Abstract
In contrast to dominant mode of ecological transition in the evolution of marine mammals, different lineages of toothed whales (Odontoceti) have repeatedly invaded freshwater ecosystems during the Cenozoic era. The so-called ‘river dolphins’ are now recognized as independent lineages that converged on similar morphological specializations (e.g., longirostry). In South America, the two endemic ‘river dolphin’ lineages form a clade (Inioidea), with closely related fossil inioids from marine rock units in the South Pacific and North Atlantic oceans. Here we describe a new genus and species of fossil inioid, Isthminia panamensis, gen. et sp. nov. from the late Miocene of Panama. The type and only known specimen consists of a partial skull, mandibles, isolated teeth, a right scapula, and carpal elements recovered from the Piña Facies of the Chagres Formation, along the Caribbean coast of Panama. Sedimentological and associated fauna from the Piña Facies point to fully marine conditions with high planktonic productivity about 6.1–5.8 million years ago (Messinian), pre-dating the final closure of the Isthmus of Panama. Along with ecomorphological data, we propose that Isthminia was primarily a marine inhabitant, similar to modern oceanic delphinoids. Phylogenetic analysis of fossil and living inioids, including new codings for Ischyrorhynchus, an enigmatic taxon from the late Miocene of Argentina, places Isthminia as the sister taxon to Inia, in a broader clade that includes Ischyrorhynchus and Meherrinia, a North American fossil inioid. This phylogenetic hypothesis complicates the possible scenarios for the freshwater invasion of the Amazon River system by stem relatives of Inia, but it remains consistent with a broader marine ancestry for Inioidea. Based on the fossil record of this group, along with Isthminia, we propose that a marine ancestor of Inia invaded Amazonia during late Miocene eustatic sea-level highs.
Highlights
In the evolution of marine mammals, the dominant mode of ecological transitions is the successful adaptation to marine life from terrestrial ancestry (Thewissen & Williams, 2002; Gingerich, 2005; Kelley & Pyenson, 2015)
Based on the fossil record of this group, along with we propose that the ancestor of Inia invaded the Brazil Craton during eustatic sea-level highs during the late Miocene
These fossil occurrences raise the question of how Inioidea evolved, and the evolutionary scenarios that led to their current distribution. 106 Here we describe a new genus and new species of Inioidea, based on a relatively complete skull, 107 mandibles, and a right scapula from the late Miocene of Panama
Summary
In the evolution of marine mammals, the dominant mode of ecological transitions (sensu Vermeij & Dudley, 2000) is the successful adaptation to marine life from terrestrial ancestry (Thewissen & Williams, 2002; Gingerich, 2005; Kelley & Pyenson, 2015). The group of extant “river dolphins” are the best exemplars of this ecological mode This non-monophyletic (i.e., paraphyletic or possibly polyphyletic) group includes four different living genera (Platanista, Lipotes, Inia, and Pontoporia) that show broad morphological similarities, including longirostral skulls and jaws, reduced orbits, flexible necks, and broad, paddle-shaped flippers (Geisler et al 2011). Fossils referred to Iniidae include a variety of taxa (e.g., Goniodelphis hudsoni, Meherrinia isoni, Ischyrorhynchus vanbenedeni), supplementing the existing data showing a much broader geographic extent for inioids in the fossil record than today These fossil occurrences raise the question of how Inioidea evolved, and the evolutionary scenarios that led to their current distribution.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
