Maoyan Zhu

Doushantuo embryos preserved inside diapause egg cysts

Phosphatized microfossils in the Ediacaran (635–542 Myr ago) Doushantuo Formation, south China, have been interpreted as the embryos of early animals. Despite experimental demonstration that embryos can be preserved, microstructural evidence that the Doushantuo remains are embryonic and an unambiguous record of fossil embryos in Lower Cambrian rocks, questions about the phylogenetic relationships of these fossils remain. Most recently, some researchers have proposed that Doushantuo microfossils may be giant sulphur-oxidizing bacteria comparable to extant Thiomargarita sp. Here we report new observations that provide a test of the bacterial hypothesis. The discovery of embryo-like Doushantuo fossils inside large, highly ornamented organic vesicles (acritarchs) indicates that these organisms were eukaryotic, and most probably early cleavage stage embryos preserved within diapause egg cysts. Large acanthomorphic microfossils of the type observed to contain fossil embryos first appear in rocks just above a 632.5 ± 0.5-Myr-old ash bed, suggesting that at least stem-group animals inhabited shallow seas in the immediate aftermath of global Neoproterozoic glaciation.

TUZOIA: MORPHOLOGY AND LIFESTYLE OF A LARGE BIVALVED ARTHROPOD OF THE CAMBRIAN SEAS

Abstract For almost 30 years, paleontologists have analyzed evolutionary sequences in terms of simple null models, most commonly random walks. Despite this long history, there has been little discussion of how model parameters may be estimated from real paleontological data. In this paper, I outline a likelihood-based framework for fitting and comparing models of phyletic evolution. Because of its usefulness and historical importance, I focus on a general form of the random walk model. The long-term dynamics of this model depend on just two parameters: the mean (μstep) and variance (σ2step) of the distribution of evolutionary transitions (or “steps”). The value of μstep determines the directionality of a sequence, and σ2step governs its volatility. Simulations show that these two parameters can be inferred reliably from paleontological data regardless of how completely the evolving lineage is sampled. In addition to random walk models, suitable modification of the likelihood function permits consideration of a wide range of alternative evolutionary models. Candidate evolutionary models may be compared on equal footing using information statistics such as the Akaike Information Criterion (AIC). Two extensions to this method are developed: modeling stasis as an evolutionary mode, and assessing the homogeneity of dynamics across multiple evolutionary sequences. Within this framework, I reanalyze two well-known published data sets: tooth measurements from the Eocene mammal Cantius, and shell shape in the planktonic foraminifera Contusotruncana. These analyses support previous interpretations about evolutionary mode in size and shape variables in Cantius, and confirm the significantly directional nature of shell shape evolution in Contusotruncana. In addition, this model-fitting approach leads to a further insight about the geographic structure of evolutionary change in this foraminiferan lineage.

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.

Sponge grade body fossil with cellular resolution dating 60 Myr before the Cambrian

Significance Phylogenomic extrapolations indicate the last common ancestor of sponges and eumetazoans existed deep in the Cryogenian, perhaps 200 million years (Myr) before the Cambrian (541 Ma). This inference implies a long Precambrian history of animals phylogenetically allied with sponges. However, there is yet little unequivocal paleontological evidence of Precambrian sponges. Here, we present a newly discovered 600-Myr-old fossil preserved at cellular resolution, displaying multiple poriferan features. The animal was covered with a dense layer of flattened cells resembling sponge pinacocytes, displaying a hollow tubular structure with apparent water inflow and outflow orifices. Although requiring additional specimens of similar form for confirmation, this finding is consistent with phylogenomic inference, and implies the presence of eumetazoan ancestors by 60 Myr before the Cambrian. An extraordinarily well preserved, 600-million-year (Myr)-old, three-dimensionally phosphatized fossil displaying multiple independent characters of modern adult sponges has been analyzed by SEM and synchrotron X-ray tomography. The fossilized animal (Eocyathispongia qiania gen. et sp. nov.) is slightly more than 1.2 mm wide and 1.1 mm tall, is composed of hundreds of thousands of cells, and has a gross structure consisting of three adjacent hollow tubes sharing a common base. The main tube is crowned with a large open funnel, and the others end in osculum-like openings to the exterior. The external surface is densely covered with flat tile-like cells closely resembling sponge pinacocytes, and this layer is punctuated with smaller pores. A dense patch of external structures that display the form of a lawn of sponge papillae has also survived. Within the main funnel, an area where features of the inner surface are preserved displays a regular pattern of uniform pits. Many of them are surrounded individually by distinct collars, mounted in a supporting reticulum. The possibility cannot be excluded that these pits are the remains of a field of choanocytes. The character set evinced by this specimen, ranging from general anatomy to cell type, uniquely indicates that this specimen is a fossil of probable poriferan affinity. So far, we have only this single specimen, and although its organized and complex cellular structure precludes any reasonable interpretation that its origin is abiogenic, confirmation that it is indeed a fossilized sponge will clearly require discovery of additional specimens.

