Junfeng Guo

Cambrian lobopodians are pivotal to understanding panarthropod early evolution, yet the functional significance of their diverse sclerites remains poorly understood. Although these discrete sclerites have generally been accepted as protective structures, whether they served as attachment sites for lobopod muscles is still under debate. Here, we report the first description of regularly distributed pit‐like microstructures on the underside of net‐like plates of Microdictyon sp. from the Shuijingtuo Formation (Cambrian Stage 3), Shaanxi, China. A concentric cluster of pit‐like microstructures is located on the internal surface of each node on the plate. Through comparative analysis with extant tardigrades, onychophorans and arthropods, alongside similar fossil structures interpreted as muscle attachment sites, we suggest that these pit‐like microstructures are attachment points for leg muscles. The plates might also provide protection for underlying musculature and probably housed specialized organs beneath their sealed holes. This study suggests that the plates of Microdictyon evolved through the integration of multiple functions, providing new insights into the functional morphology and ecology of Cambrian armoured lobopodians and the early evolution of panarthropods.

Modern cephalopods are unique among molluscs in ancestrally possessing a chambered, gas- and fluid-filled shell, the internal septa of which are penetrated by a siphuncle. Combined with jet propulsion, the siphuncle enables cephalopods to engage in a nektonic and mostly predatory lifestyle through long-term regulation of their bouyancy¹. The origin of this key innovation remains unknown, as major gaps exist between the earliest accepted cephalopod, Plectronoceras cambria² from the late Cambrian era, and molecular clock estimates placing the divergence of the lineage from other molluscs in the early Cambrian^3,4. Here we report Eoceras shaanxiense gen. et sp. nov., a millimetre-sized cephalopod with an orthoconic shell from the Shuijingtuo Formation (Cambrian Stage 3) of South China. Internally, the shell exhibits multiple septa along with a peripherally situated segmented tube that appears to bridge the septa through minute canals, indicating that the soft body migrated aperturally to form successive chambers during growth while maintaining contact with earlier chambers via the tube. Collectively, these features identify the segmented tube in E. shaanxiense gen. et sp. nov. as a candidate primordial cephalopod siphuncle, thus extending the known range of stem cephalopods back to the early Cambrian and revealing the early stages in the evolutionary assembly of a chambered phragmocone employed in buoyancy regulation.

New post‐embryonic specimens and the first known embryos of Octapyrgites elongatus , a medusozoan cnidarian in the family Olivooidae, are described from Member 5 of the Yanjiahe Formation (Cambrian Stage 2) in western Hubei Province, South China. This discovery extends the embryological record of olivooids beyond the Kuanchuanpu biota and from the Fortunian Stage to Cambrian Stage 2. Micro‐CT scans of three embryos revealed internal anatomical structures, including relict soft tissue and stacked sets of four and eight embryonic oral lobes, with the four‐lobed set situated closest to the oral pole. Together, available specimens of O. elongatus record a mostly complete developmental cycle extending from the early organogenesis stage to hatchling specimens showing 12 annulations in the aboral portion of the theca. Co‐occurring, morphologically similar spheroidal specimens probably represent the earlier cleavage, blastula, or gastrula stages. Embryos of O. elongatus are relatively large, averaging 720 μm long, though many embryos have undergone diagenetic shrinkage and deformation. As in other olivooids, development was direct, with adoral migration of the embryonic oral lobes. The early ontogeny of O. elongatus is most similar to that of Quadrapyrgites quadratacris , further suggesting that Octapyrgites and Quadrapyrgites , both of which exhibit tetraradial symmetry, were more closely related to each other phylogenetically than either was to Olivooides or to the three other, pentaradially symmetrical olivooid genera.

