Chengjiang Biota Research Articles

Early Cambrian metazoan fossil record of South China: Generic diversity and radiation patterns

South China with its rich fossil record is an important region for studying Early Cambrian metazoan diversity patterns. Compilation of a database of metazoan genera from the traditional Lower Cambrian (Cambrian Series 1–2) of South China allows a quantitative analysis of biodiversity changes. The dataset shows that about 876 genus names have been reported from the Lower Cambrian of South China. Of these only about 582 genera are now considered valid after taxonomic scrutiny. Plotting generic diversity shows a two-part, first-order trend: 1, diversity increase through the most of the Meishucunian and Qiongzhusian stages (South China usage), but punctuated by a modest decline during the upper Meishucunian; followed by 2, diversity decline in the Canglangpuan and Longwangmiaoan stages (South China usage). About 155 genera of small shelly fossils (SSFs) occur in Meishucunian rocks. The Meishucunian fauna is dominated by helcionellids, orthothecimorph hyoliths and other enigmatic fossils and is very consistent with the pretrilobitic ‘Tommotian Fauna’. Diversity increases in the Qiongzhusian stage, reaching 304 genera, the peak value known for the Cambrian. The Qiongzhusian fauna is characterized mainly by the occurrence of dominant pan-arthropods and crown-group brachiopods, representing an initial phase of the Cambrian Evolutionary Fauna. A substantial decline in generic diversity through the Canglangpuan (ca 153 genera) and Longwangmiaoan (ca 69 genera) stages is due partly to a eustatic regression, and it approximates the global pattern, reflecting an extinction event at the end of the Lower Cambrian. The observed regional biodiversity through this interval has both evolutionary and taphonomic influences: the decline in SSF diversity beginning in the upper Meishucunian was not only related to the decimation of SSFs, but also to a reduction in phosphogenesis. An apparent peak in diversity during the Qiongzhusian was somewhat exaggerated by the exceptional Chengjiang Biota.

Early Cambrian Yangtze Plate Maotianshan Shale macrofauna biodiversity and the evolution of predation

The large number of soft-part preserved fossils from the Lower Cambrian Maotianshan Shale (Chengjiang) Lagerstätten suggests particularly favourable conditions for a rich life on the shelf-zone sea bottom of the Yangtze Plate, China. This high degree of biodiversity opens an excellent window into the early radiation phase of Metazoa and represents a significant data source for the study of adaptive strategies among early animals. Feeding and locomotion are the main life strategies of organisms we investigated with regard to two major benthic macrofaunal components of the Maotianshan Shale biota, the nemathelminths and the arthropods. Our attempt was to test whether food, feeding and locomotory strategies of the benthic Lower Cambrian shallow-water communities were as diversified as it appears from the morphological diversity of the organisms present. Two major types of feeders can be discerned: suspension/micro-particle feeders – mostly epibenthic sedentary taxa – and larger-particle feeders, living in benthic to bentho-pelagic realms. Scant evidence exists for exclusive vegetarians, fungi eaters, biomat utilizers and for in-faunal vertical burrowers or grazers (bioturbators). Predation, in a wider sense, seems to be a, if not the, major feeding mode among metazoans. Nemathelminths and arthropods are amongst the best examples. In the benthic shallow-water regime, as exposed by the Maotianshan Shale biota, animals and their ontogenetic stages were the most suitable and readily available food source. At least for arthropods, we propose that improvement of predatory strategies was paralleled by the enhancement of locomotory and food manipulation structures. Accumulating evidence of late Precambrian to Early Cambrian metazoans exposing diverse morphologies and life styles indicates that, on the large scale, phylogenesis progressed gradually in the Late Precambrian. This renders an “explosive” radiation of Metazoa unlikely.

