Early Cambrian Fossils Research Articles

Correlation and stratigraphic implications of the lowermost Cambrian small shelly fossils from new sites of South China

Early Cambrian small shelly fossils (SSFs) represent some of the earliest biomineralized examples of metazoans and are often extremely abundant and diverse, so can be useful for regional and intercontinental correlation in pre-trilobitic carbonate or phosphorite deposits. New SSF assemblages chemically extracted from the Cambrian Yangjiagou Member carbonates in the Fucheng area, southern Shaanxi Province, China. The SSFs include anabaritids, protoconodonts, hyolitheminths, maikhanellids, Quadrapyrgites , Archaeooides , Olivooides and Siphogonuchites . The recovered SSFs represent two previously identified SSF assemblage zones: the Anabarites trisulcatus – Protohertzina anabarica assemblage zone and the succeeding Paragloborilus subglobosa – Purella squamulosa assemblage zone. The early Cambrian fossil assemblage zones described from this locality are correlated on a regional scale across the Yangtze Platform from eastern Yunnan, the Three Gorge region of western Hubei, northern Sichuan, the Ningqiang and Xixiang regions of southern Shaanxi and the Nemakit–Daldynian Stages of Siberia. The results reveal that the Yangjiagou Member is provisionally equivalent to the Zhongyicun Member of Yunnan, the Yanjiahe Formation of Hubei, the Maidiping Formation of Sichuan and the Kuanchuanpu Formation of Shaanxi. This paper offers new evidence and fossil data for the location of the Precambrian–Cambrian boundary in the Micangshan area of South China.

Scaphopoda is the sister taxon to Bivalvia: Evidence of ancient incomplete lineage sorting

The almost simultaneous emergence of major animal phyla during the early Cambrian shaped modern animal biodiversity. Reconstructing evolutionary relationships among such closely spaced branches in the animal tree of life has proven to be a major challenge, hindering understanding of early animal evolution and the fossil record. This is particularly true in the species-rich and highly varied Mollusca where dramatic inconsistency among paleontological, morphological, and molecular evidence has led to a long-standing debate about the group's phylogeny and the nature of dozens of enigmatic fossil taxa. A critical step needed to overcome this issue is to supplement available genomic data, which is plentiful for well-studied lineages, with genomes from rare but key lineages, such as Scaphopoda. Here, by presenting chromosome-level genomes from both extant scaphopod orders and leveraging complete genomes spanning Mollusca, we provide strong support for Scaphopoda as the sister taxon of Bivalvia, revitalizing the morphology-based Diasoma hypothesis originally proposed 50 years ago. Our molecular clock analysis confidently dates the split between Bivalvia and Scaphopoda at ~520 Ma, prompting a reinterpretation of controversial laterally compressed Early Cambrian fossils, including Anabarella, Watsonella, and Mellopegma, as stem diasomes. Moreover, we show that incongruence in the phylogenetic placement of Scaphopoda in previous phylogenomic studies was due to ancient incomplete lineage sorting (ILS) that occurred during the rapid radiation of Conchifera. Our findings highlight the need to consider ILS as a potential source of error in deep phylogeny reconstruction, especially in the context of the unique nature of the Cambrian Explosion.

The early animal radiation: insights from interpreting the Cambrian problematic fossils

Genic and genomic data have been reshaping our understanding of the earliest radiation event of metazoans, the well-known Cambrian Evolutionary Radiation, not only from the respects of reshuffling the phylogenetic topologies of some animal phyla but by deciphering the deep homologies of many morphological features. These advances, together with the continuing discoveries of the Ediacaran-Cambrian fossils, are unveiling the cladogenetic process of the early metazoans and the patterns of morphologic evolution during this biological radiation event. In this review, I focus on a small but challenging field, the problematic fossils from the early Cambrian fossil Lagerstätten, such as the Chengjiang biota, mainly on the controversies concerning their interpretation and the consequent impacts on understanding the early evolution of animals. The bizarre body plans of the early Cambrian problematica alone do not account for the difficulties in studying their biology and affinity. Instead, it is the combined action of the taphonomic artifacts and the uncertainty in homologizing the preserved characters that impede generating plausible interpretations. Despite all these issues, a testable and repeatable method for interpreting fossils has emerged and is becoming more practicable. The integration of an evolutionary-grade conceptual frame is beneficial to the interpretation of the Cambrian problematic fossils. Together with the focus on taphonomic alternation and homologic assessment, the Cambrian problematic fossils are becoming more informative nodes in the “parsing tree” of early animal evolution.

