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Isthminia panamensis, a new fossil inioid (Mammalia, Cetacea) from the Chagres Formation of Panama and the evolution of 'river dolphins' in the Americas.

Pyenson ND, Vélez-Juarbe J, Gutstein CS, Little H, Vigil D, O'Dea A - PeerJ (2015)

Bottom Line: 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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Paleobiology, National Museum of Natural History, Smithsonian Institution , Washington, DC , USA ; Departments of Mammalogy and Paleontology, Burke Museum of Natural History and Culture , Seattle, WA , USA.

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.

No MeSH data available.


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. Art by Julia Molnar.
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fig-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. Art by Julia Molnar.

Mentions: The fossil record of Inioidea reveals a far broader geographic distribution in the past than would be predicted from the extant ranges of Inia and Pontoporia. Fossil inioids outside of South America have predominantly been recovered from marine deposits representing nearshore depositional environments, although Isthminia’s recovery from rocks representing potentially a open ocean setting is consistent with ecomorphological traits that Isthminia shares with pelagic odontocetes alive today (Fig. 14). Although some freshwater Pan-Inia lineages from the late Miocene of Argentina may have been ∼4 m in total length, they are based on fragmentary remains (Cozzuol, 2010), and Isthminia is the largest marine inioid yet reported, in addition to being the only fossil inioid known from the Caribbean. Based on the available evidence, Isthminia occupied a high trophic level in a highly productive fully marine tropical Caribbean coastal ecosystem that predated the complete formation of the Panamanian Isthmus. Many of the bony fish species that are recorded in spectacular abundance from adjacent otolith assemblages in the Chagres Formation (File S2) may have formed a dominant portion of the prey resources for Isthminia, as they do for extant delphinids (see Kelley & Motani, 2015).


Isthminia panamensis, a new fossil inioid (Mammalia, Cetacea) from the Chagres Formation of Panama and the evolution of 'river dolphins' in the Americas.

Pyenson ND, Vélez-Juarbe J, Gutstein CS, Little H, Vigil D, O'Dea A - PeerJ (2015)

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. Art by Julia Molnar.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4562255&req=5

fig-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. Art by Julia Molnar.
Mentions: The fossil record of Inioidea reveals a far broader geographic distribution in the past than would be predicted from the extant ranges of Inia and Pontoporia. Fossil inioids outside of South America have predominantly been recovered from marine deposits representing nearshore depositional environments, although Isthminia’s recovery from rocks representing potentially a open ocean setting is consistent with ecomorphological traits that Isthminia shares with pelagic odontocetes alive today (Fig. 14). Although some freshwater Pan-Inia lineages from the late Miocene of Argentina may have been ∼4 m in total length, they are based on fragmentary remains (Cozzuol, 2010), and Isthminia is the largest marine inioid yet reported, in addition to being the only fossil inioid known from the Caribbean. Based on the available evidence, Isthminia occupied a high trophic level in a highly productive fully marine tropical Caribbean coastal ecosystem that predated the complete formation of the Panamanian Isthmus. Many of the bony fish species that are recorded in spectacular abundance from adjacent otolith assemblages in the Chagres Formation (File S2) may have formed a dominant portion of the prey resources for Isthminia, as they do for extant delphinids (see Kelley & Motani, 2015).

Bottom Line: 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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Paleobiology, National Museum of Natural History, Smithsonian Institution , Washington, DC , USA ; Departments of Mammalogy and Paleontology, Burke Museum of Natural History and Culture , Seattle, WA , USA.

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.

No MeSH data available.