Yuanlong Zhao

Mechanism for Burgess Shale-type preservation

Exceptionally preserved fossil biotas of the Burgess Shale and a handful of other similar Cambrian deposits provide rare but critical insights into the early diversification of animals. The extraordinary preservation of labile tissues in these geographically widespread but temporally restricted soft-bodied fossil assemblages has remained enigmatic since Walcott’s initial discovery in 1909. Here, we demonstrate the mechanism of Burgess Shale-type preservation using sedimentologic and geochemical data from the Chengjiang, Burgess Shale, and five other principal Burgess Shale-type deposits. Sulfur isotope evidence from sedimentary pyrites reveals that the exquisite fossilization of organic remains as carbonaceous compressions resulted from early inhibition of microbial activity in the sediments by means of oxidant deprivation. Low sulfate concentrations in the global ocean and low-oxygen bottom water conditions at the sites of deposition resulted in reduced oxidant availability. Subsequently, rapid entombment of fossils in fine-grained sediments and early sealing of sediments by pervasive carbonate cements at bed tops restricted oxidant flux into the sediments. A permeability barrier, provided by bed-capping cements that were emplaced at the seafloor, is a feature that is shared among Burgess Shale-type deposits, and resulted from the unusually high alkalinity of Cambrian oceans. Thus, Burgess Shale-type preservation of soft-bodied fossil assemblages worldwide was promoted by unique aspects of early Paleozoic seawater chemistry that strongly impacted sediment diagenesis, providing a fundamentally unique record of the immediate aftermath of the “Cambrian explosion.”

Eight-armed Ediacara fossil preserved in contrasting taphonomic windows from China and Australia

We report the preservation of the eight-armed Ediacara fossil Eoandromeda octobrachiata as carbonaceous compressions in the Doushantuo black shale of south China and as casts and molds in the Rawnsley Quartzite in South Australia. The contrasting preservational styles in two taphonomic windows indicate that E. octobrachiata may have had a relatively recalcitrant organic integument, which rules out its close comparison with giant agglutinated foraminifers such as xenophyophores. Its octaradial symmetry and dextrally spiraling arms suggest that it may be a diploblastic-grade animal sharing some features with cnidarians and ctenophores, although its phylogenetic affinity remains open. It is the first and only unambiguously identified Ediacaran macrofossil that occurs in two drastically different taphonomic windows, thus bridging the conventional biological and taxonomic gaps between the Ediacara and Miaohe biotas, which collectively record the earliest known macroscopic and complex life.

Direct evidence for predation on trilobites in the Cambrian.

A fossil arthropod similar to Fuxianhuia and displaying an exceptionally well–preserved alimentary canal with in situ gut contents is reported from the lower Middle Cambrian (ca. 510 Myr ago) of South China. Three–dimensionally preserved, paired phosphatic nodules, arranged in series along both sides of the gut and containing spherical bacteria, probably represent serial digestive glands. The gut is filled with fragments of the eodiscoid trilobite, Pagetia. The well-developed digestive glands and the fragmentary trilobite remains suggest (i) that the arthropod was a durophagous, possibly selective predator, and (ii) that small trilobites such as eodiscoids were a major food source in Cambrian marine ecosystems. This fossil association augments the small number of previously described associations providing more or less direct evidence of predator–prey relationships in Cambrian epibenthic communities.

A Parvancorina-like arthropod from the Cambrian of South China

Constraining the origin of animal groups is allowed, to some extent, by discoveries of Cambrian Lagerstätten that preserve both mineralizing and nonmineralizing organisms. A new species is reported here of the Cambrian arthropod Skania, which bears an exoskeleton that shares homologies with the Neoproterozoic (Ediacaran) organism Parvancorina and firmly establishes a Precambrian root for arthropods. A new monophyletic group, Parvancorinomorpha, is proposed as the first clade within the arthropod crown group demonstrably ranging across the Neoproterozoic–Paleozoic transition. The Parvancorinomorpha is interpreted to be the sister group of the Arachnomorpha. Incipient cephalization in Skania and related genera represents a step in the progression toward division of a cephalon from a large posterior trunk as shown in Cambrian arachnomorphs such as naraoiids and the addition of a pygidium and thoracic tergites as shown in the arachnomorph clade basal to trilobites. This evidence can serve as a new calibration point for estimating the divergence time for the last common ancestor of arthropods and priapulids based on molecular clock methods.

Kailidiscus, A New Plesiomorphic Edrioasteroid from the Basal Middle Cambrian Kaili Biota of Guizhou Province, China

Abstract A new genus and species of edrioasteroid grade echinoderm, Kailidiscus chinensis, is described from the Kaili Biota of the basal lower Middle Cambrian Kaili Formation from Guizhou Province, China. This echinoderm has a strong resemblance to isorophid edrioasteroids, bearing a well-developed peripheral rim, long curved ambulacra, and slightly imbricate interambulacral plating at maturity. However, the presence of pore-bearing, double biserial floor plates, tiered cover plates, lack of radially positioned oral frame plates, and unincorporated hydropore/gonopore are unknown in isorophids. Many of these features bear strong resemblance to eocrinoids and helicoplacoids, attesting to the plesiomorphic nature of this taxon. Despite the obvious anatomical differences, specimens of this species preserve a complete ontogeny that resembles that of isorophids. Juveniles show a discoidal theca with straight ambulacra that transitions to an inflated theca with strongly curved ambulacra with maturity.

