Toothed Whales, Odontocetes, are known to have colonised freshwater systems multiple times during the Neogene, resulting in four distinct lineages of 'River Dolphins' in different geographical areas, the Iniidae, Lipotidae, Platanistidae, and Pontoporiidae. The Lipotidae had a single species which persisted into modern times, Yangtze River Dolphin, Lipotes vexillifer, but which is thought to have become extinct in the late twentieth century due to Human activities. The Platanistidae comprises two species of completely freshwater-dwelling Dolphins from South Asia, Platanista minor from the Indus River system, and Platanista gangetica from the Ganges River and associated waterways. The Iniidae comprises a single genus of River Dolphins from South America, Inia, as well as a number of fossil species in four extinct genera. The single known species of Pontoporiidae, the La Plata River Dolphin, Pontoporia blainvillei, is also found in South America, but is not an obligate freshwater dweller, also venturing into coastal waters.
The two species of Platanista are among the most highly specialised Cetaceans, with large enlarged, thin and pneumatic supraorbital crests enclosing their melons (fatty organs used in echolocation), and eyes reduced to the point where they are almost blind; an evolutionary adaptation to living in sediment laden waters. A number of fossil Dolphins have been assigned to the Platanistidae, but all are from marine sediments, leaving no clear indicator as to when the group moved into freshwater systems. The South American Iniidae present a similar situation, with most known fossil species coming from marine deposits, although one freshwater species, Ischyrorhynchus vanbenedeni, has been described from the Late Miocene of Argentina.
In a paper published in the journal Science Advances on 20 March 2024, Aldo Benites-Palomino of the Department of Paleontology at the University of Zurich, and the Departamento de Paleontología de Vertebrados at the Museo de Historia Natural of the Universidad nacional Mayor de San Marcos, Gabriel Aguirre-Fernández, also of the Department of Paleontology at the University of Zurich, Patrice Baby of Geosciences-Environnements Toulouse at the Université de Toulouse, Diana Ochoa of the Centro de Investigación para el Desarrollo integral y Sostenible at the Universidad Peruana Cayetano Heredia, and the Departmento de Geología at the Universidad de Salamanca, Ali Altamirano, also of the Departamento de Paleontología de Vertebrados at the Museo de historia natural of the Universidad nacional Mayor de San Marcos, John Flynn of the Division of Paleontology at the American Museum of Natural History, the Department of Earth & Environmental Sciences at Columbia University, and the Graduate Programs in Biology and Earth and Environmental Sciences at the City University of New York, Marcelo Sánchez-Villagra, again of the Department of Paleontology at the University of Zurich, Julia Tejada, again of the Departamento de Paleontología de Vertebrados at the Museo de Historia Natural of the Universidad nacional Mayor de San Marcos, the Division of Paleontology at the American Museum of Natural History, and of the Division of Geological and Planetary Sciences at the California Institute of Technology, Christian de Muizon of the Departement Origines et Evolution at the Muséum Nation-al d’Histoire Naturelle, and Rodolfo Salas-Gismondi, once again of the Departamento de Paleontología de Vertebrados at the Museo de Historia Natural of the Universidad nacional Mayor de San Marcos, the Centro de Investigación para el Desarrollo integral y Sostenible at the Universidad Peruana Cayetano Heredia, and the Division of Paleontology at the American Museum of Natural History, describe a new species of Platanistid Dolphin from the Early to Middle Miocene Pebas Formation of Peruvian Amazonia.
The new species is named Pebanista yacuruna, where 'Pebanista' is a combination of Pebas, after the formation from which the fossil came, and Platanista, the only living genus of Platanistid Dolphin, and 'yacuruna' is a mythical water creature from the folklore of the Kichua peoples of the Peruvian Amazon. The new species is described from a single partial skull, comprising the posterior part of the rostrum, the facial region including part of the right supraorbital crest, the temporal and occipital regions.
The skull has a preserved length of 698 mm and an estimated width of 281 mm. The sutures of the skull are well fused, indicating that it was an adult at the time of death. The vertex of the skull is deviated leftwards, the premaxillae in the rostrum and facial areas is asymmetric, the braincase is anteroposteriorly shorter than wide; and the palatines lack contact and project dorsolaterally, all of which are diagnostic of the Platanistidae. Based upon the width of the skull, the living Dolphin is estimated to have been between 281 cm and 247 cm in length. Although Pebanista yacuruna undoubtedly occupied an inland, freshwater environment, it is similar in size to marine members of the group, considerably larger than living Platanista spp., and recent River Dolphins in general.
All known living members of the Platanistidae are restricted to the river systems of South Asia. Fossil Platanistid Dolphins are fairly common in the Oligocene and Miocene, when they appear to have had a global distribution and to have occupied a range of ecological niches, but all previously described fossils of the group are considered to have been marine. The group reached peak diversity in the Early Miocene, declining in numbers and diversity after this time, following the emergence of f other toothed Cetacean groups such as Delphinoids, Beaked Whales, and Physeteroids around the time of the Middle Miocene Climatic Optimum.
The Early Miocene was a time of global cooling, as well as increased subsidence in the Andean-Amazonian foreland basin system, with most of the modern west Amazon rainforest area in Colombia, Peru, and Brazil covered by a continental-scale fresh water to brackish water foreland system (the Pebas System) parallel to the Andes. During the Early Miocene there were at least two large scale marine influxes from the Caribbean Sea into this basin. This wetland ecosystem reached its maximum extent during the Middle Miocene Climatic Optimum, as a complex arrangement of terrestrial and aquatic environments rich in nutrients and prey types, inhabited by a wide range of Fish, Turtles, Crocodylians (Caimans and Gharials), and mammals (Marsupials, Sloths, Rodents, Primates, and Ungulates), among others. This resource rich ecosystem appears to have favoured large sizes in predators such as Pebanista yacuruna and Gharials, a group of Crocodilians which also have marine ancestry, but which today are restricted to South Asia.
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