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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.


Skull in ventral view.Ventral views of the type skull of Isthminia panamensis (USNM 546125) from (A) photographs and (B) orthogonal digital three-dimensional polygon model prepared from CT data, with lighting and color modifications using the Smithsonian X 3D browser. See http://3d.si.edu/explorer?s=iEpExr to measure, modify, or download this model.
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fig-4: Skull in ventral view.Ventral views of the type skull of Isthminia panamensis (USNM 546125) from (A) photographs and (B) orthogonal digital three-dimensional polygon model prepared from CT data, with lighting and color modifications using the Smithsonian X 3D browser. See http://3d.si.edu/explorer?s=iEpExr to measure, modify, or download this model.

Mentions: The skull of Isthminia is relatively complete on its dorsal aspect, although it is missing the left side of the facial bones (Fig. 3). The skull is heavily eroded along its ventral surface, and the basicranium is absent except for a small portion of the right parietal and right alisphenoid (Fig. 4). The skull preserves most of the dorsal aspect of the supraoccipital, including small portions that articulate with the vertex and nuchal and sigmoidal crests (Figs. 3A–3C). Overall, the profile of the skull is dominated by the rostrum (Fig. 5), which is complete and comprises approximately 75% of the length of the preserved skull (the rostrum length is 36.6 cm; Table 1). The anterior portion of the rostrum is slightly displaced by both an oblique and transverse fractures, likely from geologic compaction or other diagenetic factors, which displace the elements approximately 1–2 mm from their life positions. Most of the upper dentition is missing from the skull, except for the anterior teeth, some of which are complete; other more posterior teeth are incomplete, while three isolated teeth were recovered from the quarry at the type locality. Despite the heavy erosion that removed most of the left portion of this skull, sufficient anatomical details are preserved on the right side of the cranium, and along the rostrum to provide insights into the morphology of Isthminia.


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)

Skull in ventral view.Ventral views of the type skull of Isthminia panamensis (USNM 546125) from (A) photographs and (B) orthogonal digital three-dimensional polygon model prepared from CT data, with lighting and color modifications using the Smithsonian X 3D browser. See http://3d.si.edu/explorer?s=iEpExr to measure, modify, or download this model.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig-4: Skull in ventral view.Ventral views of the type skull of Isthminia panamensis (USNM 546125) from (A) photographs and (B) orthogonal digital three-dimensional polygon model prepared from CT data, with lighting and color modifications using the Smithsonian X 3D browser. See http://3d.si.edu/explorer?s=iEpExr to measure, modify, or download this model.
Mentions: The skull of Isthminia is relatively complete on its dorsal aspect, although it is missing the left side of the facial bones (Fig. 3). The skull is heavily eroded along its ventral surface, and the basicranium is absent except for a small portion of the right parietal and right alisphenoid (Fig. 4). The skull preserves most of the dorsal aspect of the supraoccipital, including small portions that articulate with the vertex and nuchal and sigmoidal crests (Figs. 3A–3C). Overall, the profile of the skull is dominated by the rostrum (Fig. 5), which is complete and comprises approximately 75% of the length of the preserved skull (the rostrum length is 36.6 cm; Table 1). The anterior portion of the rostrum is slightly displaced by both an oblique and transverse fractures, likely from geologic compaction or other diagenetic factors, which displace the elements approximately 1–2 mm from their life positions. Most of the upper dentition is missing from the skull, except for the anterior teeth, some of which are complete; other more posterior teeth are incomplete, while three isolated teeth were recovered from the quarry at the type locality. Despite the heavy erosion that removed most of the left portion of this skull, sufficient anatomical details are preserved on the right side of the cranium, and along the rostrum to provide insights into the morphology of Isthminia.

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.