Amiskwia

{{Short description|Genus of extinct, gnathiferan worms}} {{Automatic taxobox | display_parents = 3 | image = NMNH-USNM PAL 57644 Amiskwia (cropped).jpg | image_caption = Fossil of ''Amiskwia sagittiformis'' from the [[Burgess Shale]] | fossil_range = {{Fossil range|Cambrian Stage 3|Middle Cambrian}} | taxon = Amiskwia | authority = Walcott, 1911 | type_species = ''Amiskwia sagittiformis'' | type_species_authority = Walcott, 1911 | subdivision = *''A. sagittiformis'' *''A. sinica'' <small>Luo & Hu, 2002</small> }}

'''''Amiskwia''''' is a genus of soft-bodied marine [[animal]]s known from [[fossil]]s of the Middle [[Cambrian]] Lagerstätten both in the [[Burgess Shale]] in [[British Columbia]], [[Canada]] and the [[Maotianshan shales]] of [[Yunnan]] Province, [[China]]. It is interpreted as a member of the clade [[Gnathifera (clade)|Gnathifera]] ''sensu lato''<ref name="ReferenceA">{{Cite journal |doi = 10.1038/s42003-019-0388-4|title = Amiskwia is a large Cambrian gnathiferan with complex gnathostomulid-like jaws|journal = Communications Biology|volume = 2|year = 2019|last1 = Caron|first1 = Jean-Bernard|last2 = Cheung|first2 = Brittany|pmid=31069273|pmc=6499802|article-number=164}}</ref> or as a basal [[cucullophora]]n.<ref name=":0" /><ref name=":1" /><ref name="Park2024"/>

== Etymology == The scientific name ''Amiskwia sagittiformis'' derives from the Cree ''amiskwi'', "beavertail", a name of various objects in Yoho National Park, and from the Latin ''sagitta'' ("arrow") and ''formis'' ("shape"), in reference to the general appearance of the animal. "Sinica", of ''A. sinica'', refers to that species' origin from China.

== Description == [[File:Amiskwia sagittiformis diagram (cropped).png|left|thumb|Diagram, showing the arrangements of the jaws and plates in the mouth (a) morphology of the ventral plate (b) and jaws (c) in comparison to equivalent elements of [[gnathostomulid]]s (d, e) and life restoration (g)]] Known specimens of ''Amiskwia'' vary in length from {{Convert|7.4 to 31.3|mm|in}} and in width from {{Convert|0.5 to 5.5|mm|in}}. The body was somewhat flattened. The head had a pair of tentacles that emerged from the midline of the head. The tentacles had a relatively thick base and tapered to a point. Along the sides of the trunk were a pair of lateral fins, which were around one third of the total body length. The trunk terminated with a flat, rounded caudal fin. The gut was straight, and ran from the mouth to the anus, which was located on the underside of the body near the caudal fin. Within the mouth is a pair of semi-circular structures, described as "jaws" each bearing 8-10 conical spikes, which increased in size away from the midline of the structure. Two other structures, dubbed the "dorsal plate" and "ventral plate", are also present in the mouth.<ref name="ReferenceA" />

== Phylogeny ==

The following [[dendrogram]] shows the evolutionary relationships of ''Amiskwia'' as in Park et al. 2024.<ref name="Park2024">{{cite journal |doi=10.1126/sciadv.adi6678 |title=A giant stem-group chaetognath |date=2024 |last1=Park |first1=Tae-Yoon S. |last2=Nielsen |first2=Morten Lunde |last3=Parry |first3=Luke A. |last4=Sørensen |first4=Martin Vinther |last5=Lee |first5=Mirinae |last6=Kihm |first6=Ji-Hoon |last7=Ahn |first7=Inhye |last8=Park |first8=Changkun |last9=De Vivo |first9=Giacinto |last10=Smith |first10=M. Paul |last11=Harper |first11=David A. T. |last12=Nielsen |first12=Arne T. |last13=Vinther |first13=Jakob |journal=Science Advances |volume=10 |issue=1 |article-number=eadi6678 |doi-access=free |pmid=38170772 |pmc=10796117 |bibcode=2024SciA...10I6678P }}</ref>

{{clade |style=line-height:100%; |label1= |1={{clade |1={{clade |1=[[Gnathostomulida]] |2=[[Rotifera]] }} |2={{clade |1=''[[Timorebestia]]'' |2={{clade |1=''Amiskwia'' |2={{clade |1="New chaetognath"{{ref label|ref_a|a}} |2={{clade |1=''[[Sagitta (arrowworm)|Sagitta]]'' |2=''[[Spadella]]'' }} }} }} }} }} }}

:{{note label|ref_a|a|a}}<small>A yet undescribed chaetognath, as of January 2024, from [[Sirius Passet]]</small>

== Ecology == ''Amiskwia'' was likely a freely swimming (nektonic) organism that was either a predator or a scavenger.<ref name="ReferenceA" />

