A new shell-bearing organism from the Cambrian Spence Shale of Utah

A new shell-bearing organism with preserved soft tissue, Armilimax pauljamisoni n. gen. n. sp., is reported from the middle Cambrian (Miaolingian: Wuliuan) Miners Hollow locality of the Spence Shale of northern Utah. The described organism is known from a single articulated specimen and preserves a prominent shell, a slug-like body, as well as a U-shaped digestive tract. Its overall appearance is similar to halkieriids, but it does not preserve sclerites. The possible affinities of the new taxon and potential reasons for the presence of a U-shaped gut are discussed. Armilimax pauljamisoni is the first shell-bearing animal of its kind from the Great Basin and extends the diversity of body plans in the Spence Shale Fossil-Lagerstätte.     

An exceptional record of Cambrian trilobite moulting behaviour preserved in the Emu Bay Shale,South Australia

Trilobites dominate the Emu Bay Shale (EBS) assemblage (Cambrian Series 2, Stage 4, South Australia) in terms of numbers, with Estaingia bilobata Pocock 1964 being extremely abundant, and the larger Redlichia takooensis Lu 1950 , being common. Many specimens within the EBS represent complete moulted exoskeletons, which is unusual for Cambrian fossil deposits. The abundance of complete moults provides an excellent record that has allowed the recognition of various recurrent moult configurations for both species, enabling the inference of movement sequences required to produce such arrangements. Moult configurations of E. bilobata are characterized by slight displacement of the joined rostral plate and librigenae, often accompanied by detachment of the cranidium, suggesting ecdysis was achieved by anterior withdrawal via opening of the cephalic sutures. Moulting in R. takooensis often followed the same method, but configurations show greater displacement of cephalic sclerites, suggesting more vigorous movement by the animal during moulting. Both species also show rare examples of Salter's configuration, with the entire cephalon anteriorly inverted, and several other unusual configurations. These results indicate that moulting in trilobites was a more variable process than originally thought. In contrast, other Cambrian Konservat‐Lagerstätten with an abundance of trilobites (e.g. Wheeler Shale, USA, and Mount Stephen Trilobite Beds, Canada) show larger numbers of ‘axial shields’ and isolated sclerites, often interpreted as disarticulated exuviae. This points to a higher level of disturbance from factors, such as animal activity, depositional processes or water movement, compared to that of the EBS, where quiescent conditions and intermittent seafloor anoxia contributed to an unparalleled trilobite moulting record.     

Sediments of the Middle Cambrian Burgess Shale, Canada

The Phyllopod Bed of the Burgess Shale, in which Walcott found the famous soft bodied fossils, consists of thin graded beds of calcareous siltstone and mud-stone, which are probably turbidites. The Burgess Shale was deposited on a reef front submarine fan, and the preservation of the fossils is probably due to rapid burial.     

The role of experiments in investigating the taphonomy of exceptional preservation

In the last 20 years, much taphonomic experimentation has focused on the interpretation of exceptionally preserved fossils. Decay experiments have been used to interpret the features preserved in soft‐bodied fossils and to determine the sequence of character loss and its impact on phylogenetic position. Experiments on the impact of microbial communities on decay and mineralization have started to illuminate the processes involved in the fossilization of soft tissues, including embryos. The role of decay in promoting authigenic mineralization has been used to investigate the formation of Ediacaran macrofossils and concretions. Maturation experiments have shown how the constituents of animals and plants are transformed over time to a macromolecular material that converges on a similar stable composition. Other maturation experiments have explained how structural colours in fossils are altered from the original. A major area requiring investigation is the role of specific types of microbes in decay and their impact on sediment and pore water chemistry, as well as the environmental controls that determine their presence and level of activity. Microbial activity has received less attention than other factors in attempts to explain why the occurrence and nature of exceptional preservation varies in time and space through the fossil record.     

