Dynamic environmental conditions recorded by the trace fossil Teichichnus and event beds during deposition of the basal Vaca Muerta Formation in the central Neuqu´en Basin (Argentina)

The mud-dominated part of the basal Vaca Muerta Formation in the central Neuquén Basin accumulated mainly under dysoxic conditions as indicated by the absence of both fine lamination and large burrows, typical of anoxic and oxic conditions, respectively. Event beds and large, strata-bound concretions preserving a nearly uncompacted sediment fabric document rather discontinuous deposition, in particular as concretions must reside within the zone of carbonate supersaturation for a prolonged period of time to form. The mudrock hosts numerous ash layers. Among them is a bed up to 40 cm-thick that was deposited by storm-induced currents and later became calcified. It contains abundant, winding Teichichnus up to 220 cm-long. The trace producers partly reworked their own spreite and probably applied a gardening and sequestrichnial nutritional strategy (that is collecting and stowing of nutritional material in the burrow for later utilization). The spreite fill documents depositional processes that delivered argillaceous sediment and provided oxygen. The ash originally constituting the Teichichnus bed originated from the volcanic arc west of the basin. In contrast, the erosive surface limiting the Teichichnus bed is covered by mud, implying sediment delivery from the east or south where rivers entered the basin while the climate was (at least seasonally) humid, whereas aridity increased towards the north. Consequently, an estuarine circulation likely developed and prevailed during the initial transgression of the lower Vaca Muerta Formation while cold Palaeo-Pacific water flowed into the subsiding and expanding basin. Later, during prevailing dry climate, an anti-estuarine circulation was probably established

Reply: Biogenic structures of unioniform bivalves in wet-interdune deposits (Late Miocene-Early Pliocene, Argentina)

The Discussion of our paper by Martínez is very welcome because it supports our finding that bivalves may colonize wet-interdune settings and thus, trace fossils produced by them may record environmental changes—this is the main point of the contribution by Carmona et al. (2018). However, there are some comments in the Discussion by Martínez about the interpretations proposed in that paper that need to be clarified here.Fil: Carmona, Noelia Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación en Paleobiología y Geología; Argentina. Universidad Nacional de Río Negro. Sede Alto Valle. Instituto de Investigaciones en Paleobiología y Geología; ArgentinaFil: Ponce, Juan Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación en Paleobiología y Geología; ArgentinaFil: Wetzel, Andreas. Universidad de Basilea; Suiz

First Jurassic brittlestar from Neuquén Basin, Argentina

Articulated fossil ophiuroids from South America were reported for the Devonian, Cretaceous, Eocene, and Miocene. Here we report the first Jurassic record of an articulated ophiuroid from the Sierra Chacaicó Formation (early Pliensbachian– Sinemurian) in Neuquén Basin, Argentina, and discuss the taphonomic processes that allowed its preservation. The Sierra Chacaicó Formation represents the onset of the Early Jurassic extensive marine transgression in the basin. The basal section comprises shoreface and offshore Gilbert-type delta system, which was affected by hyperpycnal discharges. The middle and upper sections are represented by offshore deposits, affected by storms and eroded by hyperpycnal channel-levee systems. The ophiuroid specimen was found in levels of massive, fine, tuffaceous sandstone beds and covered by coarse sandstone containing a large amount of plant debris and organic matter. It was preserved articulated, with a complete disc and almost complete arms. Based on the microstructure of the spine-bearing lateral arm plates, the ophiuroid is assigned to Sinosura, an extinct genus of the family Ophioleucidae, widespread in the Lower Jurassic deposits of Europe but previously unknown from other parts of the world. The posture of the ophiuroid, with one arm curved distally and extended in one direction and the other four arms symmetrically oriented in the opposite direction, suggests a walking or escape movement frozen in time. This implies that the ophiuroid was buried alive by sediment thick enough to prevent successful escape. The taphonomic and sedimentologic evidence indicates that the fossil material was found in hyperpycnal deposits accumulated in offshore positions, which carried a high concentration of sediment in suspension

