This study implies a survey of a somewhat unexplored Cambrian carbonate formation in the Argentine Precordillera (AP) located in western Argentina, close to the city of San José de Jáchal. The carbonate platform of the AP is a unique piece of the South American geology and is in this study partly surveyed and compared with the Stephen Formation of northern Canada, a middle Cambrian unit renowned for its contents of exceptionally well preserved soft bodied fossils named the Burgess Shale biota. The investigated formation consists of an olistolith among the several Los Túneles Olistoliths at the Western Precordillera. The olistolith originates from the Cambrian Period and lies embedded in younger material with an age and history up for debate by several paleontologists and biostratigraphers. Shallow investigations have shown that these rocks may have similar properties to rocks of the Stephen Formation. There are only a few known rock assemblages on Earth showing Burgess Shale-type (BST) preservation and if the Los Túneles Olistolith proves to possess BST preservation it would be of great substance for the geological researchers of Argentina. The olistolith has in this study been explored by gathering samples in field and dissolving them in acid to investigate possible fossil content. The aim is thus to ascertain whether or not the Los Túneles Olistolith may contain especially well preserved fossils. This is the first study ever made of the microfauna in a middle Cambrian unit in the whole of South America, and hence it will tell if further investigations would be of interest. This survey determines that the Los Túneles Olistolith actually consists of three olistoliths encased in matrix, do not contain BST preservation and that further studies are not probable to show otherwise. The fossil findings, such as Chancelloriidae Chancelloria, Hexactinellida Recticulosa and Mollusca Hyolitha establish that the three Los Túneles Olistoliths originate from middle Cambrian while the matrix surrounding the olistolith is determined to be of Devonian age. / Den här studien är den första undersökningen någonsin som fokuserar på mikrofauna i sedimentära avlagringar från mellersta kambrium i Sydamerika. Studien utreder en tidigare bara ytligt utforskad kalkstensformation i västra Argentina, nära staden San José de Jáchal, i den argentinska precordilleran. Formationen, med namnet Los Sombreros Formationen, innehåller olistoliter från kambrium som undersökts och jämförts med den välkända Stephen Formation från norra Kanada, en formation som är känd för att innehålla exceptionellt välbevarade mjukdelar av fossil, kallat the Burgess Shale Biota. Syftet med studien är att fastställa huruvida Los Túneles Olistoliterna har potential att innehålla välbevarade fossil samt att utreda ifall ytterligare undersökningar är av intresse eller ej.                       Studien fokuserar på vad som tidigare trotts vara en av olistoliterna bland Los Túneles Olistoliterna, som är en del av Los Sombreros Formationen, men som i denna studie visat sig egentligen vara tre olika olistoliter som avsatts intill varandra. Tidigare ytliga undersökningar har visat att dessa olistoliter skulle kunna innehålla liknande fossil som the Stephen Formation. Det finns bara ett fåtal platser i världen där så pass exceptionellt välbevarade fossil tidigare hittats och om Los Túneles Olistoliterna skulle visa sig vara ett nytt fynd skulle det vara av betydelse för den fortsatta geologiska forskningen i Argentina. Genom insamling av prover som upplösts i syra och sedan undersökts i mikroskop har slutsatsen dragits att Los Túneles Olistoliterna inte innehåller Burgess Shale Biota och att ytterligare undersökningar förmodligen inte kommer visa annorlunda resultat. Fossilfynden som har gjorts, så som Chancelloriidae Chancelloria, Hexactinellida Recticulosa och Mollusca Hyolitha fastställer att de tre Los Túneles Olistoliterna härstammar från mellersta kambrium medan omkringliggande material kommer från Devon. / Este estudio se enfoca en un área poco explorada de una formación que aloja olistolitos carbonaticos del Cámbrico y Ordovícico situado en la Precordillera de Argentina en el oeste del país, cerca de San José de Jáchal, Provincia de San Juan. La plataforma carbonatada de la precordillera es una parte única de la geología de Sudamérica y es en esta investigación parcialmente estudiada y comparada con la Formación Stephen en el norte de Canadá, una unidad del Cámbrico Medio famosa por su contenido de fósiles excepcionalmente bien preservados llamado the Burgess Shale biota.                       La investigación se ha enfocado en un olistolito entre los varios que aparecen en el sector Los Túneles en el norte de la Precordillera de San Juan, oeste de Argentina. De este olistolito sa ha recuperado macrofauna indicativa del período Cámbrico. Este olistolito está alojado en rocas clásticas cuya una edad que es todavía un tema de debate entre paleontólogos y bioestratigrafos. Investigaciones superficiales han mostrado que estas rocas pueden tener los mismos atributos que las rocas de la Formación Stephen. Solo hay unas pocas formaciones sedimentarias en la Tierra que mantienen preservación del tipo de Burgess Shale (BST) y si el olistolito de estudio de la sección de Los Túneles presenta preservación de BST va a tener gran importancia para la Geología de Argentina. El estudio ha incluido un muestreo de campo de varios olistolitos, posterior tratamiento químico (desagregación física y química de las calizas en ácido para investigar possible contenido de microfósiles), y finalmente”picking” bajo lupa binocular para rescatar micropiezas fósiles. El objetivo de este es comprobar si el olistolito de la sección de Los Túneles puede tener fósiles especialmente bien preservados. Este estudio es el primer estudio de la micro fauna del Cámbrico medio en el conjunto de Sudamérica y de ahí que lo indicará si estudios adicionales serían de interés.                       Este investigación determina que el olistolito en Los Túneles Olistolitos en realidad consiste en tres olistolitos encerrados en matriz, contiene fósiles sin una preservación BST y por tanto estudios adicionales no son requeridos para demostrar lo contrario. Los hallazgos fósiles, por ejemplo Chancelloriidae Chancelloria, Hexactinellida Recticulosa y Mollusca Hyolitha, establezca que los tres olistolitos de Los Túneles Olistolitos son de Cámbrico Medio y la matriz que rodea a los olistolitos de periódo Devónico.

