Calcimicrobe-archaeocyath associations were the primary architects of early-middle Cambrian reefal ecosystems. Although these communities dispersed relatively rapidly worldwide along tropical margins during Cambrian Stages 2 to 4, their exact environmental setting and variations through space and time are still poorly constrained. These ecosystems are well-represented along the Laurentian margin, with the Sonora State of Mexico representing its easternmost extent. This study reevaluates the Cerro Rajón section (Caborca area, Northwestern Sonora), a succession of mixed carbonate-siliciclastic sediments where these reefal associations occur. By integrating field observations, petrographic analysis, and faunal evaluation, we propose a revised paleoenvironmental reconstruction and a refined taxonomic framework for the local archaeocyathan assemblages. Evidence from offshore and lower-to-upper shoreface facies, including characteristic tempestites, alongside with the dominance of skeletal grains and ooids and the absence of typical platform interior microfacies, support a ramp model for the section. Within this architecture, calcimicrobe-archaeocyathan patch-reef complexes are interpreted to have developed in the dynamic setting of the upper offshore. Furthermore, taxonomic revisions suggest that archaeocyathan diversity at this locality is lower than previously reported, primarily due to the identification of several synonyms. These results clarify the depositional context of the Sonora State and reinforce its paleogeographic continuity with the broader Laurentian margin, providing a clearer framework for understanding the environmental conditions of the Phanerozoic’s earliest metazoan reefs.

During the Ediacaran–Cambrian boundary interval, the Iberian margin of West Gondwana recorded oceanic upwelling and phosphogenesis favouring the preservation of originally phosphatic and diagenetically phosphatized shells. In the Abenójar Dome and the Alcudia Anticline of the Central Iberian Zone, the Fuentepizarra Formation comprises several phosphorite and phosphatic limestone interbeds that yield a microfossil assemblage rich in the helcionellid Anabarella plana, the halkieriid Halkieria sacciformis, the hyolith Conotheca subcurvata and chancelloriid debris. The assemblage is correlatable with the Purella antiqua Zone of the Aldan-Lena region and the Purella cristata Zone of the Anabar Uplift (Siberian Platform), slightly predating the Watsonella Zone of the Siberian Platform and the Watsonella crosbyi–Oelandiella korobkovi Zone of the northern Montagne Noire (France), both close to the (not yet globally defined) base of Cambrian Age 2. Despite the intra-Terreneuvian collapse of the Cadomian Orogen, stratigraphically associated with the onset of diachronous gaps, the base and top of the Fortunian Stage is well constrained throughout southwestern Europe based on radiometric ages and the micro- and ichnofossil record.

En el estado de Sonora afloran importantes secuencias del Paleozoico Superior con una abundante biota fósil relativamente bien preservada. El objetivo de este trabajo es estudiar los afloramientos localizados en el cerro El Yugo, ubicado en el municipio de Arivechi, noroeste de México, mediante bioestratigrafía de alta resolución basada en foraminíferos. La metodología consiste en la elaboración de secciones delegadas para microfacies así como estudio de óxidos mayores por medio de fluorescencia de rayos X (FRX), además de la extracción de conodontos mediante la disolución de caliza con ácido acético. El estudio documenta la biota presente en las rocas portadoras, lo cual permite realizar una correlación de microfacies, óxidos mayores y biota. Los resultados permiten identificar las biozonas de Nanicella gallowayi, Eogeinitzina devonica, Moravamminidae indet., Tikhinella measpis, Laxoendothyra parakosvensis y Stacheoides tenuis, lo cual permite datar la secuencia en un rango comprendido del Devónico Superior (Frasniano) al Misisípico Inferior-Medio (Tournaisiano-Viseano). El evento de extinción Kellwasser (LKW) del Devónico Superior es identificado en el presente estudio mediante la desaparición de T. measpis y E. devonica en el límite Frasniano-Famenniano. Posterior al Famenniano, la ausencia de biota fósil indica un período de recuperación de ésta.