Nonmineralized triradial conulariids from the lowermost Cambrian Stage 2 of the Olenek Uplift, Siberian Platform

AbstractIn the early Cambrian fossil record, triradial symmetry is typical for anabaritids and occurs among carinachitids. The former are an extinct group of minute benthic cnidarians covered with a calcareous tubular exoskeleton. The origin of the anabaritids is poorly understood, but previously reported triradial pyramid-shaped steinkerns and molds of the oldest conulariids, Vendoconularia, from the upper Ediacaran of the White Sea region suggested the anabaritids were closely related to conulariids. However, triradial symmetry could originate independently in different lineages in the late Ediacaran and early Cambrian. Herein we describe a new taxon, Ilankirus kessyusensis new genus new species, from the base of the Cambrian Stage 2 of the Olenek Uplift (Siberian Platform). These fossils occur as ornamented steinkerns in the shape of trilateral pyramids and lack any relics of a mineralized exoskeleton. Abundant plastic deformations and fractures of the casts suggest the organism was weakly if at all mineralized. The steinkerns are encrusted with a thin patina of iron-rich chlorite (chamosite) formed because of a multistage diagenetic replacement of authigenic glauconite (glauconite–berthierine–chamosite) under reducing conditions of oxygen-depauperate pore- and seawater. Both lacking two major autapomorphies of the crown-group conulariids (mineralized periderm and quadrate cross section of the oral region of the periderm), the late Ediacaran triradial Vendoconularia and Terreneuvian Ilankirus represent stem-group conulariids.UUID: http://zoobank.org/a2ce04fa-36a5-485a-806c-f13f4749fc7f

Detrital Zircon U‐Pb Geochronology of the Xilin Group: Constraints for the Early Paleozoic Tectonic Evolution of the Songliao Massif

AbstractThe Xilin Group, composed of the Chenming, Laodaomiaogou, Qianshan and Wuxingzhen formations, is one of the Early Paleozoic terranes in the eastern Songliao Massif, mainly consisting of thick layers of fine clastic and carbonate rocks. This study presents LA‐ICP‐MS zircon U‐Pb geochronological data for the Laodaomiaogou and Qianshan formations, further constraining their provenance and the Early Paleozoic tectonic evolution of the Songliao Massif on the eastern Central Asian Orogenic Belt. Most zircons from the Laodaomiaogou and Qianshan formations show magmatic oscillatory zoning and high Th/U ratios (0.26–2.41). Zircon U‐Pb dating results indicate that the detrital zircons from the silty mudstone of the Laodaomiaogou Formation yield peak ages of 634 Ma, 775 Ma, 820 Ma, 880 Ma and 927 Ma, as well as multi‐episodic Archean to Paleoproterozoic and Mesoproterozoic ages (1405–643 Ma), implying its deposition time is younger than ∼634 Ma. Furthermore, the occurrence of Early Cambrian fossils indicates that the Laodaomiaogou Formation was deposited during the late stage of the Early Cambrian (∼514 Ma). The zircons from the K‐bentonite of the Qianshan Formation show four peak ages of 444 Ma, 471 Ma, 489 Ma and 518 Ma and the youngest age peak of 444 ± 4 Ma (n= 6) indicates that the Qianshan Formation was deposited during the Late Ordovician. In addition, the peak ages of the detrital zircons in the silty mudstone of the Qianshan Formation are 472 Ma and 498 Ma, as well as two other concordant points with207Pb/206Pb apparent ages of 1824 Ma and 1985 Ma. The dating results in this study, together with published data, indicate the absence of Pan‐African magmatic events in the Songliao Massif prior to the initial deposition of the Xilin Group, in contrast to those distributed widely in the Jiamusi Massif. Taken together, we conclude that the depositional provenance of the Laodaomiaogou and Qianshan formations was derived from the Songliao Massif. Furthermore, the characteristics of the detrital zircon age composition and rock associations indicate that the Laodaomiaogou Formation formed in a passive continental margin environment, in contrast to the Qianshan Formation, which formed in an active continental margin environment. The above results also imply that the Songliao and Jiamusi massifs might not have collided before the Late Ordovician.

