A redescription of the Middle Cambrian wormAmiskwia sagittiformis Walcott from the Burgess Shale of British Columbia

Amiskwia sagittiformis Walcott is redescribed on the basis of the five available specimens. The dorsoventrally compressed body consists of a head bearing a pair of prominent tentacles and a trunk with lateral and caudal fins. A gut with subterminal openings, cerebral ganglia, nerve cords, blood vessels and muscles have been identified with varying degrees of confidence. An active pelagic mode of life is considered probable. The rejection by earlier workers of this worm from the arrow-worms (Chaetognatha) is confirmed, but their identification of it as a nemertean (Nemertea) cannot be supported owing to the absence of critical data. The systematic position ofA. sagittiformis remains unresolved.     

A new stalked filter-feeder from the middle Cambrian Burgess Shale, British Columbia, Canada

Burgess Shale-type deposits provide invaluable insights into the early evolution of body plans and the ecological structure of Cambrian communities, but a number of species, continue to defy phylogenetic interpretations. Here we extend this list to include a new soft-bodied animal, Siphusauctum gregarium n. gen. and n. sp., from the Tulip Beds (Campsite Cliff Shale Member, Burgess Shale Formation) of Mount Stephen (Yoho National Park, British Columbia). With 1,133 specimens collected, S. gregarium is clearly the most abundant animal from this locality.This stalked animal (reaching at least 20 cm in length), has a large ovoid calyx connected to a narrow bilayered stem and a small flattened or bulb-like holdfast. The calyx is enclosed by a flexible sheath with six small openings at the base, and a central terminal anus near the top encircled by indistinct openings. A prominent organ, represented by six radially symmetrical segments with comb-like elements, surrounds an internal body cavity with a large stomach, conical median gut and straight intestine. Siphusauctum gregarium was probably an active filter-feeder, with water passing through the calyx openings, capturing food particles with its comb-like elements. It often occurs in large assemblages on single bedding planes suggesting a gregarious lifestyle, with the animal living in high tier clusters. These were probably buried en masse more or less in-situ by rapid mud flow events.Siphusauctum gregarium resembles Dinomischus, another Cambrian enigmatic stalked animal. Principal points of comparison include a long stem with a calyx containing a visceral mass and bract-like elements, and a similar lifestyle albeit occupying different tiering levels. The presence in both animals of a digestive tract with a potential stomach and anus suggest a grade of organization within bilaterians, but relationships with extant phyla are not straightforward. Thus, the broader affinities of S. gregarium remain largely unconstrained.     

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.     

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.     

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.     

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.     

Paleoecology of the Greater Phyllopod Bed community,Burgess Shale

To better understand temporal variations in species diversity and composition, ecological attributes, and environmental influences for the Middle Cambrian Burgess Shale community, we studied 50,900 fossil specimens belonging to 158 genera (mostly monospecific and non-biomineralized) representing 17 major taxonomic groups and 17 ecological categories. Fossils were collected in situ from within 26 massive siliciclastic mudstone beds of the Greater Phyllopod Bed (Walcott Quarry — Fossil Ridge). Previous taphonomic studies have demonstrated that each bed represents a single obrution event capturing a predominantly benthic community represented by census- and time-averaged assemblages, preserved within habitat. The Greater Phyllopod Bed (GPB) corresponds to an estimated depositional interval of 10 to 100 KA and thus potentially preserves community patterns in ecological and short-term evolutionary time.The community is dominated by epibenthic vagile deposit feeders and sessile suspension feeders, represented primarily by arthropods and sponges. Most species are characterized by low abundance and short stratigraphic range and usually do not recur through the section. It is likely that these are stenotopic forms (i.e., tolerant of a narrow range of habitats, or having a narrow geographical distribution). The few recurrent species tend to be numerically abundant and may represent eurytopic organisms (i.e., tolerant of a wide range of habitats, or having a wide geographical distribution). Rarefaction curves demonstrate variation in species richness through the GPB and suggest that more stenotopic species could still be discovered with additional sampling. Comparisons between richness and evenness trends suggest that the community is relatively stable overall, despite gradual species turnover through time, especially in assemblages from younger beds. Less diverse assemblages with low species evenness possibly represent the onset of less favourable environmental conditions.Fossil occurrences in individual beds were analysed using a range of statistical techniques (Correspondence Analysis, Canonical Correspondence Analysis, Minimum Spanning Tree, Indicator Species Analysis, Mantel Test) to extract community patterns. Results suggest the presence of four fossil assemblages based on distinct species associations. The different assemblages presumably reflect variations in environmental and ecological conditions, some acting through time, leading to species turnover. “Disturbances” (e.g., changes in paleo-redox conditions), differences in substrate firmness, and limited taphonomic biases are probably the main factors contributing to community structure. The influence of ecological factors, however, is also predicted from non-random patterns of species recurrences in successive events. Preliminary comparisons with Lower Cambrian Chengjiang-type assemblages of southern China suggest that the overall structure and ecology of Cambrian communities remained relatively stable until at least the Middle Cambrian in subtidal siliciclastic soft substrate environments.Comparisons with modern marine benthic ecosystems further suggest the Burgess Shale community was probably highly dependent on immigration from a regional pool of species after each burial event. This could support the view that species availability, habitat characteristics, and recolonisation processes were more important in structuring the community in the long-term than species interactions or environmental variations at a local scale.     