The ‘Orsten’—More than a Cambrian Konservat-Lagerstätte yielding exceptional preservation

In several areas of southern Sweden, limestone nodules, locally called Orsten occur within bituminous alum shales. These shales and nodules were deposited under dysoxic conditions at the bottom of what was most likely a shallow sea during the late Middle to Upper Cambrian (ca. 500 million years ago). Subsequently, the name ‘Orsten’ has been referred to particular, mainly arthropod, fossils from such nodules, and, in a wider sense, to the specific type of preservation of minute fossil through secondarily phosphatization. This preservation is exceptional in yielding uncompacted and diagenetically undeformed three-dimensional fossils. ‘Orsten’-type preservation resulted from incrustation of a thin external layer and also by impregnation by calcium phosphate and, therefore, mineralization of the surface of the former animals during early diagenesis. Primarily, this type of preservation seems to have affected only cuticle-bearing metazoans such as cycloneuralian nemathelminths and arthropods. ‘Orsten’ preservation in this sense seems to be limited by size, in having yielded no partial or complete animals larger than 2 mm. On the other end of the scale, even larvae 100 μm long are preserved, often more complete than larger specimens, and details such as setules and pores smaller than 1 μm can be observed. Fossils preserved in such a manner are almost exclusively hollow carcasses, but can be filled secondarily; less common are completely phosphatized compact specimens. The high quality of preservation makes the Swedish ‘Orsten’ a typical Konservat-Lagerstätte. Yet, its special type of preservation is more widespread in time and geographical distribution than assumed initially, and the origin of the phosphate is not necessarily restricted just to one source. Subsequent to the first discoveries of limb fragments of Cambrian arthropods in 1975, animals in this special preservational type have been discovered in several continents and across a broad stratigraphic range including even Proterozoic strata. The latter have yielded early cleavage and metazoan embryonic stages, expanding knowledge on the preservational capacities of the ‘Orsten’. Here, we report the recent status of our research on the ‘Orsten’ and give perspectives for future exploration on a worldwide scale, particularly in light of a recently formed international research group named Center of Orsten Research and Exploration (C.O.R.E.).     

Carbonaceous preservation of Cambrian hexactinellid sponge spicules

Early fossil sponges offer a direct window onto the evolutionary emergence of animals, but insights are limited by the paucity of characters preserved in the conventional fossil record. Here, a new preservational mode for sponge spicules is reported from the lower Cambrian Forteau Formation (Newfoundland, Canada), prompting a re-examination of proposed homologies and sponge inter-relationships. The spicules occur as wholly carbonaceous films, and are interpreted as the remains of robust organic spicule sheaths. Comparable sheaths are restricted among living taxa to calcarean sponges, although the symmetries of the fossil spicules are characteristic of hexactinellid sponges. A similar extinct character combination has been documented in the Burgess Shale fossil Eiffelia. Interpreting the shared characters as homologous implies complex patterns of spicule evolution, but an alternative interpretation as convergent autapomorphies is more parsimonious. In light of the mutually exclusive distributions of these same characters among the crown groups, this result suggests that sponges exhibited an early episode of disparity expansion followed by comparatively constrained evolution, a pattern shared with many other metazoans but obscured by the conventional fossil record of sponges.     

Colour in Burgess Shale animals and the effect of light on evolution in the Cambrian

Diffraction gratings are reported from external surfaces of the hard, protective parts of Wiwaxia corrugata, Canadia spinosa and Marrella splendens from the Burgess Shale (Middle Cambrian (515 million years), British Columbia). As a consequence, these animals would have displayed iridescence in their natural environment: Cambrian animals have previously been accurately reconstructed in black and white only. A diversity of extant marine animals inhabiting a similar depth to the Burgess Shale fauna possess functional diffraction gratings. The Cambrian is a unique period in the history of animal life where predatory lifestyles and eyes capable of producing visual images were evolving rapidly. The discovery of colour in Cambrian animals prompts a new hypothesis on the initiation of the ''Big Bang'' in animal evolution which occurred during the Cambrian: light was introduced into the behavioural systems of metazoan animals for the first time. This introduction, of what was to become generally the most powerful stimulus in metazoan behavioural systems, would have triggered turbulence in metazoan evolution.     

Revision of Echmatocrinus from the Middle Cambrian Burgess Shale of British Columbia

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.     

Three-dimensional preservation of arthropod integument from the Middle Cambrian of Australia

Three-dimensional preservation of arthropod soft integument occurs in Middle Cambrian sediments of the Georgina Basin, western Queensland, Australia. The beds are referred to the Monastery Creek Phosphorite Formation, Gowers Formation, Inca Shale Formation and Devon-court Limestone Formation. The finds include arthropod type-A larvae previously described by Müller & Walossek and several complete appendages, possibly of early Palaeozoic 'ostracodes', as well as indeterminable cuticular remains. The Australian sediments were in part deposited under high water-energy conditions, whereas previously known occurrences of three-dimensional soft-integument preservation have been from environments of lower water energy. Such preservation may thus be more widespread in the Early Paleozoic than hitherto known. □ Phosphatization, arthropods, type-A larvae, appendages, Middle Cambrian, Geogina Basin, Australia, three-dimensional preservation.     