Trace fossils, algae, invertebrate remains and new U-Pb detrital zircon geochronology from the lower Cambrian Torneträsk Formation, northern Sweden

ABSTRACT Nineteen ichnotaxa, together with algal and invertebrate remains, and various pseudo-traces and sedimentary structures are described from the Torneträsk Formation exposed near Lake Torneträsk, Lapland, Sweden, representing a marked increase in the diversity of biotic traces recorded from this unit. The “lower siltstone” interval of the Torneträsk Formation contains mostly simple pascichnia, fodinichnia and domichnia burrows and trails of low-energy shoreface to intertidal settings. The assemblage has very few forms characteristic of high-energy, soft-sediment, foreshore or upper shoreface environments (representative of the Skolithos ichnofacies). Uranium-lead (U-Pb) LA-ICPMS analysis of zircon from a thin claystone layer within the “lower siltstone” interval yielded a maximum depositional age of 584 ± 13 Ma, mid-Ediacaran. Most of the zircon is represented by rounded detrital grains that yield dates between 3.3 and 1.0 Ga. Although the age of the basal sandstone-dominated interval of the Torneträsk Formation remains elusive owing to the absence of fossils, the ichnofossil suite from the overlying “lower siltstone” interval lacks deep arthropod trackways, such as Rusophycus and Cruziana, and is suggestive of a very early (Terreneuvian, possibly Fortunian) Cambrian age. The ichnofauna is otherwise similar to early Cambrian trace fossil assemblages from other parts of Baltica, regions further south in modern Europe, and from Greenland.

Precambrian and early Cambrian palaeobiology of India: Quo Vadis

Significance of Precambrian and early Cambrian palaeobiological remains recorded over the last few years on the Indian subcontinent are discussed in the present review. The importance of these remains is assessed in the backdrop of new reports from different parts of the world. Archaean, Proterozoic and early Cambrian successions of India have revealed some of the oldest rock fragments of the world, most primitive eukaryotic remains, bizarre Ediacaran forms and typical early Cambrian fossils. For the sake of brevity and easy comprehension, palaeobiological remains are grouped under Archaean, Proterozoic, Ediacaran and early Cambrian time-frames. New evidence of life forms, documented from the sedimentary basins exposed in peninsular and extra-peninsular region of the country, have challenged the various paradigms, viz. evolutionary, biostratigraphic status of different lithounits and age of the encompassing sedimentary successions. Newly published geochronological and geochemical results are also considered in this review to understand the developments in these fields and their significance in the evolution of hydrosphere, biosphere, and moderating processes over the geological timescale. Recent Indian endeavors in the field of Astrobiology and inputs provided in the areas of astrobiological studies are also discussed.

