Pebanista yacuruna: A new species of South Asian River Dolphin from the Miocene of Peru.

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.

Pebanista yacuruna, MUSM 4017. holotype skull in dorsal (A) and (B), ventral (C) and (D), left lateral (E) and (F), and anterodorsal views (F) and (G). Benites-Palomino et al. (2024).

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.

Size comparison between 'River Dolphins' and marine Platanistoids and River Dolphins. White silhouettes indicate the minimum body length calculated or recorded; gray body outlines indicate the largest size recorded or estimated in: (A) Macrosqualodephis ukupachai, (B) Zarhachis flagellator, (C) Pontoporia blainvillei, (D) Pebanista yacuruna, (E) Inia geoffrensis, (F) Platanista gangetica, and (G) Lipotex vexillifer. (H) Artistic reconstruction of Pebanista yacuruna. Jaime Bran in Benites-Palomino et al. (2024).

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.

See also...

Isthminia panamensis: A South American ‘River Dolphin’ from the Late Miocene of Panama.

There are four species of River Dolphins known today; Inia geoffrensis and Pontoporia blainvillei from South America, Platanista gangetica from South Asia and the probably extinct Lipotes vexillifer from China (Lipotes vexillifer, the Yangtze River Dolphin, is considered Critically Endangered under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species, as there were less than 50 adult individuals surviving when the species was last assessed in 2003, all living within a few hundred kilometers of one-another on a single river). These were formerly thought to comprise a single group, but are now thought to represent three distinct lineages that separately invaded freshwater environments from the sea. The fossil record of the Asian River Dolphins is poor, but the South American River Dolphins, Inioidea, have a fairly good fossil record, with freshwater fossils known from a number of sites in South America, and older marine specimens from South America, North America and Europe.

In a paper published in the journal Peer J on 1 September 2015, Nicholas Pyenson of the Department of Paleobiology at the National Museum of Natural History and the Departments of Mammalogy and Paleontology at the Burke Museum of Natural History and Culture, Jorge Vélez-Juarbe of the Department of Mammalogy at the Natural History Museum of Los Angeles County and the Florida Museum of Natural History at the University of Florida, Carolina Gutstein of the Department of Paleobiology at the National Museum of Natural History and the Comisión de Patrimonio Natural at the Consejo de Monumentos Nacionales, Holly Little, also of the Department of Paleobiology at the National Museum of Natural History, and Dioselina Vigil and Aaron O’Dea of the Smithsonian Tropical Research Institute, describe a new species of South American ‘River Dolphin’ from the Late Miocene Chagres Formation on the Caribbean coast of Panama.

The new species is named Isthminia panamensis, where ‘Isthminia’ means ‘isthmus-Inia’ a reference to the similarity between the fossil, which was found on the Isthmus of Panama, and the living Inia geoffrensis and ‘panamensis’ means ‘from Panama’. It is described from an almost complete skull, a complete set of mandibles (lower jaws) and an incomplete right scapula and two carpals. The specimen is estimated to be between 5.8 and 6.1 million years old.

Skull in dorsal, anterior, and posterior views. Dorsal views of the skull of Isthminia panamensis from (A) photographs and (B) orthogonal digital three-dimensional polygon model prepared from CT data, with lighting and colour modifications using the Smithsonian X 3D browser. (C) Anterior and (D) posterior views of the skull of Isthminia panamensis from orthogonal digital three-dimensional polygon model prepared from CT data. Anatomical Abbreviations: adif, anterior dorsal infraorbital foramen; alis, alisphenoid; fr, frontal; gf, glenoid fossa of squamosal; ju, jugal; la, lacrimal; max, maxilla; mc, maxillary crest; me, mesethmoid; na, nasal; nar, bony narial opening or naris; nuc, nuchal crest; pa, parietal; pdif, posterior dorsal infraorbital foramen; pmax, premaxilla; popf, postorbital process of the frontal; socc, supraoccipital; sopf, supraorbital process of frontal; smf, suprameatal fossa; sq, squamosal; zpsq, zygomatic process of squamosal. Pyenson et al. (2015).

Isthminia panamensis is estimated to have been about 285 cm long in life, making it large for a Inioid, though not the largest species known. It has an extremely long snout compared to the skull, though this is typical for an Inioid, being one of the more obvious distinguishing features of the group. It had 15 teeth on each side of its upper jaw and 18 teeth on each side of its lower jaw; most of these teeth are straight and lack wear facets, though the canines and premolars curve backwards slightly, and some of these do show wear.

Mandibles in ventral and lateral views. Ventral views of the mandibles of Isthminia panamensis from (A) photographs and (B) orthogonal digital three-dimensional polygon model prepared from CT data, with lighting and colour modifications using the Smithsonian X 3D browser. (C) Left lateral and (D) right lateral views of the mandibles of Isthminia panamensis from orthogonal digital three-dimensional polygon model prepared from CT data. Anatomical Abbreviations: ap, angular process of mandible; cp, coronoid process of mandible; mef, mental foramen or foramina; ms, mandibular symphysis. Pyenson et al. (2015).

Isthminia panamensis shows a number of morphological traits more consistent with a fully marine lifestyle than a freshwater one, and the Chagres Formation is interpreted as having been an open water marine environment not close to any estuary or riverine environment. This is interesting as, while marine Inioids are well known in the fossil record, the majority of these are thought to have lived before the group invaded freshwater environments, or at to have descended from members of this group that diverged from the true ‘River Dolphins’ before this invasion. Isthminia panamensis appears to have been more closely related to the living River Dolphin Inia geoffrensis than the other South American River Dolphin species, Pontoporia blainvillei. This has implications for the history of the Inioids as a group, as it implies that either the ancestors of Inia geoffrensis and Pontoporia blainvillei invaded freshwater environments separately, or that the ancestors of Isthminia panamensis moved from a marine environment to a freshwater environment and back again. Either of which scenarios undermines the simple view of a single transition from one habitat to another by a group which gave rise to all later River Dolphins in South America, and suggests a more complex history for the group.

Reconstruction of Isthminia. Life reconstruction of Isthminia panamensis, feeding on a Flatfish, which would have been abundant in the neritic zone of the late Miocene equatorial seas of Panama. Julia Molnar in Pyenson et al. (2015).

See also…

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A new species of Dolphin from the Early Miocene of Peru.
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