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Skull in left lateral view of the holotype (MUSM 3898) of Ensidelphis riveroi from the lower Miocene Chilcatay Fm (Zamaca, Pisco Basin, Peru). (a,b), complete skull; (c,d), detail of the neurocranium. Linear hatching indicates major breaks, dark shading areas covered by the sediment or dental alveoli, and beige shading reconstructed missing parts. In (b) and (d) the mandibles are shown in blue and the tympanic bulla in brown.

Skull in left lateral view of the holotype (MUSM 3898) of Ensidelphis riveroi from the lower Miocene Chilcatay Fm (Zamaca, Pisco Basin, Peru). (a,b), complete skull; (c,d), detail of the neurocranium. Linear hatching indicates major breaks, dark shading areas covered by the sediment or dental alveoli, and beige shading reconstructed missing parts. In (b) and (d) the mandibles are shown in blue and the tympanic bulla in brown.

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Several aspects of the fascinating evolutionary history of toothed and baleen whales (Cetacea) are still to be clarified due to the fragmentation and discontinuity (in space and time) of the fossil record. Here we open a window on the past, describing a part of the extraordinary cetacean fossil assemblage deposited in a restricted interval of time...

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... preorbital processes of the frontals are broken and only a posteromedial portion is preserved. In lateral view the best-preserved right process (Figure 5c,d) displays a thickening (ratio between the height of this process measured in lateral view perpendicular to the maxilla-frontal suture and the vertical distance from the lower margin of the occipital condyles to the vertex of the skull = 0.10) that is lesser than in Dilophodelphis, Furcacetus, Medocinia, Pomatodelphis, and Zarhachis, all having ratios >0.30. However, this value in Ensidelphis was measured along the broken lateral surface and a thickening of the preorbital process in its missing lateral portion cannot be excluded. ...
Context 2
... ventral view on the right side of the rostrum, 54 alveoli are visible, the six anteriormost being in the premaxilla (Figure 5a,b). However, this value does not represent the total tooth count of the upper right quadrant, considering that the posterior portion of the upper right alveolar row is covered by the mandible. ...

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... Subsequently, Lambert et al. (2019) included a unique eurhinodelphinid genus (i.e., Xiphiacetus) in a broadscale, total evidence analysis of the odontocete families [51] with the result that it was the sister group of the platanistid Zarhachis flagellator, forming a clade that was the sister group of the whole Delphinida. Later, another work found that a monophyletic Eurhinodelphinidae clade was basal to a large Platanistoidea clade [52]. In summary, it is clear that the phylogenetic relationships of the Eurhinodelphinidae within the odontocete families are still not completely resolved. ...
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    ... Squalodelphinidae are archaic longirostrine platanistoid dolphins known mostly from the early and middle Miocene of Europe, North America, and South America (Dal Piaz, 1917;Lambert et al., 2014;Nelson and Uhen, 2018). Though many archaic odontocetes in various stem and crown groups have been allied with Platanista within the Platanistoidea, Squalodelphinidae seem to be the only other family consistently forming a sister taxon relationship with the Platanistidae (Geisler et al., 2011;Lambert et al., 2014;Tanaka and Fordyce, 2015;Boersma and Pyenson, 2016;Albright et al., 2018;Bianucci et al., 2020). Squalodelphinids share maxillary crests and long rostra with the Platanistidae, though the crests are dense, massive, and dorsally convex (Lambert et al., 2014) rather than being large flanges flanking the melon (e.g., Platanista, Zarhachis). ...
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    ... The high diversity of this clade during the Early Miocene is demonstrated by the odontocete assemblage of the Chilcatay Formation as known from strata exposed at Ulluyaja, Zamaca and south of Cerro Colorado. Indeed, three new genera and species of squalodelphinids (Furcacetus flexirostrum Bianucci et al., 2020, Huaridelphis raimondii Lambert et al., 2014band Macrosqualodelphis ukupachai Bianucci et al., 2018a and a new genus and species of platanistoid basal to the clade formed by the squalodelphinids and the platanistids (Ensidelphis riveroi Bianucci et al., 2020) have been described from these localities; in addition, the squalodelphinid Notocetus vanbenedeni Moreno, 1893, a platanistid close to Araeodelphis natator Kellogg, 1957 and another unnamed platanistoid have also been reported (Lambert et al., 2014b;Bianucci et al., 2015Bianucci et al., , 2018aBianucci et al., , 2020. The high diversity of platanistoids of the Chilcatay Formation (Figs 18, 19) correlates with a high disparity in the size and shape of the skulls. ...