The basal Cambrian Kuanchuanpu Formation (southern Shaanxi, China) yields well‐preserved specimens of the medusozoan genus Olivooides, a thecate polyp with pentaradial symmetry. Described herein is a rare internal thecal (peridermal) structure, termed the ‘transverse disc‐shaped diaphragm’ (TDD), which consists of a thin, adapically convex transverse wall spanning the peridermal cavity in certain specimens of O. mirabilis and O. multisulcatus. The TDD may be a diagnostic feature distinguishing Olivooides from other genera in the family Olivooidae, but it may also have been adventitious and thus similar in origin to the schott (apical wall) of conulariids, Sphenothallus and coronate scyphopolyps. If primary in nature, the TDD may have served to increase the stability and/or feeding efficiency of the polyp within the periderm, and it may have provided additional compressional strength to the periderm. The TDD is the earliest known transverse exoskeletal element in cnidarians, although similar transverse structures in tabulate corals, conulariids, Sphenothallus, coronate scyphopolyps and Archotuba conoidalis may have arisen independently in these lineages.

Extant ecdysozoans (moulting animals) are represented by a great variety of soft-bodied or articulated organisms that may or may not have appendages. However, controversies remain about the vermiform nature (i.e. elongated and tubular) of their ancestral body plan. We describe here Beretella spinosa gen. et sp. nov. a tiny (maximal length 3 mm) ecdysozoan from the lowermost Cambrian, Yanjiahe Formation, South China, characterized by an unusual sack-like appearance, single opening, and spiny ornament. Beretella spinosa gen. et sp. nov has no equivalent among animals, except Saccorhytus coronarius, also from the basal Cambrian. Phylogenetic analyses resolve both fossil species as a sister group (Saccorhytida) to all known Ecdysozoa, thus suggesting that ancestral ecdysozoans may have been non-vermiform animals. Saccorhytids are likely to represent an early off-shot along the stem-line Ecdysozoa. Although it became extinct during the Cambrian, this animal lineage provides precious insight into the early evolution of Ecdysozoa and the nature of the earliest representatives of the group.

Extant ecdysozoans (moulting animals) are represented by a great variety of soft-bodied or articulated organisms that may or may not have appendages. However, controversies remain about the vermiform nature (i.e., elongated and tubular) of their ancestral body plan. We describe here Beretella spinosa gen et sp. nov. a tiny (maximal length 3 mm) ecdysozoan from the lowermost Cambrian, Yanjiahe Formation, South China, characterized by an unusual sack-like appearance, single opening, and spiny ornament. Beretella has no equivalent among animals, except Saccorhytus, also from the basal Cambrian. Phylogenetic analyses resolve both fossil species as a sister group (Saccorhytida) to all known Ecdysozoa, thus suggesting that ancestral ecdysozoans may have been non-vermiform animals. Saccorhytids are likely to represent an early off-shot along the stem-line Ecdysozoa. Although it became extinct during the Cambrian, this animal lineage provides precious insight into the early evolution of Ecdysozoa and the nature of the earliest representatives of the group.

Extant ecdysozoans (moulting animals) are represented by a great variety of soft-bodied or articulated organisms that may or may not have appendages. However, controversies remain about the vermiform nature (i.e., elongated and tubular) of their ancestral body plan. We describe here Beretella spinosa gen et sp. nov. a tiny (maximal length 3 mm) ecdysozoan from the lowermost Cambrian, Yanjiahe Formation, South China, characterized by an unusual sack-like appearance, single opening, and spiny ornament. Beretella has no equivalent among animals, except Saccorhytus , also from the basal Cambrian. Phylogenetic analyses resolve both fossil species as a sister group (Saccorhytida) to all known Ecdysozoa, thus suggesting that ancestral ecdysozoans may have been non-vermiform animals. Saccorhytids are likely to represent an early off-shot along the stem-line Ecdysozoa. Although it became extinct during the Cambrian, this animal lineage provides precious insight into the early evolution of Ecdysozoa and the nature of the earliest representatives of the group.

