Shell microstructure of the early bivalve Pojetaia and the independent origin of nacre within the mollusca

Abstract: Pojetaia and Fordilla are the oldest bivalve molluscs, occurring in roughly co‐eval rocks from the Tommotian, and are the only undisputed, well‐known bivalves from the Cambrian. New specimens reveal that Pojetaia had a laminar inner shell microstructure reminiscent of the foliated aragonite of modern monoplacophorans, and the same is true for Fordilla. A similar shell microstructure is seen in Anabarella and Watsonella, providing support for the hypothesis that they are the ancestors of bivalves. Foliated aragonite shares many similarities with nacre, and it may have been the precursor to nacre in bivalves. No cases of undisputed nacre occur in the Cambrian, in spite of much shell microstructure data from molluscs of this time period. Thus, although considered by many to be homologous among molluscs, we conclude that nacre convergently evolved in monoplacophorans, gastropods, bivalves, and cephalopods. This independent origin of nacre appears to have taken place during, or just prior to, the Great Ordovician Biodiversification Event and represents a significant step in the arms race between predators and molluscan prey.     

Problematic cap-shaped fossils from the Lower Cambrian of North-East Greenland

Three species ofOcruranus Liu, 1979 are described from the Bastion Formation of North-East Greenland, of late Early Cambrian (middle Dyeran of North American usage) age, representing the youngest record of a genus originally described from the earliest Cambrian Meishucunian Stage of China. An accompanying species, tentatively assigned toXianfengella He &Yang, 1982, seems also to be present in South Australia in strata of late Early Cambrian (Botoman of Siberian usage) age, although also this genus was described originally from the Meishucunian.Ocruranus andXianfengella from China have been interpreted as possibly parts of coeloscleritophoran scleritomes, perhaps halkieriids, rather than individual molluscan shells. Their shell form is not typical of helcionelloids which otherwise dominate the Early Cambrian molluscan record, but conclusive evidence of affinity is not forthcoming from the Greenland records. New taxa:Ocruranus septentrionalis n. sp. andOcruranus tunuensis n. sp.      

New data on molluscs and their shell microstructures from the Middle Cambrian Gowers Formation, Australia

Abstract: Numerous new cases of preserved shell microstructure were discovered in molluscs from the Middle Cambrian Gowers Formation (Ptychagnostus atavus/Peronopsis opimus Zone, Floran Stage) in the Georgina Basin, Australia. The new data provide further evidence that, by the Middle Cambrian, molluscan shell microstructures were diverse, and many molluscs had a complex shell with multiple types of shell microstructure. In addition, many new occurrences of laminar microstructures are described herein. For many, the nature of these laminar microstructures is not known, but in three species the microstructure is foliated calcite, and in at least two the microstructure is more likely to have been calcitic semi‐nacre, a type of microstructure known in brachiopods and bryozoans but unknown in modern molluscs. This commonality among these three closely related lophotrochozoans underscores a similar mechanism of biomineralization. Moreover, these observations suggest a prevalence of calcite‐shelled lineages among molluscs from the Middle Cambrian, a time of calcite seas. In addition, the broad occurrence of laminar, nacre‐like microstructures in many of these fossils reveals how widespread these strong (fracture‐resistant) microstructures were in Middle Cambrian molluscs. Additionally, a few specimens of Yochelcionella preserve imprints of a bilaterally symmetrical pair of muscle scars. New taxa described here include Corystos thorntoniensis gen. et sp. nov., Yochelcionella snorkorum sp. nov., Yochelcionella saginata sp. nov., and Anhuiconus? agrenon sp. nov.     

