Sidneyia

Credit: Marianne Collins.

*Sidneyia* is an extinct genus of vicissicaudate artiopod arthropod that lived in British Columbia and China from the Early to Middle Cambrian.

## History

In 1910, Charles Walcott had discovered the Burgess Shale through a slab that had slid down the mountain via a snow slide and into the nearby trail, revealing a well-preserved "crustacean" fossil in a block that traced back to an unnamed formation he would come to name the Burgess Shale. This differs from the popular story that states Helena Walcott had discovered the block after their horses slipped off the trail, where it was traced back to the Burgess Shale in the following summer, which is likely fictitious since Walcott's meticulous journals state otherwise.

Nonetheless, the first *Sidneyia* was discovered during the first day of the Walcott's exploration of the formation, and was named after his eldest son, Sidney, who aided Charles in finding and recovering the specimen. The specific epithet for the type species, *S. inexpectans*, refers to the surprise they felt when they discovered *Sidneyia*. Harry Whittington had found that most Burgess Shale specimens still retained some of their 3D form, albeit still extremely compressed, which he used when describing the fauna to chip away individual microlayers of the specimens to view their internal mechanics and to chip away bits of mud that had seeped into tight crevices during fossilization. This was done to *Sidneyia* specimens to view their internal anatomy. Zhang and Shu (2002) named *S. sinica* from the Chengjiang Lagerstätten of South China.

A fragment of a gill found uncatalogued in the Walcott Collection thought to be from *Sidneyia* was later found to be the folded and decayed gill of *Ottoia*. This was rediscovered in the collection by Dr. D.E.G. Briggs. Bruton revised and redescribed the genus and found it be be distinct in the 1981. In 2023, *Sidneyia minor* was described from the Xiaoshiba section of Kunming, Yunnan, China. It was described from the nearly-complete YKLP 12435 (holotype) and the complete YKLP 12434 (paratype), with the specific epithet refering to it's small size.

## Description

 File:Sidneyia anatomical illustration.png
*Sidneyia* has an arched semicylindrical-shaped carapace that is different from typical Burgess Shale organisms that are flatter. Some specimens have damaged carapaces that reveal the gills and legs, and many are damaged similarly due to extreme compression. The carapace was not fully hardened by mineralization, but were instead highly stiffened, or "lightly sclerotized" (as termed by Gould (1989) in his book on the Burgess Shale and evolution). On the underside, the gill branches were on the outer side and the inner for the legs, which were also used for consuming prey. Bruton revealed that - if a *Sidneyia* specimen were complete - one could strip away the microlayers containing the carapace to then encounter a full set of gills and then the walking legs.

These layers ware found directly on top of each other in specimens, and would be stripped away with a vibro-chisel. Each layer is about a micron thick, Additionally, the alimentary canal lies at the midline between the coxae and the first leg segments; one specimen bears damage to the posterior of one end, where a trilobite, the animal's last meal before being engulfed in a mudslide, is sticking out. Most specimens are compressed from above, which is good for anatomical documentation. However, this has led to reconstruction of the animal's depth to be difficult. One specimen shows A *Sidneyia* from the front is known, which made it known that the animal had a rounded profile. Small beady eyes and two antennae are attached at the head.

As one of the largest arthropods from the Burgess Shale, *Sideneyia* is noted to have been a predator or maybe partly a scavenger, since it possessed a mouth that food was passed towards via the limbs, rather than similar arthropods that have a filter. The eyes, antennae and other features of the carapace are often included as indentations on the counterpart. The gnathobases of the legs are rough, which allowed them to connect to the body in life, and each walking leg has 7 segments. On the front of the underside, the gnathobases form a circular mouth. The spiny edges of the mouth lead scientists to presume *Sidneyia* was carnivorous. Many specimens were compiled from photographs and drawings by Bruton to form a 3D model to use for study.

As it moves posteriorly, the gut widens to form a digestion chamber. and fecal retention in specimens indicate they fed often. *Sidneyia* preyed on small trilobites, smaller arthropods, brachiopods and hyoliths. Gnathobases in the feeding arms resemble *Limulus* and were likely used in the crushing of prey. It was benthic, trawling the seabed for food. Termed "appendage F" and found disarticulated at first, these were later identified as the feeding appendages of *Peytoia* after being wrongly referred to *Sidneyia*. Walcott deemed *Sidneyia* the only animal in this quarry large enough to carry such an apparatus. This and Walcott's idea of it being a relentless predator led Walcott to assume that the appendage was originally articulated with the head. However, Bruton (1981) found that the head shield did not have enough space for the appendage to be attached here.

*S. sinica* is distinguished through a semi-elliptical cephalon in the dorsal view and an elongate telson, which distinguishes it from *S. inexpectans*. Sidneyia minor is distinguished through a semicircular head shield that is short and has lateral notches, a thorax with ten imbricated tergites, a slender abdomen, limited to one (cylindrical) abdominal sclerite, paired triangular flukes on the telson, a broad and anteriorly-sitting hypostome and three post-antennal limb pairs with each having one slender walking leg and an ovular gill branch. The holotype of this species is articulate and flattened to the dorsoventral.

## Paleoecology

*Sidneyia* is part of the Burgess Shale Formation and lived from the Early through Middle Cambrian. *S. sinica* is known from the Maotianshan section of the Chengjiang Lagerstätten. Animals found in these types of quarries are typically killed, buried, preserved and crushed in mudslides. Specimens are found in a wide variety of poses, but most are found from the top-down.

## Gallery

"Appendage F", once attributed to *Sidneyia*.
## References

* https://www.tandfonline.com/doi/abs/10.1080/03115510208619239
* https://en.wikipedia.org/wiki/Sidneyia
* https://paleobiodb.org/classic/basicTaxonInfo?taxon\_no=19033
* https://www.goodreads.com/book/show/36475.Wonderful\_Life
* https://link.springer.com/article/10.1007/s11430-022-1019-8
* https://burgess-shale.rom.on.ca/fossils/sidneyia-inexpectans/