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The Cambrian stratigraphic succession of the Pyrenees (SW Europe) has undergone a complex Variscan and Alpine tectonothermal history leading to marked metamorphism and development of cleavage networks, which might partly explain the lack of Cambrian fossiliferous beds. This gap has traditionally precluded its paleobiogeographic and biostratigraphic relationships with other neighboring peri-Gondwanan units. Correlations are only based on lithostratigraphic comparisons and radiometric constraints. In this general scheme, the Terrades inlier (Gerona Province, Spain) provides the only significant and indisputable 'early Cambrian' fossil record of the Pyrenees. This predominantly siliciclastic outcrop consists of multiple patch reefs and bioherms having yielded archeocyaths dated at Cambrian Epoch 2, Age 3. This paper describes, for the first time, the microfossil assemblage included in the archeocyathan-microbial reefal complex that crops out in the Terrades inlier to clarify its age and affinities with surrounding tectonostratigraphic units. Reefal flanks of patch reefs have yielded bradoriids, brachiopods, molluscs, tommotiids, chancelloriids, hyoliths, and the problematic fossil (and chronostratigraphically significant) Rhombocorniculum cancellatum Cobbold, 1921. In addition to confirming the previously assigned age of the succession, the recovered fauna emphasizes strong affinities with the surrounding Occitan Domain (Montagne Noire, southern Massif Central, France) and Sardinia (Italy). Along with lithostratigraphic comparison and tectonic considerations, this further supports the recent reconstructions positioning the Pyrenean domain between the Montagne Noire (to the southwest) and Sardinia (further to the northeast) on the Gondwana margin during Cambrian times.

Important Upper Paleozoic sequences crop out in the state of Sonora with an abundant, relatively well-preserved, fossil biota. This work aims to study the outcrops found in Cerro El Yugo, which is located in the Municipality of Arivechi, in northwestern Mexico, through high-resolution biostratigraphy based on foraminifera. Samples were prepared for thin sections and studied for microfacies analysis. Major oxides were also studied by means of X-ray fluorescence, in addition to the extraction of conodont elements by dissolving limestone with acetic acid. The study documents the biota present in the host rocks, which allows for a correlation of microfacies, major oxides and biota. Biozones of Nanicella gallowayi, Eogeinitzina devonica, Moravamminidae indet., Tikhinella measpis, Laxoendothyra parakosvensis, and Stacheoides tenuis were identified, which allowed the dating of the sequence to between the Upper Devonian (Frasnian) and the Lower-Middle Mississippian (Tournaisian-Visean). The Late Devonian (Kellwasser) event is identified in the present study by the disappearance of T. measpis and E. devonica at the Frasnian-Famennian boundary. After the Famennian, the absence of fossil biota indicates a period of recovery of it.

The Ayios Photios Group in SW Cyprus represents a deep-water slope or base of slope sequence that was accumulated on a Mesozoic margin of the Mediterranean Tethys. The Episkopi Formation is an essentially radiolaritic sequence, the age of which was previously considered to range between the Callovian and the Valanginian. A moderately well preserved (PI 4-5) radiolarian assemblage was extracted after laboratory leaching with diluted hydrofluoric acid of a radiolarian chert bed south of the village of Episkopi. Based on the co-occurrence of Bernoullius rectispinus leporinus Conti & Marcucci, Hexasaturnalis tetraspinus (Yao) and Theocapsommella cucurbiformis (Baumgartner), the assemblage is correlated with the Middle Bathonian (Unitary Association Zone 6). Our results extend downwards the known age range of radiolarites of the Episkopi formation and reduce the previously known range of depositional hiatus with the underlying Upper Triassic formations of the Ayios Photios Group.

A small ammonoid assemblage from Carboniferous-Permian boundary strata of Kuh-e-Bagh-e-Vang in the Tabas region (east-central Iran) is reported. The new species Agathiceras irani-cum KORN & GHADERI n. sp. and Eoasianites baghevangensis KORN & GHADERI n. sp. are described. Ammonoids from Frasnian to Early Permian age occur in a conglomerate unit near the base of the Jamal Formation and indicate deep erosion and reworking in the basin.

The Camenellan tommotiid Lapworthella bornholmiensis is systematically revised based on the original type material and collections of Small Shelly Fossils at the Swedish Museum of Natural History. The species is clearly differentiated from all other species of Lapworthella by its unique surface ornamentation. Variability in sclerite shape is analyzed among the 39 specimens and four sclerite morphotypes are recognized: bilaterally subsymmetrical A sclerites with straight apertural margin and subrectangular to subelliptical cross-section; asymmetrical B sclerites with oblique to curved apertural margin and subrectangular to subelliptical cross-section; asymmetrical C sclerites with one concave surface resulting in elliptical to sub-triangular cross section; elongated asymmetrical D sclerites with subcircular cross section. The C sclerites are further divided into two sub-types. The sclerite morphotypes of Lapworthella bornholmiensis allow a detailed comparison with other lapworthellid taxa and we identify a pattern of recurring sclerite morphotypes across a range of species, allowing a new understanding of the lapworthellid scleritome structure. The stratigraphic range of Lapworthella bornholmiensis includes the Cambrian Stage 4 Gislov Formation, the Wuliuan-Drumian Forsemolla and Exsulans Limestones in the Alum Shale Formation and Guzhangian limestone fill in Proterozoic basement fissures in Bohuslan, making this long ranging species the youngest known tommotiid in the fossil record.