Paterimitra pyramidalis from South Australia: scleritome,shell structure and evolution of a lower Cambrian stem group brachiopod

The tommotiid Paterimitra pyramidalis Laurie, 1986, is redescribed based on well‐preserved material from the lower Cambrian Wilkawillina, Wirrapowie and Ajax limestones of the Flinders Ranges, South Australia. The material shows that the scleritome of Paterimitra pyramidalis includes three sclerite morphotypes (S1, S2 and L). Detailed shell microstructure studies show striking similarities with both the paterinid brachiopod Askepasma toddense and the tommotiid Eccentrotheca helenia, which strengthens the suggested evolutionary link between tommotiids and brachiopods. Based on the partly articulated specimens and similarities in shell microstructure and sclerite morphology with Eccentrotheca, Paterimitra pyramidalis is reconstructed as a tube‐dwelling, epifaunal, sessile, filter‐feeder with an organic pedicle‐like attachment structure. The proposed reconstruction of the scleritome comprises a basal unit composed of one S1 and one S2 sclerite, as well as an unresolved number of L sclerites lining a coniform tubular structure.     

THE CANAL SYSTEM IN SCLERITES OF LOWER CAMBRIAN SINOSACHITES (HALKIERIIDAE: SACHITIDA): SIGNIFICANCE FOR THE MOLLUSCAN AFFINITIES OF THE SACHITIDS

Abstract:  The halkieriids (Sachitida He, 1980 ) from the Early to Mid Cambrian possess a hollow sclerite with a complex branching canal system. An analysis of the canal system morphology in the halkieriid Sinosachites ( Thambetolepis ) delicatus ( Jell, 1981 ) from South Australia reveals similarities to the aesthete canal system in the shell plates of chitons, which has been analysed in a number of extant taxa. The compartments, referred to as macro-aesthetes in chitons, and lateral canals in halkieriids, have overlapping diameters and are constrained in morphology by the space of accommodation by maintaining a constant width, whereas length is more variable. Both canal systems are morphologically distinct from shell pores of other lophotrochozoans and known mollusc classes. Similarities in sclerite growth, microstructure and mineralogy further suggest that halkieriids, along with the other sachitids, are molluscs, most likely stem aculiferans (Polyplacophora and Aplacophora).     

Plywood‐like shell microstructures in hyoliths from the middle Cambrian (Drumian) Gowers Formation,Georgina Basin,Australia

Hyoliths are a group of Palaeozoic fossils with calcareous shells whose affinities remain controversial. As their shells were originally aragonitic, their fossils are usually coarsely recrystallized, and few data on their microstructure are available. We report hyoliths from the middle Cambrian (Drumian, Floran) Gowers Formation of the eastern Georgina Basin, Queensland. These are preserved as phosphatic internal moulds, often with the inner layers of the shell also partly replaced by phosphate. Microstructural details preserved by this early diagenetic phosphatization show that these hyolith conchs were originally composed of fibrous crystallites, c. 0.5 μm wide, parallel to one another and to the inner surface of the shell. In several species, the fibres are arranged in a plywood‐like structure composed of multiple lamellae with a different fibre orientation in each lamella: often they are transversely oriented (relative to the long axis of the conch) in the inner part of the wall and longitudinally oriented in the outer part. Opercula also show a microstructure of parallel fibres. The lamello‐fibrillar microstructure we report from hyoliths is reminiscent of microstructures of many Cambrian molluscs; that this microstructure is found in both conchs and opercula suggests that these structures are serial homologues of one another, and in this respect they resemble brachiopod valves. As with many other biological plywoods, the hyolith shell probably records self‐organization in a liquid‐crystal‐like organic matrix. This provided a straightforward way to construct a material that could resist stresses from different directions, offering an effective defence against predators.     

贵州松桃寒武系清虚洞组Mongolitubulus squamifer 壳体显微结构初步研究

本文主要对贵州松桃寒武系清虚洞组灰岩中酸泡获得的管状化石Mongolitubulus squamifer壳体显微及亚显微结构进行研究。研究结果显示, M. squamifer不仅内层广泛发育纵向纤维结构,具鳞片的外层外壁也发育有微弱的纵向纤维结构,可能解释了管体外壁广泛发育的纵向开裂现象。在一枚保存有圆卵形鳞片以下部位的标本中发现,鳞片排列形式是由基部密集的小瘤点状颗粒然后过渡为不规则的圆卵形鳞片,到管体中上部逐渐形成规则排列的三角形鳞片。通过测量部分已报道的M.squamifer管体宽度与鳞片宽度发现正常发育的棘刺,鳞片大小与管体宽度有一定相关性。贵州松桃的M.squamifer壳层原始结构为2层,包括致密的具鳞片的外层和具明显纵向纤维结构的内层,管体内外层之间发育空隙导致管体容易破损。一些标本近基部断口处的层间空隙容易被次生矿物充填,导致内层增厚致密纤维结构消失或产生中间填充层。该研究揭示了M.squamifer的鳞片排列特征和壳体微观结构,为解释其亲缘关系提供新的形态学证据。     

