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Occupation of shallow environments by rhynchonelliform brachiopods is rare during the early late Tremadocian (Tr2). However, in the NW Argentina basin the plectorthoidean Tarfaya purmamarcaensis occupied high-energy environments during this time interval. This species forms up to 15 cm thick monospecific concentrations in shoreface and offshore transition settings, and polytypic pavements in relatively deeper water offshore environments. Generation of such concentrations could be linked to the population dynamics of T. purmamarcaensis. A geometric morphometrics analysis allowed us to recognize four growth stages in the ontogeny of the species. The juvenile phases are absent in high-energy proximal environments and are scarce in the shoreface; in contrast, no representatives of the fourth phase (hypermature adults) have been found in the open platform deposits, and adults are almost absent. Although shell concentrations are usually linked to physical processes (i.e., storm events), the low taphonomic alteration of the shells suggests that transport was not a highly influential factor. Morphological differences (i.e., development of cardinal canals, ribs incurved posterolaterally) in specimens from different environments suggest that the population dynamics could have been the main cause in generating different concentrations. According to the source-sink model, high productivity of brachiopods in the shoreface environment and a passive transport of larvae to the offshore might explain not only the thicker shallow-water concentrations but also the differences in shell growth of populations inhabiting these environments. [ABSTRACT FROM AUTHOR]

The lower Paleozoic marine siliciclastic succession of the Central Andean Basin, northwestern Argentina, provides a valuable record of the onset of the Great Ordovician Biodiversification Event in western Gondwana. A new ichnospecies of rosette trace fossil, Gyrophyllites cristinae, is documented from lower and upper Tremadocian (Tr1 and Tr2) deposits of this basin. It is characterized by five to six non-overlapping petaloid lobes and can be easily differentiated from the other four formally defined ichnospecies. Gyrophyllites cristinae occurs at the top of hummocky cross-stratified sandstone regularly interbedded with mudstone. These deposits are interpreted as reflecting the alternation of high-energy storm events and low-energy fair-weather conditions immediately below the fair-weather wave base, representing deposition in offshore transition environments. Gyrophyllites has been traditionally interpreted as the product of worms of uncertain taxonomic affinity that mined the sediment in search for food (fodinichnia). The occurrence of Gyrophyllites cristinae in these Ordovician deposits records post-storm colonization. Storms may have increased oxygenation and supplied fresh organic detritus that were exploited by worm-like, surface detritus- or shallow deposit-feeders exploring the uppermost silt-rich fine-grained sediments. [ABSTRACT FROM AUTHOR]

The re-study of species previously attributed to the genusNanorthisfrom the Tremadocian of north-western Argentina has led to the recognition of the new generaGondwanorthis(type speciesNanorthis calderensisBenedetto), to which the Iranian speciesNanorthis bastamensisGhobadi Pour, Kebriaee-Zadeh & Popov is also referred, andLampazarorthis(type speciesEoorthis bifurcataHarrington), which includesNanorthis brachymyariaBenedetto andLampazarorthis alatasp.nov.New extensive collections from the Cordillera Oriental support reassignment of the upper Tremadocian speciesNanorthis purmamarcaensisto the genusTarfaya. The speciesNanorthis carinataLaurie, from Tasmania, is reassigned here toTarfaya. According to the present taxonomic revision,Nanorthisis not cosmopolitan but is a pan-tropical genus, confined to Laurentia, the Laurentian-derived Precordillera terrane, and probably to Baltica, Siberia and Kazakhstanian terranes, whereasGondwanorthisandTarfayaare Gondwanan and peri-Gondwanan endemic taxa. Cladistic analysis of early orthoids, plectorthoids and dalmanelloids, including those recorded from mid to high latitude Gondwana successions, portraysGondwanorthisandLampazarorthisas a sister group basal to the tarfayid + heterorthid clade, whereas the South American and North AfricanIncorthisappears as a basal member of this clade. Morphological and stratigraphical evidence provided in this study, along with the results of the cladistic analysis, supports the view that heterorthids could have arisen from the tarfayids by the end of Floian. The enlargement and incipient bilobation of the cardinal process inTarfaya grandisleaves this species as a potential ancestor ofTissintia, which occurs in Darriwilian beds of the Central Andean Basin (Bolivia), but could have been present in this basin since the late Floian. We speculate that punctae in the Heterorthidae evolved independently from aTarfaya-like ancestor, and that the transition from an impunctate to a punctate shell may have occurred repeatedly through rhynchonelliform evolution. http://zoobank.org/urn:lsid:zoobank.org:pub:FFE818F6-FA92-4B26-AE76-077617C3D9EA [ABSTRACT FROM AUTHOR]