    ... The high diversity of this clade during the Early Miocene is demonstrated by the odontocete assemblage of the Chilcatay Formation as known from strata exposed at Ulluyaja, Zamaca and south of Cerro Colorado. Indeed, three new genera and species of squalodelphinids (Furcacetus flexirostrum Bianucci et al., 2020, Huaridelphis raimondii Lambert et al., 2014band Macrosqualodelphis ukupachai Bianucci et al., 2018a and a new genus and species of platanistoid basal to the clade formed by the squalodelphinids and the platanistids (Ensidelphis riveroi Bianucci et al., 2020) have been described from these localities; in addition, the squalodelphinid Notocetus vanbenedeni Moreno, 1893, a platanistid close to Araeodelphis natator Kellogg, 1957 and another unnamed platanistoid have also been reported (Lambert et al., 2014b;Bianucci et al., 2015Bianucci et al., , 2018aBianucci et al., , 2020. The high diversity of platanistoids of the Chilcatay Formation (Figs 18, 19) correlates with a high disparity in the size and shape of the skulls. ...
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    ... Distribution of hyper-longirostry within a composite phylogenetic tree of crown odontocetes supplemented with schematic drawings of the dorsal view of the skull from a well-known species for each clade in which hyper-longirostry is documented. Relationships between extant odontocete families based on McGowen et al. (2020) and relationships of extinct clades (including debated relationships of Eurhinodelphinidae + Eoplatanistidae; different hypotheses indicated with dashed lines) based on Muizon (1991), Lambert (2005a), Geisler et al. (2011), Tanaka et al. (2017, and Bianucci et al. (2020). Line drawings of Eoplatanista, Parapontoporia, Zarhinocetus, and Xiphiacetus modified from Marx et al. (2016); line drawing of Ensidelphis modified from Bianucci et al. (2020). ...
    ... Relationships between extant odontocete families based on McGowen et al. (2020) and relationships of extinct clades (including debated relationships of Eurhinodelphinidae + Eoplatanistidae; different hypotheses indicated with dashed lines) based on Muizon (1991), Lambert (2005a), Geisler et al. (2011), Tanaka et al. (2017, and Bianucci et al. (2020). Line drawings of Eoplatanista, Parapontoporia, Zarhinocetus, and Xiphiacetus modified from Marx et al. (2016); line drawing of Ensidelphis modified from Bianucci et al. (2020). All skulls scaled at the same condylobasal length Goolaerts et al. 2020) of the new finds of fossil remains of Eurhinodelphinidae in the upper Miocene (middle Tortonian) Diest Formation in Belgium (North Sea Basin). ...
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    Article
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    ... One living riverine species, P. gangetica, warrants special attention. This species is the sole extant representative of an otherwise diverse fossil group, the Platanistoidea (sensu de Muizon 1987), whose contents and phylogenetic relationships are still debated (e.g., Fordyce and de Muizon 2001;Boersma et al. 2017;Tanaka and Fordyce 2017 and references therein;Viglino et al. 2018aViglino et al. ,b, 2020Gaetán et al. 2019;Bianucci et al. 2020). The earliest fossil record of this group comes from the late Oligocene, reaching a peak in diversity by the early Miocene and a marked decrease by the middle-late Miocene (e.g., de Muizon 1987;Fordyce and de Muizon 2001). ...
