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| Orientalosuchus naduongensis
Massonne, Vasilyan, Rabi & Böhme, 2019
DOI: 10.7717/peerj.7562
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Abstract
During systematic paleontological surveys in the Na Duong Basin in North Vietnam between 2009 and 2012, well-preserved fossilized cranial and postcranial remains belonging to at least 29 individuals of a middle to late Eocene (late Bartonian to Priabonian age (39–35 Ma)) alligatoroid were collected. Comparative anatomical study of the material warrants the diagnosis of a new taxon, Orientalosuchus naduongensis gen. et sp. nov. The combined presence of an enlarged fifth maxillary tooth, prominent preorbital ridges, a large supraoccipital exposure on the skull table, a palatine-pterygoid suture anterior to the posterior end of the suborbital fenestra, and a pterygoid forming a neck surrounding the choana is unique to this species. Unlike previous phylogenies, our parsimony analysis recovers a monophyletic Late Cretaceous to Paleogene East to Southeastern Asian alligatoroid group, here named Orientalosuchina. The group includes Orientalosuchus naduongensis, Krabisuchus siamogallicus, Eoalligator chunyii, Jiangxisuchus nankangensis and Protoalligator huiningensis, all of them sharing a medial shifted quadrate foramen aerum. The recognition of this clade indicates at least two separate dispersal events from North America to Asia: one during the Late Cretaceous by Orientalosuchina and one by the ancestor of Alligator sinensis during the Paleogene or Neogene, the timing of which is poorly constrained.
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| Figure 2: Skull of Orientalosuchus naduongensis (GPIT/RE/09761) (holotype), Na Duong Formation, upper Eocene, Vietnam. Skull in dorsal (A and B) and ventral (C and D) view.
Abbreviations: ai, atlas intercentrum; an, angular; ar, articular; bo, basioccipital; bs, basisphenoid; ch, choana; d, dentary; d4, dentary tooth 4; emf, external mandibular fenestra; eo, exoccipital; ect, ectopterygoid; f, frontal; fo, foramen; fae, foramen aerum; hu, humerus; if, incisive foramen; itf, infratemporal fenestra; j, jugal; l, lacrimal; mx, maxilla; mx5, maxilla tooth 5; n, nasal; oc, occipital condylus; p, parietal; pf, prefrontal; pal, palatine; pmx, premaxilla; po, postorbital; pt, pterygoid; q, quadratum; qj, quadratojugal; sa, surangular; so, supraoccipital; sp, splenial; sq, squamosal; stf, supratemporal fenestra. Scale = 5 cm.
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| Figure 3: Skull of Orientalosuchus naduongensis (GPIT/RE/09730), Na Duong Formation, upper Eocene, Vietnam. Skull in dorsal (A and B) and ventral (C and D) view.
Abbreviations: ect, ectopterygoid; f, frontal; if, incisive foramen; j, jugal; l, lacrimal; mx, maxilla; mx5, maxilla tooth 5; n, nasal; pal, palatine; pf, prefrontal; pmx, premaxilla; pmx5, premaxilla tooth 5; po, postorbital; pob, postorbital bar; pt, pterygoid. Scale = 5 cm.
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Orientalosuchus naduongensis gen. et sp. nov.
Orientalosuchus
Etymology: The name Orientalosuchus refers to the Latin word “oriens” for “east” and “suchus” the old Greek word “soukhos” for “crocodile.”
Orientalosuchus naduongensis
Etymology: The species name “naduongensis” refers to the Na Duong coal mine type locality in northeastern Vietnam.
Diagnosis: Orientalosuchus naduongensis is diagnosed by the combination of the following characters: notch between the premaxilla and maxilla; dominant maxillary ridge alongside the nasal; the fifth maxillary tooth is the largest maxillary tooth; anterior tip of frontal is acute and projects between the nasal bones; small supratemporal fenestra; large supraoccipital exposure preventing the parietal from reaching the posterior skull table in adults; quadrate foramen aerum lies on the dorsomedial angle of the quadrate; large suborbital fenestrae reaching anteriorly the level of the seventh to eighth maxillary tooth; maxilla-palatine suture forms an obtuse angle and not reaching beyond the anterior end of the suborbital fenestra; palatine-pterygoid suture lies anterior to the posterior end of the suborbital fenestra; pterygoid forms a neck surrounding the choana; dentary tooth row with only 16 teeth; laterally compressed posterior teeth; very small external mandibular fenestra; foramen aerum at the lingual margin of the retroarticular process; axis with a hypapophysis that is located near the center of the centrum; coracoid with a very large glenoid; iliac blade with a rectangular posterior outline and a dorsal indentation; dorsal osteoderms with no or only modest ridge.
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Conclusions:
Parsimony analysis finds the new late Eocene taxon from Vietnam, Orientalosuchus naduongensis, as the sister taxon to Krabisuchus siamogallicus from the Eocene of Thailand. Together they form a monophyletic extinct basal East to Southeastern Asian alligatoroid clade of Late Cretaceous origin that also included Jiangxisuchus nankangensis, Eoalligator chunyii and Protoalligator huiningensis. The current phylogeny supports at least two different dispersals from North America to Eastern Asia: one during the Late Cretaceous (Orientalosuchina) and a second during the Cenozoic (Alligator sinensis lineage). Improved fossil calibrations and taxon sampling will be vital for further constraining the timing and resolving the climatic/paleogeographical context of these dispersals.
Tobias Massonne, Davit Vasilyan, Márton Rabi and Madelaine Böhme. 2019. A New Alligatoroid from the Eocene of Vietnam highlights An Extinct Asian Clade Independent from Extant Alligator sinensis. PeerJ. 7:e7562. DOI: 10.7717/peerj.7562
