Explore the vast range of titles available.

The advent of embryophytes (land plants) is among the most important evolutionary breakthroughs in Earth history. It irreversibly changed climates and biogeochemical processes on a global scale; it allowed all eukaryotic terrestrial life to evolve and to invade nearly all continental environments. Before this work, the earliest unequivocal embryophyte traces were late Darriwilian (late Middle Ordovician; c. 463-461 million yr ago (Ma)) cryptospores from Saudi Arabia and from the Czech Republic (western Gondwana). Here, we processed Dapingian (early Middle Ordovician, c. 473-471 Ma) palynological samples from Argentina (eastern Gondwana). We discovered a diverse cryptospore assemblage, including naked and envelope-enclosed monads and tetrads, representing five genera. Our discovery reinforces the earlier suggestion that embryophytes first evolved in Gondwana. It indicates that the terrestrialization of plants might have begun in the eastern part of Gondwana. The diversity of the Dapingian assemblage implies an earlier, Early Ordovician or even Cambrian, origin of embryophytes. Dapingian to Aeronian (Early Silurian) cryptospore assemblages are similar, suggesting that the rate of embryophyte evolution was extremely slow during the first c. 35-45 million yr of their diversification. The Argentinean cryptospores predate other cryptospore occurrences by c. 8-12 million yr, and are currently the earliest evidence of plants on land.

A conodont-graptolite biostratigraphic study was carried out on the top strata of the San Juan, Las Chacritas and Las Aguaditas formations in the La Trampa Range, Precordillera of San Juan in western Argentina. Significant conodont records in the San Juan and Las Chacritas formations allow for the recognition of the Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus (Microzarkodina hagetiana and M. ozarkodella subzones) and Eoplacognathus suecicus zones of Darriwilian age. Index species and co-occurrences of graptolites and conodonts were recorded in the Las Aguaditas Formation allowing the identification of the Nemagraptus gracilis and the Pygodus anserinus zones, which represent the Sandbian Stage. These data indicate a hiatus between the Las Chacritas and the Las Aguaditas formations, corresponding to the Pygodus serra Zone and the Pterograptus elegans and Hustedograptus teretiusculus zones (upper Darriwilian). A total of 7287 identifiable conodont elements were recorded from the study section. The species frequency registered for each zone shows that Periodon and Paroistodus are the most abundant taxa, which are indicative of open marine environments. The records of particular conodont taxa, such as Histiodella, Periodon, Microzarkodina, Eoplacognathus and Baltoniodus, allow a precise global correlation with other regions such as south-central China, Baltoscandia, North America, Great Britain, Southern Australia and New Zealand. The graptolite fauna identified here are recognized worldwide in equivalent strata in the Baltic region, Great Britain, North America, China, southern Australia and New Zealand. The presence of graptolites in the ribbon limestones of the Las Chacritas Formation is documented for the first time.

Upper Cambrian to Middle Ordovician mafic volcanic rocks of the Donjek assemblage comprise the oldest exposed units of the Alexander terrane in the Saint Elias Mountains of northwestern Canada. In this study, we use the geochemical and geological characteristics of these rocks to decipher their tectonic setting, petrogenetic history, and relationship to the early Paleozoic Descon arc system of the Alexander terrane in southeastern Alaska. Donjek assemblage volcanic rocks are subdivided into three geochemical types: transitional basalt (type I), light rare earth-enriched island-arc tholeiite to calc-alkaline basalt (type II), and enriched mid-ocean ridge basalt to ocean-island basalt (type III). Simple petrogenetic models illustrate that the basalts were generated by the decompressional partial melting of enriched asthenospheric mantle and variably mixed with depleted mantle and subduction-related components.Analogous geochemical signatures for modern Sumisu Rift and Okinawa Trough lavas imply that the Donjek assemblage basalts erupted during the rifting of the Descon arc. This model provides a new comparative framework for terranes of Siberian, Baltican, and Caledonian affinity in the North American Cordillera and, in particular, suggests a paleogeographic connection to rift-related magmatism in the Seward Peninsula region of the Arctic Alaska-Chukotka terrane.

Dactylofusa velifera Cocchio 1982 is an easily recognisable, fusiform Ordovician acritarch species with a characteristic membrane. The taxonomy of this taxon is revised based on a literature review and on the investigation of new fossil assemblages from sections in South China. Morphological and biometric studies show that a subdivision into three varieties is justified. The taxonomical rank of Dactylofusa velifera var. brevis Albani 1989 is changed and its diagnosis is emended; Dactylofusa velifera var. velifera is the automatically created autonym. An additional new subspecific taxon is erected: Dactylofusa velifera var. sinensis var. nov. The stratigraphical and palaeogeographical occurrences of Dactylofusa velifera are reviewed, indicating that the species was widely distributed around the peri-Gondwanan margin during the Early Ordovician and early Middle Ordovician. Dactylofusa velifera is of biostratigraphical importance because its First Appearance Datum (FAD) can be used to indicate the late Tremadocian. Palaeobiogeographically, its distribution generally corresponds to that of the Early Ordovician 'messaoudensis-trifidum' acritarch assemblage in the peri-Gondwanan region.

The presence of six articulated exoskeletons of late holaspid specimens of the rare harpetid Eoharpes benignensis entombed under a pygidial shield of the large asaphid trilobite Nobiliasaphus repulsus from the Middle Ordovician Dobrotivá Formation of the Prague Basin, Czech Republic is interpreted as a unimodal monotaxic trilobite cluster. The sheltered preservation of the trilobites may be explained as; (1) hiding behavior associated with predation pressure; (2) storm disturbance; or (3) molting associated with feeding. It is herein suggested that these Middle Ordovician holaspid trilobites deliberately entered the restricted space under a large isolated asaphid trilobite pygidial shield to find a refuge and shared the space within restricted shelters with conspecifics. The completeness of all specimens of the rare taxon Eoharpes, combined with the presence of more than one individual in this restricted space, excludes the possibility of transportation by bottom currents. This exceptional find represents an example of “frozen behavior” and provides a new insight in the life strategy of Middle Ordovician benthic trilobites. Attack abatement, e.g., avoidance and dilution effects, is for the first time proposed as a possible explanation for this example of sheltered gregarious behavior in trilobites.