Small shelly fossils (SSFs) are highly informative of the ‘Cambrian explosion’. Their palaeobiodiversity has been documented from lower Cambrian deposits worldwide but it remains elusive in areas such as Iran, despite this region occupying a critical position on the north‐western Gondwana margin during the early Cambrian. This new study of the SSFs of the lower Cambrian of northern Iran provides a large new dataset from this understudied area. We revise the micropalaeontological signal of the Soltanieh Formation of the Alborz Mountains and introduce novel data from the Soltanieh and overlying Barut Formations of the Soltanieh Mountains. The new, solid taxonomic and stratigraphic SSF data enable us to distinguish two successive microfaunal assemblages. The first occurs in the Soltanieh Formation of the Soltanieh and Alborz Mountains and is dominated by anabaritids ( Anabarites trisulcatus , A . ex gr. trisulcatus , A . tristichus , A . dalirense sp. nov., Cambrotubulus decurvatus ) along with protoconodonts ( Protohertzina anabarica and P . unguliformis ), maikhanellids ( Maikhanella multa , Purella squamulosa and Purella sp.), Aetholicopalla adnata , indeterminate cones and irregular tubes. The second assemblage, from the Barut Formation, is dominated by a diverse assemblage of molluscs ( Oelandiella korobkovi and cap‐shaped morphotypes). Siphogonuchitid sclerites also occur in both assemblages. The two SSF assemblages are characteristic of the Terreneuvian. Our dataset enables us to assess the sequence of faunal change of the Ediacaran–Cambrian transition; in contrast to the tube–sclerite–brachiopod succession presented in the literature, the Iranian fauna changes from one dominated by tubes and sclerites, to one dominated by molluscs and sclerites.

The early Cambrian record of Small Shelly Fossils (or SSFs) of Laurentia has been relatively poorly studied despite their major importance for understanding the Cambrian explosion. They represent key biostratigraphic tools for the subdivision and correlation of the Terreneuvian and Cambrian Series 2 at regional and global scales. This study is aimed at improving our knowledge of SSF stratigraphic ranges of Laurentia by focusing on the Puerto Blanco Formation of the Cerro Rajon section, near Caborca in northwestern Sonora, Mexico. Four SSF assemblages have been recovered that include sclerites of chancelloriids (Archiasterella hirundo, A. charma, A. cf. A. pentactina, Allonnia erromenosa, A. tetrathallis and Chancelloria spp.), hyolithelminths (Hyolithellus spp.), hyoliths (Cupitheca cf. C. mira, Petasotheca sp., Hyolithid sp. and Parkula bounites), micromolluscs (Mackinnonia corrugata, Xianfengella sp., Pelagiella sp. and Pojetaia sp.), brachiopods (Eoobolus sp. and Rajonia ornata), sclerites of the lobopod Microdictyon multicavus, one unidentified bradoriid and an indeterminate fossil. Distribution of the SSFs that extend below the first trilobite and archaeocyathan demonstrates that most of the Puerto Blanco Formation is Cambrian Age 3 to 4. The SSFs provide additional clues to correlate Ediacaran-Cambrian sedimentary successions in Sonora with the southern Great Basin (USA). The described assemblages are also compared with SSF assemblages of other localities of the American Cordillera and southeastern Laurentia. At the global scale, they support faunal connections with Australia and possibly China.

The Shotori Range of east-central Iran (east of Tabas) has yielded Famennian ammonoid assemblages dominated by the family Sporadoceratidae. Four genera Maeneceras Hyatt, 1884, Iranoceras Walliser, 1966, Sporadoceras Hyatt, 1884 and Eribudites Korn, 1999 are represented. The conodont assemblage of one sample containing Iranoceras revealed an Upper marginifera Zone age. The ammonoid assemblages are characterised by comparatively large specimens; they reach conch diameters of 300 mm (including the body chamber) and the mean size is larger than 100 mm. The preservation of the material from the Shotori Range and size comparison with sporadoceratid assemblages from the Anti-Atlas of Morocco and the Rhenish Mountains of Germany suggest that hydraulic sorting has resulted in a bias towards large conchs, explaining the size distribution, rather than latitudinal differences. The new species Maeneceras tabasense is described; the genus Iranoceras is revised with a new description of the two species Iranoceras pachydiscus (Walliser, 1966) and Iranoceras pingue (Walliser, 1966).