Halkieria系统进化及其化石分类学研究进展

Halkieria是寒武纪一类躯体上披有骨质鳞片的疑难后生动物,前后各有一个大壳板,两侧对称,其钙质骨片在软躯体上排列形成几条规律的纵向带。半个世纪以来,学术界对Halkieria生物系统确切位置的讨论以及Halkieriidae不同属种亲缘关系的探索从未停止。本文总结了Halkieria化石在骨片属种分类、古地理和地层分布、生物系统分类学的研究进展,对当前的生物系统分类归纳出以下几种观点:1) Halkieria为腕足动物的祖先,并与托莫特壳类有一定的亲缘关系;2)Halkieria与Wiwaxia等骨片化石具有单源性,组成一种新的分类单元Halwaxiidae,属于软体动物干群或腕足动物和环节动物的干群;3)Halkieria为软体动物有刺亚门干群,与无板纲、多板纲更为亲近; 4) Halkieria为环节动物的祖先。目前,虽然多数人根据Halkieria与现生多板纲的骨片排列一致将Halkieria归属软体动物有刺亚门的干群,但近年来世界各地新化石材料的陆续发现及壳体发育研究使得Halkieria的系统分类位置出现新的争议。通过对化石分类学的系统厘定和生物分类研究进展的总结梳理,...     

Periodic shell decollation as an ecology-driven strategy in the early Cambrian Cupitheca

Shell decollation is a growth strategy that has been adopted by a number of invertebrate taxa to offset the metabolic and ecological disadvantages of shell growth. However, little is known about the origin and evolution of this process. We here describe well-preserved specimens of the hyolith Cupitheca decollata sp. nov. preserving the decollation process, from the early Cambrian Yu'anshan Formation (c. 518 Ma) of South China. Based on a large number of specimens collectively representing different developmental stages, we use high-resolution X-ray microtomography and scanning electronic microscopy to reconstruct the process of decollation in this taxon. Cupitheca is among the earliest known small shelly fossils, and thus our discovery confirms that periodic decollation had evolved by the onset of the Cambrian explosion, reflecting the high intensity of the predator–prey arms race in early Cambrian ecosystems. A comparison between the decollation processes of Cupitheca and other shelly invertebrates suggests that periodic decollation and the associated molecular mechanisms of calcium dissolution, uptake, allocation and deposition may have had multiple independent origins.     

A new mobergellan (small shelly fossils) from the early middle cambrian of morocco and its significance

A new mobergellan genus and species,Tateltella ranoculata, is described from the early Middle Cambrian (Agdzian Stage) of Morocco. The new taxon is characterized by only four pairs of muscle scars and is furthermore distinguished from other mobergellans by its strongly concave shape and its distally rising muscle scars. The individual specimens ofTateltella ranoculata distinctly vary in size and display different ontogenetic stages. Juvenile, intermediate, and adult stages can be distinguished by means of the development of the muscle scars that differ in Position relative to the apex, size, and distinctness between individual stages. The shell ofT. ranoculata is composed of a succession of thin phosphatic lamellae separated by interlamellar gaps, presumably originally filled by organic material. The interlamellar gaps may be divided by septum-like structures producing discrete cavities. The specimens are the youngest mobergellans known so far and correlation of their stratigraphic position suggests a correspondence with the lower part of the Amgan stage of the Siberian Platform. In addition, they are the first reported mobergellans from the present day continent Africa. Other mobergellan taxa and mobergellan-like species are briefly reviewed and the genusHippoklosma Missarzhevsky, previously assigned to the Mobergellidae, is excluded from the family due to its different shell structure. An evolutionary trend of reduction in the number of muscle scars from 14 in the early Early Cambrian to only eight in the early Middle Cambrian is apparent among mobergellans.      

