The cactus-like chancelloriids from the Middle Cambrian Burgess Shale are revised on the basis of Walcott's (1920) original collections and new material containing several hundred specimens collected by Royal Ontario Museum field expeditions from 1975 to 2000. Walcott's interpretation of chancelloriids as sponges was based on a misinterpretation of the dermal coelosclerites as embedded sponge-type spicules, an interpretation that further led to the lumping of three distinct taxa into one species, Chancelloria eros Walcott, 1920. The other two taxa are herein separated from C. eros and described as Allonnia tintinopsis n. sp. and Archiasterella coriacea n. sp., all belonging to the Family Chancelloriidae Walcott, 1920. Chancelloriids were sedentary animals, anchored to shells or lumps of debris in the muddy bottom, or to sponges, or to other chancelloriids. They had a radially symmetrical body and an apical orifice surrounded by a palisade of modified sclerites. Well-preserved integuments in Al. tintinopsis and Ar. coriacea do not show any ostium-like openings. Neither is there any evidence for internal organs, such as a gut. Partly narrowed specimens suggest that the body periodically contracted from the attached end to expel waste material from the body cavity. Chancelloriids were close in organization to cnidarians but shared the character of coelosclerites with the bilaterian halkieriids and siphogonuchitids. The taxon Coeloscleritophora is most likely paraphyletic.

One hundred years after Charles Doolittle Walcott found a wealth of Cambrian fossils in the Rocky Mountains of British Columbia, Desmond Collins reflects on the bumpy road of their classification.

As the largest predators of the Cambrian seas, the anomalocaridids had an important impact in structuring the first complex marine animal communities, but many aspects of anomalocaridid morphology, diversity, ecology, and affinity remain unclear owing to a paucity of specimens. Here we describe the anomalocaridid Hurdia , based on several hundred specimens from the Burgess Shale in Canada. Hurdia possesses a general body architecture similar to those of Anomalocaris and Laggania , including the presence of exceptionally well-preserved gills, but differs from those anomalocaridids by possessing a prominent anterior carapace structure. These features amplify and clarify the diversity of known anomalocaridid morphology and provide insight into the origins of important arthropod features, such as the head shield and respiratory exites.

More than forty specimens from the middle Cambrian Burgess Shale reveal the detailed anatomy of Isoxys, a worldwide distributed bivalved arthropod represented here by two species, namely Isoxys acutangulus and Isoxys longissimus. I. acutangulus had a non-mineralized headshield with lateral pleural folds (= "valves" of previous authors) that covered the animal's body almost entirely, large frontal spherical eyes and a pair of uniramous prehensile appendages bearing stout spiny outgrowths along their anterior margins. The 13 following appendages had a uniform biramous design-i.e., a short endopod and a paddle-like exopod fringed with marginal setae with a probable natatory function. The trunk ended with a flap-like telson that protruded beyond the posterior margin of the headshield. The gut of I. acutangulus was tube-like, running from mouth to telson, and was flanked with numerous 3D-preserved bulbous, paired features interpreted as digestive glands. The appendage design of I. acutangulus indicates that the animal was a swimmer and a Visual predator living off-bottom. The general anatomy of Isoxys longissimus was similar to that of L acutangulus although less information is available on the exact shape of its appendages and visual organs. I. longissimus is characterized by extremely long anterior and posterior spines. There are now seven Isoxys species known with soft-part preservation, I. acutangulus, I. longissimus from the Burgess Shale, I. auritus and I. curvirostratus from the Maotianshan Shale of China, I. communis and I. glaessneri from the EMU Bay Shale of Australia and I. volucris from Sirius Passet in Greenland. The frontal appendages of Isoxys strongly resemble those of other Cambrian arthropods, characterized by a single pair of "great appendages" with a shared prehensile function yet some variability in length and shape.

The morphology of Titzoia 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 (l) 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, Titzoia 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.

The collection, since 1975, of over 1500 specimens of Leanchoilia Walcott by the Royal Ontario Museum has prompted reassessment of the genus and its species from the Middle Cambrian Burgess Shale. Among new characters in Leanchoilia superlata, the type species, are the presence of two pairs of eyes, a dorsal double carina bracketing the axis of the body segments, segmentation of the gill branch of the appendages, and serration along the body edges from the posterior third of the cephalic shield to the last body segment. Leanchoilia persephone Simonetta, previously synonymized with L. superlata, is also well represented in the Burgess Shale, and is re-established as a valid species, owing to conspicuous differences from the type species. These are the absence of the diagnostic up-curving snout of the cephalic shield, the absence of carina, the shorter 'great appendages', the smooth edges of the body, and its overall shape in dorsal aspect. Leanchoilia superlata and L. persephone may be sexual dimorphs of each other. The ROM collections extend considerably the geographical distribution and stratigraphic range of Leanchoilia in western Canada.