Detrital zircon U–Pb geochronology of Xilin Group: Constraints for the early Paleozoic tectonic evolution of the Songliao Massif

The Xilin Group, composed of the Chenming, Laodaomiaogou, Qianshan and Wuxingzhen formations, is one of the Early Paleozoic terranes in the eastern Songliao Massif, mainly consisting of thick layers of fine clastic and carbonate rocks. This study presents LA-ICP-MS zircon U-Pb geochronological data for the Laodaomiaogou and Qianshan formations, further constraining their provenance and the Early Paleozoic tectonic evolution of the Songliao Massif on the eastern Central Asian Orogenic Belt. Most zircons from the Laodaomiaogou and Qianshan formations show magmatic oscillatory zoning and high Th/U ratios (0.26–2.41). Zircon U-Pb dating results indicate that the detrital zircons from the silty mudstone of the Laodaomiaogou Formation yield peak ages of 634 Ma, 775 Ma, 820 Ma, 880 Ma and 927 Ma, as well as multi-episodic Archean to Paleoproterozoic and Mesoproterozoic ages (1405–643 Ma), implying its deposition time is younger than ∼634 Ma. Furthermore, the occurrence of Early Cambrian fossils indicates that the Laodaomiaogou Formation was deposited during the late stage of the Early Cambrian (∼514 Ma). The zircons from the K-bentonite of the Qianshan Formation show four peak ages of 444 Ma, 471 Ma, 489 Ma and 518 Ma and the youngest age peak of 444 ± 4 Ma (n = 6) indicates that the Qianshan Formation was deposited during the Late Ordovician. In addition, the peak ages of the detrital zircons in the silty mudstone of the Qianshan Formation are 472 Ma and 498 Ma, as well as two other concordant points with 207Pb/206Pb apparent ages of 1824 Ma and 1985 Ma. The dating results in this study, together with published data, indicate the absence of Pan-African magmatic events in the Songliao Massif prior to the initial deposition of the Xilin Group, in contrast to those distributed widely in the Jiamusi Massif. Taken together, we conclude that the depositional provenance of the Laodaomiaogou and Qianshan formations was derived from the Songliao Massif. Furthermore, the characteristics of the detrital zircon age composition and rock associations indicate that the Laodaomiaogou Formation formed in a passive continental margin environment, in contrast to the Qianshan Formation, which formed in an active continental margin environment. The above results also imply that the Songliao and Jiamusi massifs might not have collided before the Late Ordovician.

Documentation by citizen scientists/naturalists of the ‘Cambrian explosion’ in Pennsylvania

For more than 125 years, teachers and amateur naturalists have sought early Cambrian fossils, emerging from scattered, unimpressive outcrops of shale in southeastern Pennsylvania. Prizing their finds, they have regularly made their most scientifically significant specimens available to professionals for study and incorporation into museum collections. This activity has played a key part in establishing the fauna of the Kinzers Formation as an important record of the early Cambrian diversification of marine life. Today, a new generation of collectors is building on this tradition, continuing to discover new and evolutionarily significant fossils.

The 'biomineralization toolkit' and the origin of animal skeletons.

Biomineralized skeletons are widespread in animals, and their origins can be traced to the latest Ediacaran or early Cambrian fossil record, in virtually all animal groups. The origin of animal skeletons is inextricably linked with the diversification of animal body plans and the dramatic changes in ecology and geosphere-biosphere interactions across the Ediacaran-Cambrian transition. This apparent independent acquisition of skeletons across diverse animal clades has been proposed to have been driven by co-option of a conserved ancestral genetic toolkit in different lineages at the same time. This 'biomineralization toolkit' hypothesis makes predictions of the early evolution of the skeleton, predictions tested herein through a critical review of the evidence from both the fossil record and development of skeletons in extant organisms. Furthermore, the distribution of skeletons is here plotted against a time-calibrated animal phylogeny, and the nature of the deep ancestors of biomineralizing animals interpolated using ancestral state reconstruction. All these lines of evidence point towards multiple instances of the evolution of biomineralization through the co-option of an inherited organic skeleton and genetic toolkit followed by the stepwise acquisition of more complex skeletal tissues under tighter biological control. This not only supports the 'biomineralization toolkit' hypothesis but also provides a model for describing the evolution of complex biological systems across the Ediacaran-Cambrian transition.

