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 mu m 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.

Reflecting the long-term decline in the generic diversity of order Conulariida Miller and Gurley, 1896 following its peak during Mid–Late Ordovician times (Leme et al., 2008, fig. 1; Bernad et al., 2026, figs. 1, 2), the global fossil record of conulariid cnidarians in the Mississippian System is dominated by the genus Paraconularia Sinclair, 1940 (Fig. 1; Table 1). Thus, most Mississippian conulariid faunas consist of one or two species in Paraconularia plus, in some cases, Conularia Miller in Sowerby, 1821 or, possibly, Holoconularia Hergarten, 1985. The cosmopolitan distribution of these conulariids was facilitated by the Devonian–Carboniferous, Austropanthalassic-Rheic migration corridor (e.g., Torres-Martínez and Sour-Tovar, 2023), the westernmost portion of which directly connected Gondwana to Laurentia (Fig. 1).

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 Cambrian3,4. 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.

A very high percentage of specimens of Conularia fragilis Barrande, 1867 from the Lower Devonian Prague Formation of Bohemia (Czech -Republic) show appreciable curvature or bending of the steeply pyramidal periderm, which in most other conulariids tends be straight. Curved or bent sections of the periderm in C. fragilis are invariably associated with substantial departure from normal patterns of the external ornamentation (transverse ribs) of the four faces. Based on these observations and on data on the microstructure and composition of the conulariids, we conclude that bending of the periderm occurred while the conulariids were alive. As in curved or bent rugose corals from the same formation, bending in the conulariids probably involved preferential addition of new peridermal material along that part of the oral growth margin on the outside of the curve and may have occurred in response to changes in ambient currents or shifting of a sandy (crinoidal) substrate distad or within crinoidstromatoporoid bioherms.

The abrupt appearance of abundant small shelly fossils (SSFs) near the base of the Cambrian System signals a key event in the evolutionary history of the Kingdom Metazoa. Among the probable cnidarian taxa, the SSF family Hexangulaconulariidae (Terreneuvian Series) is characterized by a fan-shaped, biradially symmetrical periderm having distinct apical and abapical portions, the latter of which bears multiple faces. Documented herein are three new hexangulaconulariid specimens from Cambrian Stage 2 on the Yangtze Platform (South China). The new specimens exhibit nine or 11 faces on each of the two sides of the abapical portion. Results of a Bayesian analysis of the phylogenetic relationships among hexangulaconulariids, carinachitids, conulariids, olivooids, and extant cnidarian genera indicate that hexangulaconulariids constitute a well-defined clade within a polytomic, stem-group Medusozoa that also includes carinachitids, conulariids and olivooids. The evolution of hexangulaconulariids featured multiple instances of increase in the total number of faces, modification of the morphology of the interfacial boundaries, and increase in the total size of the periderm. Increase in the number of faces appears to have been achieved in part by the transformation of a single ancestral face into three faces, through the development of a longitudinal furrow or ridge along each of the two adapically converging loci of inflection points (angular bends) of the prominent transverse ribs of the ancestral face.

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.

Paraconularia planicostata (Dawson, 1868) is one of two species in this genus (Sinclair, 1940) of conulariid cnidarian currently known from Mississippian strata in the Maritimes Basin in Atlantic Canada (Babcock and Feldmann, 1986). A salient characteristic of the species is the presence of a low and narrow internal carina at or near the longitudinal midline of each of the four faces of the gently tapered periderm (Van Iten et al., 2020). Paraconularia planicostata occurs in marine carbonates in the Upper Mississippian lower Windsor and lower Codroy groups in Nova Scotia and western Newfoundland, respectively (e.g., Bell, 1929; Stacy, 1953; Dix and James, 1988; Boehner et al., 1989; Boehner and Prime, 1993; Lavoie and Sami, 1998; Jutras et al., 2006; Ryan and Giles, 2017; Van Iten et al., 2020). The species is exceptionally abundant, with richly fossiliferous biohermal limestones containing up to approximately 20 specimens per 1000 cm(3) of host rock (Van Iten, unpublished observations)-a volumetric density matched by few other conulariid-bearing rock units.

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.

Sphenothallus Hall, 1847, one of the most widely distributed and longest ranging genera in the fossil record, has been documented from all systems of the Paleozoic Erathem except the Permian (Table 1), although it has been stated (e.g., Choi, 1990; Bolton, 1994; Fatka et al., 2012) that the genus also occurs in that system. At present the first appearance of this epibenthic, polypoid medusozoan cnidarian lies in Cambrian Stage 3, while the previously known youngest occurrences are in the Pennsylvanian System. Sphenothallus has been found in numerous formations on all continents except Australia and Antarctica. It occurs in a variety of marine facies ranging from shallow nearshore to deep offshore and has even been found in strata of coastal lacustrine origin, probably as an allochthonous element (Lerner and Lucas, 2011). Many of the rock units known to contain Sphenothallus also contain conulariids (Table 1), an extinct group of marine scyphozoans that may have been closely related to Sphenothallus (Van Iten et al., 1992, 1996). Van Iten et al. (1992) interpreted Sphenothallus as a medusozoan cnidarian of uncertain class-level affinities, but later Dzik et al. (2017) documented internal peridermal structures that may be homologous to similar features in the periderm of coronate scyphozoans (see for example illustrations in Van Iten, 1992, and Van Iten et al., 1996).