Epistemic values in the Burgess Shale debate

Focusing primarily on papers and books discussing the evolutionary and systematic interpretation of the Cambrian animal fossils from the Burgess Shale fauna, this paper explores the role of epistemic values in the context of a discipline (paleontology) striving to establish scientific authority within a larger domain of epistemic problems and issues (evolutionary biology). The focal point of this analysis is the repeated claims by paleontologists that the study of fossils gives their discipline a unique ‘historical dimension’ that makes it possible for them to unravel important aspects of evolution invisible to scientists who study the extant biosphere. The first part of the paper explores the shifting of emphasis in the writings of paleontologists between two strategies that employ opposing views on the classical positivist and physicalist ideal of science. The second part analyzes paleontologists’ claims of privileged access to life’s historical dimension in a situation where a theoretical upheaval occurring independent of the epistemical problem at hand completely shifts the standards for evaluating the legitimacy of various knowledge claims. Though the various strategies employed in defending the privileged historical perspective of paleontology have been disparate and, to an extent contradictory, each impinges on the acceptance of a specific epistemic ideal or set of values and success or failure of each depends on the compatibility of this ideal with the surrounding community of scientists.     

Skimming the surface with Burgess Shale arthropod locomotion

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.     

Ultrastructure of some Cambrian palynomorphs from the Bright Angel Shale, Arizona, USA

Palynomorphs with complex resistant walls have been recovered from several Cambrian deposits in the continental United States. Those recovered from the Bright Angel Shale of Arizona typically preserve both a primary wall, and an outer envelope (synoecosporal wall) that encloses multiple spore-like bodies within. At least three distinct types of primary walls, are recognized with the TEM: 1) a unilaminate wall with a smooth inner surface and a sculptured outer surface, 2) a wall of three unornamented laminae of very uniform thickness, and, 3) a thicker wall with multiple thin, lightly-staining layers embedded in a darker matrix. This third type of primary wall bears a strong resemblance to those of certain Lower Devonian hilate cryptospore monads from the Welsh Borderlands. No extant algae produce spores with walls as thick or as complex, suggesting that these Cambrian palynomorphs were the desiccation-resistant spores of cryptogams belonging to the charophyte–embryophyte lineage. Multilaminate spore walls, which are characteristic of some extant liverworts and Paleozoic cryptospores, may have evolved via the fusion of separate, multiple laminae. This appears to be the primitive plant sporoderm type, but it may have evolved asynchronously with respect to the evolution of the embryophytic development of the sporophyte in land plants.     

