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Based on mapping of laterally traceable stratigraphic discontinuities, we propose a high-resolution allostratigraphic scheme for one of the world’s foremost fossil marine vertebrate Lagerstätten: the lower Miocene strata of the Chilcatay Formation exposed along the Ica River near Zamaca, southern Peru. Measured sections combined with 1:10,000 scale mapping of a 24 km2 area provide an overview of the stratal architecture, as well as a general facies framework and interpretation of the various depositional settings. As a whole, the Chilcatay alloformation is bounded by the CE0.1 unconformity at the base and the PE0.0 unconformity at the top. An internal Chilcatay surface, termed CE0.2, splits the alloformation into two distinct allomembers (Ct1 and Ct2). The Ct1 allomember comprises three facies associations recording deposition in shoreface, offshore, and subaqueous delta settings. The Ct2 allomember comprises two facies associations, recording deposition in shoreface and offshore settings. Using these data, we place the rich marine vertebrate assemblage in a precise spatial and stratigraphic framework. The well-diversified vertebrate assemblage is dominated by cetaceans (mostly odontocetes) and sharks (mostly lamniforms and carcharhiniforms); rays, bony fish, and turtles are also present. Taxonomic novelties include the first records of baleen whales, platanistids, and eurhinodelphinids from the Chilcatay Formation.

Based on mapping of laterally traceable stratigraphic discontinuities, we propose a high-resolution allostratigraphic scheme for one of the world’s foremost fossil marine vertebrate Lagerstätten: the lower Miocene strata of the Chilcatay Formation exposed along the Ica River near Zamaca, southern Peru. Measured sections combined with 1:10,000 scale mapping of a 24 km2 area provide an overview of the stratal architecture, as well as a general facies framework and interpretation of the various depositional settings. As a whole, the Chilcatay alloformation is bounded by the CE0.1 unconformity at the base and the PE0.0 unconformity at the top. An internal Chilcatay surface, termed CE0.2, splits the alloformation into two distinct allomembers (Ct1 and Ct2). The Ct1 allomember comprises three facies associations recording deposition in shoreface, offshore, and subaqueous delta settings. The Ct2 allomember comprises two facies associations, recording deposition in shoreface and offshore settings. Using these data, we place the rich marine vertebrate assemblage in a precise spatial and stratigraphic framework. The well-diversified vertebrate assemblage is dominated by cetaceans (mostly odontocetes) and sharks (mostly lamniforms and carcharhiniforms); rays, bony fish, and turtles are also present. Taxonomic novelties include the first records of baleen whales, platanistids, and eurhinodelphinids from the Chilcatay Formation.

Based on mapping of laterally traceable stratigraphic discontinuities, we propose a high-resolution allostratigraphic scheme for one of the world’s foremost fossil marine vertebrate Lagerstätten: the lower Miocene strata of the Chilcatay Formation exposed along the Ica River near Zamaca, southern Peru. Measured sections combined with 1:10,000 scale mapping of a 24 km2 area provide an overview of the stratal architecture, as well as a general facies framework and interpretation of the various depositional settings. As a whole, the Chilcatay alloformation is bounded by the CE0.1 unconformity at the base and the PE0.0 unconformity at the top. An internal Chilcatay surface, termed CE0.2, splits the alloformation into two distinct allomembers (Ct1 and Ct2). The Ct1 allomember comprises three facies associations recording deposition in shoreface, offshore, and subaqueous delta settings. The Ct2 allomember comprises two facies associations, recording deposition in shoreface and offshore settings. Using these data, we place the rich marine vertebrate assemblage in a precise spatial and stratigraphic framework. The well-diversified vertebrate assemblage is dominated by cetaceans (mostly odontocetes) and sharks (mostly lamniforms and carcharhiniforms); rays, bony fish, and turtles are also present. Taxonomic novelties include the first records of baleen whales, platanistids, and eurhinodelphinids from the Chilcatay Formation.

