Isoxys Walcott, 1890
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https://doi.org/ 10.4202/app.2009.0024 |
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persistent identifier |
https://treatment.plazi.org/id/FA76175A-DF38-FF85-FCA4-F9C2DD8835E3 |
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treatment provided by |
Felipe |
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scientific name |
Isoxys Walcott, 1890 |
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Genus Isoxys Walcott, 1890
Type species: Isoxys chilhoweanus Walcott, 1890 , by original designation; from the lower Cambrian ( Bonnia –Olenellus Zone) Chilhowee Group, Tennessee, USA.
Species included: Isoxys acutangulus ( Walcott, 1908) ; Isoxys carbonelli Richter and Richter, 1927 ; Isoxys longissimus Simonetta and Delle Cave, 1975 ; Isoxys communis Glaessner, 1979 ; Isoxys auritus Jiang, 1982 (in Luo et al. 1982); Isoxys paradoxus Hou, 1987 ; Isoxys bispinatus Cui, 1991 (in Huo et al. 1991); Isoxys zhurensis Ivantsov, 1990 ; Isoxys volucris Williams, Siveter, and Peel, 1996 ; Isoxys curvirostratus Vannier and Chen, 2000 ; Isoxys wudingensis Luo and Hu, 2006 (in Luo et al. 2006); Isoxys glaessneri García−Bellido, Paterson, Edgecombe, Jago, Gehling, and Lee, 2009 .
Emended diagnosis.—Headshield thin, nonmineralized, flexible, folded dorsally into two equal hemispherical flaps that covered most of the body features. No true articulated hinge, pleural folds being possibly conjoined dorsally by a narrow band of cuticle. Each pleural fold with two prominent spines of variable length extending antero− and posterodorsally. Dorsal outline straight or slightly convex to form a weak to well−developed cusp anterior of headshield mid−length. Weak lateral sculpture; a small circular node and/or a chevron−like shallow furrow may be present anteriorly. Entire lateroadmarginal ridge swollen to extremely narrow. When present, external ornament is uniform micro−reticulation or longitudinal striae. Narrow to broad doublure may be present. Long segmented body attached to headshield by its most anterior part. Bulbous ocular segment bearing a pair of large spherical eyes that protrude slightly antero−ventrally. One pair of frontal appendages, long, curved, extend beyond the flap margins; possibly flexible with prehensile function; divided into a possible proximal peduncle followed by a short−segmented claw−like unit with spiny outgrowths along its anterior margin. Homonomous series of 13 pairs of biramous appendages, each with short endopods and large flap−like exopods fringed with setae. Trunk end has a tail fan formed by the flattened telson with a pair of lateral flaps and the posteriormost pairs of exopods. Tube−like midgut flanked with numerous pairs of glands. (Modified from Williams et al. 1996: 950; Vannier and Chen 2000: 311).
Discussion.— Isoxys is an arthropod known from the early and middle Cambrian of North America, South China, Siberia, South Australia and North Gondwana ( Spain: Richter and Richter 1927; France: Vannier et al. 2005). Its wide distribution is, however, restricted to tropical and subtropical regions, indicating possible temperature control on its distribution ( Williams et al. 1996). Isoxys is unique among the so−called bivalved arthropods by its stout, in some cases extremely long, cardinal spines and the lack of strong lateral relief. The majority of bivalved arthropods known from the Cambrian of various regions have either featureless rounded shields (e.g., waptiids; Taylor 2002) or shields with a strong lobation (e.g., most bradoriids; Hou et al. 2002). Isoxys differs from them in having prominent cardinal spines and no long telescopic trunk extending behind the shield thus contrasting with, for example, Waptia Walcott, 1912 , Clypecaris Hou, 1999 , Canadaspis Novozhilov, 1960 , Occacaris Hou, Bergström, Wang, Feng, and Chen, 1999 , and Forfexicaris Hou, Bergström, Wang, Feng, and Chen, 1999 (Briggs et al. 1994; Hou et al. 2004).
Isoxys is regarded here as having a short head bearing eyes and a single pair of frontal appendages. However, uncertainties remain concerning the number of head segments of this arthropod. Numerous specimens of Isoxys acutangulus ( Fig. 2A–C) clearly show a gap between the segment that bore the frontal appendage and the presumed first trunk segment that bore a biramous appendage and the first pair of midgut glands. Our material does not allow us to determine whether there were other additional head segments that may have filled this gap and if these carried any kind of (reduced?) appendages. The “bivalved carapace” of Isoxys is likely to be a headshield—i.e., a cuticular fold originating from the head section and extending laterally and posteriorly into large flap−like extensions (pleural folds).
The pair of frontal appendages of Isoxys shows strong similarities with the so−called “great appendage” of other Cambrian arthropods such as Leanchoilia Walcott, 1912 , Alalcomenaeus Simonetta, 1970 , Yohoia Walcott, 1912 , Jianfengia Hou, 1987 , Haikoucaris Chen, Waloszek, and Maas, 2004 , and Fortiforceps Hou and Bergström, 1997 (see Vannier et al. 2009). These resemblances concern the structure of the appendage itself, especially the small number of podomeres bearing outgrowths, its frontal location and prehensile shape. The presence of a smooth peduncle similar to that of other great appendage arthropods cannot be ascertained: whereas some specimens have a great appendage with its proximal part lacking endite−like projections or spines (e.g., Fig. 2B), others do show outgrowths along the entire length of their appendage (e.g., Fig. 3A, B View Fig ). Contrasting with Isoxys , “great appendage” arthropods have a tiny cap−like headshield with no lateral/posterior extensions covering the animal’s body. A headshield comparable with that of Isoxys is found in only two forms from the Chengjiang biota, namely Occacaris and Forfexicaris (see Hou et al. 2004). Both have stout “great appendages” pointing forwards and upwards, which may suggest possible relations of Isoxys with these two bivalved arthropods. “Great appendage” (or megacheiran) arthropods encompass a wide range of Cambrian forms from smaller epibenthic predators that do not exceed 10 cm in length to, according to some authors, much larger anomalocaridids ( Chen et al. 2004). Among the latter, the one that shows the closest resemblances to megacheirans based on its appendages is Parapeytoia . However, other anomalocaridids (e.g., Maas et al. 2004) have multisegmented frontal appendages curved posteriorwards and no true peduncle, which does not support a placement with the megacheirans.
The definition and status of megacheirans as a whole remains uncertain, even their monophyly. They have been assigned by some authors (e.g., Wills et al. 1998; Cotton and Braddy 2004) to the Arachnomorpha (= chelicerate−allied and trilobite−allied clades). Chen et al. (2004) tentatively placed them in the stem−lineage Chelicerata on the basis of presumed homologies between the “great appendage” and the chelicera of the crown−group Chelicerata (e.g., Recent spiders, scorpions, and horseshoe crabs). These hypotheses require more character support.
Stratigraphic and geographic range.—Lower middle Cambrian, cosmopolitan.
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