Eocrinoid echinoderms from the Lower Cambrian Guanshan Fauna in Wuding, Yunnan, China

This is a brief report of a new occurrence of eocrinoids from the Early Cambrian Wulongqing Formation in Yunnan, China. The eocrinoids from the Guanshan fauna are among the earliest known eocrinoids. Different from many other Early and Middle Cambrian eocrinoids, the Guanshan eocrinoids are char-acterized by the absence of sutural pores and epispires, the long and spiral brachioles, the extremely long stalk, and the ratio of the length of the stalk versus that of the calyx. The discovery of the eocri-noids from the Guanshan fauna not only provides new information to the investigation of the early evolution of this animal group, but also shed new light on the occurrence and migration of early eocrinoids.

Phosphatized Punctatus with mouthparts and its embryo fossils from the Lower Cambrian of Ningqiang, south Shaanxi, China

The appearance of coelenterates marks the real beginning of metazoan evolution. It therefore has a prominent position in the origin and evolutionary history of organisms, and is also a pivotal question of evolutionary biology. Punctatus is an extinct, ancient marine animal from the early stage of the Cambrian explosion, occurring at the lowermost Cambrian of both Kuanchuanpu (Ningqiang, Shaanxi) and Maidiping (Emei, Sichuan) areas. Punctatus has been studied for many years since the discovery of its fragments. Systematic and phylogenetic analysis has long been limited because of the rarity of complete specimens. In order to improve research into Punctatus, more than ten thousand globular fossils were recovered by means of “Chemistry Retting”. On the basis of the study of these globular fossils, a series of Punctatus fossils with cone parts and finely preserved soft-tissue mouthparts and fossilized metazoan embryo were recovered. Through research on characteristics such as shape, modality and structure of these fossils symbiotic with Punctatus emeiensis, the author found many possible embryo fossils including the evidence of gastrula-stage animal fossils. The sequence of fetation might have appeared on the corolliform oral region of Punctatus emeiensis. A study of the soft tissues, functional morphology and the sequence of embryo fossils shows evidence that Punctatus resembles coelenterate polyps in systematic classification. Perhaps it also represents an ancestor of the coelenterate with an original tentacle. A comparison with the real “tentacle animal” found in the Chengjiang Fauna, shows that the original tentacle is very tiny and its function range is limited. This reveals the primitive nature of the animal. Although the original tentacle is so small, it does exist, representing the first big step towards the real flexible tentacle with a strong function from the early evolving tentacle.

THE SYSTEMATICS AND PHYLOGENETIC RELATIONSHIPS OF VETULICOLIANS

Abstract: Vetulicolians have variously been considered to be unusual arthropods, stem‐group deuterostomes or relatives of the tunicates. They are known from a number of Cambrian Lagerstätten, and are particularly diverse in the Chengjiang biota of Yunnan Province, China. We recognize two classes, Vetulicolida and Banffozoa, which together form a monophyletic group. Within the Chinese collections we also identify two new species and recognize one new genus:Vetulicola monilesp. nov. andBullivetula variolagen. et sp. nov. The evidence from new and previously described specimens is used to undertake a phylogenetic analysis and to evaluate a range of hypotheses for the affinities of vetulicolians. Given the difficulties of interpreting features in enigmatic fossils and the apparently contradictory set of characters possessed by vetulicolians, it is not possible on current evidence to reach an unequivocal conclusion regarding the phylogenetic position of the group. One possibility is that they are a sister group of arthropods that lost limbs but gained gill structures analogous to those of deuterostomes, but several features remain unexplained by this model. If they are protostomes, a more generally parsimonious position is close to the kinorhynchs. An alternative is that they are deuterostomes, although a placement at the base of the clade is not supported by the evidence. If they are deuterostomes, it is more likely that they are close to the tunicates.

