Cambrian Communities Research Articles

Protomelission is an early dasyclad alga and not a Cambrian bryozoan.

The animal phyla and their associated body plans originate from a singular burst of evolution occurring during the Cambrian period, over 500 million years ago1. The phylum Bryozoa, the colonial 'moss animals', have been the exception: convincing skeletons of this biomineralizing clade have been absent from Cambrian strata, in part because potential bryozoan fossils are difficult to distinguish from the modular skeletons of other animal and algal groups2,3. At present, the strongest candidate4 is the phosphatic microfossil Protomelission5. Here we describe exceptionally preserved non-mineralized anatomy in Protomelission-like macrofossils from the Xiaoshiba Lagerstätte6. Taken alongside the detailed skeletal construction and the potential taphonomic origin of 'zooid apertures', we consider that Protomelission is better interpreted as the earliest dasycladalean green alga-emphasizing the ecological role of benthic photosynthesizers in early Cambrian communities. Under this interpretation, Protomelission cannot inform the origins of the bryozoan body plan; despite a growing number of promising candidates7-9, there remain no unequivocal bryozoans of Cambrian age.

Quantitative paleoecology of Cambrian (Series 2–Miaolingian) communities from central Sonora, Mexico

This work presents a paleoecological approach to study benthic communities preserved in Cambrian strata exposed at the Sahuaral locality in central Sonora, northwestern Mexico. Based on species richness and abundance, this work analyzed 627 individuals corresponding to 33 species from five distinctive Cambrian formations: the lowermost Proveedora Formation characterized by bioturbation horizons that record the occurrence of a benthic-infaunal biota from coastal shallow seas, followed by the Buelna Formation which contains reef deposits dominated by autotrophic epibenthic communities. The Cerro Prieto Formation was deposited in a high-energy shallow marine environment, dominated by primary consumers-epifaunal and suspensions feeders. The uppermost unit, the El Gavilán Formation, is an intercalation of shale and fossiliferous limestone characterized by the fossil remains of suspension feeders, detritus feeders and nektobenthic organisms of low energy deep water environments. The paleobiota consist of ichnofossils, microbial oncolites, chancellorids, brachiopods, hyolithids, and trilobites in a sequence of increasing diversity and complexity. Paleoecological indices (Simpson Dominance, Shannon-Weaver Index, Evenness Index, and Total Richness) were determined and compared in the study area. In addition, the effective number of species and the true diversities were obtained, which are similar between each station distributed in each formation. However, it is possible that the high degree of dominance is a factor to indicate an inequality between the true diversity and the species richness identified. These results provide new analytical and paleoecological criteria for better understanding depositional environments and fossil associations where the spatial distribution of benthic organisms, richness and abundance provide a valued window to Cambrian biotas. The structure of the marine community of the El Sahuaral area has a great affinity with other Cambrian communities, such as those from the Stephen Formation (Burgess Shale, British Columbia) and the Chengjiang biota of the Yangtze Shelf deposits in South China.

A giant nektobenthic radiodont from the Burgess Shale and the significance of hurdiid carapace diversity.

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 that Titanokorys bridges 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 known Pahvantia. Despite distinct carapaces, Titanokorys shares similar rake-like appendages for sediment-sifting with Cambroraster, 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.

Paleoecology of cambrian communities of central Sonora, Mexico: Paleoenvironmental and biostratigraphic considerations

Cambrian marine ecosystems in Mexico are constituted by diverse taxonomic groups, including trace fossils, poriferans, brachiopods, hyolithids, trilobites, echinoderms and other problematic fauna. The present research aims to identify and analyze the paleoecological characteristics of the marine ecosystems of central Sonora, Mexico; specifically, in the San José de Gracia area where the Proveedora, Buelna, Cerro Prieto and El Gavilán formations are exposed. In this study some ecological indices have been applied, such as Simpson's dominance and evenness, Shannon H′ diversity, Menhinick species richness and Chao total species richness. Guilds and adaptive strategies show how different environments were occupied through time. The present work constitutes the first paleoecological work for the marine ecosystems of Cambrian strata in Mexico.The Proveedora Formation consists of fine-grained quartzarenite with high bioturbation and trace fossils, it shows a low species richness (4) and evenness, where suspension feeders are dominating. The paleoenvironmental analysis of this unit indicates a sandy environment, such as a coastal and sublittoral areas, sandbars or delta front. The Buelna Formation is composed of limestone interbedded with shale, containing algae, brachiopods, salterellids, hyolithids, trilobites and echinoderms. Mostly it possesses a medium species richness (9) and intermediate evenness. Functional groups are dominated by primary producers and suspension feeders. The paleoenvironmental analysis of this unit indicates a shallow marine environment. The Cerro Prieto Formation consists of an oolitic limestone strongly recrystallized with few fossils, mainly brachiopods and trilobites. It has a low species richness (2) distributed in an unevenness way since only one species occupies the whole site. It indicates a shallow marine environment of high energy. The El Gavilán Formation consists of a thick sequence of shale interbedded with limestone, having abundant fossils like poriferans, brachiopods, hyolithids and trilobites, with a species richness (16) and with a high evenness, exhibiting a great variety of functional groups, dominating the suspension feeders and nektobenthic forms. The paleoenvironmental analysis of this unit shows an open shelf environment.

