Search

Fig. S3: Biotite Mg/Mg+Fe+Mn vs Al diagram of P2 ash layers.

Fig. S4: Biotite Mg/Mg+Fe+Mn vs Mn diagram of P2 ash layers.

Fig. S5: Biotite Mg/Mg+Fe+Mn vs K+Na+Ca diagram of P2 ash layers.

The upper Neogene Pisco Formation of Peru is known worldwide as one of the most significant Cenozoic marine vertebrate Konservatt-Lagerstatten, even featuring cetacean specimens that retain remains of soft tissues or stomach contents. Previous works showed that biomediated precipitation of dolomite concretions around large-sized decaying carcasses was one of the most relevant processes responsible for exceptional fossil preservation. In turn, little is known about the modes of fossilization of well-preserved small-sized vertebrates, which are rather common in the Pisco Formation, but mostly do not exhibit dolomite concretions. We report on a cetacean specimen, identified as belonging to the extinct short-snouted, small dolphin species Brachydelphis mazeasi (Pontoporiidae), preserved within a late Miocene sandy deposit at the site of Pampa Corre Viento. This specimen consists of a moderately disarticulated partial skeleton exhibiting well-mineralized bones; it is not enclosed within a dolomite concretion, being however delimited by an evident dark boundary in the host sediment. Scanning electron microscopy and microanalytical investigations identify Mn-oxides and apatite as early diagenetic minerals around the skeleton. We argue that a rapid burial of the specimen was pivotal for the preservation of the bones, and allowed the early establishment of anoxic processes for degradation of organic matter. Coupled with availability of P in porewater, the reducing conditions and the lowered pH allowed precipitation of Ca-phosphate while increasing Mn solubility close to the pontoporiid carcass. Mn-oxides precipitated at the redox boundary, the latter defining the outer edge of the volume of sediment affected by altered chemical conditions due to the decaying processes. The permeability of the sediment and the small size of the carcass were possible factors unfavorable to extensive sulfate reduction, thus preventing the formation of a dolomite concretion and allowing bone phosphatization. This record emphasizes the role of conditions favorable to bone mineralization in absence of an isolating carbonate concretion, in cases of high quality preservation of small-sized vertebrates observed in the Pisco Formation. The observation of patterns in the distribution of diagenetic minerals in the sediment enclosing vertebrate remains without, or with limited carbonate concretions provides insights into early taphonomic processes. RESUMEN La Formación Pisco de Perú, del Neógeno tardío, es conocida a nivel mundial como uno de los principales Konservatt-Lagerstatten, donde se preservan vertebrados marinos, presentando incluso, restos de tejidos blandos y contenido estomacal de los fósiles. Trabajos previos han demostrado que las precipitaciones de dolomita, mediante actividad biológica, alrededor de grandes cadáveres en descomposición, fue uno de los procesos principales en la preservación de estos fósiles. Sin embargo, son poco conocidos los procesos de fosilización de los vertebrados de pequeño tamaño, los cuales se encuentran bien conservados y son bastante comunes en la Formación Pisco, pero en su mayoría no muestran concreciones de dolomita. En este estudio presentamos un pequeño espécimen de cetáceo, perteneciente a la especie extinta de nariz corta Brachydelphis mazeasi (Pontoporiidae), conservado dentro de un depósito arenoso del Mioceno tardío, en el sitio de Pampa Corre Viento. Este espécimen consiste de un esqueleto parcialmente desarticulado que exhibe una buena mineralización, no se encuentra dentro de una concreción de dolomita, pero el sedimento de su en torno posee una coloración negra bien evidenciada. Mediante microscopía electrónica se han identificado óxidos de Mn y apatito alrededor del esqueleto como minerales producto de diagénesis temprana. Argumentamos que un enterramiento rápido del espécimen fue fundamental para la preservación de los huesos, y permitió el pronto establecimiento de condiciones anóxicas para la degradación de la materia orgánica. La disponibilidad de P en el agua de poro, las condiciones reductoras y disminución del pH, permitieron la precipitación de Ca-fosfato, a la vez que aumentaba la solubilidad de Mn cerca de la carcasa del pontoporiido. Los oxidos de Mn precipitaron en el límite redox y definieron el borde externo del sedimento químicamente afectado por los procesos de descomposición. La permeabilidad del sedimento y el tamaño pequeño de la carcasa fueron posiblemente factores desfavorables para la reducción de sulfato, evitando así la formación de una concreción de dolomita y permitiendo la fosfatación ósea. Este registro enfatiza el papel de las condiciones favorables para la mineralización ósea en ausencia de la formación de una concreción aislante de carbonato, en casos de preservación de alta calidad de vertebrados de pequeño tamaño de la Formación Pisco. La observación de patrones de distribución de minerales diagenéticos en el sedimento puede dar una idea acerca de los procesos tafonómicos tempranos.

