euarthropod A, Nielsen, 1995
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publication ID |
https://doi.org/ 10.1038/s41467-022-34204-w |
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DOI |
https://doi.org/10.5281/zenodo.7346858 |
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persistent identifier |
https://treatment.plazi.org/id/03FD8143-EC04-FFAA-FF75-FA0BFF0F9E2A |
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treatment provided by |
Tatiana |
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scientific name |
euarthropod A |
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Castle Bank euarthropod A
Figures 6 – 9 View Fig View Fig View Fig View Fig , Supplementary Fig. 5 View Fig
Material, locality, and horizon. NMW.2021.3 G.8 , known from part and counterpart. Collected from the Darriwilian (Middle Ordovician, Didymograptus murchisoni Biozone ) Gilwern Volcanic Formation at Castle Bank, near Llandrindod, Powys ( UK) 33, 34.
Description. NMW.2021.3 G.8 is a complete specimen preserved compressed to givea lateral viewand measures ~3 mm alongthe dorsal margin ( Fig. 6 View Fig ).
The head region preserves evidence for what appear to be two lateral cephalic sclerites proximal to an annulated proboscis ( Figs. 6 View Fig , 7 View Fig ). A pair of trapezoidal sclerites <1 mm in length, which are compressed and superimposed on each other, have been rotated forwards to cover the anterior of the head and proximal part of the proboscis (Supplementary Fig. 5 View Fig ). The preserved portion of the proboscis is less than 1 mm in length. The cephalic sclerites preserve a marginal rim, and the element covering the right side of the head also displays numerous triangular spines on the anterior and ventral margins ( Fig. 7a View Fig , Supplementary Fig. 5 View Fig ). The proboscis, which curves ventrally and is likely incomplete distally, displays annulations and a dark linear feature that runs parallel to the long axis, interpreted as an internal canal, as well as regularly spaced (approximately one per three to four annulations) stout triangular dorsal spines ( Figs. 7 View Fig , 8c View Fig ). The proboscis is more similar in preservation to the trapezoidal sclerites than to the trunk.
The trunk, which measures ca. 2 mm along the dorsal margin, is curved, narrow and tapers slightly to the posterior. The trunk displays a wrinkled texture. At the anterior (just behind the lateral carapace elements), faint subrectangular elements sit dorsal to the body region ( Fig. 7a View Fig ; Supplementary Fig. 5 View Fig ). Further to the posterior, indents in the dorsal margin and faint transverse lineations indicate the position of intersegmental furrows and associated subrectangular lateral swimming flaps ( Figs. 6 View Fig , 9 View Fig ). Fainter lines overlain by wrinkles preserved towards the middle of the trunk are also tentatively identified as dorsal furrows and lateral flaps, and indicate the presence of at least 12 furrows and flaps ( Fig. 6 View Fig ).
The body terminates in a caudalfan ~0.25 mm long, composed of triangular caudal blades with spinose lateral and posterior margins. Three blades that widen to the posterior can be observed on the near (right) side, whereas two are observed on the left side ( Figs. 8b View Fig , 9 View Fig ).
Remarks. The wrinkled texture of the body, change in slope on the dorsal margin and orientation of the paired head sclerites (rotated forwards), as well as the lack of internal structures (e.g. guts, nervous system) suggest that this specimen may represent a moult rather than a carcass 39, though this is not conclusive. The presence of an internal cavity does not refute this, as similar structures have been reported from isolated radiodont appendages (e.g. ref. 15 Fig. 13). If this specimen does represent a moult, then it is possible that it bore only a single cephalic sclerite. The appearance of a paired sclerite may have been caused by folding, shearing and breakage of a single, larger, sclerite during the moulting process. The smaller ‘ rectangular elements ’ posterior to this apparently paired sclerite may represent additional broken fragments.
If the presence of multiple head sclerites in NMW.2021.3 G.8 (rather than one in NMW.2021.3 G.7) is confirmed, then this difference and the distinct morphology of the dorsal spines on the proboscis together suggest that NMW.2021.3 G.7 may represent a distinct species. Other potentially diagnostic features of NMW.2021.3 G.8, such as the spinose caudal fan, are not preserved in NMW.2021.3 G.7, but are present in opabiniids Opabinia and Utaurora 36.
Alternatively, NMW.2021.3 G.8 could alsobe regarded asan earlier ontogenetic stage of Mieridduryn bonniae than NMW.2021.3 G.7. At ~3 mm, euarthropod A falls within the size range of some larger crustacean larvae (e.g. decapods, remipedes) 40; thus, postembryonic morphological changes such as differing spine form on the proboscis and numberofheadsclerites couldexplain the observeddifferences in the two specimens. Within radiodonts the number of head sclerites is stable at the family level, and does not change during ontogeny (in at least one species, Lyrarapax unguispinus 41), but the number of carapace elements does change during ontogeny in other euarthropods. For example, in the fossil phosphatocopine Dabashanella and the living ostracod Manawa , a univalved carapace develops into a bivalved one 42, 43. Metamorphosis is expected to be ancestral for panarthropods 44 and has been observed in some members of the upper stem group (e.g. the megacheiran Leanchoilia 45, but not all, see ref. 46); hence a comparatively minor morphological change during ontogeny for NMW.2021.3 G.7 and NMW.2021.3 G.8 cannot be ruled out with the current data. Thus,we leave the smaller specimen in open nomenclature, and consider both possibilities (a distinct species or an earlier ontogenetic stage of Mieridduryn nov. gen.) for this specimen in our phylogenetic analyses (section below).
Further discussion of the relationship of Mieridduryn nov. gen. and euarthropod A to opabiniids is provided in Supplementary Information.
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