Cretaceous soft-shelled turtles (Trionychidae) of Mongolia: new diversity, records and a revision
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This paper is devoted to the description and revision of material of Cretaceous soft-shelled turtles (Trionychidae) of Mongolia. It includes the description of seven trionychid species, six of which are new, and two new genera: the cyclanorbine Nemegtemys conflata gen. et sp. nov. from the Nemegt Formation (Maastrichtian), and the trionychines Gobiapalone breviplastra gen. et sp. nov. from the Nemegt and Barungoyot (Campanian) formations, G. orlovi from the Baynshire Formation (Cenomanian–Santonian), ‘Trionyx’ baynshirensis sp. nov. from the Baynshire Formation, ‘T.’ gilbentuensis sp. nov. from the Nemegt Formation, ‘T.’ gobiensis sp. nov. from the Nemegt Formation, and ‘T.’ shiluutulensis sp. nov. from an unknown formation (Campanian). In addition, one shell from the ?Baynshire Formation of Khermin Tsav is assigned to Gobiapalone sp. The type material of Amyda menneri is considered to be Trionychidae indet. and Amyda menneri to be a nomen dubium. Finally, we revise other available materials of Cretaceous trionychids from 45 localities in Mongolia. Nemegtemys conflata, if correctly assigned, is the earliest known member of Cyclanorbinae. The two species of the new genus Gobiapalone are included in two phylogenetic analyses of Trionychidae. In both analyses Gobiapalone is monophyletic. In the first analysis, Gobiapalone is placed within Apalonina. In the second analysis, Gobiapalone is sister to Apalonina. Thus, the results of both analyses show that Apalonina, which is a rather advanced and well-supported trionychid clade, or its closest sister taxon (stem-Apalonina), were present in the Late Cretaceous of Asia. These results suggest that most other supra-generic clades of modern trionychids had been established in Asia by the Late Cretaceous. That suggestion is supported by the discovery of a cyclanorbine Nemegtemys conflata in the Late Cretaceous of Mongolia. Finally we summarize the latest data on temporal and geographical distributions of Cretaceous Trionychidae of Asia and North America.