Correlation of stable oxygen isotope temperature record with light attenuation profiles in reef-dwellingTridacna shells

Molluscs are known to record environmental changes in their carbonate shells in detail. This paper reports the findings of a high-resolution analysis of stable oxygen isotopic compositions and light transmission properties of a shell of the reef-dwelling Pacific giant clamTridacna gigas. Our findings reveal that the annual growth rates and the longevity ofTridacna specimens can be readily determined by measuring the annual light attenuation pattern within the shell. Annual seasonal changes in water temperature are reflected with high resolution in the stable oxygen isotope ratios and in the light attenuation values of the aragonite shell. The inner shell ofT. gigas deposited below the pallial line revealing undisturbed shell accretion with high growth rates shows the maximum seasonal oxygen isotope range and the highest resolution in light attenuation changes. We suggest that this is the best part of the shell to reconstruct former seasonal surface water temperatures in tropical environments. Scanning electron microscopy (SEM) studies suggest that the annual growth patterns observed in transmitted light are generated by a complex pattern of daily growth increments with varying sizes of skeletal crystallites and varying amounts of organic carbon.     

STENOTHECOIDA,A PROPOSED NEW CLASS OF CAMBRIAN MOLLUSCA

Cambridium, Bagenovia, and Stenothecoides, composing the Family Cambridiidae, a monotypic superfamily and an order, were in 1960 assigned (although with a query) to the molluscan class Monoplacophora. The basic error of this assignment, according to the author, was the assumption that these specimens are univalves. One specimen from Siberia and a second from Alaska demonstrate that Stenothecoides is bivalved; Bagenovia was first described as a bivalve, but the implication of two valves was ignored. Short internal ridges normal to the shell margin in Cambridium and Stenothecoides, described by Rasetti and Horný, show little resemblance to features of pelecypod shells. These markings are not homologous to paired muscle scars of monoplacophorans. The asymmetric bivalved shell and internal furrows are interpreted as features of class-rank significance; the extinct class Stenothecoida is proposed to accommodate these genera. These animals are most common in Lower Cambrian, but range into Middle Cambrian. They may have been functionally similar to brachiopods, but were unable to compete with those more efficient bivalves.     

A spinose stem group brachiopod with pedicle from the Middle Cambrian Burgess Shale

A new genus and species of a Middle Cambrian stem group brachiopod, Acanthotretella spinosa n. gen. and n. sp., is described from the Burgess Shale Formation. Most of the 42 specimens studied came from the Greater Phyllopod bed (Walcott Quarry) and were collected from five bed assemblages, each representing a single obrution event. Specimens are probably preserved within their original habitat. In contrast to all brachiopods known from the Burgess Shale, the shells of the new stem group brachiopod are often deformed and do not show signs of brittle breakage, which suggests that the valves were originally either entirely organic in composition or, more likely, had just a minor mineral component. Acanthotretella spinosa differs from all the other described Cambrian brachiopods in that it is covered by long, slender and possibly partly mineralized spines that are posteriorly inclined at an oblique angle away from the anterior margin. The spines penetrate the shell and are mainly comparable with the thorn‐like organic objects that have been inferred from early siphonotretoid brachiopods. The pedicle was slender and was composed of a central coelomic region and emerged from an apical foramen at the end of an internal pedicle tube. The finding of a pedicle attached to the macrobenthic algae Dictyophycus and other epibenthos implies that A. spinosa did not have an infaunal mode of life. The visceral region and interior characters are poorly preserved.     

A first report of Sphenothallus Hall, 1847 in the Cambrian of Variscan Europe

Ten specimens of two phosphatic fossils have been recently discovered in lower and middle portions of Middle Cambrian Jince Formation in the Czech Republic. They are attributed to the genus Sphenothallus Hall, 1847 and described as two separate species; comparatively small conchs are described as S. kozaki sp. nov., the much larger specimens characterized by its smooth and partly flexible organo-phosphatic walls of shell are determined as ?S. kordulei sp. nov. Sphenothallus is known to range from Cambrian to Permian and accommodates numerous species. However, its Cambrian distribution is considerably restricted. Generally rare specimens have been described from Lower to Middle Cambrian of Laurentia and from the Lower Cambrian of Gondwana and peri-Gondwana. The new record of Sphenothallus from the Jince Biota represents a notable extension of their geographic range.     