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Two specimens of Metaconularia manni (Roy, 1935) from the lower Middle Silurian Scotch Grove Formation (eastern Iowa) exhibit well-defined, relict soft parts replicated in silica. One of these specimens bears phosphatic periderm, whereas the other specimen is a mold. Present within the erect, undistorted apical region of the specimen preserving periderm, on opposite sides of the peridermal cavity, are two small, elongate masses of silica located near the midlines of two of the four faces. Present in the central portion of the other specimen, at a somewhat greater distance from the apex, are five pairs of hollow, elongate, keeled pouch-like bodies (hereafter pouches), the long axes of which converge on the center of the fossil. Each pair of pouches is associated with a short, narrow, gently curved or broadly U-shaped tube, also composed of silica. Additionally, two of the pouch/tube combinations are associated with a pair of rectilinear furrows that correspond to the paired internal carinae that straddled the conulariid's facial midlines. We interpret the paired pouches and short tubes in the moldic specimen as relic conulariid soft parts homologous, respectively, to the interradial gonads and retractor muscles of extant, stauromedusan and polypoid scyphozoan cnidarians. Unlike most conulariids, which exhibit four faces, this individual had five faces, an aberrant morphology known in one other conulariid. The two small masses in the other specimen are more difficult to interpret, but they, too, could be relic gonads or longitudinal muscles. These interpretations suggest that, as in certain extant scyphozoans, at least one conulariid lost the free-living, sexual medusoid life phase.

Five quartzose sandstone slabs hosting small groups or mass associations of Anaconularia anomala are described from the Upper Ordovician Letna Formation in the Prague Basin. The slabs contain from 4 to 59, mutually adjacent or contiguous conulariids ranging from 5 to 89 mm in length and situated on a single bedding plane. Associated with the conulariids are small mudstone intraclasts and other fossils, mostly disarticulated brachiopods and trilobites. All 101 studied conulariid specimens are oriented parallel to bedding and show strong preferential alignment, with the apical ends pointing in the same general direction. Sixteen of the conulariids terminate adapically in a probabe schott and/or exhibit a possible internal schott, while five specimens preserve one or two apertural lappets. The investigated conulariids lived in clumps and were buried catastrophically following alignment by unidirectional currents or flows acting on bodies that may have been leaning in the down-current direction and/or which had their center of mass displaced toward their apical end. Neither these nor 3000 additional specimens of A. anomala from the Sandbian Letna and Zahorany formations show any evidence of clonal budding; however, the hypothesis that clumping resulted from asexual proliferation cannot be ruled out. Finally, the periderm of A. anomala was compaction-resistant and smooth, lacking both corrugation (transverse ribs) and nodes, but whether the angular groove present at the facial midline of casts is a mould of a midline sulcus or of an internal carina remains unclear.

Storm-dominated siliciclastic shelf deposits in the upper half of the Upper Tiouririne Formation (Upper Ordovician, upper Berounian-c. Katian 1-2) of southern Morocco contain large specimens of at least two species of Archaeoconularia (A. aff. consobrina and A. cf. imperialis). Pseudoconularia cf. grandissima occurs in basal beds of the formation, but is far less abundant than Archaeoconularia. The large size of the Archaeoconularia (>500 mm long in some cases) suggests gigantism induced by cold, nutrient-rich waters. Specimens preserving the apical end terminate in a schott, indicating that the conulariids were severed near the apex prior to final burial. Many of the Archaeoconularia occur in monospecific aggregations in which the component specimens exhibit preferential alignment, probably owing to entrainment and burial by storm currents or flows. Some Archaeoconularia are encrusted by edrioasteroids and/or craniid brachiopods, which in certain cases are so closely spaced as to interfere with growth. The edrioasteroids and brachiopods were probably epibionts attached to living, erect or semi-erect conulariids that were partially infaunal. More generally, Late Ordovician conulariid assemblages of South Polar Gondwana and adjacent terranes exhibit low generic diversity relative to those of low- to mid-latitude terranes (Baltica and Laurentia), and are dominated numerically by Archaeoconularia.

?Holoconularia rossica sp. nov. is described from interstratified shale and siltstone beds in the Dashkovka Member of the Upper Mississippian (early Serpukhovian) Gurovo Formation in the Moscow Basin, Central Russia. The status of the genus Holoconularia, originally described from the Devonian of Germany, is discussed, and it is argued that at least one species previously assigned to it may belong in Paraconularia instead. Another species, P. mosquensis from the Middle Pennsylvanian of Central Russia, may be more closely related to ?H. rossica than to species in Paraconularia. Comparisons are also made with Conularia, Ctenoconularia, and Tasmanoconularia, three genera that are very similar to certain species in Holoconularia. The well-preserved periderm of ?H. rossica sp. nov. exhibits microscopic circular pores (micropores) and (in one specimen) malformed, cleft or scalloped ornamentation. The micropores in this and other conulariids appear to be primary anatomical features that may have housed an extremely slender, very short pillar of non-mineralized (organic) periderm connecting successive microlamellae composed predominantly of organic material. Cleft and scalloped features may represent healed injuries inflicted by predators, possibly nautiloids and/or gnathostomes.