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Radiodonts, stem-group euarthropods that evolved during the Cambrian explosion, were among the largest and most diversified lower palaeozoic predators. These animals were widespread geographically, occupying a variety of ecological niches, from benthic foragers to nektonic suspension feeders and apex predators. Here, we describe the largest Cambrian hurdiid radiodont known so far,Titanokorys gainesi, gen. et sp. nov., from the Burgess Shale (Marble Canyon, Kootenay National Park, British Columbia). Estimated to reach half a metre in length, this new species bears a very large ovoid-shaped central carapace with distinct short posterolateral processes and an anterior spine. Geometric morphometric analyses highlight the high diversity of carapace shapes in hurdiids and show thatTitanokorysbridges a morphological gap between forms with long and short carapaces. Carapace shape, however, is prone to homoplasy and shows no consistent relationship with trophic ecology, as demonstrated by new data, including a reappraisal of the poorly knownPahvantia. Despite distinct carapaces,Titanokorysshares similar rake-like appendages for sediment-sifting withCambroraster,a smaller but much more abundant sympatric hurdiid from the Burgess Shale.The co-occurrence of these two species on the same bedding planes highlights potential competition for benthic resources and the high diversity of large predators sustained by Cambrian communities.

The origin of the arthropod carapace, an enlargement of cephalic tergites, can be traced back to the Cambrian period. However, its disparity and evolution are still not fully understood. Here, we describe a new ‘bivalved’ arthropod,Fibulacaris nereidisgen. et sp. nov., based on 102 specimens from the middle Cambrian (Wuliuan Stage) Burgess Shale, Marble Canyon area in British Columbia's Kootenay National Park, Canada. The laterally compressed carapace covers most of the body. It is fused dorsally and merges anteriorly into a conspicuous postero-ventrally recurved rostrum as long as the carapace and positioned between a pair of backwards-facing pedunculate eyes. The body is homonomous, with approximately 40 weakly sclerotized segments bearing biramous legs with elongate endopods, and ends in a pair of small flap-like caudal rami.Fibulacaris nereidisis interpreted as a suspension feeder possibly swimming inverted, in a potential case of convergence with some branchiopods. A Bayesian phylogenetic analysis places it within a group closely related to the extinct Hymenocarina.Fibulacaris nereidisis unique in its carapace morphology and overall widens the ecological disparity of Cambrian arthropods and suggests that the evolution of a ‘bivalved' carapace and an upside-down lifestyle may have occurred early in stem-group crustaceans.

The in situ preservation of animal behaviour in the fossil record is exceedingly rare, but can lead to unique macroecological and macroevolutionary insights, especially regarding early representatives of major animal clades. We describe a new complex ecological relationship from the middle Cambrian Burgess Shale (Raymond Quarry, Canada). More than 30 organic tubes were recorded with multiple enteropneust and polychaete worms preserved within them. Based on the tubicolous nature of fossil enteropneusts, we suggest that they were the tube builders while the co-preserved polychaetes were commensals. These findings mark, to our knowledge, the first record of commensalism within Annelida and Hemichordata in the entire fossil record. The finding of multiple enteropneusts sharing common tubes suggests that either the tubes represent reproductive structures built by larger adults, and the enteropneusts commonly preserved within are juveniles, or these enteropneusts were living as a pseudo-colony without obligate attachment to each other, and the tube was built collaboratively. While neither hypothesis can be ruled out, gregarious behaviour was clearly an early trait of both hemichordates and annelids. Further, commensal symbioses in the Cambrian may be more common than currently recognized.

Bayesian phylogenetic analysis using a Markov k model on a morphological dataset with 90 taxa and 213 characters (see Methods). Numbers next to nodes are posterior probabilities. Yellow box highlights the group Hymenocarina. Loricicaris may represent a junior synonym of Protocaris (3). Green box highlights the group Hymenocarina.