Some small shelly fossils are important index fossils for global stratigraphic subdivisions and correlations of the Cambrian Terreneuvian. The first appearance datum (FAD) of the cosmopolitan mollusk Aldanella attleborensis has been suggested as one of the potential markers for defining the base of Cambrian Stage 2. Aldanella fossils were uncommon in South China, and if occurring, were often described as Aldanella yanjiaheensis, A. attleborensis, or indeterminate species in the literature, while A. yanjiaheensis was often taken as a junior synonym of A. attleborensis. Nevertheless, a detailed taxonomic revision of A. yanjiaheensis based on material from its type locality awaits to be made. In this study, we systematically re-evaluated A. yanjiaheensis based on the numerous specimens collected from the base of Member 5 of the Yanjiahe Formation in the Three Gorges area, western Hubei Province of South China. Detailed taxonomic comparison further substantiates that A. yanjiaheensis is a junior synonym of A. attleborensis, signifying its strong potential for a global correlation across paleocontinents. Morphological parameter analyses indicate that the length and width of shell tube of A. attleborensis shows allometric growth. The nearly cosmopolitan distribution and characteristic morphology of A. attleborensis indicate that it can play a significant role in the subdivision and correlation of Cambrian Stage 2. The co-occurrence of A. attleborensis and Watsonella crosbyi from the base of Member 5 of the Yanjiahe Formation corroborates that Member 5 belongs to Cambrian Stage 2.

Anabarella, a conspicuous taxon of early mollusc, is widely distributed in the early Cambrian strata and is considered an important link in the evolutionary lineage that reflects a transitional form from helcionelloids to bivalves. In South China,Anabarellahas mainly been documented from Yunnan, Shaanxi, Sichuan, and Hubei provinces. However, the taxonomy ofAnabarellais questionable, which has implications for the interpretation of the genus’ temporal and spatial distribution. New and abundant well-preserved specimens of the helcionelloid molluscAnabarellawere recovered from the Member 5 of the Yanjiahe Formation in the Three Gorges area. Through morphological study, these specimens can be definitely identified asA. plana. On the basis of this new material, the species ofAnabarellapreviously reported in the literature from South China were taxonomically revised, and, with the exception ofA. planafrom the Yanjiahe Formation, other species should be assigned toIgorella. Therefore, at present,A. planais the only valid species of the genusAnabarellain South China and is limited to Cambrian Stage 2. Study of the available specimens ofA. planareveal three types of microstructures: convex polygonal impressions, concave polygons, and lamello-fibrillar microstructure. In addition, the thicker shell of the sub-apical area and the three different structures of the sub-apical area provide more evidence thatA. planamight have adapted a semi-infanual mode of life and indicate thatAnabarellais a likely ancestor ofWatsonella.

Hexangulaconulariids, a family of biradially symmetrical medusozoan cnidarians, have been widely reported from the Lower Cambrian of South China. The four currently recognized genera of hexangulaconulariids differ from each other mainly in the number of faces in the abapical region of the periderm. However, previously published illustrations of the monospecific type genus, Hexangulaconularia, clearly show two distinct morphotypes, one with six faces and the other with 10. Specimens with 10 faces are herein reassigned to the genus Decimoconularia. In addition, the new species D. anisfacialis is described from the Kuanchuanpu Formation (Cambrian Fortunian Stage) in the Kuanchuanpu and Shizhonggou sections in Ningqiang County, southern Shaanxi Province, China. Also described are additional specimens of H. formosa from the same formation in the Zhangjiagou section in Xixiang County, southern Shaanxi Province, and from Member 2 of the Yanjiahe Formation (Cambrian Fortunian Stage) in western Hubei Province. The discovery of D. anisfacialis extends the known stratigraphical range of Decimoconularia, now composed of two species, downward from Cambrian Stage 2 into the Fortunian Stage. Additionally, certain specimens previously assigned to H. formosa are reassigned to D. anisfacialis. The diagnoses of Hexangulaconularia, Decimoconularia, and Hexangulaconulariidae are emended accordingly. In accordance with the rule of time priority, the previously designated type genus and species, Hexaconularia He and Yang, 1986 and Hexaconularia sichuanensis He and Yang, 1986, are replaced herein by Hexangulaconularia He, in Xing et al., 1983.