Comments on: Periodicity in the extinction rate and possible astronomical causes – comment on mass extinctions over the last 500 myr: an astronomical cause? (Erlykin et al.)

Comments on: Periodicity in the extinction rate and possible astronomical causes – comment on mass extinctions over the last 500 myr: an astronomical cause? (Erlykin <i>et al</i>.)

Self-regulation of trilobite diversity in Murero (middle Cambrian, Spain) due to compensatory extinction

As species accumulate in a community, competition for available ecospace is expected to prevent the addition of new species and to facilitate species extinction, thus producing a dynamic equilibrium of diversity. This mechanism remains under debate since its empirical support comes mainly from indirect or partial evidence, with very few direct tests at the species level. Here a new method is described to detect the presence of selfregulation feedbacks between species richness and turnover rates. It consists of Monte Carlo simulations which randomize the distribution of species ranges among stratigraphic intervals, providing predictions which allow the detection of genuine self-regulation feedbacks in the real data. Since the simulations include any potential bias due to preservation, sampling, or change in depositional environment, and these biases would also affect the real dataset, they are thus ruled out as explanations for any difference found. This method is applied to one of the best known fossiliferous sequences worldwide, the Rambla de Valdemiedes in Murero (RV1 section, middle Cambrian, Spain), a classic locality that has been studied for more than 150 years and which stands out due to its excellent sampling density, continuous deposition, and homogeneous fossil preservation. The results show that trilobite species richness was self-regulated due to positive feedback with extinction rate, which implies that compensatory extinction regulated this fauna in spite of the on-going Cambrian radiation. The lack of evidence of any origination feedback suggests ecological opportunities were not limiting for new species to colonize this Cambrian community

Complexity and diversity of eyes in Early Cambrian ecosystems

Here we report exceptionally preserved non-biomineralized compound eyes of a non-trilobite arthropod Cindarella eucalla from the lower Cambrian Chengjiang Lagerstätte, China. The specimen represents the oldest microanatomical evidence confirming the occurrence of highly developed vision in the early Cambrian, over 2,000 ommatidia in each eye. Moreover, a quantitative analysis of the distribution of eyes related to life habit, feeding types, and phyla respectively, from the Chengjiang biota indicates that specimens with eyes mostly belong to the arthropods, and they usually were actively mobile epifaunal and nektonic forms as hunters or scavengers. Arthropods took the lead in evolution of ‘good vision' and domination in Cambrian communities, which supports the hypothesis that the origin and evolution of ‘good vision' was a key trait that promoted preferential diversification and formed the foundation of modern benthic ecosystems in the early Cambrian ocean.

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.

Composition and tiering of the Cambrian sponge communities

Sponges are dominant sessile suspension feeders in the Cambrian metazoan community; thus, sponge tiering and composition in the Cambrian communities will provide important information for elucidating the evolutionary history of the epifaunal community. Based on more than two thousand articulated sponge specimens from the early Cambrian Chengjiang fauna, the present study demonstrates that the Chengjiang sponge community is dominated by demosponges with a well-developed three-level tiering, consisting of 0–5cm, 5–15cm and 15–30cm tiers. The majority of the sponges are restricted to the 0–15cm tier, suggesting rich nutrients in the bottom seawater of the Cambrian ocean. Tiering in the Cambrian sponge communities is consistent and similar to that of the Ediacaran and Phanerozoic epifaunal communities, but the fourth tier (50–100cm) in these latter communities is absent in the Cambrian communities. Demosponges dominate the shallow-water sponge communities and occur in all tiers. Hexactinellids are relatively rare and limited in the lower tiers in the shallow-water community, but dominate in the deep-water communities. The composition variation in the Cambrian sponge communities may be controlled by the differences in skeleton architecture between the Demospongiae and Hexactinellida.

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.

A giant Ordovician anomalocaridid

Anomalocaridids, giant lightly sclerotized invertebrate predators, occur in a number of exceptionally preserved early and middle Cambrian (542-501 million years ago) biotas and have come to symbolize the unfamiliar morphologies displayed by stem organisms in faunas of the Burgess Shale type. They are characterized by a pair of anterior, segmented appendages, a circlet of plates around the mouth, and an elongate segmented trunk lacking true tergites with a pair of flexible lateral lobes per segment. Disarticulated body parts, such as the anterior appendages and oral circlet, had been assigned to a range of taxonomic groups--but the discovery of complete specimens from the middle Cambrian Burgess Shale showed that these disparate elements all belong to a single kind of animal. Phylogenetic analyses support a position of anomalocaridids in the arthropod stem, as a sister group to the euarthropods. The anomalocaridids were the largest animals in Cambrian communities. The youngest unequivocal examples occur in the middle Cambrian Marjum Formation of Utah but an arthropod retaining some anomalocaridid characteristics is present in the Devonian of Germany. Here we report the post-Cambrian occurrence of anomalocaridids, from the Early Ordovician (488-472 million years ago) Fezouata Biota in southeastern Morocco, including specimens larger than any in Cambrian biotas. These giant animals were an important element of some marine communities for about 30 million years longer than previously realized. The Moroccan specimens confirm the presence of a dorsal array of flexible blades attached to a transverse rachis on the trunk segments; these blades probably functioned as gills.