A treasure trove of Cambrian secrets

Cambrian Explosion Animal life exploded in diversity and form during the Cambrian period about 500 million years ago. Fu et al. describe an early Cambrian fossil site in China that contains a variety of specimens, more than half of which are previously undescribed (see the Perspective by Daley). The site rivals previously described Cambrian sites, such as the Burgess Shale, and should help to elucidate biological innovation and diversification during this period. Science , this issue p. [1338][1]; see also p. [1284][2] [1]: /lookup/doi/10.1126/science.aau8800 [2]: /lookup/doi/10.1126/science.aaw8644

The Qingjiang biota-A Burgess Shale-type fossil Lagerstätte from the early Cambrian of South China.

Burgess Shale-type fossil Lagerstätten provide the best evidence for deciphering the biotic patterns and magnitude of the Cambrian explosion. Here, we report a Lagerstätte from South China, the Qingjiang biota (~518 million years old), which is dominated by soft-bodied taxa from a distal shelf setting. The Qingjiang biota is distinguished by pristine carbonaceous preservation of labile organic features, a very high proportion of new taxa (~53%), and preliminary taxonomic diversity that suggests it could rival the Chengjiang and Burgess Shale biotas. Defining aspects of the Qingjiang biota include a high abundance of cnidarians, including both medusoid and polypoid forms; new taxa resembling extant kinorhynchs; and abundant larval or juvenile forms. This distinctive composition holds promise for providing insights into the evolution of Cambrian ecosystems across environmental gradients.

Northwest Africa’s Ediacaran to early Cambrian fossil record, its oldest metazoans and age constraints for the basal Taroudant Group (Morocco)

The Anti-Atlas area in southern Morocco preserves one of the most complete latest Ediacaran to Cambrian shallow-marine carbonate records in the world and serves as a standard for global chemostratigraphic δ13Ccarb curves. Contrary to the excellent chemostratigraphic coverage, neither metazoan fossils (only stromatolites and calcified algae) nor radiometric age constraints were hitherto available for the basal part of this carbonate record, the Tifnout Member of the Adoudou Formation of the Taroudant Group. We report an Ediacara-type fauna from shallow-marine sandstones from the basal Tabia Member and microfossils from dolostones from both the Tabia and the Tifnout members of the Adoudou Formation. The former are discoidal fossils and share many characteristics with the form genus Aspidella and exhibit a peculiar style of preservation, pointing towards an early diagenetic two-stage (carbonate followed by silica) cementation history, leading to good preservation of these supposedly once soft-bodied organisms. The dolostone-hosted microfossils are preserved as sparite-filled molds enclosed by a thin opaque envelope. Despite their small size, they resemble late Ediacaran calcifying metazoans such as Namacalathus and tubular small shelly fossils. Together, these fossils constitute the oldest known direct evidence for presumably metazoan life from Northwest Africa. We also report new time constraints for the Adoudou Formation and fossils enclosed therein based on combined LA-ICP-MS and high-precision CA-ID-TIMS U-Pb zircon geochronology of detrital and volcanic zircons. Together with new chemostratigraphic (δ13Ccarb) and literature data, these new age constraints suggest a late Ediacaran age (time span between ca. 561 and 542 Ma) for the discoidal Ediacara-type fossils from the basal Tabia Member, and a latest Ediacaran to earliest Cambrian (time span between ca. 544 and 540 Ma) and an early Cambrian age (time span between ca. 528 and 523 Ma) for the two microfossil faunas from the Tifnout Member.

