Uncus dzaugisi

For the body part, see Uncus.

Uncus dzaugisi Temporal range: late Ediacaran,
Artist's interpretation of U. dzaugisi
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Superphylum:	Ecdysozoa
Genus:	†Uncus
Species:	†U. dzaugisi
Binomial name
†Uncus dzaugisi Hughes et al., 2024

Uncus dzaugisi was a species of animal which lived approximately 560 to 550 million years ago during the late Ediacaran in what is now Southern Australia. Its smooth cylindrical shape, with one end being wider than the other, suggests that it was a member of the clade Ecdysozoa (a group containing arthropods, nematodes and tardigrades, amongst others). This makes it the oldest ecdysozoan known so far, as well as one of the oldest known bilaterians. It is currently the only member of the genus Uncus.

Discovery and name

Fossil bed at Nilpena Ediacara National Park, 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.

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 uncus, meaning '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.

Description and paleobiology

Uncus was a soft-bodied, smooth and cylindrical organism that grew from 6 to 31 mm (0.2 to 1.2 in) in length, with a width of 0.7 to 2.4 mm (0.0 to 0.1 in). 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.

Paleoecology

Further information: 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. 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. The microbial mats also likely aided in the preservation of several Ediacaran organisms by stabilizing the impressions left in the sediment.

Affinity

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. 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.