Fig. S2: Stratigraphic logs of the studied sites.

Fig. S1: 39Ar–40Ar age spectrum of the biotite separate of the tephra layer LB-T11.

The sequence stratigraphic framework and a summary of the fossil fauna of the upper Miocene portion of the Pisco Formation exposed along the western side of the Ica River (southern Peru) is presented through a new geological map encompassing an area of about 200 km2 and detailed chronostratigraphic analyses. Extensive field mapping and sedimentological study of outcrop sections have shown that the Pisco Formation is a cyclical sediment unit composed of at least three fining-upward, unconformity-bounded depositional sequences, designated P0, P1, and P2 from oldest to youngest. In the study area, these sequences progressively onlap a composite basal unconformity from southwest to northeast. Integration of biostratigraphic and tephrochronologic age determinations constrains the ages of the three Pisco sequences within the study area. Based on the age of surrounding sediments, a conservative estimate of the age of P0 suggests deposition of these strata between 17.99 ± 0.10 Ma and 9.00 ± 0.02 Ma, whereas diatom biostratigraphy and calculated 40Ar/39Ar ages converge to indicate that strata of the P1 sequence were deposited sometime between 9.5 Ma and 8.9 Ma and that those of the P2 sequence are younger than 8.5 Ma and older than 6.71 ± 0.02 Ma. Our survey for both vertebrate and macro-invertebrate remains in the three sequences confirms the outstanding paleontological value of the Pisco Formation and contributes to depict regional faunal shifts in the fossil assemblage.

The Mio – Pliocene Pisco Formation, Peru, is a world renowned Konservat-Lagerstätte. Beside its potential in preserving important information on the history of the Pacific Ocean during the Miocene, the Pisco Fm. provides palaeontologists with the unique opportunity to study in detail the evolution of marine vertebrate fauna during the Neogene. Because diatomites are one of the main lithologies of the Pisco Fm., previous authors have hypothesised exceptionally high sedimentation rates related to the deposition of diatom frustules as the main cause leading to the formation of the Lagerstätte. However, sedimentation rates were never calculated. With the perspective to build a solid stratigraphic framework for the whole Pisco Fm., we started to investigate two important fossiliferous sites: Cerro Colorado and Cerro Los Quesos. Within these two areas, measurement of multiple stratigraphic sections combined with extensive field mapping of a number of distinct marker beds was carried out, which allowed a high-resolution correlation and creation of a local lithostratigraphic framework. Integrated tephrostratigraphy,40Ar/39 Ar dating and diatom biostratigraphy permitted an accurate chronostratigraphy to be established. Both biostratigraphic and radiometric ages converge to indicate a late Miocene age for the two sites, amending previous attribution of Cerro Colorado and Cerro Los Quesos to the middle Miocene. Age models for the site of Cerro Los Quesos reveal that sedimentation rates are high but in line with those of high-productivity environments, thus invalidating the hypothesis of exceptional sedimentation rates as the main cause for the formation of the Lagerstätte. These results are extremely important also in the perspective of building a complete stratigraphic framework for the Pisco Fm.

International audience; One-hundred and ninety-two fossil marine vertebrate specimens, preserved as bone elements cropping out at Cerro Los Quesos (Pisco Basin, Peru), are identified and reported on a 1:4,000 scale geological map and in the corresponding stratigraphic section. All the fossils originate from the Pisco Formation, which is dated in this area to the late Miocene (from 7.55 Ma to ≥6.71 Ma, based on 40 Ar/ 39 Ar analyses of three volcanic ash layers along the section). Specimens are particularly concentrated near the top of the two main hills, where the geologically youngest portion of the examined section crops out. The impressive fossil assemblage includes cetaceans (91.6%), represented by mysticetes (balaenopteroids and cetotheriids) and odontocetes (phocoenids, physeteroids, and ziphiids, including the holotype of Nazcacetus urbinai). Seals, a crocodile, a seabird, bony fish, and sharks are also reported. Isolated large teeth of Carcharocles and Cosmopolitodus are common and, in several instances, associated to mysticete skeletons. Together with a similar work recently published for the other late Miocene locality of Cerro Colorado, this work represents a case study for the detailed inventory of the extraordinary paleontological heritage of the Pisco Basin. As such, it constitutes the basis for future taphonomic, paleoecological, and systematic studies, as well as for a much-needed conservation effort. ARTICLE HISTORY

