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…
A fossil Porpoise from the Early Pliocene of Antwerp Harbour.Porpoises, Phocoenidae, are small Dolphins, Delphinida, found today across most of the world’s oceans, but with a fossil record restricted almost entirely to the North Pacific. Only a single fossil species from outside the Pacific Basin has been described,Septemtriocetus bosselaersi from Pliocene sediments at Verrebroek Dock in Antwerp...
A fossil Dolphin from the Late Miocene of Hokkaido, Japan, which may be the oldest True Dolphin.Dolphins, Delphinidae, are the most diverse group of Whales (or indeed any form of Marine Mammals) alive today, with 36 described species including charismatic animals such as Bottlenose Dolphins and Killer Whales. Historically a large number of fossils have also been...
A new species of Dolphin from the Early Miocene of Peru.The Squalodelphinidae are a small group of small to medium-sized Toothed Whales known from the Miocene of Europe and North...
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