Utaurora

Credit: Franz Anthony.

*Utaurora* is an extinct genus of opabiniid euarthropod that lived in Utah during the Middle Cambrian.

## History

*Utaurora* is known from specimen KUMIP 314087, a well-preserved but somewhat degraded full-body euarthropod specimen. In 2022, it was named. The name is a portmanteau of Utah, it's place of discovery, and Aurora, a Roman goddess who is the goddess of dawn and turned her lover into an insect, based on it being a euarthropod close to the origin of Arthropoda. However, the specimen was misidentified as a radiodont several times before being assigned to Opabiniidae in 2022. Briggs et al. (2008) and Robison et al. (2015) placed it in *Anomalocaris* sp.. Pates et al. (2021) place the specimen as incertae sedis in a table.

## Description

The holotype, KUMIT 314087, is a part compressed in the dorso-laterally. It was collected by a P. Reese from the upper Wheeler Formation of Carpoid Quarry. Though the face is degraded, it preserves 13 segments in the proboscis and likely 15 in life. The trunk is slender. The setal structures form blocks that cover the width of the trunk and the proximal of the anterior margin of the lateral flaps; this feature coins the specific epithet, *comosa*, Latin for "hairy" or "leafy", reflecting the many hair-like structures over the body. At least 7 pairs of elongate, pointed caudal blades comprise the tail fan. These three characters distinguish *Utaurora* from *Opabinia*. The body has a sagittal length of 29 millimeters and a total length (based on scale bars) of roughly 30 millimeters. Much of the specimen is obscured, but fine anatomy can be observed.

The head is ~10% the entire body length and the eyes, mouth and proboscis are faintly preserved. Two circular red structures under the head are interpreted as a mouth or possibly a damaged eye. Below the head is a proboscis that preserves what is interpreted as the internal structure. The trunk is ~72% of the body length, with corrugated dorsal margins that bear indents that connect them to the dorsal intersegmental furrows. The setal blocks end to a round, subtriangular tip that hangs over the proximal of the flaps. Minimum 14 and probably 15 lateral flaps are on the right of the body. These body flaps have a smooth surface with no ornamentation and have no evidence of strengthening rays or any other structures within. A small triangular setal block on the posterior flap distinguishes from the caudal blades to form part of the caudal fan, with thin structures protruding from under flaps 12 and 13. These may be poorly-preserved ventral lobopodous limbs, broken margins of the flaps or taphonomically associated with the matrix. The fan is ~18% of the body length, and, in life, probably bore pairs of 8 blades.

## Classification

As an opabiniid, *Utaurora* was a euarthropod close to the origin of Arthropoda. This was determined by placing it into a morphological matrix (of 43 fossil and 11 extant arthropod taxa), where 68% of trees recover it and *Opabinia* closely related, with only 0.04% recovering it as a rdiodont. This would make *Utaurora* an opabiniid, being the first of the group to be named in over 100 years. They used a system that gives strong outputs. This would mean that *Opabinia* was not such a unique animal, and that Utaurora was several million years younger, making the family prevailing. The posterior oriented mouth in *Opabinia* and deuteropods and convergent or lost in radiodonts and the dorsally-transverse furrows delineating segmented boundaries found in all opabiniids may be a precursor to arthrorized tergites in deuteropods and possibly lost in radiodonts and secondly evolved, or convergent. These traits indicate how opabiniids connect to related groups. They recover:

They speculate that opabiinids descended from an ancestor with two separate appendages that gradually merged into a proboscis with a 'mouthed' end through a series of convergences, acquisition and reversals. Dorsal expression of the trunk segmentation may have affected this process. This would require either secondary loss of arthropodized appendages or convergently evolved arthropodized appendages. However, this has not been greatly considered in recent research, so it is still relatively hazy.

## References

* https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2021.2093
* https://phys.org/news/2022-02-century-opabiniid.html