Uncus dzaugisi#Affinity

File:Fossil_bed_at_Nilpena_Ediacara_National_Park.jpg, where Uncus was discovered]]

The systematic excavation that led to the discovery of Uncus began in 2018 in the Ediacara Member, a rock unit of the Rawnsley Quartzite within the Nilpena Ediacara National Park, South Australia, which is known for its well preserved Ediacaran fossils. During the excavation, the team, led by Mary Droser, noted the presence of deep, hook-shaped indentations in the rock. At first, they were not looked into with much interest, but after discovering dozens of similar specimens, the team began to investigate their true identity. The discovery of trace fossils in nearby sediments supported the hypothesis that the indentations were indeed fossils of a previously unknown organism.{{cite journal |last1=Hughes |first1=Ian V. |last2=Evans |first2=Scott D. |last3=Droser |first3=Mary L. |title=An Ediacaran bilaterian with an ecdysozoan affinity from South Australia |journal=Current Biology |date=18 November 2024 |volume=34 |issue=24 |pages=5782–5788.e1 |doi=10.1016/j.cub.2024.10.030|pmid=39561775 |bibcode=2024CBio...34.5782H }}

Uncus dzaugisi was scientifically described in 2024 by Ian V. Hughes and colleagues. The genus name that was chosen, Uncus, is derived from the Latin word {{wikt-lang|la|uncus}}, meaning {{gloss|hooked}}, in reference to the shape of many specimens. The specific name dzaugisi is the Latinisation of the surname of Mary, Matthew and Peter Dzaugis and honors their contributions to fieldwork at the Nilpena Ediacara National Park.

Uncus was a soft-bodied, smooth and cylindrical organism that grew from {{cvt|6|to|31|mm|1}} in length, with a width of {{cvt|0.7|to|2.4|mm|1}}. Specimens of Uncus have varying degrees of curvature, with one end being wider than the other. The wider end is suspected to be the posterior (rear) end as seen in other ecdysozoans. Unlike most other Ediacaran organisms which leave relatively thin impressions and deform easily, Uncus possibly had a sturdy external membrane, as evidenced by 77 out of the 82 specimens showing little to no deformation, and also leaving very deep impressions.

The sharpness of the fossil edges also suggests that Uncus was free from the organic mats that covered the seabeds of Ediacaran Australia, showing that it was most likely motile, unlike sessile organisms which have softer margins due to the microbial mat growing on them. Its motility is further supported by the existence of the ichnogenus Multina on one of the fossil beds where Uncus specimens were found, which it is proposed to be the maker of. Several specimens have also been found overlaying other Ediacaran organisms such as Funisia, and even the feeding traces of other motile organisms such as Dickinsonia.

{{Further|Ediacaran biota}}

Uncus was discovered in the Ediacara Member of the Rawnsley Quartzite in the Nilpena Ediacara National Park, South Australia. The sediments that make up the fossil bed where deposited sometime around 560–550 million years ago, and are mostly composed of quartzite and sandstone. During that time, South Australia would have been a shallow marine environment with an abundance of microbial mats.{{cite magazine |last=Narbonne |first=Guy M. |year=1998 |title=The Ediacara biota: A terminal Neoproterozoic experiment in the evolution of life |url=https://www.geosociety.org/gsatoday/archive/8/2/pdf/gt9802.pdf |magazine=GSA Today |pages=1–6 |volume=8 |issue=2 |issn=1052-5173}}{{Cite journal |last1=Grazhdankin |first1=Dmitriy |last2=Gerdes |first2=Gisela |date=1 September 2007 |title=Ediacaran microbial colonies |url=https://www.scup.com/doi/full/10.1111/j.1502-3931.2007.00025.x |journal=Lethaia |volume=40 |issue=3 |pages=201–210 |doi=10.1111/j.1502-3931.2007.00025.x |bibcode=2007Letha..40..201G |issn=0024-1164}} With such a nutrition-rich environment, there were also many other organisms like Spriggina, Dickinsonia and Kimberella, which, much like Uncus, would probably have fed on the bountiful microbial mats.{{Citation |last1=Fedonkin |first1=M. A. |title=New data on Kimberella, the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications |date=2007-01-01 |work=The Rise and Fall of the Ediacaran Biota |volume=286 |pages=0 |editor-last=Vickers-Rich |editor-first=Patricia |url=https://pubs.geoscienceworld.org/gsl/books/edited-volume/1658/chapter-abstract/107472549/New-data-on-Kimberella-the-Vendian-mollusc-like?redirectedFrom=fulltext |access-date=2025-04-08 |publisher=Geological Society of London |isbn=978-1-86239-233-5 |last2=Simonetta |first2=A. |last3=Ivantsov |first3=A. Y. |editor2-last=Komarower |editor2-first=Patricia}}{{Cite journal |last1=Ivantsov |first1=Andrey Yu |last2=Zakrevskaya |first2=Maria |date=23 February 2023 |title=Body plan of Dickinsonia, the oldest mobile animals |url=https://www.cambridge.org/core/journals/earth-and-environmental-science-transactions-of-royal-society-of-edinburgh/article/abs/body-plan-of-dickinsonia-the-oldest-mobile-animals/FF87B06D1EC07F41DDCF10AE54BF50F1 |journal=Earth and Environmental Science Transactions of the Royal Society of Edinburgh |language=en |volume=114 |issue=1–2 |pages=95–108 |doi=10.1017/S175569102300004X |bibcode=2023EESTR.114...95I |issn=1755-6910}} The microbial mats also likely aided in the preservation of several Ediacaran organisms by stabilizing the impressions left in the sediment.{{Cite journal |author=Gehling, J.G. |year=1987 |title=Earliest known echinoderm – A new Ediacaran fossil from the Pound Subgroup of South Australia |journal=Alcheringa |volume=11 |issue=4 |pages=337–345 |doi=10.1080/03115518708619143|bibcode=1987Alch...11..337G }}