Exceptionally preserved jellyfishes from the Middle Cambrian

Cnidarians represent an early diverging animal group and thus insight into their origin and diversification is key to understanding metazoan evolution. Further, cnidarian jellyfish comprise an important component of modern marine planktonic ecosystems. Here we report on exceptionally preserved cnidarian jellyfish fossils from the Middle Cambrian (approximately 505 million years old) Marjum Formation of Utah. These are the first described Cambrian jellyfish fossils to display exquisite preservation of soft part anatomy including detailed features of structures interpreted as trailing tentacles and subumbrellar and exumbrellar surfaces. If the interpretation of these preserved characters is correct, their presence is diagnostic of modern jellyfish taxa. These new discoveries may provide insight into the scope of cnidarian diversity shortly after the Cambrian radiation, and would reinforce the notion that important taxonomic components of the modern planktonic realm were in place by the Cambrian period.     

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.     

Tentaculate Fossils from the Cambrian of Canada (British Columbia) and China (Yunnan) Interpreted as Primitive Deuterostomes

Molecular and morphological evidence unite the hemichordates and echinoderms as the Ambulacraria, but their earliest history remains almost entirely conjectural. This is on account of the morphological disparity of the ambulacrarians and a paucity of obvious stem-groups. We describe here a new taxon Herpetogaster collinsi gen. et sp. nov. from the Burgess Shale (Middle Cambrian) Lagerstätte. This soft-bodied vermiform animal has a pair of elongate dendritic oral tentacles, a flexible stolon with an attachment disc, and a re-curved trunk with at least 13 segments that is directed dextrally. A differentiated but un-looped gut is enclosed in a sac suspended by mesenteries. It consists of a short pharynx, a conspicuous lenticular stomach, followed by a narrow intestine sub-equal in length. This new taxon, together with the Lower Cambrian Phlogites and more intriguingly the hitherto enigmatic discoidal eldoniids (Cambrian-Devonian), form a distinctive clade (herein the cambroernids). Although one hypothesis of their relationships would look to the lophotrochozoans (specifically the entoprocts), we suggest that the evidence is more consistent with their being primitive deuterostomes, with specific comparisons being made to the pterobranch hemichordates and pre-radial echinoderms. On this basis some of the earliest ambulacrarians are interpreted as soft-bodied animals with a muscular stalk, and possessing prominent tentacles.     

Lapworthella filigrana n.sp. (incertae sedis) from the Lower Cambrian of the Cassiar Mountains,northern British Columbia,Canada, with comments on possible levels of competition in the early Cambrian

Two samples from the base of the Rosella Formation(Nevadella Zone) in the Cassiar Mountains, British Columbia, yielded a new species of the enigmatic genusLapworthella, L. filigrana n. sp., that is characterized by denticulate growth ridges and ornamented inter-ridge areas. As with other species ofLapworthella, intra-specific variation is considerable. One variant is comparable toL. lucida Meshkova 1969, from northern Siberia, but shows differences in the ornamentation of the more apical inter-ridge areas. The high degree of morphological variation inLapworthella finds parallels in certain other Lower Cambrian organisms. This variability may be the result of a low level of competition rather than a previous proposal of a primitive and inefficient genetic mechanism.     

A Web of Controversies: Complexity in the Burgess Shale Debate

Using the Burgess Shale controversies as a case-study, this paper argues that controversies within different domains may interact as to create a situation of “com- plicated intricacies,” where the practicing scientist has to navigate through a context of multiple thought collectives. To some extent each of these collectives has its own dynamic complete with fairly negotiated standards for investigation and explanation, theoretical background assumptions and certain peculiarities of practice. But the intellectual development in one of these collectives may “spill over” having far reaching consequences for the treatment of apparently independent epistemic problems that are subject of investigation in other thought collectives. For the practicing scientist it is necessary to take this complex web of interactions into account in order to be able to navigate in such a situation. So far most studies of academic science have had a tendency to treat the practicing scientist as members of a single (enclosed) thought collective that stands intellectually isolated from other similar entities unless the discipline was in a state of crisis of paradigmatic proportions. The richness and complexity of Burgess Shale debate shows that this encapsulated kind of analysis is not enough.