== History of research == ''Amiskwia'' was originally categorized by [[Paleontology|paleontologist]] [[Charles Doolittle Walcott|Charles Walcott]]. Walcott thought he saw three buccal spines in the fossils, and therefore categorized ''Amiskwia'' as a [[chaetognatha|chaetognath]] worm (arrow worm). However, ''Amiskwia'' appears to lack the characteristic grasping spines and teeth of other Burgess fossil arrow worms. Later scientists suggested an affinity with the [[nemertea]]ns (ribbon worms), but the evidence for this was somewhat inadequate.<ref name=SCM1977>{{cite journal | issn = 0031-0220 | last = Conway Morris | first= S. | year = 1977 | title = A redescription of the Middle Cambrian worm ''Amiskwia sagittiformis'' Walcott from the Burgess Shale of British Columbia| journal = Paläontologische Zeitschrift|volume =51|issue=3–4|doi=10.1007/BF02986576|pages =271–287| bibcode = 1977PalZ...51..271M | s2cid = 84554285 }}</ref> [[Simon Conway Morris|Conway Morris]], on re-examining of the Burgess Shale fauna in the 1970s, described it as being the single known species in an otherwise unknown [[phylum (biology)|phylum]], given that it has two tentacles near its mouth, rather than the characteristic single tentacle of true nemerteans. (Nemerteans do not have a single tentacle. However, a pair of antero-lateral tentacles is present in two of the many genera of pelagic nemerteans. Nemerteans do have a single eversible—normally internal—proboscis, which when everted could resemble an anterior median tentacle if fossilized. Whether retracted or everted, the proboscis is the only structure in pelagic nemerteans likely to fossilize, as it is the only structure with substantial connective tissue and muscle. The body wall has almost no muscle or connective tissue and is exceedingly unlikely to fossilize; hence, a pelagic nemertean fossil would be only the proboscis).<ref name=SCM1977/> [[Nicholas Butterfield|Butterfield]] implies from the appearance of the fossils that the organisms may have lacked a cuticle:<ref name=Butterfield1990>{{Cite journal| first = N. J.| title = Organic Preservation of Non-Mineralizing Organisms and the Taphonomy of the Burgess Shale| journal = Paleobiology| volume = 16| issue = 3| last = Butterfield| pages = 247–399| date = 1 July 1990 | issn = 0094-8373| doi = 10.1017/s0094837300009994 | jstor = 2400788| bibcode = 1990Pbio...16..272B| s2cid = 133486523}}</ref> while this is also true of the nemerteans, these organisms lack a [[coelom]] and are thus unlikely to fossilise. He goes on to argue that the absence of cuticle is characteristic of the [[chaetognath]]s; whilst teeth would be expected, a similar fossil, ''[[Wiwaxia]]'', shows such structures in only 10% of the expected instances, and [[anomalocaridid]]s are often found detached from their mouthparts, so the absence may be taphonomic rather than genuine. The absence of spines could simply mean that the fossils represent young organisms — or that later chaetognath evolution involved [[paedomorphosis]].<ref>Kasatkina, A. P. 1982. Ŝetinkočelustnyje morej SSSR i sopredel'nyh vod. 136 pp. Nauka, Leningrad. :Cited in {{cite journal | author = Doguzhaeva, L. A. | author2= Mutvei, H.|author3= Mapes, R. H. | year = 2002 | title = Chaetognath grasping spines from the Upper Mississippian of Arkansas (USA) | journal = [[Acta Palaeontologica Polonica]] | volume = 47 | issue = 3 | pages = 421–430 | url = http://www.app.pan.pl/archive/published/app47/app47-421.pdf | access-date = 2007-08-19}}</ref>

Two studies published in 2019 redescribed ''Amiskwia.'' Vinther and Parry (2019) argued that ''Amiskwia'' was a stem-group chaetognath,<ref name=":0">{{Cite journal |last1=Vinther |first1=Jakob |last2=Parry |first2=Luke A. |date=March 2019 |title=Bilateral Jaw Elements in Amiskwia sagittiformis Bridge the Morphological Gap between Gnathiferans and Chaetognaths |journal=Current Biology |language=en |volume=29 |issue=5 |pages=881–888.e1 |doi=10.1016/j.cub.2019.01.052|pmid=30799238 |s2cid=72332845 |doi-access=free |bibcode=2019CBio...29E.881V |hdl=1983/51b1b6c1-0220-4469-977f-480e847a9101 |hdl-access=free }}</ref> while Caron and Cheung (2019) suggested that the organism was a total group [[Gnathifera (clade)|gnathiferan]], based on the presence of gnathiferan-like jaws and ventral plates within the mouth. Its precise affinity within this group is difficult to resolve, they suggested that if it fell in the stem lineage of any extant phylum then it would be a [[gnathostomulid]].<ref name="ReferenceA"/> A 2022 study supported a stem-chaetognath interpretation, suggesting that gnathiferan-like jaws were lost in the ancestor of chaetognaths.<ref name=":1">{{Cite journal |last1=Bekkouche |first1=Nicolas |last2=Gąsiorowski |first2=Ludwik |date=2022-12-31 |title=Careful amendment of morphological data sets improves phylogenetic frameworks: re-evaluating placement of the fossil Amiskwia sagittiformis |url=https://www.tandfonline.com/doi/full/10.1080/14772019.2022.2109217 |journal=Journal of Systematic Palaeontology |language=en |volume=20 |issue=1 |article-number=2109217 |doi=10.1080/14772019.2022.2109217 |bibcode=2022JSPal..2009217B |s2cid=252747552 |issn=1477-2019}}</ref> A 2024 study again supported a stem-chaetognath position.<ref name="Park2024" />

== See also == {{Portal|Paleontology}} * [[Paleobiota of the Burgess Shale]]

==References== {{Reflist}}

== External links == * {{Cite web|date=2011|title=''Amiskwia sagittiformis''|work=Burgess Shale Fossil Gallery|publisher=Virtual Museum of Canada|url=http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=20|archive-url=https://web.archive.org/web/20201112025257/http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=20|archive-date=2020-11-12|access-date=2023-01-21}} *[http://paleobiology.si.edu/burgess/amiskwia.html Graphic of Amiskwia in motion]

[[Category:Cambrian invertebrates]] [[Category:Burgess Shale fossils]] [[Category:Maotianshan shales fossils]] [[Category:Chaetognatha]] [[Category:Controversial taxa]] [[Category:Cambrian genus extinctions]]