Isoxys

Abstract: Palaeoscolecidan worms are an important component of many Lower Palaeozoic marine assemblages, with notable occurrences in a number of Burgess Shale-type Fossil-Lagerstatten. In addition to material from the lower Cambrian Kinzers Formation and Latham Shale, we also describe two new palaeoscolecidan taxa from the lower Cambrian Sirius Passet Fossil-Lagerstatte of North Greenland: Chalazoscolex pharkus gen. et sp. nov and Xystoscolex boreogyrus gen. et sp. nov. These palaeoscolecidans appear to be the oldest known (Cambrian Series 2, Stage 3) soft-bodied examples, being somewhat older than the diverse assemblages from the Chengjiang Fossil-Lagerstatte of China. In the Sirius Passet taxa the body is composed of a spinose introvert (or proboscis), trunk with ornamentation that includes regions bearing cuticular ridges and sclerites, and a caudal zone with prominent circles of sclerites. The taxa are evidently quite closely related; generic differentiation is based on degree of trunk ornamentation, details of introvert structure and nature of the caudal region. The worms were probably infaunal or semi-epifaunal; gut contents suggest that at least X. boreogyrus may have preyed on the arthropod Isoxys. Comparison with other palaeoscolecidans is relatively straightforward in terms of comparable examples in other Burgess Shale-type occurrences, but is much more tenuous with respect to the important record of isolated sclerites. These finds from Greenland provide further evidence that palaeoscolecidans possessed a complex anterior introvert directly comparable to a number of priapulid-like taxa from other Burgess Shale-type assemblages. Although these palaeoscolecidans have been allied with the nematomorphs, molecular data in conjunction with our observations suggest that this hypothesis is untenable. Palaeoscolecidans and similar priapulid-like taxa are probably primitive cycloneuralians and as such may indicate the original bodyplan of this important group of ecdysozoans. In addition, we describe another sclerite-bearing fossil from the Sirius Passet Fossil-Lagerstatte that may be related to the cambroclaves.

Abstract: The morphology of Tuzoia is reinterpreted in the light of abundant new specimens from the Middle Cambrian Burgess Shale (British Columbia, Canada) and Kaili (Guizhou, China) Lagerstatten. Tuzoia was a very large (up to 180 mm long) bivalved arthropod with a nonmineralized domelike carapace strengthened by prominent pointed features and often flanked by a lateral ridge bearing a spiny frill. The reticulate pattern of Tuzoia is comparable with that of present-day crustaceans (e.g., myodocope ostracods) and is interpreted as a structural compromise between exoskeletal lightness and high resistance to mechanical stress. Tuzoia had a pair of large, stalked, spherical, possibly compound eyes facing forward. Flagella-like antennae protruded through the anterior notch. No other appendages are known except possible filamentous setae underlying the carapace. Tuzoia typically occurs as laterally (lc) or dorsoventrally (dvc) compacted carapaces or single valves. Each type (lc or dvc) emphasizes particular aspects of the morphology (e.g., spiny lateral ridge, ventral margin) that were often interpreted as specific differences by previous authors. A revision of Tuzoia validates only 7 of the 23 named species. Tuzoia is placed tentatively within a group of large bivalved arthropods along with Isoxys and the possible ancestors of Thylacocephala (Lower Cambrian–Upper Cretaceous). In the Middle Cambrian, Tuzoia occurs across Laurentia, South and North China, and the Perigondwanan area (Bohemia) within a relatively narrow subtropical belt, indicating a high dispersal capability and possible latitudinal control on its distribution. Functional morphology, taphonomy, and the distributional pattern indicate that Tuzoia was a free-swimming arthropod.