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.

QUANTITATIVE ANALYSIS OF TAPHOFACIES AND PALEOCOMMUNITIES IN THE EARLY CAMBRIAN CHENGJIANG LAGERSTÄTTE

Abstract Detailed sedimentological and quantitative taphonomical analyses of 11,974 fossil specimens from an early Cambrian (Stage 3) Chengjiang-type deposit near Haikou, Yunnan, reveals significant relationships between the original depositional environments and the composition and preservation of their respective fossil assemblages. In general, the Maotianshan Shale is characterized by superimposed couplets of laminated background and thin event mudstone layers representing two distinct taphofacies, A and B, respectively. Fossils in taphofacies A consist predominantly of indeterminate organic elements and fecal or algal strings with few, poorly preserved, soft-bodied animals. Among those, disarticulated arthropods account for 84.3% of specimens (mostly isolated valves of Kunmingella douvillei) and 51.4% of species. Poriferans represent 7.4% of specimens and 22.9% of species. Fossils in this taphofacies have undergone significant pre- or syn-burial decay and represent limited time-averaged assemblages exhibiting low species richness. By contrast, taphofacies B contains greater numbers of species and specimens and better preserved soft-bodied animals. Taphofacies B represents mostly smothered organisms by distal tempestites. Arthropods are also dominant in taphofacies B, both in terms of species richness (41%) and abundance of specimens (44%). Poriferans, priapulids, lobopods, and brachiopods exhibit similar low species richness (6–8% each), but poriferans and lobopods are numerically rare, at around 1% each, whereas priapulids and brachiopods make up 26% and 24% of specimens, respectively. The arthropod Kunmingella douvillei (19%), the priapulid Cricocosmia jinningensis (19%), and the brachiopod Diandongia pista (18%) are the most abundant species in taphofacies B. Fossil assemblages in taphofacies A and B have similar recurrent and abundant species and similar temporal trends in evenness and richness, but taphofacies A captures only a portion of the species that are preserved in taphofacies B. These results suggest that the fossil assemblages present in both taphofacies represent a single local community subjected to two different taphonomic processes and imply similar recurrent environmental conditions within the section studied.

Community structure and composition of the Cambrian Chengjiang biota

Based on previously published species data (228 species in over 18 phyla) and field sampling (114 species and 18406 individuals) in the Chengjiang-Haikou-Anning area, we analyzed quantitatively the paleocommunity composition and structure of the Cambrian Chengjiang biota (Cambrian Series 2, eastern Yunnan, China). Arthropods dominate the community both in species diversity (species: 37%) and in abundance (individuals: 51.8%). Priapulids (individuals: 22.6%) and brachiopods (individuals: 16.3%) follow in abundance rank. The arthropod Kunmingella douvillei (26.2%), the priapulid Cricocosmia jinning-ensis (15.4%), and the brachiopod Diandongia pista (11%) are the three most abundant species. Ecological analyses show that the community was dominated by epifaunal organisms (species: 63%, individuals: 68.4%) followed by infaunal organisms (species: 11.9%, individuals: 25.9%), nektobenthic organisms (species: 11.5%, individuals: 2.6%), and pelagic organisms (species: 5.3%, individuals: 3.1%). The diverse feeding strategies, dominated by suspension feeders (species: 35.6%, individuals: 26.1%) and hunter/scavengers (species: 31.1%, individuals: 40.4%), indicate the former existence of a complex food chain and intense competition. Epifaunal vagrant omnivores (28.2%), infaunal vagrant hunter/scavengers (19.8%), epifaunal sessile suspension feeders (17.7%), and epifaunal vagrant hunter/scavengers (15.3%) were the most abundant ecological groups, represented primarily by arthropods, poriferans, priapulids, and brachiopods. Ecological group analyses reveal that the early Cambrian Chengjiang biota is similar in community patterns and functional relations to modern biotas in shallow marine settings.