Trilobites have traditionally been considered fully marine. Through the integration of ichnological, palaeobiological and sedimentological datasets within a sequence-stratigraphic and strati­graphic palaeobiology framework, we challenge this assumption. This analysis is based on the study of incised fluvio-estuarine valley deposits from the Furongian Tilcara Member (TM) and the latest Furongian Pico de Halcón Member (PHM) of the Santa Rosita Formation, the early late Tremadocian Cardonal Formation (CF), and the Dapingian–Darriwilian Alto del Cóndor Formation (ACF), from Cordillera Oriental of northwest Argentina. These valleys were incised into wave-dominated shallow-marine strata and filled with transgressive deposits that accumulated in tide-dominated estuaries. Whereas the TM lacks any body or trace fossil evidence of the presence of trilobites in estuarine settings, the other three units reveal that trilobites were able to inhabit these settings. The PHM and CF are host to trilobite trace fossils in outer estuarine facies, both containing various ichnospecies of Cruziana (e.g., C. omanica and C. semiplicata in the TM) and Rusophycus (e.g., R. latus in both units). In addition, the PHM also contains body fossils of the olenid trilobite Neoparabolina frequens argentina in the same deposits in which the trace fossils are preserved, as well as from middle estuarine facies. The ACF displays trilobite trace fossils of the C. rugosa group in inner, middle, and outer estuarine deposits, illustrating further landward incursions. This unit also contains body fossils of the asaphid trilobite Ogyginus sp. Accordingly, our data indicate two attempts of landward exploration via brackish water: phase 1 in which the outer to middle portion of estuaries were colonized by olenids (Furongian–early late Tremadocian) and phase 2 involving exploration of the inner, middle, and outer estuarine zones by asaphids (Dapingian–Darriwilian). Our study indicates that these trilobites were tolerant to salinity stress and able to make use of the ecological advantages offered by marginal-marine environments migrating up-estuary, following salt wedges either reflecting amphidromy or as euryhaline marine wanderers. It is suggested that tolerance to salinity stress arose independently among different trilobite groups as a result of the broad array of behaviors and adaptations of trilobites during the Ordovician radiation. We speculate that the assumption that all trilobites were stenohaline may have resulted in the misinterpretation of some tide-dominated estuarine deposits as fully marine. [ABSTRACT FROM AUTHOR]

The eoorthid brachiopod Apheoorthina is reported for the first time from the Lower Ordovician of NW Argentina. It is represented by a species similar to A. ferrigena from the Tremadocian of the Prague Basin, increasing the faunal affinities between the Central Andean Basin and the South European microcontinents, in particular the Bohemian region (Perunica). Nine out of the fourteen brachiopod genera reported from the Tremadocian of the Central Andean Basin (~64%) are shared with the Mediterranean region, four of which (~28%) have been recorded in the Prague Basin, and two (Kvania and Apheoorthina) are restricted to the Central Andes and Perunica. Dispersal pathways around Gondwana are analyzed in the light of major factors affecting large-scale distribution of brachiopods (environment, larval capacity for dispersal, oceanic currents). The presence in Apheoorthina aff. ferrigena of a well-preserved larval protegulum measuring 420 µm in width and 210 µm in length strongly suggests that this species had planktotrophic larvae capable of long-distance dispersal. According to recent ocean-atmosphere general circulation models for the Ordovician Period, the Central Andean margin was dominated by the cold-water Antarctica Current. Despite the complex non-zonal pattern produced by current deflections around the peri-Gondwanan microcontinents, the general westward circulation sense favoured larval dispersal from the Andean region to North Africa, Avalonia, the Armorican Terrane Assemblage, and Perunica. On the other hand, the eastwards flowing Gondwana Current connected the North Gondwana waters with the South American epicontinental seas, which could explain the reversed migration of some brachiopods. [ABSTRACT FROM AUTHOR]

Estuarine deposits of the Furongian Pico de Halcón Member of Argentina contain trace and body fossils that allow assessing the role of evolutionary and environmental factors on early colonization of marginal-marine environments. The outer region of the estuary was characterized by a subtidal sandbody flanked by tidal flats. Vertical burrows of suspension feeders and probably passive predators are abundant in the high-energy areas of the outer estuary. Horizontal trilobite and vermiform trace fossils record the activities of mobile deposit, detritus, and suspension feeders in more protected settings. Arthropod trackways, vertical burrows, and horizontal vermiform burrows have been recorded in middle estuarine deposits. The occurrence of trackways attests the ability of some late Cambrian marine arthropods to periodically migrate inland following tidal channels or via salt-water wedges. No trace and body fossils have been found in inner valley areas. Outer and middle estuarine deposits contain the trilobite Neoparabolina frequens argentina. Its record in brackish-water settings is envisaged as a result of its generalist nature and ability (physiologic or behavioral) to cope with a wide array of conditions, flexible resource requirements, and successful spreading. Although trace-fossil distribution along the salinity gradient reflects the basics of the "brackish-water model", the remarkably low ichnodiversity in the middle estuary and the lack of trace fossils in the inner estuary are interpreted as reflecting evolutionary controls. Teichichnus assemblages, forming monospecific occurrences or with Planolites , in fine-grained, heterolithic facies seem to be remarkably persistent through geologic time. The studied trace fossils yield insights on attempts of some clades to expand into stressful brackish-water settings early in their evolutionary histories. In addition, comparison of the environmental tolerance of the Furongian benthos with other occurrences in younger early Paleozoic estuarine settings allows to track early steps of the colonization of brackish-water environments as it developed during the Ordovician Radiation. • Furongian estuarine deposits from Argentina host ichnofauna and an olenid trilobite. • Benthos in outer and middle estuary record early invasion of brackish-water. • Generalist trilobites account for a remarkable habitat transition. • Ecosystems yielded a truncated Eltonian pyramid of producers and first-tier consumers. • Known adaptive possibilities at the onset of the Ordovician radiation are expanded. [ABSTRACT FROM AUTHOR]