Earliest ontogeny of early Cambrian acrotretoid brachiopods \u2014 first evidence for metamorphosis and its implications

BackgroundOur understanding of the ontogeny of Palaeozoic brachiopods has changed significantly during the last two decades. However, the micromorphic acrotretoids have received relatively little attention, resulting in a poor knowledge of their ontogeny, origin and earliest evolution. The uniquely well preserved early Cambrian fossil records in South China provide a great new opportunity to investigate the phylogenetically important ontogeny of the earliest acrotretoid brachiopods, and give new details of the dramatic changes in anatomy of acrotretoid brachiopods during the transition from planktotrophic larvae to filter feeding sedentary juveniles.ResultsWell preserved specimens of the earliest Cambrian acrotretoid brachiopods Eohadrotreta zhenbaensis and Eohadrotreta? zhujiahensis (Cambrian Series 2, Shuijingtuo Formation, Three Gorges area, South China) provide new insights into early acrotretoid ontogeny, and have significance for elucidating the poorly understood early phylogeny of the linguliform brachiopods. A more comprehensive understanding of the applied terminology based on new observation, especially in definition of the major growth stages (embryo, planktotrophic larva, post-metamorphically sessile juvenile and adult), is established. The so-called acrotretoid “larval shell” of both valves of Eohadrotreta demonstrates evidence for metamorphosis (shedding of the larval setae and transitions of shell secretion), during the planktotrophic stage. Therefore, it is here termed the metamorphic shell. The inferred early acrotretoid larval body plan included a bivalved protegulum, secreted at the beginning of the pelagic stage, which later developed two pairs of larval dorsal setal sacs and anterior–posterior alignment of the gut during metamorphosis.ConclusionThe primary larval body plan of acrotretoid Eohadrotreta is now known to have been shared with most early linguliforms and their relatives (including paterinates, siphonotretoids, early linguloids, the problematic mickwitziids, as well as many early rhynchonelliforms). It is suggested that this type of earliest ontogeny can be considered as plesiomorphic for the Brachiopoda and probably first evolved in stem group brachiopods with subsequent heterochronic changes.

The early Cambrian fossil embryo Pseudooides is a direct-developing cnidarian, not an early ecdysozoan.

Early Cambrian Pseudooides prima has been described from embryonic and post-embryonic stages of development, exhibiting long germ-band development. There has been some debate about the pattern of segmentation, but this interpretation, as among the earliest records of ecdysozoans, has been generally accepted. Here, we show that the ‘germ band’ of P. prima embryos separates along its mid axis during development, with the transverse furrows between the ‘somites’ unfolding into the polar aperture of the ten-sided theca of Hexaconularia sichuanensis, conventionally interpreted as a scyphozoan cnidarian; co-occurring post-embryonic remains of ecdysozoans are unrelated. We recognize H. sichuanensis as a junior synonym of P. prima as a consequence of identifying these two form-taxa as distinct developmental stages of the same organism. Direct development in P. prima parallels the co-occuring olivooids Olivooides, and Quadrapyrgites and Bayesian phylogenetic analysis of a novel phenotype dataset indicates that, despite differences in their tetra-, penta- and pseudo-hexa-radial symmetry, these hexangulaconulariids comprise a clade of scyphozoan medusozoans, with Arthrochites and conulariids, that all exhibit direct development from embryo to thecate polyp. The affinity of hexangulaconulariids and olivooids to extant scyphozoan medusozoans indicates that the prevalence of tetraradial symmetry and indirect development are a vestige of a broader spectrum of body-plan symmetries and developmental modes that was manifest in their early Phanerozoic counterparts.