The Mio-Pliocene Pisco Formation (Ica Desert, Peru) is known for its vast outcrops, which preserve one of the richest concentrations of fossil marine vertebrates on Earth. During the last thirty years, punctual studies carried out on several exceptionally well preserved individual specimens highlighted the extraordinary diversity of vertebrates that populated the area: cetaceans, pinnipeds, turtles, sea birds, and fishes are just a few examples. Through our successive fieldwork campaigns, for the first time a detailed taphonomic and stratigraphic study of the fossil assemblages from localities of the oldest strata (middlelate Miocene) of the Pisco Fm. was undertaken. Stratigraphy was used to correctly assign specimens to well-defined strata and, consequently, to evaluate the temporal variation in the composition and concentration of the fossils along the investigated stratigraphical sequence. GPS data were collected for all the fossil vertebrates and a preliminary systematic determination was carried out in the field for all the specimens, while the most interesting and well-preserved ones were taken in laboratory for preparation and detailed study. Taphonomic observations were also carried out: orientation of the skeletons, skeletal completeness, bone articulation, associated invertebrates, associated shark teeth, and evidence of bioerosion. Sediment samples enclosing the fossils were collected along the continuous vertical diatomaceous mudstone sequences and were examined through different analyses (grain size analyses, micropalaeontological analyses, major and minor elements analyses, etc.) to understand the condition of burial and to obtain a more precise dating of the fossiliferous layers, using calcareous nannoplankton (where present), diatoms, silicoflagellates, radiolarians, and dinoflagellates as biostratigraphic markers. Some of these microfossils were also used to measure as proxies to infer on paleo-depth and water paleo-temperature. Samples for radiometric Argon-Argon dating were collected in discrete tephra-beds identified in some of the investigated sequences. All the information obtained through stratigraphical and taphonomic analyses will be used and integrated to better understand the context, both physical and ecological, in which these exceptional fossil assemblages were deposited and preserved

Laminated diatomaceous mudstones are the most characteristic lithology of the Mio- Pliocene Pisco Formation, Ica desert, Peru; discrete tuff layers intercalate to them. The Pisco Fm. deposited during the Neogene in the East Pisco Basin, one of the Andean forearc basins which run eastward of the Peru-Chile trench. Nowadays, the Pisco Fm. is one of the most famous Fossillagerstätte in the world. Its popularity is due to the great abundance of exceptionally well preserved marine vertebrates, especially cetaceans, lying in the diatomaceous mudstone. Many specimens are indeed fully articulated and often concentrated in horizontal accumulation layers. This study is meant to understand how diatom deposition may have positively interfered in the genesis of this Fossillagerstätte, by: 1) analyzing diatom laminae and their different assemblages and obtaining information on the seasonality affecting the water column during deposition of the Pisco Fm.; 2) recognizing and evaluating any evidence of exceptional harmful algal blooms (HABs) coinciding with the fossil vertebrate accumulation levels; HABs were recently evoked to explain four discrete horizons showing fossil marine vertebrates accumulation in the late Miocene of Atacama desert, Chile (Pyenson et al., 2014); 3) investigating the hypothesis that diatom and diatom mats deposition may have favored the preservation of vertebrates by lowering the level of dissolved oxygen on the sea floor; indeed, these conditions would have prevented the development of a whale-fall community that generally leads to the fast decay of the carcasses and the final disarticulation of skeletons; 4) evaluating the sedimentation rates of the diatomaceous mudstones to understand how long it would have taken for a carcass to be completely buried, thus, verifying whether fossil preservation is mainly due to the high sedimentation rates of diatomaceous mudstones as suggested by Brand et al. (2004); Moreover, we are investigating diatom assemblages close to tuff levels to verify whether the deposition of volcanic ashes may have enhanced primary production in the Pisco Fm.; indeed, anomalous algal blooms recently observed on the ocean surface following volcanic explosive events have been related to the release of macronutrients such as Fe, NH+ 4,NO-3, NO-2, PO3-4, Si by volcanic ashes reaching the ocean surface (Duggen et al., 2007).