Based on fossils of ''Microdictyon sinicum'', the trunk was elongate, up to {{convert|77|mm}} in length (although other species may have been smaller or larger), with 10 pairs of tube-like limbs and 9 pairs of netted sclerite plates. Each limb pair was associated with a single pair of plates, except for the posterior-most set of limbs which shared a sclerite pair with the penultimate leg pair. The legs are undifferentiated, tipped with a pair of small, curved claws. The legs were finely [[Annulus (zoology)|annulated]], and ornamented further with small [[Onychophora#Skin_and_muscle|papillae]]. The trunk itself was also annulated and dotted with small papillae between the limb pairs. At each sclerite plate pair, the body swells slightly to accommodate it, starting at a negligible height and becoming larger posteriorly as the plates increase in diameter. These plate-bearing mounds, and the area where the limbs attach below them, are devoid of annulation. The sclerite plates are subcircular and convex outwardly. They are composed of a fine, roughly hexagonal mesh of rounded hollow pits and specialized nodes at the mesh intersections. The nodes vary in morphology between species, but are typically mushroom-shaped.<ref name="junyuan1995" /> The walls of the sclerite were thin, and the sclerotized bars between nodes may have further bumps on their surface, and the entire mesh is surrounded by a thin marginal rim. Rarely, the plates may develop large spines randomly on their surface.<ref name="zhang2007">{{cite journal |last1=Zhang |first1=Xi-Guang |last2=Aldridge |first2=Richard J. |title=Development and Diversification of Trunk Plates of the Lower Cambrian Lobopodians |journal=Palaeontology |date=2007 |volume=50 |issue=2 |pages=401–415 |doi=10.1111/j.1475-4983.2006.00634.x |language=en |issn=1475-4983|doi-access=free |bibcode=2007Palgy..50..401Z }}</ref> The head and neck were smooth or only finely annulated, with a small mouth set slightly ventrally at the anterior of the head. No evidence for eyes, cephalic appendages, or any structure resembling teeth or jaws has been reported. The gut was simple and straight, with the anus found at the end of a small posterior tail-like projection.<ref name="junyuan1995" />

== Ecology ==
''Microdictyon'' was [[epibenthic]], walking along the seafloor using is [[Lobopodia|lobopodous]] limbs. The anterior portion of the body was the most flexible - fossils showing ''Microdictyon'' strongly curled up indicate that the cause of death was [[Anoxic waters|anoxia]], inferred from studies on modern [[Arthropod|arthropods]] and [[Annelid|annelids]].<ref name="junyuan1995" /> The claws of ''Microdictyon'' are not particularly suited for walking on muddy substrate, but rather for climbing or grabbing — it was once proposed that, based on frequent association with fossils of ''[[Eldonia]]'', and a presumed [[Pelagic zone|pelagic]] lifestyle for the [[jellyfish]]-like ''Eldonia'', that ''Microdictyon'' would use its claws to grab onto the large disc-shaped animal as it swam in order to feed on it. However, ''Eldonia'' is no longer thought to be pelagic, and probably lived gregariously on the seafloor. The frequent association with ''Microdictyon'' still likely indicates an ecological link, with ''Microdictyon'' possibly using its claws to feed on ''Eldonia'' or other carcasses — a similar relationship has been proposed for ''[[Paucipodia]]''. ''Paucipodia'' and ''Microdictyon'' have also been found preserved alongside mass concentrations of living, dead, and decaying ''[[Cricocosmia]]'' worms.<ref name="vannier2017">{{cite journal |last1=Vannier |first1=Jean |last2=Martin |first2=Emmanuel L. O. |title=Worm-lobopodian assemblages from the Early Cambrian Chengjiang biota: Insight into the "pre-arthropodan ecology"? |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=15 February 2017 |volume=468 |pages=373–387 |doi=10.1016/j.palaeo.2016.12.002 |bibcode=2017PPP...468..373V |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018216308136 |issn=0031-0182}}</ref>

