Sidneyia

[2][3] The appendages bear heavily sclerotised spined basal segments (basipods) called gnathobases, used to process food.[2] 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.[1] S. inexpectans had three pairs of digestive glands within the head shield and front of the thorax, adjacent to the central gut tube.[1] Sidneyia is thought to have been seafloor dwelling (epibenthic) generalist durophagous predator and/or scavenger that used its gnathobases (which closely resemble those of horseshoe crabs) 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 (representing around 6% of the stomach contents of S. inexpectans), but possibly also softer animals like worms or soft bodied arthropods like bradoriids.Today, both Sidneyia and Emeraldella are placed as part of the clade Vicissicaudata within Artiopoda, which includes trilobites and other arthropods with similar bodyforms.
Size comparison of Sidneyia species
Biramous limb of Sidneyia inexpectans Key: gnathobasic basipod (pink, labelled pr) exopod (yellow, labelled ex), endopod (orange, with terminal podomere labelled pd7 in yellow)
PreꞒScientific classificationEukaryotaAnimaliaArthropodaArtiopodaVicissicaudataWalcottType speciesSpeciesarthropodfossilsBurgess ShaleBritish Columbiahypostometergitesbiramoustelsonsclerotisedpodomeresexopodepibenthicdurophagoushorseshoe crabstrilobitesbrachiopodsbradoriidsCharles WalcottPhyllopod bedChengjiang BiotaSpence ShaleSirius PassetWuliuanCambrian Stage 3EmeraldellaPaleobiota of the Burgess ShaleBibcodeEcdysozoaPanarthropodaBailongiaFalcatamacarisMolariaSquamaculaThulaspisUrokodiaAcanthomeridionZhiweniaCarimersaKodymirusTardisiaCheloniellidaCheloniellonAglaspididaAngarocarisAustralaglaspisBeckwithiaTremaglaspididaeCyclopitesAglaspididaeAglaspisAglaspoidesTrilobitomorphaArthroaspisKwanyinaspisPygmaclypeatusRetifaciesXandarellidaCindarellaLuohuilinellaPhytophilaspisSinoburiusXandarellaNektaspidaEmucarididaeEmucarisKangacarisNaraoiidaeMisszhouiaNaraoiaLiwiidaeBuenaspisSoomaspisTariccoiaConcilitergaHelmetiaKuamaiaSaperionTegopelteTrilobitaWikidataOpen Tree of LifePaleobiology Database