A new turban-shaped gogiid eocrinoid from the Kaili Formation (Kaili Biota), Balang, Jianhe County, Guizhou Province, China

A rare new turban-shaped basal Middle Cambrian gogiid eocrinoid, Turbanicystis inflatus new genus and species is described from the Kaili Formation (Kaili Biota), Guizhou Province, China. It co-occurs with three other markedly different gogiids, Sinoeocrinus, Globoeocrinus, and Balangicystis, and is part of a unique Middle Cambrian community that is dominated by echinoderms.

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.

Cambrian food web: A brief review

Abstract Cambrian trophic web guilds consist of 1/Primary producers: calcified and non calcified bacteria, phytoplankton (acritarchs) and dissolved organic matter and 2/Primary consumers: mainly filter/suspension-feeders, some carnivorous, probable deposit feeders and rare grazers; parasites can only be surmised. Trophic modes are illustrated by examples taken in the different Cambrian communities.

The Middle Cambrian metazoanWiwaxia corrugata(Matthew) from the Burgess Shale andOgygopsisShale, British Columbia, Canada

A detailed redescription of the Middle Cambrian metazoanWiwaxia corrugata(Matthew) is given with the aid of photographs, mostly taken under ultraviolet radiation, and explanatory camera lucida drawings.Wiwaxiais known only from the Stephen Formation with four localities within theBathyuriscus-ElrathinaZone, the celebrated Burgess Shale (Phyllopod bed) andOgygopsisShale and two localities that appear to expose strata relatively high in the Formation, and a new locality in the underlyingGlossopleuraZone. Specimens from theOgygopsisShale consist almost entirely of isolated sclerites, whereas in the Phyllopod bed the species is also known from entire specimens, semi-isolated assemblages and isolated soft-parts. T he description here is based almost entirely on Phyllopod bed m aterial.Wiwaxiawas originally largely covered by sclerites. On the basis of shape and arrangement five distinct zones are recognizable: dorsal, upper lateral, lower lateral, anterior and ventro-lateral. In addition, there was a row of elongate dorso-lateral spinose sclerites running along either side of the body. Each sclerite was inserted separately via a root-like structure. Growth of the animal appears to have occurred by moulting of the sclerites; one unique juvenile specimen appears to be preserved in the act of moulting. The ventral surface was apparently an area of soft tissue devoid of sclerites. Little is known of the internal anatomy, although anteriorly there was a feeding apparatus consisting of two rows of posteriorly directed teeth. Most stages of growth are known varying from presumed juveniles, which at their smallest may have lacked spines, to adults over 50 mm long.Wiwaxiaappears to have been an epifaunal deposit feeder that crawled across the sea-bed, although smaller juveniles may have been infaunal. The dorso-lateral spines may have provided protection against predators and the existence of broken spines suggests the successful deterrence of attack. The affinities ofWiwaxiado not appear to lie with the polychaetous annelids. While the possibilities of convergence cannot be eliminated, there appears to be a significant similarity betweenWiwaxiaand molluscs that suggests a related derivation from a turbellarian ancestor. Nevertheless, Wiwaxia has a distinctive bodyplan and as such cannot be accommodated in any known phylum. WhileWiwaxiais unique to the Stephen Formation isolated sclerites from Lower Cambrian rocks appear to represent earlier wiwaxiids, although these sclerites show differences in their size and composition as com pared withWiwaxia.Wiwaxiaenhances knowledge of early metazoan evolution and ecology in the Cambrian. In particular, it gives fresh insights into the origin and relative success of certain metazoan bodyplans, the origin of trace fossils, and the importance of predation in Cambrian communities.

Earliest cavity-dwelling organisms (coelobionts), Lower Cambrian Poleta Formation, Nevada

The record of cavity-dwelling organisms (coelobionts) is long ranging, and is known to extend as far back as the upper Lower Cambrian. Older coelobionts, from the middle Lower Cambrian Poleta Formation of western Nevada, have been found, however, in framework cavities in archaeocyathid patch reefs, thus extending the record of the coelobiontic community even further.The Poleta Formation coelobiontic community is less diverse than that found in the upper Lower Cambrian community (from cavities in archaeocyathid patch reefs of the Forteau Formation of Labrador); it is dominated by algae, in particular Epiphyton. Renalcis and encrusting mammillary algae play a secondary role. Juvenile archaeocyathids, soft sponges, Archaeotrypa, and microtubules make up the remainder of the skeletal cavity biota. Sediments that floor the cavities contain a low-diversity but abundant infauna represented by meandering burrows; horizontal feeding or locomotion traces were produced on the surface of the cavity sediments.