[[File:Secondarily phosphatized cuticular plates in Microdictyon sp. from the early Cambrian Xinji Formation, Shuiyu section, Shanxi Province, China.png|thumb|right|''Microdictyon'' sclerite plates from the Cambrian Xinji Formation.<ref name="chen2024" />]]

The purpose and development of ''Microdictyon'''s sclerotized plates remains a subject of debate. Some authors, beginning with Jerzy Dzik in 2003,<ref name="dzik2003">{{cite journal |last1=Dzik |first1=Jerzy |title=Early Cambrian lobopodian sclerites and associated fossils from Kazakhstan |journal=Palaeontology |date=2003 |volume=46 |issue=1 |pages=93–112 |doi=10.1111/1475-4983.00289 |bibcode=2003Palgy..46...93D |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/1475-4983.00289 |language=en |issn=1475-4983|url-access=subscription }}</ref> propose that based on a similar proposed method of sclerite growth and a similar morphology, these sclerites could be interpreted as large [[Compound eye|compound eyes]], similar to [[Phacopida|phacopid trilobites]]<ref name="eyessssss">{{cite journal |last1=Schoenemann |first1=Brigitte |last2=Clarkson |first2=Euan N. K. |title=The median eyes of trilobites |journal=Scientific Reports |date=8 March 2023 |volume=13 |issue=1 |pages=3917 |doi=10.1038/s41598-023-31089-7 |pmid=36890176 |language=en |issn=2045-2322|pmc=9995485 |bibcode=2023NatSR..13.3917S }}</ref> — this idea has been widely rejected.<ref name="Zhang2007">{{Cite journal | last1 = Zhang | first1 = X. -G. | last2 = Aldridge | first2 = R. J. | doi = 10.1111/j.1475-4983.2006.00634.x | title = Development and Diversification of Trunk Plates of the Lower Cambrian Lobopodians | journal = Palaeontology | volume = 50 | issue = 2 | pages = 401 | year = 2007 | bibcode = 2007Palgy..50..401Z | s2cid = 85293118 | doi-access = free }}</ref> The other proposals, that being defense and or muscle attachment, are viewed as much more plausible. Evidence from a small number of specimens, including both complete fossils from the Chengjiang as well as isolated microfossil plates, indicate that ''Microdictyon'' and other eoconchariids [[Ecdysis|moulted]] their plates asynchronously, with the new, larger plates forming beneath the old ones, starting with the plates in the middle of the body and spreading to the anterior and posterior plates later. The new plates were not formed all at once, and the old plates remained overtop of the new ones for some time before the moult was completed, with the old cuticle rupturing along the dorsal surface, and the animal emerging with already hardened plates.<ref name="zhang2007" /><ref name="dzik2003" /><ref name="chen2024" /> This is similar to the biphasic moulting of modern [[Isopoda|isopods]] (the posterior half of the body moults first, followed later by the anterior half), as opposed to most [[Crustacean|crustaceans]], which moult their entire exoskeleton in a single step.<ref name="chen2024">{{cite journal |last1=Chen |first1=Ailin |last2=Vannier |first2=Jean |last3=Guo |first3=Jin |last4=Wang |first4=Deng |last5=Gąsiorek |first5=Piotr |last6=Han |first6=Jian |last7=Ma |first7=Wenjiao |title=Molting in early Cambrian armored lobopodians |journal=Communications Biology |date=5 July 2024 |volume=7 |issue=1 |page=820 |doi=10.1038/s42003-024-06440-x |pmid=38969778 |pmc=11226638 |language=en |issn=2399-3642}}</ref>

