Editing Marrella (section)

==Morphology==
[[File:Marrella splendens by Haug et al.png|thumb|350px|''Marrella splendens'' by Haug et al. [[2012 in paleontology|2012]]<ref>Haug, J. T., Castellani, C., Haug, C., Waloszek, D., Maas, A. (2012). A Marrella−like arthropod from the Cambrian of Australia: a new link between "Orsten"−type and Burgess Shale assemblages. Acta Palaeontologica Polonica 58: 629–639. doi:10.4202/app.2011.0120</ref><br />Top left– dorsal view on a rendered 3D model<br />
top right and centre right– micrographs under polarized light
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top right – well preserved specimen USNM 83486f with the exopods in a "rusty" preservation (cf. García−Bellido and Collins 2006)
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bottom left – stereo image of specimen USNM 139665. Exopods of preceding limbs are super−imposing each other, separated by a thin layer of sediment
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bottom right – detail of specimen ROM 56766A in "rusty" preservation. Here the spines on the lateral side of the exopod ringlets are well preserved
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centre right – one of the smallest specimens of ''M. splendens'' USNM 219817e that possesses preserved appendage remains Black bars for centre right image = 0.6mm, rest = 1mm|left]]
Specimens of ''Marrella'' range from {{Convert|2.4 to 24.5|mm|in}} in length. The head shield had two pairs of long posteriorly curved projections/spines, the posterior pair of which had a serrated keel. There is no evidence of eyes. On the underside of the head was a pair of long and sweeping flexible [[antenna (biology)|antennae]], composed of about total 30 segments, projecting forward at an angle of 15 to 30 degrees away from the midline. On part of the antennae, the joints between segments bear [[seta]]e. Behind and slightly above the antennae attached a pair of short and stout paddle-like swimming appendages, composed of one long basal segment and five shorter segments, the edges of the latter of which were fringed with [[seta]]e (hair-like structures).<ref name=":0" /><ref name="Whittington1971" />

The body had a minimum of 17 segments ([[Tagma (biology)|tagma]]), increasing to over 26 segments in larger specimens, each with a pair of [[biramous]] (two-branched) appendages. The lower branches of each appendage (the [[endopod]]) were elongate and leg-like with 5 segments/podomeres excluding the basal segment/basipod, with the terminal segments being tipped with claws. The endopods sequentially decreased in size posteriorly, with the size reduction accelerating beyond the 9th pair. The upper branch (the [[exopod]]), which functioned as gill was segmented and bore thin filamentous structures. There is a tiny, button-like [[telson]] at the end of the [[Thorax (arthropod anatomy)|thorax]].<ref name=":0">García-Bellido, Diego & Collins, Desmond. (2006). A new study of Marrella splendens (Arthropoda, Marrellomorpha) from the Middle Cambrian Burgess Shale, British Columbia, Canada. Canadian Journal of Earth Sciences. 43. 721-742. 10.1139/e06-012.</ref><ref name="Whittington1971" />

A 1998 paper suggested that striations present on the front projection of well-preserved specimens of ''Marrella'' represented a [[diffraction grating]] pattern, that in life would have resulted in an [[iridescent]] sheen.<ref name="Parker1998">{{cite journal |last= Parker|first= A. R.|year= 1998|title= Colour in Burgess Shale animals and the effect of light on evolution in the Cambrian|journal= Proceedings of the Royal Society B: Biological Sciences|volume= 265|issue= 1400|pages= 967–972|doi=10.1098/rspb.1998.0385|pmc= 1689164}}</ref> However the conclusions of the paper regarding other animals with supposed iridescent diffraction gratings have been questioned by other authors.<ref>{{Cite journal |last=Smith |first=Martin R. |date=January 2014 |editor-last=Lane |editor-first=Phil |title=Ontogeny, morphology and taxonomy of the soft-bodied Cambrian 'mollusc' Wiwaxia |journal=Palaeontology |language=en |volume=57 |issue=1 |pages=215–229 |doi=10.1111/pala.12063 |bibcode=2014Palgy..57..215S |s2cid=84616434 |quote=The full width of each sclerite [of ''Wiwaxia''] is striated by finely spaced longitudinal lineations. Parker (1998) argued that these were superficial – although they are not visible on surfaces imaged under SEM and do not exhibit interference under transmitted light, so might be better interpreted as internal channels indicating microvillar secretion.|doi-access=free }}</ref><ref>{{Cite journal |last1=Parry |first1=Luke |last2=Caron |first2=Jean-Bernard |date=2019-09-06 |title=Canadia spinosa and the early evolution of the annelid nervous system |journal=Science Advances |language=en |volume=5 |issue=9 |pages=eaax5858 |doi=10.1126/sciadv.aax5858 |issn=2375-2548 |pmc=6739095 |pmid=31535028 |bibcode=2019SciA....5.5858P |quote=In Canadia, longitudinal striations along chaetae, which have previously interpreted as external evidence for iridescence, are concordant with the dimensions of microvilli and represent internal rather than external features.}}</ref> Dark stains are often present at the posterior regions of specimens, probably representing extruded waste matter<ref name="Whittington1978">{{Cite journal | last1 = Whittington | first1 = H. B. | title = The Lobopod Animal ''Aysheaia pedunculata'' Walcott, Middle Cambrian, Burgess Shale, British Columbia | pages = 165–197 | journal = Phil. Trans. R. Soc. Lond. B | volume = 284| issue = 1000| doi =  10.1098/rstb.1978.0061| year = 1978 |bibcode = 1978RSPTB.284..165W | doi-access = free }}</ref> or [[hemolymph]].<ref>{{cite conference| url =http://gsa.confex.com/gsa/2010RM/finalprogram/abstract_171740.htm| title = Synchrotron Imaging of Burgess Shale Fossils: Evidence for Biochemical Copper (Hemocyanin) in the Middle Cambrian Arthropod ''Marrella splendens'' | last1 = Pratt |first1 = Brian R.| last2 = Pushie | first2 = M. Jake | last3 = Pickering | first3 = Ingrid J.| last4 = George | first4 = Graham N. | archive-url = https://web.archive.org/web/20160309172618/http://gsa.confex.com/gsa/2010RM/finalprogram/abstract_171740.htm | archive-date = 2016-03-09 }}</ref> A single specimen caught in the act of [[ecdysis]] ([[moulting]]) is known, which shows that the exoskeleton split at the front of the shield.<ref>{{Cite journal |last1=Drage |first1=Harriet B. |last2=Legg |first2=David A. |last3=Daley |first3=Allison C. |date=2023-08-21 |title=Novel marrellomorph moulting behaviour preserved in the Lower Ordovician Fezouata Shale, Morocco |journal=Frontiers in Ecology and Evolution |volume=11 |doi=10.3389/fevo.2023.1226924 |issn=2296-701X|doi-access=free }}</ref><ref>{{Cite journal |last1=García-Bellido |first1=Diego C. |last2=Collins |first2=Desmond H. |date=May 2004 |title=Moulting arthropod caught in the act |journal=Nature |language=en |volume=429 |issue=6987 |pages=40 |doi=10.1038/429040a |issn=0028-0836|doi-access=free |pmid=15129272 |bibcode=2004Natur.429...40G }}</ref>