The Cambrian Stage 4 upper Bastion Formation of Albert Heim Bjerge and CH Ostenfeld Nunatak, NE Greenland, yielded 34 excellently preserved sclerites of Lapworthella schodackensis among other small shelly fossils. Lapworthellids have been interpreted as members of the camenellans, a basal tommotiid group. Little is known about this group although the morphological and ultrastructural features of their sclerites allow a potential reconstruction of a lophophorate body plan. The exquisite material from Greenland provides significant new data for the revision of the species taxonomy, but also for the comprehension of the scleritome structure of lapworthellids and the mode of formation of their sclerites. Two morphotypes of L. schodackensis sclerites are identified: one with a simple apex, occurring in sinistral and dextral forms; and one bilaterally symmetrical sclerite with two apices. All bear a similar ornamentation constructed of repeated growth sets consisting of a reticulate inter-rib groove with tubercles, a densely denticulate rib and a striated sub-rib area. The new data on the ornamentation and observations of the laminar shell microstructure of L. schodackensis enable us to improve the reconstruction of growth in lapworthellids. Finally, the morphological features of the two types of sclerites provide new information for the reconstruction of the bilaterally symmetrical multi-component lapworthellid scleritome with evidence of the fusion of adjacent sclerites during early ontogeny.

Trabajo presentado en el International Symposium on the Ediacaran-Cambrian Transtion (ISECT), celebrado en St. John's, Newfoundlan (Canada), del 15 al 29 de junio de 2017

The lowermost carbonate beds of the Cerro San Pedro (San Pedro de la Cueva, Sonora State, Mexico) previously assigned to the Mississippian, belong in fact to the lower Ordovician. The limestone of this succession is often dolomitized and chertified, and displays gastropods, pelmatozoans, sponges, and trilobites as major bioclastic components. The different microfacies show that high-energy grainstones, proximal tempestites, and distal tempestites dominated the sedimentation. The paleoenvironments of deposition correspond to an inner ramp, a mid-ramp, and perhaps the upper part of an outer ramp. The strata are characterized by the incertae sedis cyanobacteria Nuia sibirica. A taxonomic revision and discussion of these cyanobacteria, often confused with ooidic grains, is emphasized. Some data are presented on other microfossils, such as primitive, monothalamous foraminifers: Rauserina sp., Vicinesphaera sp., and Neoarchaesphaera sp., leperditicopida and their endolithic microperforations, and primitive chaetetids. Compared to the contemporaneous deposits of the USA, some paleobiological components, e.g., the lithistid siliceous sponges Archaeoscyphia, stromatoporoids? Pulchrilamina, and receptaculacean algae Calathium, are quite rare in the studied section of Sonora, but the predominance of tempestites in the carbonate succession shows that boundstones formed before were systematically eroded and resedimented. The paleogeographic implications are the following: (1) a lower Ordovician intertropical Nuia Province is newly defined in the western part of the lower Ordovician intertropical belt; (2) San Pedro de la Cueva constitutes one of the westernmost outcrops of this new Nuia Province; (3) from Sonora, Nuia extends eastward as far as South China; (4) due to its westernmost paleoposition, San Pedro de la Cueva was frequently affected by tropical storms; and (5) northern lower Ordovician terranes of Mexico (Sonora, Chihuahua, and Baja California) are entirely distinct from the southern ones (Oaxaca). The paleopositions and paleogeographic connections of these northern Mexican terranes with Laurentia, Avalonia, and peri-Gondwanan parts of South America, through the Iapetus and Rheic oceans, still remain disputable or unknown.

To infer the early evolution of mollusc shell microstructures we must know the most ancient fossil record of molluscs. Fortunately the shells of many early molluscs are preserved via internal coatings and replacements by apatite that record sub-micrometer structural details that otherwise would be lost during diagenetic recrystallization. We herein discuss the methodology by which one can infer original shell microstructure from phosphatized fossils, pointing out the main problems and solutions in interpreting these traces of original shell crystal morphology. We also review the information these fossils have provided about the earliest evolution of the mollusc shell. Our long-term goal is to create a dataset of microstructures in early molluscs, which will be useful in understanding the incipient evolutionary arms race between molluscs and their predators, and will help elucidate how the mollusc biomineralization toolkit was built through time.