Abstract:Identification of the primary constituents of small shelly fossil (SSF) shells is important for explaining the evolution of SSF faunas. The characteristics and constituents of Terreneuvian tubular SSFs found in north‐east Sichuan, China, are revealed using light microscopy, scanning electron microscopy and energy dispersive X‐ray spectroscopy. Petrographic thin sections indicate that the chemical composition of the shells is mainly calcium carbonate with smaller amounts of phosphorus, silica and pyrite. Most o… Show more
“…1). Figure 1.Map of localities of SSF-bearing sections in northeast Sichuan, China (modified from Pang et al, 2010, 2017b). …”
Section: Geological Settingmentioning
confidence: 99%
“…Cambrian small shelly fossils of the Yangtze Platform generally have undergone diagenesis, e.g., secondary phosphatization (Zhu et al, 1996;Chen et al, 2016;Pang et al, 2017b;Ji et al, 2019). Therefore, their primary shell ultrastructure is mostly obliterated by diagenesis and generally the soft tissues have decayed.…”
Section: Morphological Terminology and Featuresmentioning
Cap-shaped skeletal fossils are the earliest undisputed body fossils of mollusks appearing in the basal Cambrian. A study on the morphometry of cap-shaped fossils from the Nanjiang area (North Sichuan, China) is undertaken to understand the origin and evolution of the early mollusks. The distribution of these fossil cap-shaped mollusks indicates a stepwise increase in their diversity during the early Cambrian. Maikhanella Zhegallo in Voronin et al., 1982 co-occuring with the spinose sclerites of siphogonuchitids, is regarded as the earliest scleritized mollusk. It is followed by other maikhanellids, e.g., Purella Missarzhevsky, 1974 and Yunnanopleura Yu, 1987, which co-occur with the earliest univalved helcionellids, e.g., Igorella Missarzhevsky in Rozanov et al., 1969. Cluster analysis of their morphometric characteristics shows that the Maikhanella group is similar to the Purella and Yunnanopleura groups, but is less comparable with univalved helcionellids. The maikhanellids are interpreted as representatives of the stem group Aculifera, although it remains uncertain if one or two larger cap-shaped shell plates were present on the elongate slug-like body, comparable to those of Halkieria Poulsen, 1967 or Orthrozanclus Conway Morris and Caron, 2007. Maikhanellids are characterized by the prominent protrusions or scales on the cap-shaped shell plates arranged in a concentric pattern around the shell apex. Evolutionarily, the protrusions or scales are reduced in younger strata, whereas subsequently a typically concentric ornament developed, the cap-shaped shell plates developed higher profiles, and the apical region became increasingly bare of scales. Meanwhile, the cap-shaped shell plates gradually evolved into a helcionellid-like appearance with an anteroposteriorly inclined apex. The morphological evolution of the earliest sclerotized mollusks reflects biotic evolution and environmental adaption among the stem-group mollusks during the early Cambrian.
“…1). Figure 1.Map of localities of SSF-bearing sections in northeast Sichuan, China (modified from Pang et al, 2010, 2017b). …”
Section: Geological Settingmentioning
confidence: 99%
“…Cambrian small shelly fossils of the Yangtze Platform generally have undergone diagenesis, e.g., secondary phosphatization (Zhu et al, 1996;Chen et al, 2016;Pang et al, 2017b;Ji et al, 2019). Therefore, their primary shell ultrastructure is mostly obliterated by diagenesis and generally the soft tissues have decayed.…”
Section: Morphological Terminology and Featuresmentioning
Cap-shaped skeletal fossils are the earliest undisputed body fossils of mollusks appearing in the basal Cambrian. A study on the morphometry of cap-shaped fossils from the Nanjiang area (North Sichuan, China) is undertaken to understand the origin and evolution of the early mollusks. The distribution of these fossil cap-shaped mollusks indicates a stepwise increase in their diversity during the early Cambrian. Maikhanella Zhegallo in Voronin et al., 1982 co-occuring with the spinose sclerites of siphogonuchitids, is regarded as the earliest scleritized mollusk. It is followed by other maikhanellids, e.g., Purella Missarzhevsky, 1974 and Yunnanopleura Yu, 1987, which co-occur with the earliest univalved helcionellids, e.g., Igorella Missarzhevsky in Rozanov et al., 1969. Cluster analysis of their morphometric characteristics shows that the Maikhanella group is similar to the Purella and Yunnanopleura groups, but is less comparable with univalved helcionellids. The maikhanellids are interpreted as representatives of the stem group Aculifera, although it remains uncertain if one or two larger cap-shaped shell plates were present on the elongate slug-like body, comparable to those of Halkieria Poulsen, 1967 or Orthrozanclus Conway Morris and Caron, 2007. Maikhanellids are characterized by the prominent protrusions or scales on the cap-shaped shell plates arranged in a concentric pattern around the shell apex. Evolutionarily, the protrusions or scales are reduced in younger strata, whereas subsequently a typically concentric ornament developed, the cap-shaped shell plates developed higher profiles, and the apical region became increasingly bare of scales. Meanwhile, the cap-shaped shell plates gradually evolved into a helcionellid-like appearance with an anteroposteriorly inclined apex. The morphological evolution of the earliest sclerotized mollusks reflects biotic evolution and environmental adaption among the stem-group mollusks during the early Cambrian.
“…Pang et al . () described what they interpreted as partially preserved prismatic microstructures from cylindrical fossils they suggested may be orthothecimorph hyoliths. Finally, there have also been reports of orthothecimorph conchs that consistently exfoliate into two or three distinct layers, sometimes with differing ornamentation (Ford ; Walcott , ; Malinky ).…”
Section: Previous Work On Hyolith Microstructurementioning
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.
“…The focus of most research into SSF-style preservation has been Cambrian SSF occurrences (Creveling et al, 2014 a, b;Pang et al, 2017). The Cambrian prevalence of this style of preservation has been linked to unusual oceanographic conditions during a "phosphatization window" (Porter, 2004;Donoghue et al, 2006;Creveling et al, 2014 a, b) that closed during the Furongian (Porter, 2004), immediately before the onset of events that led to the Great Ordovician Biodiversification Event (GOBE; e.g.…”
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