Representatives of the family Inaniguttidae dominate a diverse and well-preserved radiolarian assemblage from Kazakhstan. The fauna was extracted from a limestone sample of the Shundy Formation, a limestone sequence that accumulated on the slope of a carbonate platform and which crops out in the Aksuran Mountains (North Balkhash Region). The family Inaniguttidae is represented in the studied assemblage by five genera and 14 species (including two new species, Triplococcus aksuranensis and Inanibigutta maletzi). The genus Triplococcus is particularly abundant, representing half of the Inaniguttids in the studied assemblage. Based on the presence of species Haplentactinia juncta, the fauna can be correlated with the upper Darriwilian Haplentactinia juncta–Inanigutta unica assemblage. Identified trilobites (Endymionia semielliptica and Porterfieldia sp. aff. P. delicata) found in the same sample also suggest a late Darriwilian age, which agrees with the age suggested previously by graptolites.

An unusual, new, giant edrioasteroid Bizarroglobus medusae n. gen. n. sp. is described from the Middle Ordovician Kanosh Shale of west-central Utah. This species has a pattern of ambulacral branching with side ambulacra arising alternately from a main ambulacral trunk, previously undocumented in edrioasteroids. This pattern is interpreted as a strategy for allometrically increasing the feeding surface during ontogeny. Bizarroglobus further differs from other isorophid edrioasteroids in the plating of the peripheral rim, and the presence of pores in the interambulacral plates primarily along the edges of the ambulacra.

Two new species of ostracods, Conchoprimitia cassidula n. sp. and Sorornanopsis avalonensis n. gen. n. sp., represent the first described Middle Ordovician ostracods from western Avalonia. They were recovered as phosphatized carapaces dissolved out of a late early Darriwilian (ca. 467 Ma) limestone boulder from the Triassic Lepreau Formation of New Brunswick, Canada. The ostracods form a low-diversity component of a higher energy, near-shore, shelf marine fauna dominated by the trilobites Neseuretus and Stapleyella and by the conodonts Drepanoistodus and Baltoniodus. The low diversity of this Avalonian ostracod fauna contrasts with more diverse (tens of species), coeval ostracod faunas from Laurentia and Baltica. The association of Darriwilian ostracods and trilobites from New Brunswick demonstrates continuing exchange of open marine, cool water biota between Avalonia, Baltica, and West and North Gondwana that began in the late early Cambrian.

The Romaine Formation of the Mingan Islands, Québec, contains a limited trilobite fauna of five genera (Bolbocephalus, Peltabellia, Petigurus, Strigigenalis, Strotactinus) with Lower Ordovician Ibexian (Floian) affinity, succeeded by two genera (Acidiphorus, Pseudomera) of early Middle Ordovician Whiterockian (Dapingian) affinity. Trilobites of later Whiterockian (Darriwillian) affinity are more abundant in the overlying Mingan Formation, with 29 genera (Amphilichas, Bathyurus, Calyptaulax, Ceraurinella, Cybeloides, Dolichoharpes, Encrinuroides, Eobronteus, Eorobergia, Failleana, Glaphurina, Glaphurus, Hibbertia, Hyboaspis, Illaenus, Isotelus, Kawina, Nieskowskia, Phorocephala, Pandaspinapyga, Pliomerops, Remopleurides, Sphaerexochus [including S. valcourensis n. sp.], Sphaerocoryphe, Stenopareia, Thaleops, Thulincola, Uromystrum, and Vogdesia). Lectotypes are selected for Mingan species of Illaenus, Peltabellia, Petigurus, Sphaerexochus, Stenopareia, and Thaleops. A number of genera form biofacies which can be related to lithofacies. Taken together with Chazy Group and Northwest Territories trilobite faunas, Mingan trilobites provide critical information on late Whiterockian trilobite distributions in Laurentia in far more detail than seen in the stratotype area of Nevada.

Storm-generated event beds are an important source of paleoecological information, especially in Paleozoic strata. Storm deposition and subsequent physical and biological modification can potentially alter the diversity structure of death assemblages significantly. To examine the effects of storm deposition on fossil assemblage composition, storm beds are compared with co-occurring beds representing background sedimentation in 67 samples from six Ordovician mixed carbonate-clastic units deposited above the maximum storm wave base. In the great majority of pairwise comparisons, evenness and sampled richness are higher in storm beds than in background beds. This effect is not explained by differences in lithification, skeletal fragmentation, or in the proportions of aragonitic or multielement skeletons. The elevated diversity of storm beds can result from homogenization of fine-scale faunal patchiness preserved in background beds or may be due to taphonomic feedback. The relative importance of these two end-member scenarios can be evaluated with detrended correspondence analysis. In shallow, carbonate-dominated environments, the former appears to predominate, while the latter is more important in a deeper setting dominated by fine-grained clastics. The disparity between background beds and storm beds suggests that, at least in the Lower Paleozoic, background beds may record a higher-resolution paleoecological signal while storm beds record a more complete census of alpha diversity. Because post–Middle Ordovician increases in the depth and intensity of bioturbation may have diminished the temporal resolution and increase the faunal completeness of background beds, this disparity is not necessarily expected in younger strata.