Bayesian analysis using a Markov k model on a morphological dataset with 90 taxa and 209 characters (without characters 206, 207, 208, 209 and 205 as originally defined (28) (see Methods, See Supplementary Material 3). Numbers next to nodes are posterior probabilities. Loricicaris may represent a junior synonym of Protocaris (3).

Size of the measured specimens (n=50), lengths and heights (mm). Most specimens follow a similar proportion of length-width. Although some variation is present regarding widths, the number of measured specimens is too small to detect any morphotype patterns. The specimen that appears as an outlier (at 15L, 10H) is ROMIP 65386.

Clusters of individuals of Fibulacaris nereidis, preserved in multiple views ROMIP 65457. A) Part. B) Counterpart. C) Telson of specimen 4, figured in Figure 4 C. The total number is approximately 11 individuals, as indicated. Abbreviations: ca) carapace, c.r) caudal rami, ro) rostrum, th) thorax.

Three-dimensional internal structures and comparisons with other arthropods from the Burgess Shale. A) Odaraia, lateral view ROMIP 61121. Odaraia was also referred as an invert swimmer and suspension feeder and shares multiple traits with Fibulacaris nereidis, including the absence of developed antennae and a highly multisegmented body. B, C) F. nereidis, ROMIP 65363, focus on the indeterminate rostrum tissue. D) Elemental maps of F. nereidis, ROMIP 65363: aluminium, carbon, calcium, phosphorous. E) Elemental maps of Marrella splendens ROMIP 60748: carbon, phosphorous. The tissue seen in the cephalic spines of M. splendens could represent cavities or unrecognised organs (79), and may have been similar to the internal tissue on the rostrum of F. nereidis. Abbreviations: ca) carapace, ey) eye, gu) gut, in?) indeterminate, le) legs, ro) rostrum. Scale bars: 10 mm (A), 2 mm (B), 1 mm (C, D, E).

Includes: Supp. Material 1-Specimens figured, measured or referenced. Supp. Material 2: added taxa. Supp. Material 3: list character matrix. Supp. Material 4: applied constraints

S1- W. fieldensis, geographic and stratigraphic distribution. S2- Nebalia bipes (Phyllocarida), decay experiments. S3- W. fieldensis, fossil assemblage. S4- W. fieldensis, synonymy list. S5- W. fieldensis, type specimens. S6- W. fieldensis, measurements (1). S7- Nebalia bipes (Phyllocarida), morphology. S8- W. fieldensis, frontal part. S9- Vargula hilgendorfii (Ostracoda), labrum. S10- Cirolana harfordi (Isopoda), mandibles. S11- W. fieldensis, mandibles and maxillules. S12- W. fieldensis, post-maxillular cephalothoracic appendages. S13- W. fieldensis, post-maxillular cephalothoracic appendages (frontal view). S14- W. fieldensis, lamellate appendages. S15- Crangon crangon (Decapoda), gills. S16- W. fieldensis, measurements (2). S17- W. fieldensis, digestive system (1). S18- W. fieldensis, digestive system (2). S19- Cladogram (adult taxa). S20- List of characters. S22- Cladogram (larval and adult taxa).

Electronic Supplementary Material Figure 2. Cross sections of a Tuzoia carapace. ROM 64825, Stanley Glacier, Kootenay National Park. (a) Low angle light emphasizing the presence of burrows. (b) Specimens photographed using cross-polarized light after cuts – the line in red represents area mapped. (c) Polished cross-section of the part and counterpart. The carapace occurs at the interface along an horizontal yellow zone which likely represents a weathering effect. (d) Maps of Ca, Ca+Si and Ca+Fe showing a weathered layer poor in Ca (framed area in c). (e) Different elemental maps showing no evidence of variations in mineralogy between the traces fossils and the matrix. The bright carbon zone at the interface between the part and the counterpart is glue holding the two pieces together. The carapace itself is not visible, and the Carbon film has probably weathered out or is too thin to be detected in cross-section. Note that part of the cut probably included both valves, but it is impossible to tell based on cross sections. Scale bars are 1 cm in (a, b) 1 mm in (c) and 0.5 mm in (d, e).