Study of over 1000 specimens of Marrella splendens Walcott, 1912, out of the more than 9000 collected by the Royal Ontario Museum (ROM) since 1975, has produced new information on the anatomy, functional morphology, and behaviour of this most common arthropod in the Burgess Shale fauna. Among the new features recognized is the distinction between the alimentary canal and circulatory system; where the former is generally three-dimensional and slightly reflective, the latter never presents any relief and is very reflective. A larger range of size is now known, from 2.4 to 24.5 mm in length, with small individuals possessing 17 body segments to large specimens with more than 26 body segments, representing an almost complete ontogenetic series. The second pair of "antennae" is now interpreted as swimming appendages, since the five distal segments are dorsoventrally compressed, fringed with setae and with a considerable blood supply, providing a paddlelike appendage capable of producing a considerable propelling force. The ROM collections extend the geographical distribution of Marrella 13 km to the southeast and the stratigraphical range through the lowest five members of the Burgess Shale Formation.

The family Lyracystidae n.fam. and genus Lyracystis n.gen. are proposed for the holotype and one paratype of the Middle Cambrian eocrinoid Gogia? radiata Sprinkle, the Burgess Shale "Arms" from the same unit, and many additional partial and more complete specimens of this eocrinoid collected from the Burgess Shale since 1975. A second species, Lyracystis reesei n.gen. and n.sp. is described from a single partly complete specimen from the similar-aged Spence Shale of northern Utah. Lyracystis has three wide V-shaped arms bearing numerous long straight brachioles in the notch, a partly organized theca having larger and smaller ridged plates with epispires, and a very long multiplated stalk made up of rounded or spiny small plates. Lyracystis is the longest-stalked, suspension-feeding echinoderm known from the Middle Cambrian. The three remaining paratypes of Gogia? radiata and four new specimens with possible branched brachioles from the Burgess Shale are renamed Gogia stephenensis n.sp.

A Cambrian fossil confirms that early arthropods shed their coats just as they do today.

West of the Fossil Gully Fault Zone on Mount Stephen, the three lowest members only of the Burgess Shale Formation are preserved: the Kicking Horse Shale, the Yoho River Limestone, and the Campsite Cliff Shale. The formation rests unconformably upon the Takakkaw Tongue Formation, whose dark basinal limestones conformably overlie paler shelf-like limestones of the Mount Whyte Formation. Mapping has resolved a long-standing problem and shown that the stratigraphical position of the famous Mount Stephen Trilobite Beds lies within the Campsite Cliff Shale. It has also revealed some of the complexities of the Fossil Gully Fault Zone, among which different periods and directions of component fault movements are indicated. Faunal evidence shows that the Plagiura–Kochaspis to Albertella and Albertella to Glossopleura zonal boundaries lie within the Takakkaw Tongue sequence. Within the Burgess Shale, three distinct soft-bodied communities occur at different stratigraphical levels on this mountain slope. The oldest, characterized by the arthropod Alalcomenaeus and chelicerate Sanctacaris, occurs low in the Kicking Horse Shale Member and is best known from Collins Quarry. The others lie within the Campsite Cliff Shale Member. The Trilobite Beds, characterized by claws of the dinocarid Anomalocaris and moults of the trilobite Ogygopsis klotzi, onlap the sloping top of a proximal bench facies of the Yoho River Limestone Member close to the Cathedral Escarpment. Slightly older and farther out in the basin are beds characterized by the dinocarid Laggania and a tulip-like animal related to Dinomischus, excavated about 12 m above the top of a thin distal wedge facies of the Yoho River Limestone at the S7 site. Among the illustrated trilobites, a new corynexochine from the Campsite Cliff Shale Member is figured.

Sphenothallus Hall, 1847 is a widespread Paleozoic marine taxon that has been interpreted most recently as a tubiculous annelid or other ‘worm’ or as a thecate hydrozoan or scyphozoan cnidarian (e.g., Mason and Yochelson, 1985; Feldmann et al., 1986; Van Iten et al., 1992, 1996; Neal and Hannibal, 2000; Zhu et al., 2000). Members of this genus are characterized by a very gently tapered, finely lamellar apatitic tube bearing a closed subconical holdfast and a pair of robust longitudinal thickenings situated at the ends of the tube's greatest diameter (Zhu et al., 2000). Rarely, the tube exhibits an internal transverse wall that extends adaperturally along the inner surface of the tube proper and may also exhibit a subcylindrical terminal protuberance (Van Iten et al., 1992, figs. 5, 6). The transverse wall of Sphenothallus is essentially identical in gross morphology and microstructure to the schott (apical wall) of conulariids, an extinct group of thecate cnidarians (Van Iten, 1991, 1992a, 1992b; Jerre, 1994; Van Iten et al., 1996; Hughes et al., 2000) that may have been closely related to Sphenothallus (Van Iten et al., 1992). Clarke (1913, pl. 26, figs. 16-18; see also Van Iten et al., 1992, fig. 3) illustrated a compound specimen of S. sica consisting of numerous “daughter” tubes arranged in a highly regular manner (in opposition and evenly spaced) along a single, relatively large “parent” tube whose test wall appears to be confluent with the base of the “daughter” tubes. This compound specimen is distinctly different from the more common associations of epibiontic, holdfast-bearing Sphenothallus arranged in a less orderly fashion on Sphenothallus tubes or other shells (see for example Feldmann et al., 1986, fig. 2), and probably is best interpreted as a clonal colony. Sphenothallus tubes, both solitary and branched, are most similar to thecae of solitary and colonial hydrozoan and scyphozoan polyps, differing from these relatively delicate structures mainly in being mineralized and in bearing a pair of longitudinal thickenings. Like conulariids, Sphenothallus occurs in rocks originally deposited in a broad spectrum of marine facies (open carbonate shelf to restricted shale basin), but is perhaps most conspicuous in restricted basin and shelf slope facies represented by dark shales and lime mudstones.