Ichnology of the miocene marine deposits from San Jorge Gulf

Este estudio analiza el contenido icnológico de los depósitos marinos someros de la Formación Chenque (Mioceno temprano). La mayoría de los icnogéneros se desarrollaron en fondos blandos, excepto 3 de ellos que fueron hallados exclusivamente en sustratos firmes. Las características icnológicas y sedimentológicas permitieron reconocer dos tipos de ambientes de depositación principales: depósitos con dominio de oleaje y depósitos dominados por acción mareal. Las trazas fósiles presentes en sustratos firmes revelaron la presencia de superficies de discontinuidad. Estudios icnológicos detallados permitieron detectar subdivisiones dentro de los principales ambientes identificados. Por ejemplo, el análisis icnológico ayudó a diferenciar entre depósitos de shoreface inferior distal y shoreface inferior proximal. De la misma manera, las características icnológicas y sedimentológicas permitieron identificar al menos tres tipos de canales mareales con diferentes niveles energéticos. Algunas estructuras biogénicas de la Formación Chenque fueron encontradas en asociación con los cuerpos fósiles de sus organismos productores (e.g. excavaciones de erizos irregulares, estructuras de equilibrio de bivalvos bisados y excavaciones de bivalvos de la subfamilia Pholadinae). En estos casos se realizaron análisis morfo-etológicos determinando el modo de construcción de los icnofósiles. Finalmente se desarrolló un análisis comparativo de los sistemas de excavaciones a través del Fanerozoico desde la perspectiva de la paleobiología evolutiva. Se confeccionó una base de datos con las referencias de todas las estructuras biogénicas. Estos sistemas de galerías son asignados a la actividad de decápodos en depósitos post-Paleozoicos preferentemente. De este modo se compararon las tendencias obtenidas a partir de la base de datos con la información aportada por los registros de cuerpos fósiles de estos organismos. Así se determinó que la abundancia de registros icnológicos observados a partir del Mesozoico se correlaciona claramente con las tendencias postuladas a partir de los cuerpos fósiles de decápodos. Los registros paleozoicos probablemente reflejen convergencias etológicas. Asimismo se concluyó que las asociaciones icnofaunísticas miocenas de la Formación Chenque reflejan ampliamente la actividad endobentónica de los organismos de la Fauna Evolutiva Moderna, con una alta diversidad y un intenso uso del ecoespacio infaunal.This study analyzes the ichnological content of the shallow marine deposits of the Chenque Formation (early Miocene). 23 ichnogenera were identified. Most of the ichnotaxa developed in softgrounds, but three ichnogenera occur only in firmgrounds. Two main depositional environments were recognized: wave-dominated deposits and tide-dominated deposits. Trace fossils present in firmground substrates reveal the presence of discontinuity surfaces. Detailed ichnological studies allow the identification of subenvironments within the major environments recognized. For example, ichnologic analysis helps to distinguish distal lower shoreface deposits from proximal lower shoreface deposits. In addition, the ichnological and sedimentological characteristics allowed identification of at least three types of tidal channels with different energy levels. Some biogenic structures were found in association with their producers (e.g. excavations of irregular urchins, equilibrium structures produced by byssate bivalves and excavations produced by pholadid bivalves). In these examples morphoethological analyses were done in order to determine the constructional processes involved in the formation of the ichnofossils. Finally, a comparative analysis of burrow systems through geologic time is undertaken from the perspective of evolutionary paleobiology. A database summarizing trace fossil occurrences through the Phanerozoic was compiled. These burrow systems are interpreted as decapod excavations in post-Paleozoic strata. The information obtained from the database was interpreted with respect to the life history of burrowing decapods, as inferred from the body fossil record. The trace fossil abundance found from the Mesozoic to the Cenozoic correlates with the trends postulated for the decapod body fossils. Paleozoic occurrences most likely reflect behavioral convergence. The ichnological association from the Miocene Chenque Formation clearly reflects the activity of the components of the Modern Evolutionary Fauna, with high diversity and intense use of the infaunal ecospace.Fil: Carmona, Noelia B.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

<i>Scolicia,</i> ichnotaxonomic practices, and the limits of behavioural convergence

Many studies convincingly documented the diagnostic morphology of Scolicia, establishing a functional link with the burrowing activities of heart urchins, common components of the marine mobile faunas that emerged during the Mesozoic Marine Revolution. However, this ichnotaxon has been repeatedly mentioned in both continental successions and Paleozoic marine strata, even though its diagnostic characteristics were absent. This approach may reflect the implicit belief that behavioural convergence is rampant in the ichnologic record and can be uncritically adopted in almost all cases. We underscore the importance of a definition of Scolicia that is based on diagnostic morphologic features and supported by a solid analysis from the perspective of function and mode of construction. This view of Scolicia highlights its value in paleoenvironmental reconstructions and its significance to unravel macroevolutionary trends. Attribution to an ichnotaxon should involve understanding of morphologic features in terms of its mode of production rather than the mere observation of superficial similarities.</p