Early Cambrian origin of modern food webs: evidence from predator arrow worms

Although palaeontological evidence from exceptional biota demonstrates the existence of diverse marine communities in the Early Cambrian (approx. 540-520 Myr ago), little is known concerning the functioning of the marine ecosystem, especially its trophic structure and the full range of ecological niches colonized by the fauna. The presence of a diverse zooplankton in Early Cambrian oceans is still an open issue. Here we provide compelling evidence that chaetognaths, an important element of modern zooplankton, were present in the Early Cambrian Chengjiang biota with morphologies almost identical to Recent forms. New information obtained from the lowermost Cambrian of China added to previous studies provide convincing evidence that protoconodont-bearing animals also belonged to chaetognaths. Chaetognaths were probably widespread and diverse in the earliest Cambrian. The obvious raptorial function of their circumoral apparatuses (grasping spines) places them among the earliest active predator metazoans. Morphology, body ratios and distribution suggest that the ancestral chaetognaths were planktonic with possible ecological preferences for hyperbenthic niches close to the sea bottom. Our results point to the early introduction of prey-predator relationships into the pelagic realm, and to the increase of trophic complexity (three-level structure) during the Precambrian-Cambrian transition, thus laying the foundations of present-day marine food chains.

LONG STALKED EOCRINOIDS IN THE BASAL MIDDLE CAMBRIAN KAILI BIOTA, TAIJIANG COUNTY, GUIZHOU PROVINCE, CHINA

Long-stemmed eocrinoids are limited to two species in the basal Middle Cambrian Kaili Biota, which occupies the middle portion of the Kaili Formation, Taijiang County, Guizhou Province, China. The Kaili Biota contains preserved soft-bodied organisms shared with either the Chengjiang Fauna (Southwest China) or the Burgess Shale Fauna (British Columbia) or with both. Echinoderms are preserved as limonitic external molds that produce excellent latex casts. Sinoeocrinus lui Zhao et al., 1994 has a complex ontogenetic development, which is described in terms of morphology of holdfast, number of thecal plate circlets, addition and morphology of thecal pores, ambulacral arrangement, and number of brachioles relative to thecal height. Because of the complex ontogeny the following species are now seen to be synonymous with S. lui: S. curtobrachiolus Zhao et al.,1994; S. lepidus Zhao et al., 1994; S. longus Zhao et al., 1994; S. minus Zhao et al., 1994; Paragogia globosa Zhao et al., 1994; and Curtoeocrinus guizhouensis Zhao et al., 1994. A second and rare eocrinoid of undetermined familial and ordinal placement, Balangicystis rotundus n. gen. and sp., has an unusually long holdfast and poreless thecal plates with prominent radial ridges. Sinoeocrinus lui and Balangicystis rotundus inhabited the outer shelf in disaerobic fine-grained shales and mudstones. Megascopic infauna in their community is not present.

Distinguishing anomalocaridids from arthropods and priapulids

AbstractThe Chengjiang fossil lagerstätte has yielded four described species of dinocaridids. These have been interpreted either as arthropods or as arthropod progenitors, occasionally with the anomalocaridid Parapeytoia yunnanensis serving as a link. This view is rejected both because anomalocaridids have a number of derived characters absent from early arthropods, and also because a shared origin gives a more parsimonious picture. Sclerites thought to belong to the mouth of P. yunnanensis differ fundamentally from those of its holotype and from those of other dinocaridids. It appears that they belong to a new large priapulid, described here as Omnidens amplus gen. et sp. nov. Copyright © 2006 John Wiley & Sons, Ltd.

Palaeontology of sponges — a review

The fossil record of sponges is a very old one, and begins in the Precambrian, but is rather incomplete. Only those sponges with a rigid skeleton, i.e., Hexactinosida and Lychniscosida (both hexactinellids), lithistids (demosponges with desmas), and sponges with a massive calcareous skeleton (polyphyletic demosponges and Calcarea) have a more or less continuous fossil record that is, however, inadequately studied, especially from the Tertiary. The beginning of sponge diversification during the Cambrian is relatively well known thanks to their very good preservation, from the Chengjiang fauna in China and Burgess Shale in Canada, where even sponges with unfused spicules occur. The majority of palaeontologic studies are concerned with taxonomic aspects of fossil sponge faunas, but investigations of the microstructure of the calcareous skeleton, of phylogeny (especially of the early forms), and of various aspects of their ecology have produced important results. Future research still has to fill gaps in the knowledge of fossil faunas, especially "soft" sponges, and in the phylogeny of sponges (especially for the polyphyletic groups like lithistids), but new approaches such as studies of biomarkers are already emerging.