== Taxonomy ==
Except for ''Microdictyon sinicum'' from the Chengjiang Biota and ''Microdictyon'' sp. from the [[Kaili Formation|Kaili Biota]], all ''Microdictyon'' species have been established on the basis of isolated sclerite plates. As of the publication of Topper et al. (2011),<ref name="topper2011">{{cite journal |last1=Topper |first1=Timothy P. |last2=and Paterson |first2=John R. |title=Microdictyon plates from the lower Cambrian Ajax Limestone of South Australia: Implications for species taxonomy and diversity |journal=Alcheringa: An Australasian Journal of Palaeontology |date=1 September 2011 |volume=35 |issue=3 |pages=427–443 |doi=10.1080/03115518.2011.533972 |bibcode=2011Alch...35..427T |url=https://www.tandfonline.com/doi/abs/10.1080/03115518.2011.533972 |issn=0311-5518|url-access=subscription }}</ref> 9 species of ''Microdictyon'' were recognized as valid, and after Wotte and Sundberg (2017),<ref name=wotte2017>{{Cite journal|author1=Thomas Wotte |author2=Frederick A. Sundberg |year=2017 |title=Small shelly fossils from the Montezuman–Delamaran of the Great Basin in Nevada and California |journal=Journal of Paleontology |volume=91 |issue=5 |pages=883–901 |doi=10.1017/jpa.2017.8 |bibcode=2017JPal...91..883W |doi-access=free }}</ref> 11 species, with a large number of specimens around the world still left in open nomenclature. Because the shape and outline of the plate varies down the length of the animal, and size will be determined by age, only node morphology and the arrangement of the holes is considered effective in determining species.<ref name="topper2011" /><ref name="wotte2017" />

* [[Type species]] '''''Microdictyon effusum''''' <small>Bengston, Matthew, and Missarzhevsky, 1981</small> (Cambrian Series 2, Stage 3) — Known from [[Kazakhstan]], [[siberia|Russia]], and [[Sweden]], possibly [[China|South China]] and the [[United Kingdom|UK]].
** {{extinct}}'''''M. rhomboidale''''' <small>Bengston, Matthew, and Missarzhevsky, 1986</small> (Cambrian Series 2, Stage 3) — Known from [[Uzbekistan]] and Russia, possibly South China, [[Canada]], and the [[United States|USA]].
** {{extinct}}'''''M. robisoni''''' <small>Bengston, Matthew, and Missarzhevsky, 1986</small> (Cambrian Series 3, Stage 5) — Known from Russia and the USA.
** {{extinct}}'''''M. chinense''''' <small>Hao and Shu, 1987</small> (Cambrian Series 2, Stage 3) — Known from South China.
** {{extinct}}'''''M. sphaeroides''''' <small>Hinz, 1987</small> (Cambrian Series 2, Stage 3 to Cambrian Series 3, Stage 5) — Known from Russia and the UK.
** {{extinct}}'''''M. sinicum''''' <small>Chen, Hou, and Lu, 1989</small> (Cambrian Series 2, Stage 3) — Known from South China ([[Chengjiang Biota]]).<ref name="junyuan1995" />
** {{extinct}}'''''M. depressum''''' <small>Bengston ''et al''., 1990</small> (Cambrian Series 2, Stage 3 to Cambrian Series 2, Stage 4) — Known from [[Australia]], possibly [[Greenland]].
** {{extinct}}'''''M. fuchengense''''' <small>Li and Zhu, 2001</small> (Cambrian Series 2, Stage 3) — Known from South China.
** {{extinct}}'''''M. jinshaense''''' <small>Zhang and Aldridge, 2007</small> (Cambrian Series 2, Stage 3) — Known from South China.<ref name="zhang2007" />
** {{extinct}}'''''M. montezumaensis''''' <small>Wotte and Sundberg, 2017</small> (Cambrian Series 2, Stage 3) — Known from the USA.<ref name="wotte2017" />
** {{extinct}}'''''M. cuneum''''' <small>Wotte and Sundberg, 2017</small> (Cambrian Series 2, Stage 3) — Known from the USA.<ref name="wotte2017" />

Additional, unnamed species are also known from North China, [[Mongolia]], [[Turkey]], and [[Mexico]].<ref name="topper2011" />

==References==
{{reflist}}

{{portal|Paleontology}}

{{Lobopodia}}
{{taxonbar|from1=Q20818056}}
[[Category:Prehistoric protostome genera]]
[[Category:Maotianshan shales fossils]]
[[Category:Burgess Shale animals]]
[[Category:Paleozoic life of New Brunswick]]
[[Category:Paleozoic life of Nova Scotia]]
[[Category:Cambrian genus extinctions]]