Carbon and oxygen isotopic composition of Lower to Middle Cambrian sediments at Taijiang, Guizhou Province, China

Secular variations in the carbon isotopic composition of organic and carbonate carbon characterize the Lower to Middle Cambrian transition that is exposed on the Yangtze Platform at Taijiang, Guizhou Province, southern China. δ 13 C values for organic matter range between −33.4 and −26.5‰. The carbon isotopic composition for carbonate carbon fluctuates between −2.7 and +3.1‰. A progressive decrease in the isotopic difference (δ) between these two isotope records reflects a decrease in the proportional contribution of bacterial biomass to the total sedimentary organic matter. In general, the observed changes are interpreted to reflect primary depositional values, notably variations in the burial rates of organic matter. These, in turn, are linked to biological changes across the Lower to Middle Cambrian transition. No distinct shift in the carbon isotopic composition marks the proposed Lower-Middle Cambrian boundary.

Basal Middle Cambrian Short-Stalked Eocrinoids from the Kaili Biota: Guizhou Province, China

Abstract Gogiid eocrinoids (Echinodermata) are the most abundant coelomate invertebrates in the Middle Cambrian (Taijiangian) Kaili Biota, Taijiang County, Guizhou Province, China. Both long- and short-stalked taxa are represented. The short-stalked Globoeocrinus globulus n. gen. and sp. has a globular theca, with well-developed sutural pores on all thecal plates in older mature specimens, short, thick stalk composed of small polygonal to rounded platelets, a large flattened attachment disc, and up to ten long, thin, helically coiled brachioles with tall, pointed cover plates. Some younger mature specimens have varying patches of non-pored plates that usually occur on the upper (probably) posterior portion of the theca. This species is over three times more abundant than the other co-occurring long-stalked gogiid Sinoeocrinus lui Zhao, Huang and Gong, 1994. Most specimens of G. globulus (ca. 80%) attach to acrotretid brachiopods, trilobite exuviae, or other skeletal remains. They are also capable of attaching directly to the seafloor.

Taphonomy, palaeoecological implications, and colouration of Cambrian gogiid echinoderms from Guizhou Province, China

Based on rich material (381 specimens examined) from two Cambrian echinoderm faunas, the early Cambrian Balang fauna and middle Cambrian Kaili fauna in Guizhou Province, South China, the taphonomy of gogiid echinoderms is described in detail, and the preservation of stereomic microstructure and organic remains of Cambrian gogiid echinoderms is reported here for the first time. Taphonomic considerations include entombment patterns, decay sequences, individual-specific diagenetic histories, unusual burial postures, selective disarticulation patterns, and postmortem elongation. In particular, five categories of gogiid entombment patterns are proposed to describe the multi-directional orientations recorded at the burial time of articulated gogiids. Gogiid-bearing slabs of Guizhou material primarily (70 %) display the type 2 entombment pattern (articulated gogiids preserved with fan-shaped brachioles); thus, most Guizhou gogiids were buried with brachioles preserved in feeding posture during obrution events. Balang gogiid faunas contain the oldest evidence of palaeoecological interactions among echinoderms and other indigenous taxa. In addition to pre-burial and post-burial decay, other potential causes for unusual disarticulation patterns exhibited by the gogiids from the lower Cambrian Balang Formation include pre-burial bio-disturbance and post-burial bioturbation based on ichnogenera, including Rusophycus and Planolites. Chemical analyses reveal that carbon, calcium, manganese and iron are the major elements responsible for the variety of colours exhibited by Guizhou gogiids. Three-dimensional stereomic microstructure (mean stereom pore size = 8.4–8.7 μm; average trabecular thickness = 4.5–4.6 μm) occurs on the external surfaces of thecal plates in two gogiid species. Stereom preservation in calcite suggests that the dissolution of calcareous echinoderm plates, yielding characteristic mouldic preservation, is sub-Recent (after lithificaiton and exposure of gogiid-bearing, marine sedimentary successions on or near the land surface).

Coupled oceanic oxygenation and metazoan diversification during the early–middle Cambrian?

The early–middle Cambrian (Fortunian to Age 4) is characterized by a significant increase in metazoan diversification. Furthermore, this interval is marked by a prominent environmental and ecological expansion of arthropod- and echinoderm-rich biotas. Recent redox work has suggested that this shift occurred during stable or decreasing marine oxygen levels, suggesting that these paleobiological and paleoecological transformations were decoupled from a redox control. We tested this idea by conducting new paleoredox analyses on Age 2–Age 4 Cambrian outer shelf (Jiuqunao-Wangjiaping), slope (Wuhe-Geyi), and basinal (Zhalagou) sections of the South China Craton. Multiple sections indicate that mid-depth waters transitioned from anoxic conditions during Cambrian Age 2 to stable oxic conditions during Cambrian Age 4. These findings suggest a stepwise expansion of oxic waters from shallow to deep settings during the early–middle Cambrian, consistent with a redox control of metazoan diversification and ecological expansion. More broadly, despite the surge in redox work over the past decade, this study highlights the need for continued coupled redox and paleontological studies to directly test models about the links between the evolution of animals, ecosystems, and marine redox conditions.

Revision of lyracystid eocrinoids from the Middle Cambrian of South China and Western Laurentia

Abstract The Middle Cambrian eocrinoid genera Lyracystis Sprinkle and Collins, 2006, from western Laurentia and Balangicystis Parsley and Zhao, 2006, from South China, described in the same year, have turned out to be closely related genera assigned to the Family Lyracystidae. Both have erect, lyre-shaped, arm-like, brachiole-bearing, feeding appendages, here termed exothecal ambulacra, that are not homologous to crinoid arms. They also have a long, multiplated stalk to elevate the theca and feeding appendages well above the sea floor, making them among the highest tiered echinoderm suspension feeders known from the Middle Cambrian. The long stalk was either inserted a short distance into the muddy sediment, or attached to rare skeletal fragments lying on the sea floor. Both genera seem well adapted to quiet-water or slow-current conditions in deeper water (150–200 m) on the outer shelf or upper slope of their respective continents.