New observations of the lobopod-like worm Facivermis from the early cambrian Chengjiang Lagerstätte

Facivermis yunnanicus (Hou & Chen, 1989), from the Early Cambrian Chengjiang Lagerstatte, a worm-like fossil with 5 pairs of tentacles and a perceived shrunken end, has been regarded as related to polychaetes, later it has been variously interpreted as lobopods, Pentastoma and lophophorates. Newly discovered complete specimens by the ELI field team show that the taxon has, in addition to the 5 pairs of appendages, a pear-shaped trunk end bearing two or three circles of hooks. Accordingly, based on these important morphological characters, reconsideration of its affinities is provided and the taxonomy is remedied herein. Because the five pairs of appendages of Facivermis yunnanicus resemble the appendages of the fore-trunk of lobopod Miraluolishania (Liu & Shu, 2004), it seems that they are homologous structures. Therefore, the affinities of Facivermis are regarded here as being close to the lobopods, in addition, the lobe-like appendages of Facivermis are very crucial to exploring the origin of the appendages of lobopods and arthropods.

A new theca-bearing Early Cambrian worm from the Chengjiang Fossil Lagerstätte, China

An elongate worm from the Early Cambrian Chengjiang fossil Lagerstätte, Laojieella thecata gen. et sp. nov., is described on the basis of two specimens. Its unique body plan features a tapering, spinose proboscis, an elliptical theca on the posterior part of the trunk, and a caudal appendage. It is considered to be most closely related to cycloneuralians, especially priapulids, but cannot be firmly assigned to any known phylum. Some of the common characteristics like the spinose proboscis, shared by other proboscis-bearing fossil worms in the Early and Middle Cambrian, suggest that the common ancestor for cycloneuralians had a zoned proboscis. Based on the presence of sand grains in the gut, the animal is viewed as a detritus feeder probably with occasional preying.

Kaili Biota: A Taphonomic Window on Diversification of Metazoans from the Basal Middle Cambrian: Guizhou, China

Abstract A Burgess Shale‐type biota is, in part, characterized by a wide diversity of taxa and soft‐part preservation. Each provides unique historical insights into early metazoan evolution. Among the more than 40 globally distributed biotas, the early Cambrian Chengjiang and Middle Cambrian Burgess‐type biotas are the largest. The Kaili Biota, from the earliest Middle Cambrian of Guizhou, China, contains representatives of 110 metazoan genera belonging to 10 phyla. It contains many well‐persevered soft‐bodied specimens. This Chinese biota has become the third most taxonomically diverse Burgess Shale‐type fauna. Because the Kaili Biota formed in an outer‐shelf environment, its main faunal character is large numbers of eocrinoids and planktoic trilobites. The Kaili is younger than the Chengjiang Biota but older than the Canadian Burgess Shale Biota it shares 30 genera with the Chengjiang and 38 genera with the Burgess Biota. The Kaili Biota displays a taphonomic window to the diversification and evolution of marine offshore organisms covering 5.13 million years between the Early and Middle Cambrian.

The evolutionary history of crustacean segmentation: a fossil‐based perspective

The evolution of segmentation in Crustacea, that is, the formation of sclerotized and jointed body somites and arrangement of somites into tagmata, is viewed in light of historical traits and functional constraints. The set of Early to Late Cambrian 'Orsten' arthropods have informed our current views of crustacean evolution considerably. These three-dimensionally preserved fossils document ancient morphologies, as opposed to purely hypothetical models and, because of the unusual preservation of larval stages, provide us with unparalleled insight into the morphogenesis of body somites and their structural equipment. The variety of evolutionary levels represented in the 'Orsten' including lobopodians, tardigrades, and pentastomids also allows phylogenetic interpretations far beyond the Crustacea. The 'Orsten' evidence and data from representatives of the Lower Cambrian Chengjiang biota in southwestern China, including phylogenetically earlier forms, form the major source of our morphology-based review of structural and functional developments that led toward the Crustacea. The principal strategy of arthropods is the simultaneous development of head somites, as expressed in a basal "head larva," and a successive addition of postcephalic somites from a preterminal budding zone with progressive maturation of metameric structures. This can be recognized in the developmental patterns of extant and fossil representatives of several euarthropod taxa, particularly crustaceans, trilobites, and chelicerates (at least basally). The development of these taxa points to an early somite-poor and free-living hatching stage. Embryonic development to a late stage within an egg, as occurring in recent onychophorans and certain in-group euarthropods, is regarded as achieved several times convergently.

