A new testudinoid turtle from the middle to late Eocene of Vietnam

Abstract

BackgroundTestudinoidea is a major clade of turtles that has colonized different ecological environments across the globe throughout the Tertiary. Aquatic testudinoids have a particularly rich fossil record in the Tertiary of the northern hemisphere, but little is known about the evolutionary history of the group, as the phylogenetic relationships of most fossils have not been established with confidence, in part due to high levels of homoplasy and polymorphism.MethodsWe here focus on describing a sample of 30 testudinoid shells, belonging to a single population that was collected from lake sediments from the middle to late Eocene (35–39 Ma) Na Duong Formation in Vietnam. The phylogenetic placement of this new material is investigated by integrating it and 11 other species of putative geoemydids from the Eocene and Oligocene to a recently published matrix of geoemydid turtles, that embraces the use of polymorphic characters, and then running a total-evidence analysis.ResultsThe new material is highly polymorphic, but can be inferred with confidence to be a new taxon, Banhxeochelys trani gen. et sp. nov. It shares morphological similarities with other southeastern Asian testudinoids, Isometremys lacuna and Guangdongemys pingi, but is placed phylogenetically at the base of Pan-Testuguria when fossils are included in the analysis, or as a stem geoemydid when other fossils are deactivated from the matrix. The vast majority of other putative fossil geoemydids are placed at the base of Pan-Testuguria as well.DiscussionThe phylogenetic placement of fossil testudinoids used in the analysis is discussed individually and each species compared to Banhxeochelys trani gen. et sp. nov. The high levels of polymorphism observed in the new taxon is discussed in terms of ontogenetic and random variability. This is the first time that a large sample of fossil testudinoids has its morphological variation described in detail.

A new alligatoroid from the Eocene of Vietnam highlights an extinct Asian clade independent from extantAlligator sinensis

“…During sedimentation, the area was in a transitional stage from shallow ponds to an anoxic lake. Further, tomistomine and Asiatosuchus -like crocodiles, as well as many fish taxa and two different turtle species occurred sympatrically with the herein described alligatoroid (Böhme et al, 2012; Garbin, Böhme & Joyce, 2019).…”

A nomenclature for fossil and living turtles using phylogenetically defined clade names

“…1). Composition-Although results are still preliminary, recent phylogenies suggest that numerous fossil testudinoids from the Eocene of the Northern Hemisphere traditionally classified as "geoemydids" may represent the stem lineage of Geoemydidae (Garbin et al 2019 (Linnaeus, 1758), or the platysternid Platysternon megacephalum Gray, 1831b (Fig. 4a).…”

“…Composition-At present, 62 extant species are recognized in many tropical to temperate regions across the globe, primarily Asia (TTWG 2017). Fossils are known from Asia (Claude et al 2012;Garbin et al 2019), Europe Not established phylogenetic definitions-Bataguridae Gray, 1870 [Joyce et al, 2004]; Geoemydidae, Theobald 1868 [Vlachos, 2018]. Diagnostic Apomorphies-Although there is strong molecular support for the monophyly of geoemydids (e.g., Pereira et al 2017), osteological apomorphies are currently lacking that uniquely diagnose the group.…”

A taxonomic revision of geoemydid turtles from Siwalik-age of India and Pakistan

“…All examined material was scored in the morphological matrix of 96 shell characters for geoemydid species from Garbin et al (2019) that extensively covers the use of polymorphic characters for this group (see Supplementary File). The fossils were then compared with material of extant Indian geoemydid species through photographs and based on the scoring of the characters in the matrix.…”