A death assemblage of priapulid worms from the Middle Cambrian Burgess Shale

A slab of Burgess Shale (Middle Cambrian), displaying an incomplete exoskeleton of the large arthropod Sidneyia inexpectans and encompassed by nine specimens of the priapulid worm Ottoia prolifica, is interpreted as a death assemblage, with the worms once living off or feeding around a carcass or freshly moulted instar of Sidneyia. Death is thought to have been caused by an obrution event that preserved the organisms in situ.     

The nature and significance of the appendages of Opabinia from the Middle Cambrian Burgess Shale

Opabinia regalis has long been regarded as a curious animal, with its five eyes, its long flexible anterior process, and gill lamellae carried on the outside of overlapping lateral lobes. More recently, Opabinia has been reconstructed with lobopod limbs lying adaxial but separate from the lateral lobes. This version of Opabinia represented a lobopod–arthropod transition and prompted a hypothesis for the origin of the biramous limb that involved uniting the lobopod limb with a lateral lobe. New evidence of elemental maps is consistent with previous interpretations of the triangular structures in Opabinia as lateral extensions of the gut; there is no convincing evidence for the presence of lobopod limbs. Re-examination of critical specimens reveals that the gill lamellae are not on the outside of the lateral lobes. The limbs of Opabinia resemble the phyllopodous exopod of arthropods; the posterior margin is fringed with blades. Opabinia remains on the stem of euarthropods but not as a part of a paraphyletic Lobopodia. The Lobopodia is a clade of Cambrian armoured lobopods and onychophorans. A new hypothesis for the origin of the arthropod biramous limb from an exopod like that in Opabinia is presented, which involves an endite-bearing phyllopodous limb as an intermediate stage.     

Beyond the Burgess Shale: Cambrian microfossils track the rise and fall of hallucigeniid lobopodians

Burgess Shale-type deposits are renowned for their exquisite preservation of soft-bodied organisms, representing a range of animal body plans that evolved during the Cambrian ‘explosion’. However, the rarity of these fossil deposits makes it difficult to reconstruct the broader-scale distributions of their constituent organisms. By contrast, microscopic skeletal elements represent an extensive chronicle of early animal evolution—but are difficult to interpret in the absence of corresponding whole-body fossils. Here, we provide new observations on the dorsal spines of the Cambrian lobopodian (panarthropod) worm Hallucigenia sparsa from the Burgess Shale (Cambrian Series 3, Stage 5). These exhibit a distinctive scaly microstructure and layered (cone-in-cone) construction that together identify a hitherto enigmatic suite of carbonaceous and phosphatic Cambrian microfossils—including material attributed to Mongolitubulus, Rushtonites and Rhombocorniculum—as spines of Hallucigenia-type lobopodians. Hallucigeniids are thus revealed as an important and widespread component of disparate Cambrian communities from late in the Terreneuvian (Cambrian Stage 2) through the ‘middle’ Cambrian (Series 3); their apparent decline in the latest Cambrian may be partly taphonomic. The cone-in-cone construction of hallucigeniid sclerites is shared with the sclerotized cuticular structures (jaws and claws) in modern onychophorans. More generally, our results emphasize the reciprocal importance and complementary roles of Burgess Shale-type fossils and isolated microfossils in documenting early animal evolution.     

Graptolite (Hemichordata,Pterobranchia) preservation and identification in the Cambrian Series 3

Due to inadequate preservation, pterobranchs are often difficult to identify in the fossil record, and a better understanding of preservational modes and diagenetic and metamorphic effects is needed for their recognition. Pterobranch hemichordates are common in Cambrian Stage 5 and younger sedimentary rocks, but are frequently overlooked. Often, pterobranch hemichordate colonies have been considered to be algal remains or hydroids. Re‐examination of Cambrian Burgess Shale algae reveals that the genera Yuknessia and Dalyia can be recognized as putative early representatives of pterobranch hemichordates. Distinct fusellar construction of the individual zooidal tubes and branching of the creeping proximal part of the colonies are found in the morphologically similar rhabdopleurid pterobranch genus Sphenoecium. The erect tubes of Sphenoecium do not branch and can reach a length of several centimetres. The development of the fusellar construction in this taxon shows a highly irregular development of the suture patterns, but a fairly consistent height of the individual fuselli. The taxon is widely distributed in the Cambrian Series 3, but has regularly been identified as a hydroid or an alga. Sphenoecium wheelerensis from the Cambrian Wheeler Shale of Utah is described as new.     