Soft-bodied fossils from the Shipai Formation, Lower Cambrian of the Three Gorge area, South China

Mudstones and shales in the Lower Cambrian Shipai Formation in the Three Gorge area, Hubei, China, are richly fossiliferous, containing common shelly fossils and some soft-bodied fossils. The latter provide important new information about the nature and variety of Cambrian soft-bodied organisms. Identifiable, non-mineralized taxa include components of the Chengjiang fauna, such as Vetulicola Hou, 1987, a palaeoscolecidan referable to Maotianshania Sun & Hou, 1987, and a brachiopod Diandongiapista with pedicle preserved (not illustrated). Cambrorhytium Conway Morris & Robison, 1988, co-occurring both in the Burgess Shale and the Chengjiang Lagerstätte, was also recovered. Additionally, a new worm is described based on two specimens. This taxon, in common with many other fossil worms, has a slender, cylindrical body with annulations, but it is characterized by each annulus bearing an elevated ridge and lacking surface ornamentation. The occurrence of exceptional preservation in the Shipai Formation has likely been overlooked due to the relatively poor resolution of soft-bodied fossils; nevertheless, this occurrence is an important extension of the Burgess Shale-type biotas in China, over 1500 km northeast of the provenance of the Chengjiang Lagerstätte.

On the Phylum Vetulicolia

The origins and radiations of metazoans, bilaterians, deuterostomes and vertebrates have been recognized as the four most important milestone events in animal history and, conversely four of the toughest and puzzling questions in evolutionary biology. Among them, the first two occurred in the deep time of the Precambrian and have left little information in the fossil records. Fortunately, intensive investigation into the Early Cambrian Chengjiang faunas in the past two decades have revealed numerous significant data on the main episode of the Cambrian Explosion, particularly information on the various earliest-known deuterostomes, leading to a much better understanding of the last two events. The “first fish”Haikouichthys andMyllokunmingia have been proved the oldest-known vertebrates, and to be among the most primitive ones, representing a key transitional group from acraniates to craniates. Indeed, “Paleontological work of Shu and collaborators is revising our understanding of the early evolution of chordates”. Moreover, the discovery of the Phylum Vetulicolia may also throw new light on the origin of deuterostomes. This extinct group with simple gill slits might represent one of the roots in the deuterostome lineage, as the evidence of molecular biology and developmental biology has predicted. Here the first discovery of tiny vetulicolians from the Chengjiang Lagerstatte is reported. The evolutionary trend from the giant to the small might represent a special adaptation in early animal history. Since all vetulicolians share similar characters in their anterior section (pharynx), and on the basis of differences in their posterior section, a new classification system is proposed, with a new class, the Heteromorphida erected. As both the “root” group vetulicolians and the “top” group vertebrates in the early deuterostome lineage, as well as those members in between, such as vetulocystids, yunnanozoans, cephalochordates and urochordates, have been recovered, a relatively complete early deuterostome phylogeny, which is compatible with phylogenies of extant deuterostomes, is nearly mature.

