Hurdia was one of the largest organisms in the Cambrian oceans,H. victoria reached between 18.3–30.5 cm (7.2–12.0 in) in length, whileH. triangulata reached up to just 8.1 cm (3.2 in) long.[1] Its head bore a pair of frontal appendages. These frontal appendages had 9 or more rarely 10 or 11 segments/podomeres, which were approximately rectangular and decreased in size towards the end of the appendage. The upper surface of the appendage was convexly curved. Podomeres 2 to 6 bore long downward pointing spines (ventral spines) with forward-curving tips. These ventral spines themselves bore up to 9 equally spaced forward-facing spines dubbed auxiliary spines, with podomeres 7 and 8 bearing shorter, smooth forward curving spines. The frontal appenages were used to bring food to its ring-shaped mouth (oral cone), in which four large plates are present, with inner rows of spines inside the main cone.[2] Like other hurdiids,Hurdia bore a large frontal carapace protruding from its head composed of threesclerites: a central component known as the H-element and two lateral components known as P-elements. Originally, it is estimated that body flaps ran along the sides of the organisms, from which largegills were suspended.[3] However, anatomy ofAegirocassis clarified thatHurdia had both ventral and dorsal flaps, and gills were on trunk segments.[4]
Movement range of the frontal appendage
Interpretations of placements (A based on articulated specimens, B based on speculation fromCambroraster) of the frontal appendages and mouthparts ofHurdia
Hurdia is either suggested to have used its frontal appendages to sift small prey from sediment, or to have used them as a trap to capture largerbenthic (seafloor dwelling) prey.[5]
Hurdia had cosmopolitan distribution; two described species has been recovered from theBurgess Shale in British Columbia, Canada. In addition,H. victoria is also known from theSpence Shale in Utah, USA.[6] Unnamed species are known fromQingjiang biota in Hubei, China,Pioche Shale in Nevada, USA, andWheeler Shale in Utah, USA.[6][7]Huangshandongia yichangensis andLiantuoia inflata from theShuijingtuo Formation in Hubei, China, andProboscicaris hospes from theJince Formation of the Czech Republic (which is identified asHurdia hospes in some papers[8]), and unnamed fossil fromOrdovicianFezouata Formation could represent species ofHurdia as well.[3][2]
Hurdia was named in 1912 byCharles Walcott, with two species, the type speciesH. victoria and a referred species,H. triangulata.[9] The genus name refers toMount Hurd.[9] It is possible that Walcott had described a specimen the year prior asAmiella, but the specimen is too fragmentary to identify with certainty, soAmiella is anomen dubium.[10] Walcott's original specimens consisted only of H-elements of the frontal carapace, which he interpreted as being the carapace of an unidentified type of crustacean. P-elements of the carapace were described as a separate genus,Proboscicaris, in 1962.
In 1996, then-curator of theRoyal Ontario MuseumDesmond H. Collins erected the taxon Radiodonta to encompassAnomalocaris and its close relatives, and included bothHurdia andProboscicaris in the group.[11] He subsequently recognized thatProboscicaris andHurdia were based on different parts of the same animal, and recognized that a specimen previously assigned toPeytoia was also a specimen of the species.[10] He presented his ideas in informal articles,[12][13] and it was not until 2009, after three years of painstaking research, that the complete organism was reconstructed.[3][14][15][16]
Sixty-nine specimens ofHurdia are known from the GreaterPhyllopod bed, where they comprise 0.13% of the community.[17]
^abWalcott, Charles D. (1912-03-13). "Middle Cambrian Branchiopoda, Malacostraca, Trilobita, and Merostomata".Smithsonian Miscellaneous Collections.57 (6).
^abDaley, Allison C.; Budd, Graham E.; Caron, Jean-Bernard (2013). "Morphology and systematics of the anomalocaridid arthropodHurdia from the Middle Cambrian of British Columbia and Utah".Journal of Systematic Palaeontology.11 (7):743–787.Bibcode:2013JSPal..11..743D.doi:10.1080/14772019.2012.732723.S2CID86465719.
^D. Collins, in North American PaleontologicalConvention, Chicago, Abstracts with Programs, S. Lidgard,P. R. Crane, Eds. (The Paleontological Society, Special Publication 6, Chicago, IL, 1992), p. 66,11.
^D. Collins (1999). "Dinocarids: the first monster predators on earth".Rotunda. Vol. 32. Royal Ontario Museum. p. 25.