Reconsideration of the supposed naraoiid larva from the Early Cambrian Chengjiang Lagerstätte, South China

The Chengjiang Lagerstätte in the Lower Cambrian of South China yields a small, larva‐like arthropod, which was considered to be a protaspis of naraoiids by many authors. The discovery of a large number of well‐preserved specimens from many new localities has allowed the original study to be revised. The relatively large size, stable morphology and unusual structure of the appendages indicate that these specimens represent adults of a new arthropod, Primicaris larvaformis. The larva‐like outline is considered to have arisen by the heterochronic process of progenesis. In addition, this animal displays primitive aspects of bodyplan and limb morphology that suggest a basal position within arachnomorphs, or perhaps even arthropods, and the similarities to the Vendian arthropod‐like animal Parvancorina probably provide an evolutionary link between Vendian forms and Cambrian arthropods.     

Early Palaeozoic diversification of chitons (Polyplacophora, Mollusca) based on new data from the Silurian of Gotland, Sweden

Isolated, well-preserved silicified sclerites of an unusually diverse Silurian paleoloricate assemblage from Gotland preserve morphological features that are important in interpreting palaeobiology. The typically granular dorsal ornament is comparable with Recent chitons, and is hence possibly linked functionally with sensory aesthetes. Ventral structures particularly in thickened shells indicate major muscle attachment sites sub-apically or, equivalently, beneath the rim of the apical area, and also marginally. In early chitons such as Cambrian Matthevia, deep ventral cavities represent comparable sub-apical sites. Three Gotland genera with an unusual, holoperipheral shell growth style apparently represent plated aplacophorans (cf. Acaenoplax), which coexisted with paleoloricate chitons in shallow inshore carbonate shelf environments. Sclerite features of all the Gotland genera are discussed together since they share most characteristics. The new family Heloplacidae includes those genera and Acaenoplax, which in a preliminary cladistic analysis form a sister group to other Lower Palaeozoic paleoloricates. Multiplated skeletons in paleoloricates and this group of aplacophorans represent parallel evolution of dorsal armour, which in chitons resulted in overlapping, articulating sclerites. The diversity of the mid-Silurian Gotland assemblage is examined against early evolutionary diversification of polyplacophorans, aplacophorans, and in relation to the overall record of Palaeozoic paleoloricate and neoloricate chitons. Peaks in diversity in early Ordovician, mid-Silurian and early Carboniferous times correspond to periods with widespread development of low latitude carbonate shelves. Neoloricates, apparently with an additional shell layer that contributed articulatory plates, appeared in the Devonian from where the fossil record remains poorly known.     

Ecology and phylogenetic affinity of the early Cambrian tubular microfossil Megathrix longus

Abundant, exquisitely preserved specimens of the enigmatic tubular microfossil Megathrix longus are reported from the early Cambrian Yanjiahe Formation in the Yangtze Gorges area. Studies of their morphology and taphonomy reveal that the inner cross‐wall of Megathrix longus is topographically flat, the trichome terminus is blunt and closed, and incomplete cross‐walls are regularly intercalated between complete ones. The deformation of trichomes, as well as the corrugation of their cross‐walls, are postmortem features caused by compaction, rather than biological features as previously interpreted. Statistical analysis indicates that chamber division is accomplished by the closure of incomplete cross‐walls. This process, which contributed to trichome growth, is extremely similar to that of modern Oscillatoriaceae. Trichome fragmentation was also observed in a series of specimens, which closely resembles that of cyanobacteria, particularly the Oscillatoriaceae. The termini of Megathrix longus suggest that it was planktonic. In general, Megathrix longus bears a close affinity to the extant Oscillatoriaceae (Cyanophyta).     