The middle Cambrian (Drumian) Rockslide Formation is a deeper-water succession of mixed carbonates and siliciclastics. At the Ravens Throat River location it hosts a Burgess Shale-type (BST) deposit. The BST units are found in two separate 1m thick horizons of green-coloured calcareous mudstone and contain a biota similar to that of the Wheeler and Marjum formations of Utah, and to some extent the Burgess Shale itself. The biota is low in diversity and preserves mainly robust soft-tissue parts. The lithologically heterogeneous composition of the formation (shale, dolomite, lime mudstone, sandstone, mudstone) and absence of metamorphism offer an opportunity to analyze the depositional environment and taphonomy of this deeper water unit. Geochemical analysis including, trace elements, organic carbon, biomarker, and synchrotron were attempted and yielded varying results. Trace element ratios, particularly V/Sc, indicate low oxygenation of the bottom waters during the deposition of the fossil-bearing interval. These results differ from most other BST deposits, which suggest that oxygenated bottom waters were maintained throughout sedimentation. In addition, organic matter in the sediments and δ13Corg values suggest the presence of benthic microbial mats. Silver is concentrated mostly in nm to μm-sized particles in the fossilifereous calcareous mudstone, suggesting elevated levels in the depositional environment. Organic films and the evidence for hyoliths feeding on them suggest an in situ preservation of the biota, perhaps from sudden and short anoxic events and quick burial under either microbial mats or the muddy sediment. This is also supported by the presence of only very rare trace fossils.

Approximately 600 to 500 million years ago, a major evolutionary radiation called the “Cambrian Explosion” gave rise to nearly all of the major animal phyla known today. This radiation is recorded by various fossil lagerstätten, such as the Burgess Shale in Canada, where soft-bodied animals are preserved in exquisite detail. Many Cambrian fossils are enigmatic forms that are morphologically dissimilar to their modern descendants, but which still provide valuable information when interpreted as stem-group taxa because they record the actual progression of evolution and give insight into the order of character acquisitions and homologies between living taxa. One such group of fossils is the anomalocaridids, large presumed predators that have had a complicated history of description. Their body has a trunk with a series of lateral lobes and associated gills, and a cephalic region with a pair of large frontal appendages, a circular mouth apparatus, stalked eyes and a cephalic carapace. Originally, two taxa were described from the Burgess Shale, Anomalocaris and Laggania, however data presented herein suggests that the diversity of the anomalocaridids was much higher. Newly collected fossil material revealed that a third Burgess Shale anomalocaridid, Hurdia, is known from whole-body specimens and study of its morphology has helped to clarify the morphology and systematics of the whole group. Hurdia is distinguished by having mouthparts with extra rows of teeth, a unique frontal appendage, and a large frontal carapace. Two species, Hurdia victoria and Hurdia triangulata were distinguished based on morphometric shape analysis of the frontal carapace. A phylogenetic analysis placed the anomalocaridids in the stem lineage to the euarthropods, and examination of Hurdia’s well-preserved gills confirm the homology of this structure with the outer branches of limbs in upper stem-group arthropods. This homology supports the theory that the Cambrian biramous limb formed by the fusion of a uniramous walking limb with a lateral lobe structure bearing gill blades. In this context, new evidence is present on the closely allied taxon Opabinia, suggesting that it had lobopod walking limbs and a lateral lobe structure with attached Hurdia-like gills. The diversity of the anomalocaridids at the Burgess Shale is further increased by two additional taxa known from isolated frontal appendages. Amplectobelua stephenensis is the first occurrence of this genus outside of the Chengjiang fauna in China, but Caryosyntrips serratus is an appendage unique to the Burgess Shale. To gain a better understanding of global distribution, a possible anomalocaridid is also described from the Sirius Passet biota in North Greenland. Tamisiocaris borealis is known from a single appendage, which is similar to Anomalocaris but unsegmented, suggesting this taxon belongs to the arthropod stem-lineage, perhaps in the anomalocaridid clade. Thus, the anomalocaridids are a widely distributed and highly diverse group of large Cambrian presumed predators, which provide important information relevant to the evolution of the arthropods.

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