An early Cambrian chelicerate from the Emu Bay Shale,South Australia

The Emu Bay Shale Lagerstätte (Cambrian Series 2, Stage 4) occurs on the north coast of Kangaroo Island, South Australia. Over 50 species are known from here, including trilobites and non‐biomineralized arthropods, palaeoscolecids, a lobopodian, a polychaete, vetulicolians, nectocaridids, hyoliths, brachiopods, sponges and chancelloriids. A new chelicerate, Wisangocaris barbarahardyae gen. et sp. nov., is described herein, based on a collection of some 270 specimens. It is up to 60 mm long, with the length of the cephalic shield comprising about 30% that of the exoskeleton. The cephalic margin has three pairs of bilaterally‐symmetrical small triangular spines. A pair of small eyes is placed well forwards on the ventral margin of the cephalic shield. The trunk comprises 11 segments that increase in length while narrowing posteriorly, each possibly bearing a pair of biramous appendages; the most posterior segment is almost square whereas the others are transversely elongated. The spatulate telson is proportionately longer than in taxa such as Sanctacaris, Utahcaris and Leanchoilia. Up to eight (?four pairs) of 3 mm‐long elements bearing alternating inward‐curving short and long spines beneath the cephalic shield are interpreted as basipodal gnathobases, part of a complex feeding apparatus. A well‐developed gut includes a stomach within the cephalic shield; it extends to the base of the telson. In a few specimens there are shell fragments within the gut, including those of the trilobite Estaingia bilobata (the most common species in the biota); these fragments have sharp margins and extend across the gut lumen. The species may have been a predator or a scavenger, ingesting material already broken up by a larger predator/scavenger. The morphology of this taxon shares many overall body features with Sanctacaris, and some with Sidneyia, particularly its gnathobasic complex. These chelicerate affinities are corroborated by phylogenetic analyses.     

A spinose stem group brachiopod with pedicle from the Middle Cambrian Burgess Shale

A new genus and species of a Middle Cambrian stem group brachiopod, Acanthotretella spinosa n. gen. and n. sp., is described from the Burgess Shale Formation. Most of the 42 specimens studied came from the Greater Phyllopod bed (Walcott Quarry) and were collected from five bed assemblages, each representing a single obrution event. Specimens are probably preserved within their original habitat. In contrast to all brachiopods known from the Burgess Shale, the shells of the new stem group brachiopod are often deformed and do not show signs of brittle breakage, which suggests that the valves were originally either entirely organic in composition or, more likely, had just a minor mineral component. Acanthotretella spinosa differs from all the other described Cambrian brachiopods in that it is covered by long, slender and possibly partly mineralized spines that are posteriorly inclined at an oblique angle away from the anterior margin. The spines penetrate the shell and are mainly comparable with the thorn‐like organic objects that have been inferred from early siphonotretoid brachiopods. The pedicle was slender and was composed of a central coelomic region and emerged from an apical foramen at the end of an internal pedicle tube. The finding of a pedicle attached to the macrobenthic algae Dictyophycus and other epibenthos implies that A. spinosa did not have an infaunal mode of life. The visceral region and interior characters are poorly preserved.     

Composition and preservation of the Chengjiang fauna –a Lower Cambrian soft-bodied biota

The Lower Cambrian Chengjiang fauna is reviewed and shown to be closely comparable with the younger Burgess Shale fauna. but with various differences in detail. A diverse group of more or less annulated lobopod animals including 'armoured lobopods' are regarded as representatives of the phylum Onychophora. 'Trilobitomorphs' include several new types. Probable protaspides of the trilobitomorph Naraoia are described. No molluses or deuterostomes have been identified. The preservational orientations of the various taxa are reviewed and compared with orientations of the Burgess Shale taxa. Orientation in the sediment is found to be closely correlated to the original shape of individuals. Several new genera and species are described: the segmented. worm-shaped Yunnanozoon lividum gen. et sp.n., the 'armoured lobopods' Onychodictyon ferox gen. et sp.n. and Cardiodictyon catenulum gen. et sp.n. and the arthropods Saperion glumaceum gen. et sp.n., Sinoburius Iunaris gen. et sp.n., and Xandarella spectaculum gen. et sp.n.