Electronic Supplementary Material Figure 1. Trace fossils associated with Tuzoia carapaces. (a) General view of two specimens of Tuzoia sp.; annulated burrow in fragmented specimen (white box), ROM 64814, Tuzoia Bed, Fossil Ridge. (b) Close-up of annulated burrow (arrow) shown in a. Scale bar is 0.5 cm. (c) General view of a high-density trace-fossil assemblage associated with Tuzoia sp. Note two clear sets of trace fossils: thin simple trails and burrows mostly confined to the carapace outline and U-shaped burrows clearly visible as positive ridges. These two distinctive set of structures are interpreted as recording a palimpsest assemblage. Although polarity of the bed was not recorded (this is talus material), based on the preservation of U-shaped burrows it is suggested that this surface records a bed sole. Accordingly, U-shaped burrows are overlying the Tuzoia carapace and possibly superimposed due to the compactional effect. U-shaped burrows, although not as visible, are also present in the host rock. ROM 57533, Walcott Talus, Fossil Ridge. Specimen most likely coming from levels of the Raymond Quarry or higher in the stratigraphic section. Scale bar is 1 cm.

S1- W. fieldensis, geographic and stratigraphic distribution. S2- Nebalia bipes (Phyllocarida), decay experiments. S3- W. fieldensis, fossil assemblage. S4- W. fieldensis, synonymy list. S5- W. fieldensis, type specimens. S6- W. fieldensis, measurements (1). S7- Nebalia bipes (Phyllocarida), morphology. S8- W. fieldensis, frontal part. S9- Vargula hilgendorfii (Ostracoda), labrum. S10- Cirolana harfordi (Isopoda), mandibles. S11- W. fieldensis, mandibles and maxillules. S12- W. fieldensis, post-maxillular cephalothoracic appendages. S13- W. fieldensis, post-maxillular cephalothoracic appendages (frontal view). S14- W. fieldensis, lamellate appendages. S15- Crangon crangon (Decapoda), gills. S16- W. fieldensis, measurements (2). S17- W. fieldensis, digestive system (1). S18- W. fieldensis, digestive system (2). S19- Cladogram (adult taxa). S20- List of characters. S22- Cladogram (larval and adult taxa).;S23. Waptia fieldensis Walcott, 1912 from the middle Cambrian (Series 3, Stage 5) Burgess Shale, British Columbia, Canada; animated reconstruction (L. Fields); general view.;S24. Waptia fieldensis Walcott, 1912 from the middle Cambrian (Series 3, Stage 5) Burgess Shale, British Columbia, Canada; animated reconstruction (L. Fields); anterior part.

Detailed information on the numerical model, additional figures for the numerical model and table summarizing parameters used for geomechanical modelling of carapace deformation

Radiodonts, a clade of Cambro-Devonian stem group euarthropods, have classically been regarded as nektonic apex predators. However, many aspects of radiodont morphology and ecology have remained unclear because of the typically fragmentary nature of fossil material. Here, we describe a new hurdiid radiodont based on abundant and exceptionally well-preserved fossils from the Burgess Shale (Marble Canyon area, British Columbia, Canada). Cambroraster falcatus gen. et sp. nov . is characterized by an extra-large horseshoe-shaped head carapace, bearing conspicuous posterolateral spinous processes, and partially covering a short trunk with eight pairs of lateral flaps. Each of the pair of frontal appendages possess five mesially curving rake-like endites equipped with a series of anteriorly directed hooked spines, altogether surrounding the oral cone. This feeding apparatus suggests a micro to macrophagous sediment-sifting feeding ecology. Cambroraster illuminates the evolution of Hurdiidae and evinces the exploitation of the diversifying infauna by these large and specialized nektobenthic carnivores in the aftermath of the Cambrian explosion.