Transient deep-water oxygenation in the early Cambrian Nanhua Basin, South China

Many late Neoproterozoic to early Cambrian fossils of multicellular eukaryotes, including those of benthic animals, are found preserved under anoxic and even euxinic bottom-water conditions, which is contradictory to the consensus that oxygen is essential to eukaryotes. To investigate this conundrum, we conducted an integrated study of iron speciation, redox-sensitive trace elements, and Mo isotopes (δ98Mo) on the black shale interval of the lower Cambrian Hetang Formation (∼535–521Ma) at Lantian, South China, in which benthic sponge fossils are abundant in the lower member (LM) but absent in the upper member (UM). Iron speciation data point to uniformly anoxic-ferruginous conditions in the LM and euxinic conditions in the UM, whereas the trace-element and δ98Mo data show greater secular variation in redox conditions. The LM shows higher mean trace element concentrations (Mo: 108ppm, U: 36ppm, V: 791ppm) and lower and more variable δ98Mo (+0.13 to +1.76‰) relative to the UM (Mo: 45ppm, U: 18ppm, V: 265ppm, δ98Mo: +1.59 to +1.67‰), and ratios of redox-sensitive trace element concentrations to total organic carbon are significantly more variable and higher on average in the LM relative to the UM. The appearance of sponge fossils and lower δ98Mo values correlate strongly with gray (i.e., lighter-colored) layers in the LM. These patterns can best be interpreted as recording mainly euxinic conditions throughout deposition of the study units, with more intense background euxinia in the LM relative to the UM, but also with frequent transient oxygenation events in the LM that do not appear in the UM. The transient oxygenation events of the LM led to the initial colonization of the deep Nanhua Basin by sponges, and the termination of these events in the UM caused sponge faunas to disappear until a general rise in O2 levels later in the Cambrian permitted their return to deeper-water habitats. Our study also illustrates that multiple geochemical and paleobiological proxies exhibit different responses in ‘poikiloredox’ environments (i.e., characterized by small-scale spatial and high-frequency temporal variations), which can lead to apparent contradictions between metazoan fossil occurrences and their inferred watermass redox conditions.

Arthropod eyes: The early Cambrian fossil record and divergent evolution of visual systems

Four types of eyes serve the visual neuropils of extant arthropods: compound retinas composed of adjacent facets; a visual surface populated by spaced eyelets; a smooth transparent cuticle providing inwardly directed lens cylinders; and single-lens eyes. The first type is a characteristic of pancrustaceans, the eyes of which comprise lenses arranged as hexagonal or rectilinear arrays, each lens crowning 8–9 photoreceptor neurons. Except for Scutigeromorpha, the second type typifies Myriapoda whose relatively large eyelets surmount numerous photoreceptive rhabdoms stacked together as tiers. Scutigeromorph eyes are facetted, each lens crowning some dozen photoreceptor neurons of a modified apposition-type eye. Extant chelicerate eyes are single-lensed except in xiphosurans, whose lateral eyes comprise a cuticle with a smooth outer surface and an inner one providing regular arrays of lens cylinders. This account discusses whether these disparate eye types speak for or against divergence from one ancestral eye type. Previous considerations of eye evolution, focusing on the eyes of trilobites and on facet proliferation in xiphosurans and myriapods, have proposed that the mode of development of eyes in those taxa is distinct from that of pancrustaceans and is the plesiomorphic condition from which facetted eyes have evolved. But the recent discovery of enormous regularly facetted compound eyes belonging to early Cambrian radiodontans suggests that high-resolution facetted eyes with superior optics may be the ground pattern organization for arthropods, predating the evolution of arthrodization and jointed post-protocerebral appendages. Here we provide evidence that compound eye organization in stem-group euarthropods of the Cambrian can be understood in terms of eye morphologies diverging from this ancestral radiodontan-type ground pattern. We show that in certain Cambrian groups apposition eyes relate to fixed or mobile eyestalks, whereas other groups reveal concomitant evolution of sessile eyes equipped with optics typical of extant xiphosurans. Observations of fossil material, including that of trilobites and eurypterids, support the proposition that the ancestral compound eye was the apposition type. Cambrian arthropods include possible precursors of mandibulate eyes. The latter are the modified compound eyes, now sessile, and their underlying optic lobes exemplified by scutigeromorph chilopods, and the mobile stalked compound eyes and more elaborate optic lobes typifying Pancrustacea. Radical divergence from an ancestral apposition type is demonstrated by the evolution of chelicerate eyes, from doublet sessile-eyed stem-group taxa to special apposition eyes of xiphosurans, the compound eyes of eurypterids, and single-lens eyes of arachnids. Different eye types are discussed with respect to possible modes of life of the extinct species that possessed them, comparing these to extant counterparts and the types of visual centers the eyes might have served.