Reappraising the early evidence of durophagy and drilling predation in the fossil record: implications for escalation and the Cambrian Explosion

The Cambrian Explosion is arguably the most extreme example of a biological radiation preserved in the fossil record, and studies of Cambrian Lagerstätten have facilitated the exploration of many facets of this key evolutionary event. As predation was a major ecological driver behind the Explosion – particularly the radiation of biomineralising metazoans – the evidence for shell crushing (durophagy), drilling and puncturing predation in the Cambrian (and possibly the Ediacaran) is considered. Examples of durophagous predation on biomineralised taxa other than trilobites are apparently rare, reflecting predator preference, taphonomic and sampling biases, or simply lack of documentation. The oldest known example of durophagy is shell damage on the problematic taxon Mobergella holsti from the early Cambrian (possibly Terreneuvian) of Sweden. Using functional morphology to identify (or perhaps misidentify) durophagous predators is discussed, with emphasis on the toolkit used by Cambrian arthropods, specifically the radiodontan oral cone and the frontal and gnathobasic appendages of various taxa. Records of drill holes and possible puncture holes in Cambrian shells are mostly on brachiopods, but the lack of prey diversity may represent either a true biological signal or a result of various biases. The oldest drilled Cambrian shells occur in a variety of Terreneuvian‐aged taxa, but specimens of the ubiquitous Ediacaran shelly fossil Cloudina also show putative drilling traces. Knowledge on Cambrian shell drillers is sorely lacking and there is little evidence or consensus concerning the taxonomic groups that made the holes, which often leads to the suggestion of an unknown ‘soft bodied driller’. Useful methodologies for deciphering the identities and capabilities of shell drillers are outlined. Evidence for puncture holes in Cambrian shelly taxa is rare. Such holes are more jagged than drill holes and possibly made by a Cambrian ‘puncher’. The Cambrian arthropod Yohoia may have used its frontal appendages in a jack‐knifing manner, similar to Recent stomatopod crustaceans, to strike and puncture shells rapidly. Finally, Cambrian durophagous and shell‐drilling predation is considered in the context of escalation – an evolutionary process that, amongst other scenarios, involves predators (and other ‘enemies’) as the predominant agents of natural selection. The rapid increase in diversity and abundance of biomineralised shells during the early Cambrian is often attributed to escalation: enemies placed selective pressure on prey, forcing phenotypic responses in prey and, by extension, in predator groups over time. Unfortunately, few case studies illustrate long‐term patterns in shelly fossil morphologies that may reflect the influence of predation throughout the Cambrian. More studies on phenotypic change in hard‐shelled lineages are needed to convincingly illustrate escalation and the responses of prey during the Cambrian.     

Pre-Columbian impact on terrestrial, intertidal, and marine resources, San Salvador, Bahamas (A.D. 950–1500)

Zooarchaeological remains from San Salvador, Bahamas, reveal trends in pre-Columbian exploitation of terrestrial, intertidal, and marine resources during the period A.D. 950–1500. Significant declines in quantities and weights of Gecarcinidae (land crabs), weights of mollusks, such as Cittarium pica (West Indian top shell) and Chiton tuberculatus/Acanthopleura granulata (chitons), and body sizes of both Sparisoma viride (stoplight parrotfish) and Serranidae (sea basses and groupers) all indicate prehistoric overexploitation. Declines in total number of identified marine taxa as well as average trophic levels of exploited marine vertebrates suggest reduced species diversity and “fishing down the marine food web.” These findings indicate that a relatively minor prehistoric human population (500–1000 people) can have significant environmental impacts, especially on small, vulnerable island ecosystems.     

MUMMPIKIA GEN. NOV. AND THE ORIGIN OF CALCITIC-SHELLED BRACHIOPODS

Abstract:  Phosphatised limestone-hosted and shale-hosted specimens of the obolellid Mummpikia nuda gen. et comb. nov. and two further unidentified obolellids from the Early Cambrian Mural Formation (Jasper National Park, Canadian Rocky Mountains) provide novel insights into the shell microstructure of obolellids and the nature of their pedicle. The shell was penetrated by abundant canals of a sub- μ m diameter and the anterior tip of the delthyrium forms a projection into the body cavity that is penetrated by a thin canal. It is argued that both shell microstructure and posterior margin are linguliformean characters and that obolellids hold a position basal in the rhynchonelliform stem-group.