Cambrian anemones with preserved soft tissue from the Chengjiang biota, China

Hou, X.-G., Stanley Jr, G.D., Zhao, J. & Ma, X.-Y. 2005 09 12: Cambrian anemones withpreserved soft tissue from the Chengjiang biota, China. Lethaia, Vol. 38, pp. 193–203.Oslo. ISSN 0024-1164.The group Cnidaria includes ‘jellyfish’, soft-bodied anemone and anemone-like formsand calcified corals. These diploblastic organisms have a fossil record extending back tothe earliest metazoans of the Neoproterozoic; however certain cnidarians of the subclassZoantharia, characterized by soft-bodied anemone-like forms, are absent or poorlyrepresented in the fossil record. Despite the paucity of fossils, it is thought that calcifi-cation by soft anemone-like animals was responsible for producing the skeleton thatallowed the preservation of the first corals. We report discovery of an abundant assem-blage of in situ soft-bodied polyps with tissues. They are preserved in exquisite detail andcome from the well-known Lower Cambrian Chengjiang biota of Yunnan, China. Thesoft-bodied polyps display a simple anatomy that is comparable to some extant anemonesof the order Actinaria. The new fossils are assigned to Archisaccophyllia kunmingensis n.gen. et n. sp. Their simple and conservative form suggests that these fossils may representsome kind of ancestral rootstock. The preserved life assemblage provides a uniquesnapshot of Lower Cambrian anemone life and provides clues for relationships withextant actiniarians as well as calcified corals. & Actiniaria, anemone, Chengjiang biota,China, Lower Cambrian, soft-bodied fossils.Xian-guang Hou (corresponding author) [xghou@ynu.edu.cn], Jie Zhao [biofeature@hotmail.com] and Xiao-ya Ma [littleya@vip.sina.com], Yunnan Key Laboratory forPalaeobiology, Yunnan University, Kunming 650091, China; George D. Stanley, Jr.[george.stanley@umontana.edu], Department of Geology, The University of Montana,Missoula, MT 59812, USA; 19th August 2004, revised 11th April 2005.

An Early Cambrian problematic fossil:Vetustovermisand its possible affinities

The Early Cambrian problematic fossil Vetustovermis (Glaessner 1979 Alcheringa3, 21-31) was described as an annelid or arthropod. Anatomical analysis of 17 new specimens from the Lower Cambrian Maotianshan Shale at Anning, Kunming (South China) does not support its affinities with annelids or arthropods. Anatomical features instead resemble other animal groups including modern flatworms, nemertines and molluscs. The presence of a pelagic slug-like form and ventral foot, as well as a head with eyes and tentacles indicates a possible affinity with molluscs, but these characters are not present only in molluscs; some of them are shared with other animal groups, including flatworms and nemertines. For example, a ventral foot-like structure is found in nemertines, 'turbellarians', and some polychaete groups. The well differentiated head is seen in separate bilaterian groups, but among molluscs it did not occur before the evolutionary level of the Conchifera. Unlike the ctenia-gills in molluscs, the gills in Vetustovermis are bar-like. All the characters displayed in this 525 million-year old soft-bodied animal fail to demonstrate clear affinity with molluscs or any other known extant or extinct animal groups, but argue for representing an independently evolved animal group, which flourished in Early Cambrian and possibly in Middle Cambrian time.

New Vetulicoliids from the Lower Cambrian Guanshan Fauna, Kunming

Abstract The Guanshan Fauna, a soft‐bodied fauna intermediate between the Chengjiang Fauna and the Kaili Fauna and also the Burgess Shale Fauna stratigraphically, consists of trilobites, trilobitoides, Tuzoia, Vetulicola, Paleoscolex, brachiopods and sponges. The discovery and research of this fauna is of great significance in understading the “Cambrian Explosion” and the evolution of early life. The occurrence of vetulicoliids from the Guanshan Fauna not only adds new members to the taxonomic list, but also provides new information to the evolution of this animal group. This paper describe Vetulicola gantoucunensis Luo, Fu et Hu sp. nov. from the Lower Cambrian Wulongqing Formation in the Kunming area. Also presented are the amended description of Vetulicola and the comparisons with related genera within Vetulicoliids. The affinity, distribution, as well as evolution of vetulicoliids are discussed.

