Abstract
Resolving the phylogeny of sea turtles is uniquely challenging given the high potential for the unification of convergent lineages due to systematic homoplasy. Equivocal reconstructions of marine turtle evolution subsequently inhibit efforts to establish fossil calibrations for molecular divergence estimates and prevent the accurate reconciliation of biogeographic or palaeoclimatic data with phylogenetic hypotheses. Here we describe a new genus and species of marine turtle, Asmodochelys parhami, from the Upper Campanian Demopolis Chalk of Alabama and Mississippi, USA represented by three partial shells. Phylogenetic analysis shows that A. parhami belongs to the ctenochelyids, an extinct group that shares characteristics with both pan-chelonioids and pan-cheloniids. In addition to supporting Ctenochelyidae as a sister taxon of Chelonioidea, our analysis places Protostegidae outside of the Chelonioidea crown group and recovers Allopleuron hofmanni as a stem dermochelyid. Gap excess ratio (GER) results indicate a strong stratigraphic congruence of our phylogenetic hypothesis; however, the highest GER value is associated with the phylogenetic hypothesis of marine turtles which excludes Protostegidae from the Cryptodira crown group. Ancestral range estimations derived from our phylogeny imply a European or North American origin of Chelonioidea in the middle-to-late Campanian, approximately 20 Myr earlier than current molecular divergence studies suggest.
Highlights
Recent studies have demonstrated that the incorporation of palaeontological data improves forecasts of biodiversity responses to climate change [1,2,3]
With an extensive fossil record spanning more than 90 Myr, sea turtles (Chelonioidea) are the oldest living marine tetrapod lineage [4], and with modern species being focal taxa for global conservation efforts, chelonioids provide a prime model for this type of integrative approach to biodiversity risk assessment
As almost all known Late Cretaceous non-protostegid chelonioids are North American, the exclusion of protostegids from crown group Chelonioidea resolves the biogeographic issues associated with the placement of protostegids as derived dermochelyoids
Summary
Recent studies have demonstrated that the incorporation of palaeontological data improves forecasts of biodiversity responses to climate change [1,2,3]. The true relationship between protostegids and crown chelonioids can only be resolved through the further refinement of turtle character-taxon matrices and the inclusion of additional fossil chelonioids into global phylogenetic studies. We describe a new genus and species of fossil chelonioid, Asmodochelys parhami, from the Upper Campanian Demopolis Chalk (79–74.5 Ma [17]) of the Gulf Coastal Plain, USA This new taxon is included in an expanded phylogenetic analysis of turtles which indicates that Asmodochelys belongs to the extinct Ctenochelyidae, a pan-chelonioid group characterized by a laterally serrated shell, extensive costal and plastral fontanelles, and the presence of epineurals positioned at various intervals along the neural series. Our analysis recovers a novel phylogeny for marine turtles that, when combined with stratigraphic and biogeographic evidence, supports a North American or European origin of crown group Chelonioidea in the middle-to-late Campanian
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