Abstract:The earliest growth of post-metamorphic (postlarval) shells in two species of Eohadrotreta is described from the Cambrian Shuijingtuo Formation of South China. Two different growth patterns can be observed by quantifying developmental variations in size and shape of successive stages of post-metamorphic shell growth (including the pedicle foramen forming stage, pedicle foramen enclosing stage and intertrough increasing stage) of Eohadrotreta zhenbaensis and Eohadrotreta? zhujiahensis. The pedicle foramen is never enclosed within the metamorphic shell of E. zhenbaensis, while the enclosed pedicle foramen of E.? zhujiahensis is located directly outside the metamorphic shell after the pedicle foramen enclosing stage. A strongly allometric growth pattern of E. zhenbaensis is demonstrated by the early enclosure of the pedicle foramen; an accelerated lengthening of the ventral intertrough is associated with the development of a more complex dorsal median septum during the intertrough increasing stage. By contrast, E.? zhujiahensis demonstrates possible paedomorphic development by delayed enclosure of pedicle foramen and an associated decreased lengthening of ventral intertrough during the intertrough increasing stage. This ontogenetic developmental sequence represents the marginal accretionary formation and growth of the pedicle foramen, which resembles that of linguloid brachiopods. Furthermore, the developmental process of the pedicle foramen of Eohadrotreta seems to recapitulate the likely evolutionary transition from the Botsfordiidae, with open delthyrium, to the Acrotheloidea, with an enclosed foramen. This study provides a unique opportunity to obtain a complete understanding of the ontogenetic development of the earliest acrotretoids, and casts new light on the phylogeny of lingulate brachiopods.

Bryozoans (also known as ectoprocts or moss animals) are aquatic, dominantly sessile, filter-feeding lophophorates that construct an organic or calcareous modular colonial (clonal) exoskeleton1–3. The presence of six major orders of bryozoans with advanced polymorphisms in lower Ordovician rocks strongly suggests a Cambrian origin for the largest and most diverse lophophorate phylum2,4–8. However, a lack of convincing bryozoan fossils from the Cambrian period has hampered resolution of the true origins and character assembly of the earliest members of the group. Here we interpret the millimetric, erect, bilaminate, secondarily phosphatized fossil Protomelission gatehousei9 from the early Cambrian of Australia and South China as a potential stem-group bryozoan. The monomorphic zooid capsules, modular construction, organic composition and simple linear budding growth geometry represent a mixture of organic Gymnolaemata and biomineralized Stenolaemata character traits, with phylogenetic analyses identifying P. gatehousei as a stem-group bryozoan. This aligns the origin of phylum Bryozoa with all other skeletonized phyla in Cambrian Age 3, pushing back its first occurrence by approximately 35 million years. It also reconciles the fossil record with molecular clock estimations of an early Cambrian origination and subsequent Ordovician radiation of Bryozoa following the acquisition of a carbonate skeleton10–13.

Parasite-host systems are pervasive in nature but are extremely difficult to convincingly identify in the fossil record. Here we report quantitative evidence of parasitism in the form of a unique, enduring life association between tube-dwelling organisms encrusted to densely clustered shells of a monospecific organophosphatic brachiopod assemblage from the lower Cambrian (Stage 4) of South China. Brachiopods with encrusting tubes have decreased biomass (indicating reduced fitness) compared to individuals without tubes. The encrusting tubes orient tightly in vectors matching the laminar feeding currents of the host, suggesting kleptoparasitism. With no convincing parasite-host interactions known from the Ediacaran, this widespread sessile association reveals intimate parasite-host animal systems arose in early Cambrian benthic communities and their emergence may have played a key role in driving the evolutionary and ecological innovations associated with the Cambrian radiation.

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The early Cambrian Guanshan biota of eastern Yunnan, China, contains exceptionally preserved animals and algae. Most diverse and abundant are the arthropods, of which there are at least 11 species of trilobites represented by numerous specimens. Many trilobite specimens show soft-body preservation via iron oxide pseudomorphs of pyrite replacement. Here we describe digestive structures from two species of trilobite, Palaeolenus lantenoisi and Redlichia mansuyi. Multiple specimens of both species contain the preserved remains of an expanded stomach region (a "crop") under the glabella, a structure which has not been observed in trilobites this old, despite numerous examples of trilobite gut traces from other Cambrian Lagerstätten. In addition, at least one specimen of Palaeolenus lantenoisi shows the preservation of an unusual combination of digestive structures: a crop and paired digestive glands along the alimentary tract. This combination of digestive structures has also never been observed in trilobites this old, and is rare in general, with prior evidence of it from one juvenile trilobite specimen from the late Cambrian Orsten fauna of Sweden and possibly one adult trilobite specimen from the Early Ordovician Fezouata Lagerstätte. The variation in the fidelity of preservation of digestive structures within and across different Lagerstätten may be due to variation in the type, quality, and point of digestion of food among specimens in addition to differences in mode of preservation. The presence and combination of these digestive features in the Guanshan trilobites contradicts current models of how the trilobite digestive system was structured and evolved over time. Most notably, the crop is not a derived structure as previously proposed, although it is possible that the relative size of the crop increased over the evolutionary history of the clade.

An accurate, transferrable, and computationally efficient potential energy surface (PES) is of paramount importance for all molecular mechanics simulations. In this work, using water as example, we demonstrate how one can construct a reliable force field by combining the advantages of both physically-motivated and data-driven machine learning (ML) methods. Different to the existing water models based on molecular many-body expansion, we adopt a separation scheme purely based on distances, which is more convenient for generic molecular systems. The new potential provides a physically interpretable energy decomposition, and in the meantime, is much more accurate than the conventional physically-motived potentials. Most importantly, through this study, we show that the information we learn from small clusters can be extrapolated into larger systems, thus providing a general recipe for the intermolecular force field development at CCSD(T) level of theory in future.

Micromorphic acrotretide brachiopods first appeared during Cambrian Epoch 2 and subsequently experienced a rapid diversification. However, our knowledge of acrotretide origins and early evolution is hampered by our poor understanding of their earliest representatives. Here, we describe one of the oldest known acrotretides from the Cambrian Series 2 Shuijingtuo Formation of southern Shaanxi and western Hubei, South China. The new genus Palaeotreta gen. nov. can be distinguished from all other genera, mainly by its unusual ontogeny and morphology; in Palaeotreta the pedicle foramen is located mostly outside of the metamorphic shell, the ventral intertrough is very short and the inclination of the ventral pseudointerarea changes from catacline to apsacline. Two ontogenetic stages (T1-T2) of post-metamorphic shell growth can be distinguished in the low cap-like Palaeotreta shannanensis gen. et sp. nov.; this type of development with a relatively long pedicle foramen forming stage (T1), a simple pedicle opening, weakly developed ventral apical process and dorsal median septum, can be considered to represent a plesiomorphy for the acrotretides. By contrast, the more derived Palaeotreta zhujiahensis (Li & Holmer, 2004) is characterized by paedomorphosis, expressed in an early enclosure of pedicle foramen and greatly decreased growth of the ventral intertrough during the intertrough-increasing stage (T3), producing a cap-like shape and retaining catacline inclination of the ventral pseudointerarea. Thus, heterochrony may have played an important role in the morphological divergence of acrotretides during the Cambrian evolutionary radiation, and promoted the subsequent radiation of acrotretides. The ontogenetic study of Palaeotreta gen. nov. also supports the notion that the ventral intertrough of acrotretide brachiopods may have formed by the 'rolling up' of the lingulide propareas.