    ... For the purposes of this study, we have defined the clade Platanistoidea as including the genera Waipatia, Otekaikea, Awamokoa, Zarhachis, Notocetus, Aondelphis, and Platanista (Viglino et al. 2018a,b). These genera represent families that are usually recovered within this clade: Waipatiidae, Squalodelphinidae, and Platanistidae (for details on specimens used, see Table 1) (but note that some authors recovered waipatiids as stem Odontoceti; e.g., Lambert et al. 2018;Bianucci et al. 2020). For further details on the still-debated systematic and phylogenetic aspects of Platanistoidea, see, for example, Fordyce (1994), Lambert et al. (2014), Fordyce (2015, 2017), Boersma and Pyenson (2016), Boersma et al. (2017), Viglino et al. (2018aViglino et al. ( ,b, 2020, Gaetán et al. (2019), and Bianucci et al. (2020), as well as references therein. ...
    ... These genera represent families that are usually recovered within this clade: Waipatiidae, Squalodelphinidae, and Platanistidae (for details on specimens used, see Table 1) (but note that some authors recovered waipatiids as stem Odontoceti; e.g., Lambert et al. 2018;Bianucci et al. 2020). For further details on the still-debated systematic and phylogenetic aspects of Platanistoidea, see, for example, Fordyce (1994), Lambert et al. (2014), Fordyce (2015, 2017), Boersma and Pyenson (2016), Boersma et al. (2017), Viglino et al. (2018aViglino et al. ( ,b, 2020, Gaetán et al. (2019), and Bianucci et al. (2020), as well as references therein. The sample also included representatives of two putative platanistoids, Squalodon calvertensis and Prosqualodon australis (for details on the phylogenetic position of these taxa, please see de Muizon [1994] and Gaetán et al. [2019]). ...
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    ... Basal odontocetes with double-rooted cheek teeth (e.g., Inticetus), platanistoids (e.g., Huaridelphis, Macrosqualodelphis), and archaic delphinidans (e.g., Incacetus, Atocetus, 'Kentriodontidae') experienced a demise at ~ 10 Ma or much earlier within the EPB (Colbert 1944;Muizon 1988;Lambert et al. 2017;Bianucci et al. 2018Bianucci et al. , 2020. Some platanistoids and 'kentriodontids', however, did survive longer in isolated marine and freshwater settings (Ichishima et al. 1994;Marx et al. 2017), as the South Asian river dolphin Platanista, the only extant representative of the platanistoids (Barnes et al. 2006). ...
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    The highly productive waters of the Humboldt Current System (HCS) host a particular temperate ecosystem within the tropics, whose history is still largely unknown. The Pisco Formation, deposited during Mio-Pliocene times in the Peruvian continental margin has yielded an outstanding collection of coastal-marine fossils, providing an opportunity to understand the genesis of the HCS ecosystem. We present a comprehensive review, completed with new results, that integrates geological and paleontological data from the last 10 My, especially focusing on the southern East Pisco Basin (Sacaco area). We discuss the depositional settings of the Pisco Formation and integrate new U/Pb radiometric ages into the chronostratigraphic framework of the Sacaco sub-basin. The last preserved Pisco sediments at Sacaco were deposited ~ 4.5 Ma, while the overlying Caracoles Formation accumulated from ~ 2.7 Ma onwards. We identified a Pliocene angular unconformity encompassing 1.7 My between these formations, associated with a regional phase of uplift. Local and regional paleoenvironmental indicators suggest that shallow settings influenced by the offshore upwelling of ventilated and warm waters prevailed until the early Pliocene. We present an extensive synthesis of the late Miocene–Pleistocene vertebrate fossil record, which allows for an ecological characterization of the coastal-marine communities, an assessment of biodiversity trends, and changes in coastal-marine lineages in relation to modern HCS faunas. Our synthesis shows that: (i) typical endemic coastal Pisco vertebrates persisted up to ~ 4.5 Ma, (ii) first modern HCS toothed cetaceans appear at ~ 7–6 Ma, coinciding with a decline in genus diversity, and (iii) a vertebrate community closer to the current HCS was only reached after 2.7 Ma. The genesis of the Peruvian coastal ecosystem seems to be driven by a combination of stepwise transformations of the coastal geomorphology related to local tectonic pulses and by a global cooling trend leading to the modern oceanic circulation system.