Extant arthropods are diverse and ubiquitous, forming a major constituent of most modern ecosystems. Evidence from early Palaeozoic Konservat Lagerstätten indicates that this has been the case since the Cambrian. Despite this, the details of arthropod origins remain obscure, although most hypotheses regard the first arthropods as benthic predators or scavengers such as the fuxianhuiids or megacheirans (‘great-appendage’ arthropods). Here, we describe a new arthropod from the Tulip Beds locality of the Burgess Shale Formation (Cambrian, series 3, stage 5) that possesses a weakly sclerotized thorax with filamentous appendages, encased in a bivalved carapace, and a strongly sclerotized, elongate abdomen and telson. A cladistic analysis resolved this taxon as the basal-most member of a paraphyletic grade of nekto-benthic forms with bivalved carapaces. This grade occurs at the base of Arthropoda (panarthropods with arthropodized trunk limbs) and suggests that arthrodization (sclerotization and jointing of the exoskeleton) evolved to facilitate swimming . Predatory and fully benthic habits evolved later in the euarthropod stem-lineage and are plesiomorphically retained in pycnogonids (sea spiders) and euchelicerates (horseshoe crabs and arachnids).

The first arthropod trackways are described from the Middle Cambrian Burgess Shale Formation of Canada. Trace fossils, including trackways, provide a rich source of biological and ecological information, including direct evidence of behaviour not commonly available from body fossils alone. The discovery of large arthropod trackways is unique for Burgess Shale-type deposits. Trackway dimensions and the requisite number of limbs are matched with the body plan of a tegopeltid arthropod. Tegopelte , one of the rarest Burgess Shale animals, is over twice the size of all other benthic arthropods known from this locality, and only its sister taxon, Saperion , from the Lower Cambrian Chengjiang biota of China, approaches a similar size. Biomechanical trackway analysis demonstrates that tegopeltids were capable of rapidly skimming across the seafloor and, in conjunction with the identification of gut diverticulae in Tegopelte , supports previous hypotheses on the locomotory capabilities and carnivorous mode of life of such arthropods. The trackways occur in the oldest part (Kicking Horse Shale Member) of the Burgess Shale Formation, which is also known for its scarce assemblage of soft-bodied organisms, and indicate at least intermittent oxygenated bottom waters and low sedimentation rates.

Agnostids (agnostinids and eodiscinids) are a widespread and biostratigraphically important group of Cambro-Ordovician euarthropods whose evolutionary affinities have been highly controversial. Their dumbbell-shaped calcified tergum was traditionally suggested to unite them with trilobites, but agnostinids have alternatively been interpreted as stem-crustaceans, based on Orsten larval material from the Cambrian of Sweden. We describe exceptionally preserved soft tissues from mature individuals of the agnostinids Peronopsis and Ptychagnostus from the middle Cambrian (Wuliuan Stage) Burgess Shale (Walcott Quarry and Marble Canyon, British Columbia, Canada), facilitating the testing of alternative hypotheses. The digestive tract includes conspicuous ramifying cephalic diverticulae. The cephalon carries one pair of elongate spinous antennules projecting to the front, two pairs of appendages with distally setose, oar-like exopods, and three pairs of presumably biramous appendages with endopods sporting club-shaped exites. The trunk bears five appendage pairs, at least the first two of which are similar to the posteriormost cephalic pairs. The combined evidence supports a nektobenthic and detritivorous lifestyle for agnostinids. A head with six appendiferous segments contrasts strikingly with the four known in trilobites and five typical of mandibulates. Agnostinids are retrieved as the sister group to polymeroid trilobites in our phylogeny, implying that crustacean-like morphologies evolved homoplastically. This result highlights the variability in segmental composition of the artiopodan head. Finally, our study emphasizes the continued role of Burgess Shale-type fossils in resolving the affinities of problematic biomineralizing taxa.