The Cenozoic succession of the East Pisco Basin preserves the sedimentary record of several episodes of deformation of the forearc crust along the Peruvian margin. The 1:50,000 scale geological map presented here encompasses an area of about 1,000 km2 lying astride the Ica River, and contributes to our understanding of the timing and mode of basin filling and deformation. Our novel two-fold megasequence framework provides a sound basis for establishing a first-order tectono-stratigraphic setting of the mid-Eocene–upper Miocene succession exposed in the study area. We interpret that the mid-Eocene to lower Oligocene succession studied in this work (megasequence P) was deposited in a single forearc basin, which was dissected into the present-day West and East Pisco basins by a fault-bounded basement high during the late Oligocene, and subsequently overlain by the Miocene fill of the East Pisco basin (megasequence N).

Strata of Chilcatay and Pisco formations exposed in the Ica Desert (East Pisco Basin, southern Peru) preserve one of the most complete and rich records of Miocene marine vertebrates of the world. Despite its exceptional importance, the chronostratigraphy of these fossil-bearing deposits has been only sporadically studied in the literature until recently. This work presents a detailed reconstruction of the chronostratigraphic framework, achieved by mapping and measuring of seven sections (totaling a height of almost 1000 m) of the Miocene Chilcatay and Pisco formations along the western side of the Ica River. The Chilcatay Formation consists of two allomembers, namely Ct1 and Ct2, bounded at the base by unconformities CE0.1 and CE0.2, respectively. Similarly, the immediately overlying Pisco Formation is divided into allomembers P0, P1, and P2, bounded at the base by unconformities PE0.0, PE0.1 and PE0.2, respectively. The new 39Ar–40Ar results presented here, combined with ages of previous work, provide precise constraints on the age of several stratigraphically referenced volcanic ash layers intercalated in the studied fossil-bearing succession, placing its vertebrate fossil fauna within a refined temporal framework and laying the solid ground for its detailed regional and global comparison. The ages of the allomembers, and thus their associated faunas, can be reliably estimated by the combination of 39Ar–40Ar dating on tephra layers with diatom biostratigraphy. In the study area, two methods are mutually consistent and constrain the deposition of the Chilcatay Formation between 19.2 and 18.0 Ma, that of P1 between 9.5 and 8.6 Ma, and that of P2 between 8.4 and 6.7 Ma. In the absence of direct dating of the P0 allomember, which lacks both preserved tephra suitable for 39Ar–40Ar dating and microfossils, its age can be constrained to the temporal gap between the youngest age available from the underlying Chilcatay strata (18.0 Ma) and the oldest age available from the overlying P1 strata (9.5 Ma).

Along the Peruvian coast, the sedimentary succession of the East Pisco Basin is exposed in the Ica Desert. At Cerro Los Quesos, laterally continuous dolomite layers characterise the diatomaceous sediments of the P2 sequence of the Miocene Pisco Formation, where a large number of marine vertebrates are exceptionally preserved, many enclosed in dolomite nodules. In this work, cemented layers from this sequence were described and sampled for petrographic, chemical, microscopic and isotopic analyses. Dolomite occurs in continuous 10–50 cm thick well cemented layers, formed by sediment of different nature: biogenic, terrigenous, volcanoclastic, and phosphatic. The underlying sediments exhibit a yellow layer with sparse dolomite crystals, a black layer with abundant Mn-oxides, and a reddish layer rich in Fe-oxides, indicating redox-related fronts. Two generations of dolomite can be recognised: an early diagenetic microcrystalline one, and a sparry one, filling the large cavities. As observed in both thin sections and on broken surfaces, microcrystalline dolomite also fills the inner spaces of the diatom areolae replicating their finest ultrastructure, such as foramina and cribra and replace calcite shells of foraminifera. δ18O and δ13C values from the microcrystalline dolomite of two layers, selected based on the absence of other carbonate phases (e.g. calcite) and the lack of sparry cement, are in agreement with those reported for the Peru margin and fall in the fields of either sulphate-reduction or methanogenesis. All the data point to dolomite precipitation associated with low-temperature early diagenesis that typically occurs in upwelling settings, where high surface water productivity is responsible for high rates of organic carbon flux to the sea bottom and for the cyclical oxygen depletion at the bottom. Such conditions also promote high abundances of marine vertebrates and the exceptional preservation of their skeletons in the sediments.

