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| Isthminia panamensis
Pyenson, Vélez-Juarbe, Gutstein, Little, Vigil & O’Dea, 2015
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. |
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.
Keywords: Ecology, Evolutionary Studies, Marine Biology, Paleontology, Zoology
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| Figure 1: Map of fossil and living Inioidea. Global map of living and fossil inioids, projected onto an orthographic globe, centered on 15°N, 45°W. Extant distributions of Inia geoffrensis (teal and black waterways) and Pontoporia blainvillei (dark gray), follow data from the IUCN (International Union for Conservation of Nature) (2013) and Secchi, Ott & Danilewicz (2003), respectively. Occurrences for fossil data derive from location of type localities for each taxon, except for reports for the Northern Europe by Pyenson & Hoch (2007), Western South America by Gutstein et al. (2015), and Amazonia and Eastern South America by Cozzuol (2010). Major fossil localites for enumerated inioids identified at least to the generic level, are listed alphabetically by region, and represented by teal or blue dots, for freshwater and marine deposits, respectively.
Base map generated by Indiemapper (http://indiemapper.com).
DOI: 10.7717/peerj.1227 |
Systematic paleontology
Cetacea Brisson, 1762
Odontoceti Flower, 1867
Delphinida Muizon, 1988a
Inioidea Gray, 1846 sensu Muizon, 1988a
Pan-Inia (NCN) (panstem-based version of Inia (Blainville, 1817))
Isthminia, gen. nov.
LSID: urn:lsid:zoobank.org:act:83F6A9B4-289D-45DE-A3D1-C361DAAAF973.
Definitions ‘Pan-Inia’ refers to the panstem that includes crown Inia (CCN), and all other lineages closer to Inia than to Pontoporia, such as Isthminia and Ischyrorhynchus. Subjective synonymies of Pan-Inia include: Iniidae Gray, 1846; Iniinae Flower, 1867; Saurocetidae Ameghino, 1891; Iniidae Muizon, 1984; Ischyrorhynchinae (Cozzuol, 1996); Iniidae Cozzuol, 2010; Iniidae Gutstein, Cozzuol & Pyenson, 2014b. Crown group Inia refers to the crown clade arising from the last common ancestor of all named species of Inia, including Inia boliviensis d’Orbigny, 1834 and Inia araguaiaensis Hrbek et al., 2014. Although we follow the suggestions of the Society for Marine Mammalogy’s Committee on Taxonomy (2014) in provisionally recognizing two sub-species of Inia geoffrensis (I. g. geoffrensis and I.g. humboldtiana Pilleri & Gihr, 1977), the phylogenetic definition of Inia can accommodate a plurality of species and subspecies.
Type and only known species. Isthminia panamensis, sp. nov.
Etymology. Isthm- reflects the type specimen’s provenance from the Isthmus of Panama and the crucial role that the formation of this isthmus played in Earth history and evolution of the biota of the Americas. This epithet follows in the tradition of another fossil cetacean from the Chagres Formation, Nanokogia isthmiaVelez-Juarbe et al., 2015. The feminine generic epithet Inia reflects its similarities to the living Amazon River dolphin (Inia geoffrensis). Pronunciation: ‘Ist-min-ee-a,’ with the emphasis on the second syllable.
Isthminia panamensis sp. nov.
Type locality. STRI locality 650009 (9°16′55.4880″N, 80°02′49.9200″W), less than 100 m northeast of the main road in the town of Piña, along the Caribbean Sea coastline of the Republic of Panama (Fig. 2).
Formation. Piña Facies of the Chagres Formation.
Etymology. The species epithet recognizes the Republic of Panama, its people, and the many generations of scientists who have studied its geological and biological histories.
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| Figure 14: 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. |
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| Figure 15: Stratigraphically calibrated phylogenetic tree of Inioidea. Time calibrated phylogenetic tree of select Delphinida, pruned from our consensus cladogram in Fig. 13, including Isthminia, with Delphinoidea collapsed. Stratigraphic range data derives from published accounts for each taxon, including global ranges. Geologic time scale based on Cohen et al. (2013). Calibration for major nodes depths follow mean divergence date estimates by McGowen, Spaulding & Gatesy (2009: table 3) for the following clades: a, Delphinida (24.75 Ma); b, Inioidea + Lipotes (22.15 Ma); c, Delphinoidea (18.66 Ma); and Inioidea (in open white circle, 16.68 Ma). All minor node depths are graphical heuristics, and not intended to reflect actual divergence dates. Arc indicates stem-based clade, Pan-Inia. Ecological habitat preference is based on depositional environment or extant habitat.
Abbreviations: Aquitan., Aquitanian; H., Holocene; Langh., Langhian; Mess., Messinian; P., Piacenzian; Ple., Pleistocene; Plioc., Pliocene; Serra., Serravallian; Zan., Zanclean. DOI: 10.7717/peerj.1227
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| Isthminia panamensis
Pyenson, Vélez-Juarbe, Gutstein, Little, Vigil & O’Dea, 2015
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.
DOI: 10.7717/peerj.1227
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Nicholas D. Pyenson, Jorge Vélez-Juarbe, Carolina S. Gutstein, Holly Little, Dioselina Vigil and Aaron O’Dea. 2015. Isthminia panamensis, A New Fossil inioid (Mammalia, Cetacea) from the Chagres Formation of Panama and the Evolution of ‘River Dolphins’ in the Americas. PeerJ. 3:e1227 DOI: 10.7717/peerj.1227