Occurrence of Byronia Matthew and Sphenothallus Hall in the Lower Cambrian of China

KurzfassungSeltene phosphatische röhrchenförmige Fossilien aus der unter- bis mittelkambrischen Kaili- Formation aus der Provinz Guizhou (Kiautschau) in Südchina wurden als nicht-calcifizierte Algen oder als, Würmer’ (ScoleciellusLiu) identifiziert. Eine erneute Untersuchung dieser Fossilien erwies, daß die Fundstücke, die als nicht-verkalkte Algen bestimmt wurden, zuSphenothallus taijiangensis n. sp. zu stellen sind, während jene Exemplare, die alsScoleciellus identifiziert wurden, zuByronia natus (Liu) gehören.Sphenothallus taijiangensis undByronia natus aus den unterkambrischen Schichten der Kaili-Formation sind die ältesten Vertreter dieser Gattungen.Byronia natus ist, nebenB. AnnulataMatthew (Mittelkambrium von Britisch Kolumbien), die einzige bekannte Art dieser Gattung. CambrovitusMao et al., ein röhrchenförmiges Fossil aus den mittelkambrischen Schichten der Kaili-Formation, wurde ursprünglich zu den Hyolithiden gestellt. Die Entdeckung eines nahezu vollständigen Exemplars mit einer apikalen Haftscheibe deutet darauf hin, daßCambrovitus, ähnlich wieByronia undSphenothallus, wahrscheinlich ein thekentragender cnidarer Polyp war.AbstractRare phosphatic tubular fossils from the Lower-Middle Cambrian Kaili Formation of Guizhou Province, southern China were originally identified as non-calcified algae or ‘worms’ (ScoleciellusLiu). Re-examination of these fossils indicates that specimens identified as non-calcified algae areSphenothallus taijiangensis n. sp., while specimens identified asScoleciellus belong toByronia natus (Liu).Sphenothallus taijiangensis andByronia natus from Lower Cambrian strata in the Kaili Formation are the oldest known representatives of their genera. In addition,B. natus (Liu) is the only known species ofByronia with the exception ofB. annulataMatthew (Middle Cambrian, British Columbia).CambrovitusMao et al., a tubular fossil from Middle Cambrian strata in the Kaili Formation, originally was classified as a hyolithid. However, the discovery of a nearly complete specimen possessing an apical attachment disk shows thatCambrovitus, likeByronia andSphenothallus, probably was a thecate cnidarian polyp.

Diversity and species abundance patterns of the Early Cambrian (Series 2, Stage 3) Chengjiang Biota from China

Abstract Lagerstätten from the Precambrian–Cambrian transition have traditionally been a relatively untapped resource for understanding the paleoecology of the “Cambrian explosion.” This quantitative paleoecological study is based on 10,238 fossil specimens belonging to 100 animal species, 11 phyla, and 15 ecological categories from the lower Cambrian (Series 2, Stage 3) Chengjiang biota (Mafang locality near Haikou, Yunnan Province, China). Fossils were systematically collected within a 2.5-meter-thick sequence divided into ten stratigraphic intervals. Each interval represents an induced time-averaged assemblage of various event (obrution) beds of unknown duration. Overall, the different fossil assemblages are taxonomically and ecologically similar, suggesting the presence of a single community type recurring throughout the Mafang section. The Mafang community is dominated by epibenthic vagile hunters or scavengers, sessile suspension feeders, and infaunal vagile hunters or scavengers represented primarily by arthropods, brachiopods, and priapulids, respectively. Most species have low abundance and low occurrence frequencies, whereas a few species are numerically abundant and occur frequently. Overall, in structure and ecology the Mafang community is comparable to the Middle Cambrian (Series 3, Stage 5) Burgess Shale biota (Walcott Quarry, Yoho National Park, British Columbia, Canada). This suggests that, despite variations in species identity within taxonomic and ecological groups, the structure and ecology of Cambrian Burgess Shale-type communities remained relatively stable until at least the Middle Cambrian (Series 3, Stage 5) in subtidal to relatively deep-water offshore settings in siliciclastic soft-substrate environments.

Decimetre-scale multicellular eukaryotes from the 1.56-billion-year-old Gaoyuzhuang Formation in North China

Fossils of macroscopic eukaryotes are rarely older than the Ediacaran Period (635–541 million years (Myr)), and their interpretation remains controversial. Here, we report the discovery of macroscopic fossils from the 1,560-Myr-old Gaoyuzhuang Formation, Yanshan area, North China, that exhibit both large size and regular morphology. Preserved as carbonaceous compressions, the Gaoyuzhuang fossils have statistically regular linear to lanceolate shapes up to 30 cm long and nearly 8 cm wide, suggesting that the Gaoyuzhuang fossils record benthic multicellular eukaryotes of unprecedentedly large size. Syngenetic fragments showing closely packed ∼10 μm cells arranged in a thick sheet further reinforce the interpretation. Comparisons with living thalloid organisms suggest that these organisms were photosynthetic, although their phylogenetic placement within the Eukarya remains uncertain. The new fossils provide the strongest evidence yet that multicellular eukaryotes with decimetric dimensions and a regular developmental program populated the marine biosphere at least a billion years before the Cambrian Explosion.