Bioapatite of fossil bone and teeth is susceptible to alteration and ion exchange during burial and diagenesis, varying its Sr content through the geological time. Nevertheless, fossil shark teeth are a powerful proxy for both chronostratigraphic and paleoecological reconstructions, thanks to the presence of the enameloid, a hard outer layer consisting of resistant fluorapatite crystallites. Here, we analyze fossil shark teeth from the Miocene sediments of the Chilcatay Formation of the Pisco Basin (southwestern Peru) with the aim of dating poorly constrained strata in this region. (Ultra)structural and compositional analyses of fossil lamniform and carcharhiniform teeth are performed through macroscopical observations, optical microscopy and SEM-EDS for evaluating the preservation state of the collected teeth. Shark teeth display a compact and well preserved outer enameloid layer formed by highly ordered bundles of crystallites that is distinctly separated by a more porous and heterogeneous inner core of dentine featuring diagenetic artefacts and microborings. Compositional mapping highlights differences in the distribution of Ca, P, F, and S in the enameloid and dentine, and chemical results show a Sr content that is consistent with the range reported for extant shark teeth. The best preserved teeth were selected for Strontium Isotope Stratigraphy (SIS), measuring the 87Sr/86Sr values in the enameloid and obtaining numerical (absolute) age estimates. In the Ica River Valley, SIS dates the Chilcatay strata to the Burdigalian (between 19.1 and 18.1 Ma), in agreement with previous radiometric, isotopic and biostratigraphic ages obtained in the same region. At Media Luna, the Chilcatay strata are dated herein for the first time, resulting in a slightly older age of 21.8–20.1 Ma (late Aquitanian–early Burdigalian). These results strengthen the notion that the Sr-ratio of shark teeth can be successfully applied for obtaining reliable age estimates via SIS.

International audience; Based on mapping of laterally traceable stratigraphic discontinuities, we propose a high-resolution allostratigraphic scheme for one of the world's foremost fossil marine vertebrate Lagerstätten: the lower Miocene strata of the Chilcatay Formation exposed along the Ica River near Zamaca, southern Peru. Measured sections combined with 1:10,000 scale mapping of a 24 km 2 area provide an overview of the stratal architecture, as well as a general facies framework and interpretation of the various depositional settings. As a whole, the Chilcatay alloformation is bounded by the CE0.1 unconformity at the base and the PE0.0 unconformity at the top. An internal Chilcatay surface, termed CE0.2, splits the alloformation into two distinct allomembers (Ct1 and Ct2). The Ct1 allomember comprises three facies associations recording deposition in shoreface, offshore, and subaqueous delta settings. The Ct2 allomember comprises two facies associations, recording deposition in shoreface and offshore settings. Using these data, we place the rich marine vertebrate assemblage in a precise spatial and stratigraphic framework. The well-diversified vertebrate assemblage is dominated by cetaceans (mostly odontocetes) and sharks (mostly lamniforms and carcharhiniforms); rays, bony fish, and turtles are also present. Taxonomic novelties include the first records of baleen whales, platanistids, and eurhinodelphinids from the Chilcatay Formation. ARTICLE HISTORY

Exquisitely preserved fossils of a new reef-building vermetid species from shallow-marine lower Miocene (Burdigalian) deposits of the Chilcatay Formation and upper Miocene (Tortonian) sediments of the Pisco Formation of Peru are here reported and described in detail for the first time. These finds are assigned to the living genus Thylacodes and recognized as representatives of a new species, Thylacodes devriesi sp. nov. This new taxon is known by long, almost straight tube-like shells that display peculiar ornamentations in form of striated lamellae and are arranged in an organ-pipe fashion. This discovery represents an important addition to the knowledge of the systematics and distribution of Thylacodes in South America in the geological past. Paleoenvironmental and taphonomic inferences drawn by the fossil remains of this reef-forming species are herein discussed for both the Chilcatay and Pisco Formations in the broader framework of the South American fossil record of Vermetidae.