It was suggested that Uncus was a member of Ecdysozoa, with possible relations to nematoids. This is supported by the sturdy membrane around the organism, which is possible evidence of an early cuticle. The overall morphology of the body lines up with both extant and fossil nematodes, especially ones that shallowly burrow within substrates. Even the hooked and rigid nature of the trace fossils are akin to the postmortem positions of modern nematodes. Alongside this, the differing widths of the trace fossils Multina, attributed to Uncus, bear resemblance to the locomotion of nematodes, which contract their muscles longitudinally to move along. However, a stylet or esophagus have not been preserved in any known specimens, which makes its assignment to Nematoida still contentious.

Ecdysozoans are not the only animals with a similar morphology to Uncus; three phyla within Spiralia—Annelida, Nemertea and Platyhelminthes (flatworms)—have members with a similar form. Despite the superficial similarity, Uncus has several distinguishing traits that makes affinity with these phyla unlikely. Annelids, more specifically Sipuncula and Echiura, rarely reach the sizes seen in Uncus. Uncus specimens also lack the defining feature of these clades, an introvert (proboscis-like structure). In Nemertea and flatworms, members are usually quite flat, whilst the deep imprints of Uncus suggest it was relatively girthy. The suggested contracting locomotion of Uncus also differs greatly from the smooth, gliding, locomotion seen in flatworms.

Many molecular clocks have estimated the origin of Ecdysozoa in the Ediacaran.{{cite journal |last1=Reis |first1=Mario dos |last2=Thawornwattana |first2=Yuttapong |last3=Angelis |first3=Konstantinos |last4=Telford |first4=Maximilian J. |last5=Donoghue |first5=Philip C.J. |last6=Yang |first6=Ziheng |title=Uncertainty in the Timing of Origin of Animals and the Limits of Precision in Molecular Timescales |journal=Current Biology |date=21 November 2024 |volume=25 |issue=22 |pages=2939–2950 |doi=10.1016/j.cub.2015.09.066|pmid=26603774 |pmc=4651906 |hdl=1983/829338e6-fcd5-4794-8028-3924ecba0929 |hdl-access=free }}{{cite journal |last1=Rehm |first1=Peter |last2=Borner |first2=Janus |last3=Meusemann |first3=Karen |last4=von Reumont |first4=Björn M. |last5=Simon |first5=Sabrina |last6=Hadrys |first6=Heike |last7=Misof |first7=Bernhard |last8=Burmester |first8=Thorsten |title=Dating the arthropod tree based on large-scale transcriptome data |journal=Molecular Phylogenetics and Evolution |date=December 2011 |volume=61 |issue=3 |pages=880–887 |doi=10.1016/j.ympev.2011.09.003|pmid=21945788 |bibcode=2011MolPE..61..880R }}{{cite journal |last1=Howard |first1=Richard J. |last2=Giacomelli |first2=Mattia |last3=Lozano-Fernandez |first3=Jesus |last4=Edgecombe |first4=Gregory D. |last5=Fleming |first5=James F. |last6=Kristensen |first6=Reinhardt M. |last7=Ma |first7=Xiaoya |last8=Olesen |first8=Jørgen |last9=Sørensen |first9=Martin V. |last10=Thomsen |first10=Philip F. |last11=Wills |first11=Matthew A. |last12=Donoghue |first12=Philip C. J. |last13=Pisani |first13=Davide |title=The Ediacaran origin of Ecdysozoa: integrating fossil and phylogenomic data |journal=Journal of the Geological Society |date=July 2022 |volume=179 |issue=4 |page=jgs2021-107 |doi=10.1144/jgs2021-107|bibcode=2022JGSoc.179..107H |hdl=2445/186596 |hdl-access=free }} If Uncus is indeed an ecdysozoan, it would be the first known Ediacaran example and would bridge the gap between the molecular clock estimates and the fossil record of Ecdysozoa, as well as the gap in the fossil record between the early bilaterians of the Ediacaran and the ecdysozoans like arthropods and priapulids otherwise first known from the Cambrian.

{{Reflist}}

{{Taxonbar|from1=Q131388581|from2=Q133866265}}

Category:Ecdysozoa

Category:Ediacaran

Category:Nematoida

Category:Extinct fauna of South Australia

Category:Animals described in 2024