More than 300 specimens of the previously rare arthropod Alalcomenaeus cambricus Simonetta have been collected from a new Burgess Shale locality in the Glossopleura Zone on Mount Stephen, British Columbia. This new material provides much more complete information on its morphology. The cephalon was covered by a shield. A pair of pedunculate eyes and three median eyes were followed by a large anterior appendage, the ‘great appendage’, bearing three long flagella. The two posterior head appendages, like those of the trunk, were biramous. They consisted of a segmented, inner branch, and a flap‐like outer branch, fringed with long filaments. The trunk consisted of 11 somites, each protected by a tergite and bearing a pair of biramous limbs. The telson was paddle‐like and fringed posteriorly with wide flat spines. Alalcomenaeus was probably a predator, moving mainly by swimming. It is now known to be one of the more abundant, widely distributed and longest ranging of Burgess Shale arthropod genera. Its affinities lie with the Arachnomorpha.

Echmatocrinus from the Middle Cambrian Burgess Shale of British Columbia was originally described as the earliest crinoid(?) known from the fossil record. Recently, Conway Morris and Ausich & Babcock have questioned whether Echmatocrinus is in fact an echinoderm, comparing it instead to cnidarians with a polyp-like body and pinnate tentacles, and other authors are beginning to use this reinterpretation. We studied the well-preserved holotype of Echmatocrinus brachiatus, two paratypes, and 18 new specimens recovered from different levels in the Burgess Shale sequence at three localities. All are preserved as pyrite films in dark shale with relatively little relief, suggesting a lightly skeletized body. Complete specimens have a long, slightly tapering, large-plated attachment stalk, a conical cup or calyx with numerous small to medium-sized irregular plates, and 7–10 short arms with heavier plating and (in the holotype) soft appendages alternating from opposite sides of several arms. Several morphologic features indicate that Echmatocrinus is an echinoderm and has crinoid affinities: (1) Sutured plates, shown by darker depressed sutures, slightly raised plate centers, and oriented plate ornament, cover all major parts of the body; (2) reticulate surface ornament in the pyrite film on the plates of all specimens matches the ornament in the Burgess Shale edrioasteroid Walcottidiscus, an undoubted echinoderm, but not the pyritized surfaces of other metazoans in the fauna; (3) this distinctive ornament may represent the surface expression of microporous stereom; (4) possible ligament or muscle pads are present between the arm ossicles to fold and unfurl the more heavily plated arms. Within the echinoderms, only crinoids commonly have a calyx attached by a stalk or stem to the substrate and bear erect, moveable, uniserial arms for feeding. Although Echmatocrinus shows some resemblance to octocorals in overall body shape as an attached suspension feeder, almost all the details are different, indicating that Echmatocrinus is most likely unrelated to this group. All complete specimens of Echmatocrinus are attached to hard substrates, either another fossil or skeletal debris. The new specimens indicate that Echmatocrinus was twice as common (about 0.02%) in the Burgess Shale fauna as previously recorded and represents one of the earliest attached, medium-level, skeletized, suspension feeders or microcarnivores in the fossil record.

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The remarkable ''evolution'' of the reconstructions of Anomalocaris, the extraordinary predator from the 515 million year old Middle Cambrian Burgess Shale of British Columbia, reflects the dramatic changes in our interpretation of early animal life on Earth over the past 100 years. Beginning in 1892 with a claw identified as the abdomen and tail of a phyllocarid crustacean, parts of Anomalocaris have been described variously as a jellyfish, a sea-cucumber, a polychaete worm, a composite of a jellyfish and sponge, or have been attached to other arthropods as appendages. Charles D. Walcott collected complete specimens of Anomalocaris nathorsti between 1911 and 1917, and a Geological Survey of Canada party collected an almost complete specimen of Anomalocaris canadensis in 1966 or 1967, but neither species was adequately described until 1985. At that time they were interpreted by Whittington and Briggs to be representatives of ''a hitherto unknown phylum.''Here, using recently collected specimens, the two species are newly reconstructed and described in the genera Anomalocaris and Laggania, and interpreted to be members of an extinct arthropod class, Dinocarida, and order Radiodonta, new to science. The long history of inaccurate reconstruction and mistaken identification of Anomalocaris and Laggania exemplifies our great difficulty in visualizing and classifying, from fossil remains, the many Cambrian animals with no apparent living descendants.

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