Tephra layers are a unique tool for stratigraphy. Their geologically instantaneous deposition together with their dispersion on wide areas make them a powerful instrument for dating and correlating simultaneous events at different localities. In this paper, we present and discuss the application of tephra fingerprinting in the upper Miocene portion of the Pisco Formation. The Pisco Formation is an important marine vertebrate Fossil-Lagerstätte that crops out in the Ica Desert, on the Peruvian Coast. The paleontological relevance of this formation has entailed the necessity of reconstructing a chronostratigraphic framework, using 40Ar/39Ar dating and correlations on volcanic ashes. Tephra layers from the volcanic activity of the Peruvian Andes are very frequent in the Pisco Formation and many represent a primary air-fall deposition on the sea floor. Therefore, many samples from tephra layers within measured stratigraphic sections at different sites in the study area were fingerprinted with a combined approach, using petrographic and chemical analyses on phenocrysts and volcanic glasses, as well as glass shard morphology and granulometric analyses, performed on 53 selected samples, collected from different localities. Based on the results obtained, we correlate some tephra layers in two localities at several km distance, where the correlation between the measured sections was previously only hypothetical. Major element chemistry of biotite proved a valuable tool to discriminate different tephra in the upper Miocene portion of the Pisco Formation, due to the presence of peraluminous as well as metaluminous compositions. This work shows that it is possible to apply tephra fingerprinting and tephrostratigraphy in a complex situation such as that represented by the Pisco Formation, where a high number of apparently similar volcanic ash layers were deposited in a shallow marine environment. The application of this correlation method allows us to increase the chronostratigraphic detail in the studied interval of the Pisco Formation, providing an accurate framework in which fossil marine vertebrates are stratigraphically located. The obtained high-resolution chronostratigraphy will greatly clarify the evolutionary history of Cetaceans.

The late Miocene Pisco Formation of Peru is an outstanding example of richness and high-quality preservation of fossil marine vertebrates. In order to reconstruct the fossilization path, we present new textural, mineralogical and Sr-isotope data of diagenetic minerals formed in correspondence of fossil specimens such as marine vertebrates and mollusks. These fossil specimens were found at Cerro los Quesos, in the Ica Desert, within the diatomaceous strata of the Pisco Formation. Dolomite, gypsum, anhydrite and Mn minerals are the main phases found, while the calcium carbonate originally forming the mollusk valves is replaced by gypsum. An early formation of dolomite and of Mn minerals, triggered by the modifications of the geochemical environment due to organic matter degradation, is suggested by the textural relationships and is confirmed by the Sr isotopic ratio of dolomite, which agrees with that of seawater at the time of sedimentation. Instead, gypsum Sr isotopic ratios indicate a pre-Miocene seawater-derived brine circulating within the sedimentary sequence as a source for Sr. Oxidation of diagenetic sulfide causing a lowering of the pH of porewater is proposed as an explanation for Ca-carbonate dissolution. The diagenetic chemical environment was, nevertheless, favorable to bone preservation.

International audience; The Miocene Pisco Formation, broadly exposed in the Ica Desert of southern Peru, is among the most outstanding Cenozoic marine Fossil-Lagerstä tten worldwide. It is renowned for its exceptional preservation and abundance of vertebrate fossils, including a rich assemblage of whales and dolphins (Cetacea). Here, we integrate taphonomic data on 890 marine vertebrate fossils, gathered through 16 different localities. Our observations range from the taxonomic distribution, articulation, completeness, disposition and orientation of skeletons, to the presence of bite marks, associations with shark teeth and macroinvertebrates, bone and soft tissue preservation, and the formation of attendant carbonate concretions and sedimentary structures. We propose that the exceptional preservation characterising many Pisco vertebrates, as well as their exceptionally high abundance, cannot be ascribed to a single cause like high sedimentation rates (as proposed in the past), but rather to the interplay of several favourable factors including: (i) low levels of dissolved oxygen at the seafloor (with the intervention of seasonal anoxic events); (ii) the early onset of mineralisation processes like apatite dissolution/recrystallisation and carbonate mineral precipitation; (iii) rapid burial of carcasses in a soupy substrate and/or a novel mechanism involving scour-induced self-burial; and (iv) original biological richness. Collectively, our observations provide a comprehensive overview of the taphonomic processes that shaped one of South America's most important fossil deposits, and suggest a model for the formation of other marine vertebrate Fossil-Lagerstä tten.