Skeletal Microstructures of the ChitonIschnochiton hakodadensisand a Comparison with the CambrianHalkieria

Halkieria is an Early Cambrian fossil genus with proposed phylogenetic affinities ranging from stem group lophotrochozoan to stem-group aculiferan. Skeletal similarities are often cited as evidence in support of a mollusk interpretation for Halkieria and related Cambrian fossils. The skeletal microstructures of the polyplacophoran mollusk (chiton) Ischnochiton hakodadensis, which bears perinotum elements resembling sclerites of halkieriids and other coeloscleritophoran small shelly fossils (SSFs), were studied to evaluate the interpretations. There are both similarities and differences between chiton skeletal structures and halkieriid sclerites: 1) the head valve of I. hakodadensis and the dorsal shell of Halkieria evangelista share a similar pattern of fine concentric growth lines, but halkieriid dorsal shells lack other polyplacophoran valve characteristics, including V-shaped indentation, radiate ribs; 2) the arrangement of I. hakodadensis perinotum elements in three zones is somewhat similar to the arrangement pattern of halkieriid sclerites; 3) the inner perinotum scales of I. hakodadensis (∼200 μm in diameter) have a base and a blade, remarkably similar in morphology to halkieriid palmate sclerites (∼400 μm in diameter). However, I. hakodadensis perinotum scales are nearly solid whereas halkieriid sclerites are hollow. These observations add a microstructural dimension to the skeletal comparison between mollusks and halkeriids, and may prove to be useful in resolving the phylogenetic affinity of Halkieria and the Halwaxiidae.

Specialized appendages in fuxianhuiids and the head organization of early euarthropods

The organization of the head provides critical data for resolving the phylogenetic relationships and evolutionary history of extinct and extant euarthropods. The early Cambrian-period fuxianhuiids are regarded as basal representatives of stem-group Euarthropoda, and their anterior morphology therefore offers key insights for reconstructing the ancestral condition of the euarthropod head. However, the paired post-antennal structures in Fuxianhuia protensa remain controversial; they have been interpreted as both 'great appendages' and as gut diverticulae. Here we describe Chengjiangocaris kunmingensis sp. nov. and Fuxianhuia xiaoshibaensis sp. nov. from a new early Cambrian (Stage 3) fossil Lagerstätte in Yunnan, China. Numerous specimens of both species show a unique 'taphonomic dissection' of the anterodorsal head shield, revealing the cephalic organization in detail. We demonstrate the presence of a pair of specialized post-antennal appendages (SPAs) in the fuxianhuiid head, which attach at either side of the posteriorly directed mouth, behind the hypostome. Preserved functional articulations indicate a well-defined but restricted range of limb movement, suggestive of a simple type of sweep feeding. The organization of the SPAs in fuxianhuiids is incompatible with the (deutocerebral) anterior raptorial appendages of megacheirans, and argue against the presence of protocerebral limbs in the fuxianhuiids. The positions of the fuxianhuiid antennae and SPAs indicate that they are segmentally homologous to the deutocerebral and tritocerebral appendages of crown-group Euarthropoda respectively. These findings indicate that antenniform deutocerebral appendages with many podomeres are a plesiomorphic feature of the ancestral euarthropod head.

The problematic early Cambrian fossil Tumulduria incomperta represents the detached ventral interarea of a paterinid brachiopod

The organophosphatic early Cambrian (Terreneuvian, Cambrian Stage 2) fossil Tumulduria incomperta has been problematic ever since its original description in 1969. Comparison of abundant specimens from the Lower Cambrian of Siberia with co-occurring brachiopod valves show that T. incomperta represents the central portion of the ventral interarea of a paterinid brachiopod similar to Cryptotreta neguertchenensis, and that the domed central portion of typical Tumulduria specimens represents the ridge-like pseudodeltidium of the interarea.