Early Cambrian Food Chain: New Evidence from Fossil Aggregates in the Maotianshan Shale Biota, SW China

Abstract Three categories of fossil aggregates are recognized in the Lower Cambrian Maotianshan Shale biota from SW China: (1) elliptical aggregates with randomly distributed exoskeletal remains of typically small- to medium-size bivalved arthropods (e.g., ostracode-like bradoriids, phyllocarid-like waptiids), hyoliths, and trilobites are interpreted as coprolites, possibly produced by anomalocaridids (e.g., waptiid-rich coprolites) and/or unknown epibenthic predators; (2) elongate, ribbon-like aggregates composed of oriented small hyolith shells, interpreted as the feces of infaunal carnivorous worms such as priapulids; and (3) concentric aggregates, typically with a central nucleus (e.g., remains of medusoid eldoniids or bivalved arthropod carapaces) and peripheral exoskeletal fragments, possibly generated by bottom currents whirling around carcasses. These new coprolite data add to morphofunctional information obtained from fossil organisms and indicate that predation occurred at different levels of th...

Paleoecology of benthic metazoans in the Early Cambrian Maotianshan Shale biota and the Middle Cambrian Burgess Shale biota: evidence for the Cambrian substrate revolution

As the depth and intensity of bioturbation increased through the Proterozoic–Phanerozoic transition, the substrates on which marine benthos lived changed from being relatively firm with a sharp sediment–water interface to having a high water content and blurry sediment–water interface. Microbial mats, once dominant on normal marine Proterozoic seafloors, were relegated to stressed settings lacking intense metazoan activity. This change in substrates has been termed the agronomic revolution [Seilacher, A., Pflüger, F., 1994. From biomats to benthic agriculture: a biohistoric revolution. In: Krumbein, W.E. (Ed.), Biostabilization of Sediments. Bibliotheks and Informationsystem del Carl von Ossietzky Universitat, Oldenburg, 97–105], and the impact of the subsequent development of the mixed layer on benthic metazoans has been termed the Cambrian substrate revolution [Bottjer, D.J., Hagadorn, J.W., Dornbos, S.Q., 2000. The Cambrian substrate revolution. GSA Today 10 (9), 1–7]. Because the Early Cambrian was a transitional time in this substrate revolution, it is hypothesized that benthic metazoans adapted to typical Proterozoic-style soft substrates co-existed with benthic metazoans adapted to more typical Phanerozoic-style soft substrates. Paleoecological examination of the benthic metazoans of the Early Cambrian Maotianshan Shale (Chengjiang) biota and Middle Cambrian Burgess Shale biota, combined with examination of core samples from the rocks in which the Maotianshan Shale biota is preserved, was performed in order to test this hypothesis. Results of the core analysis reveal that the rocks in which the Maotianshan Shale biota was preserved, and therefore the substrate on which it lived, contains almost no evidence for bioturbation of any kind, while the immediately underlying rocks of similar lithology are dominated by intermediate levels of horizontal bioturbation and contain some evidence for the beginning of mixed layer development. These results suggest that benthic metazoans of the Maotianshan Shale biota lived on a typical Proterozoic-style soft substrate, and that environmental conditions associated with the exceptional preservation of the Maotianshan Shale biota may have also suppressed bioturbation levels. Results of the paleoecological analysis indicate that the Maotianshan Shale and Burgess Shale biotas contain mobile and sessile benthic metazoans adapted to typical Proterozoic-style soft substrates and others adapted to typical Phanerozoic-style soft substrates. In addition, the Maotianshan Shale biota is dominated by benthic suspension feeding genera adapted to typical Proterozoic-style soft substrates, indicating that this biota lived during the early stages of the Cambrian substrate revolution. The Burgess Shale biota, however, contains more benthic suspension feeding genera adapted to typical Phanerozoic-style soft substrates than the Maotianshan Shale biota, which suggests that the soft substrate on which they lived had more advanced levels of mixed layer development. This research suggests that the adaptive radiation of benthic metazoans during the “Cambrian explosion” was driven in part by the Cambrian substrate revolution, as benthic metazoans were forced to adapt to the development of the mixed layer in subtidal siliciclastic soft substrate environments during the Cambrian.