We report on bite marks incising fossil mammal bones collected from upper Miocene deposits of the Pisco Formation exposed at Aguada de Lomas (southern Peru) and attributed to the giant megatooth shark Carcharocles megalodon. The bitten material includes skull remains referred to small-sized baleen whales as well as fragmentary cetacean and pinniped postcrania. These occurrences, the first in their kind from the Southern Hemisphere, significantly expand the still scarce record of bite marks for C. megalodon; moreover, for the first time a prey (or scavenging item) of C. megalodon is identified at the species level (as Piscobalaena nana, a diminutive member of the extinct mysticete family Cetotheriidae). Due to the fragmentary nature of the studied material, the exact origin of the detected marks (i.e., by scavenging or by active predation) cannot be ascertained. Nevertheless, relying on actualistic observations and size-based considerations, we propose that diminutive mysticetes (e.g., cetotheriids) were some of the target prey of adult C. megalodon, at least along the coast of present-day Peru. C. megalodon is thus here interpreted as an apex predator whose trophic spectrum was focused on relatively small-sized prey. Lastly, we propose a link between the recent collapse of various lineages of diminutive mysticetes (observed around 3 Ma) and the extinction of C. megalodon (occurring around the end of the Pliocene).

Fossil bones, together with teeth, are the most common remains of vertebrates that could manage to get preserved over geological times, providing information on the diagenetic and fossilization processes that occurred in the depositional paleoenvironment. Fossil bones from the marine vertebrate Konservat-Lagerstatte in the East Pisco Basin and Sacaco area (Peru) show a high variety of different textural and chemical features, suggestive of different processes variably contributing to the fossilization path. At the macroscopic scale, bone samples can be grouped into six different categories on the basis of the color (red to gray to white) and hardness (which relates to the mineralization degree); a variety of case studies can be found between these categories. Microscopically, the original microstructure of the bone tissue, both compact and cancellous, is well preserved in all the studied samples, with differences in cavity fillings, distribution of microcracks, and presence of Fe oxides in the diverse bone types. The bone composition and mineralogy correspond to fluorapatite. Differences in color, mineralization degree and geochemistry can be interpreted in terms of different fossilization paths, from burial at the seafloor to exposure in the present-day desert environment. The fossilization paths are strongly conditioned by the factors controlling the interplay of the mechanisms of apatite dissolution-recrystallization and dolomite precipitation (formation of carbonate concretions) as well as the fixation of iron in finely disseminated sulfides in the very early stages of fossilization.

The upper Neogene Pisco Formation of Peru is known worldwide as one of the most significant Cenozoic\ud marine vertebrate Konservatt-Lagerstätten, even featuring cetacean specimens that retain remains of soft tissues or\ud stomach contents. Previous works showed that biomediated precipitation of dolomite concretions around large-sized\ud decaying carcasses was one of the most relevant processes responsible for exceptional fossil preservation. In turn, little is\ud known about the modes of fossilization of well-preserved small-sized vertebrates, which are rather common in the Pisco\ud Formation, but mostly do not exhibit dolomite concretions. We report on a cetacean specimen, identified as belonging\ud to the extinct short-snouted, small dolphin species Brachydelphis mazeasi (Pontoporiidae), preserved within a late\ud Miocene sandy deposit at the site of Pampa Corre Viento. This specimen consists of a moderately disarticulated partial\ud skeleton exhibiting well-mineralized bones; it is not enclosed within a dolomite concretion, being however delimited\ud by an evident dark boundary in the host sediment. Scanning electron microscopy and microanalytical investigations\ud identify Mn-oxides and apatite as early diagenetic minerals around the skeleton. We argue that a rapid burial of the\ud specimen was pivotal for the preservation of the bones, and allowed the early establishment of anoxic processes for\ud degradation of organic matter. Coupled with availability of P in porewater, the reducing conditions and the lowered\ud pH allowed precipitation of Ca-phosphate while increasing Mn solubility close to the pontoporiid carcass. Mn-oxides\ud precipitated at the redox boundary, the latter defining the outer edge of the volume of sediment affected by altered\ud chemical conditions due to the decaying processes. The permeability of the sediment and the small size of the carcass\ud were possible factors unfavorable to extensive sulfate reduction, thus preventing the formation of a dolomite concretion\ud and allowing bone phosphatization. This record emphasizes the role of conditions favorable to bone mineralization in\ud absence of an isolating carbonate concretion, in cases of high quality preservation of small-sized vertebrates observed\ud in the Pisco Formation. The observation of patterns in the distribution of diagenetic minerals in the sediment enclosing\ud vertebrate remains without, or with limited carbonate concretions provides insights into early taphonomic processes.