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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.


Transverse CT slices through the skull.Computed tomography (CT) slices through the vertex of Isthminia panamensis (USNM 546125) across four slightly sub-transverse planes that pass anterior to posterior (A–D). CT slices (A–D) represent respective CT slices numbers 20566, 20625, 20655, and 20708, available for download on the Smithsonian X 3D browser (http://3d.si.edu). Numbers 1 and 2 denote facial and endocranial sagittal midlines, respectively, showing the sinistral displacement of the facial bones typical in many odontocetes (Geisler & Sanders, 2003; Mead & Fordyce, 2009).
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fig-7: Transverse CT slices through the skull.Computed tomography (CT) slices through the vertex of Isthminia panamensis (USNM 546125) across four slightly sub-transverse planes that pass anterior to posterior (A–D). CT slices (A–D) represent respective CT slices numbers 20566, 20625, 20655, and 20708, available for download on the Smithsonian X 3D browser (http://3d.si.edu). Numbers 1 and 2 denote facial and endocranial sagittal midlines, respectively, showing the sinistral displacement of the facial bones typical in many odontocetes (Geisler & Sanders, 2003; Mead & Fordyce, 2009).

Mentions: Frontal. Dorsally, the frontal is mostly covered by the maxilla, but it is exposed along the posterior and posteromedial edges of the skull roof (Figs. 3 and 5–7). In Isthminia, the right and left frontals form the highest part of the vertex, and together form a pair of rounded, rectangular knobs with a slight midline cleft (Figs. 3A–3C, 5 and 6). This topographic high for the frontals at the vertex is similar in Inia, Ischyrorhynchus or Meherrinia, and even Pontoporia and Lipotes, although the frontals in Isthminia are small and low by comparison with Pan-Inia. Unlike Inia and Meherrinia, the midline cleft between the right and the left frontals at the vertex does not show participation of an anterior supraoccipital (or possibly interparietal) wedge externally nor in internal CT scan data (Fig. 7 and Video S1). The dorsal surface of the vertex is lightly rugose, but not as strongly as in adult specimens of Inia.


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)

Transverse CT slices through the skull.Computed tomography (CT) slices through the vertex of Isthminia panamensis (USNM 546125) across four slightly sub-transverse planes that pass anterior to posterior (A–D). CT slices (A–D) represent respective CT slices numbers 20566, 20625, 20655, and 20708, available for download on the Smithsonian X 3D browser (http://3d.si.edu). Numbers 1 and 2 denote facial and endocranial sagittal midlines, respectively, showing the sinistral displacement of the facial bones typical in many odontocetes (Geisler & Sanders, 2003; Mead & Fordyce, 2009).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig-7: Transverse CT slices through the skull.Computed tomography (CT) slices through the vertex of Isthminia panamensis (USNM 546125) across four slightly sub-transverse planes that pass anterior to posterior (A–D). CT slices (A–D) represent respective CT slices numbers 20566, 20625, 20655, and 20708, available for download on the Smithsonian X 3D browser (http://3d.si.edu). Numbers 1 and 2 denote facial and endocranial sagittal midlines, respectively, showing the sinistral displacement of the facial bones typical in many odontocetes (Geisler & Sanders, 2003; Mead & Fordyce, 2009).
Mentions: Frontal. Dorsally, the frontal is mostly covered by the maxilla, but it is exposed along the posterior and posteromedial edges of the skull roof (Figs. 3 and 5–7). In Isthminia, the right and left frontals form the highest part of the vertex, and together form a pair of rounded, rectangular knobs with a slight midline cleft (Figs. 3A–3C, 5 and 6). This topographic high for the frontals at the vertex is similar in Inia, Ischyrorhynchus or Meherrinia, and even Pontoporia and Lipotes, although the frontals in Isthminia are small and low by comparison with Pan-Inia. Unlike Inia and Meherrinia, the midline cleft between the right and the left frontals at the vertex does not show participation of an anterior supraoccipital (or possibly interparietal) wedge externally nor in internal CT scan data (Fig. 7 and Video S1). The dorsal surface of the vertex is lightly rugose, but not as strongly as in adult specimens of Inia.

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.