Sidneyia is an extinct arthropod known from fossils found from the Early to the Mid Cambrian of China and the Mid Cambrian Burgess Shale of British Columbia, Canada.

Description 

Sidneyia inexpectans reached lengths of at least , while the largest known specimen of S. minor is around  long and  wide. The head shield is short, with notches present on the sides to accomodate stalked eyes, with the underside having a hypostome. The head has a pair of segmented antennae, as well as three pairs of post-antennal appendages. This was followed by a thorax, which had either nine (S. inexpectans) or ten (S. minor) segments/tergites, each associated with a pair of biramous appendages, this was followed with one (S. minor) to three (S. inexpectans) abdomen segments/tergites, with the body terminating with a telson, which comprised a pair of tail flukes. The appendages bear heavily sclerotised spined basal sediments (basipods) called gnathobases, used to process food. In S. minor, the biramous appendages have 8 podomeres/segments on the endopod, with the last segment being a terminal claw. The exopod of these limbs is flattened, and bears lamellae. In S. inexpectans, the endopods of the biramous limbs have seven podomeres, with the first four of these each bearing a number of thin inward projecting spines, while the outer three podomeres bore more stout claw-like spines, with the fourth to ninth pairs of post antennal limbs bearing exopods with blade-like lamellae, which are thought to have been used as gills. S. inexpectans had three pairs of digestive glands adjacent to the central gut tube.

Ecology 
Sidneyia is thought to have been seafloor dwelling (epibenthic) generalist durophagous predator and/or scavenger that used its gnathobases to crush and shred prey items, including hard-shelled organisms like juvenile trilobites (which are abundantly preserved as stomach contents in S. inexpectans) and brachiopods, but possibly also softer animals like worms.

Taxonomy 
Sidneyia was discovered in 1910 during the first day of Charles Walcott's exploration of the Burgess Shale. He named it after his elder son, Sidney, who had helped to locate the site and collect the specimen. The species name, Sidneyia inexpectans, is derived from the meaning of "Sidney's surprise".

144 specimens of Sidneyia are known from the Greater Phyllopod bed, where they comprise 0.27% of the community.

Sidneyia sinica was named in 2002 from a specimen found in the Maotianshan Shales. However, it has since been rejected from the genus, and other indeterminate specimens assigned to the genus from the Spence Shale and Sirius Passet lack key diagnostic characters. Specimens that can confidently assigned to the genus include Sidneyia cf. inexpectans, known from the Wuliuan Mantou Formation of North China, as well as Sidneyia minor from the Early Cambrian (Cambrian Stage 3) Xiaoshiba Biota of Yunnan, China. Sidneyia minor differs from the type species by having "ten thoracic tergites, only one abdominal sclerite and a telson that bears narrow triangular tail flukes with prominent ridge-like lateral margins".

In 1923, Sidneyia, was placed, along with Emeraldella, as part of the group "Xenopoda". Today, both Sidneyia and Emeraldella are placed as part of the clade Vicissicaudata within Artiopoda, which includes trilobites and other arthropods with similar bodyforms. However, Sidneyia and Emeraldella are usually not recovered as each others closest relatives within Vicissicaudata, rendering "Xenopoda" invalid.

See also 
 Paleobiota of the Burgess Shale

References

External links

Further reading

Prehistoric arthropod genera
Burgess Shale fossils
Maotianshan shales fossils
Wheeler Shale
Artiopoda
Cambrian genus extinctions
Fossil taxa described in 1911