''Sidneyia'' is an extinct marine

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

known from

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s found from the Early to the Mid Cambrian of China and the Mid Cambrian

Burgess Shale The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fos ...

British Columbia British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...

, Canada.

Description

Sidneyia size comparison.svg, Size comparison of ''Sidneyia'' species Rsos211134f08 d.jpg, Biramous limb of ''Sidneyia inexpectans'' ''Sidneyia inexpectans'' reached lengths of at least . The largest known specimen of ''S. minor'' is around long and wide, while the largest specimen of ''S. malongensis'' is long and wide. The head shield is short, with notches present on the sides to accommodate stalked eyes, with the underside having a hypostome. The head has a pair of segmented antennae, as well as three pairs of post-antenal appendages. This was followed by a thorax, which had eight to ten segments/ tergites, each associated with a pair of

biramous The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, ...

(two branched) appendages, this was followed with one to three abdomen segments/tergites, with the body terminating with a

telson The telson () is the hindmost division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment (biology), segment of the arthropod body, or an additional division that is not a true segm ...

, which comprised a pair of tail flukes. The appendages bear heavily sclerotised spined basal segments (basipods) called gnathobases, used to process food. In ''S. minor'', the biramous appendages have 8 podomeres/segments on the endopod (the inner leg like branch), with the last segment being a terminal claw. The

exopod The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip (a ...

(the outer branch) 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 within the head shield and front of the thorax, adjacent to the central gut tube.

Ecology

''Sidneyia'' is thought to have been seafloor dwelling ( epibenthic) generalist

durophagous Durophagy is the eating behavior of animals that consume hard-shelled or exoskeleton-bearing organisms, such as corals, shelled mollusks, or crabs. It is mostly used to describe fish, but is also used when describing reptiles, including fossil t ...

predator and/or scavenger that used its gnathobases (which closely resemble those of

horseshoe crabs Horseshoe crabs are arthropods of the family Limulidae and the only surviving xiphosurans. Despite their name, they are not true crabs or even crustaceans; they are chelicerates, more closely related to arachnids like spiders, ticks, and scorpi ...

) to crush and shred prey items, including hard-shelled organisms like juvenile

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

s (which are abundantly preserved as stomach contents in ''S. inexpectans'') and

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear e ...

s (representing around 6% of the stomach contents of ''S. inexpectans''), but possibly also softer animals like worms or soft bodied arthropods like bradoriids.

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

Chengjiang Biota The Maotianshan Shales () are a series of Early Cambrian sedimentary deposits in the Chiungchussu Formation or Heilinpu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized orga ...

of South China. However, it has since been rejected from the genus, and other indeterminate specimens assigned to the genus from the Spence Shale and

Sirius Passet Sirius Passet is a Cambrian Lagerstätte in Peary Land, Greenland. The Sirius Passet Lagerstätte was named after the Slædepatruljen Sirius, Sirius sledge patrol that operates in North Greenland. It comprises six places in Nansen Land, on the eas ...

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, ''Sidneyia minor'' from the Early Cambrian (

Cambrian Stage 3 Cambrian Stage 3 is the still unnamed third stage of the Cambrian. It succeeds Cambrian Stage 2 and precedes Cambrian Stage 4, although neither its base nor top have been formally defined. The plan is for its lower boundary to correspond approxima ...

) Xiaoshiba Biota of Yunnan, China, and a valid species of ''Sidneyia'' from Chengjiang, ''Sidneyia malongensis''. In 1923, ''Sidneyia'', was placed, along with ''

Emeraldella ''Emeraldella'' is a genus of arthropod known from the Middle Cambrian of North America. The type species ''E. brocki'' was described in 1912 from the Burgess Shale. 21 specimens of ''Emeraldella'' are known from the Greater Phyllopod bed, where th ...

'', as part of the group "Xenopoda". Today, both ''Sidneyia'' and ''Emeraldella'' are placed as part of the clade

Vicissicaudata Vicissicaudata is an unranked group of artiopodan arthropods, containing Cheloniellida, Aglaspidida and several other genera outside these groups like ''Sidneyia'' and ''Emeraldella.'' Description Vicissicaudatans are similar to most other ...

within

Artiopoda Artiopoda is a clade of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomor ...

, 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. Cladogram of Artiopoda including ''Sidneyia'' after Berks et al. 2023. Cladogram of Vicissicaudata after McCoy et al. 2025:

Description

Ecology

Taxonomy

See also

Notes

References

External links

Further reading