The question of what are turtles has been a source of a lively scientific debate over the past decades. Until recently, the phylogenetic placement of turtles within Amniota was uncertain and controversial. Molecular studies at the genome level confirm their sister relationship to archosaurs and rejected their relationship to lepidosaurs. However, relationships of lineages of turtles have not been studied using genomic techniques.
In a forthcoming paper in Molecular Phylogenetics and Evolution Crawford and colleagues (in press 2014) provide the first genome-scale analysis of turtle phylogeny. They sequenced 2381 ultraconserved element (UCE) loci representing a total of 1,718,154 bp of aligned sequence. The sampling includes 32 turtle taxa representing all 14 recognized turtle families and six additional outgroups.
This robust phylogeny shows that proposed phylogenetic names correspond to well-supported clades, and this topology is more consistent with the temporal appearance of clades and paleobiogeography.
The ultraconserved element (UCE) loci phylogeny supports the monophyly of Cryptodira, with Trionychia as the sister taxon to all other cryptodires. The clade including non-trionychian cryptodires was previously phylogenetically defined as ‘Durocryptodira’ by Danilov and Parham. The topology from ultraconserved elements and other molecular studies support the monophyly and the recognition of Durocryptodira, which is in contrast with the morphological hypothesis.
Combining the UCE phylogeny with the known fossil record of turtles allows reconstruction of some global biogeographic patterns. Intercontinental dispersal of turtles is common, usually involving a limited number of species.
The earliest fossils of stem testudinoids, stem trionychians, and stem cryptodires are from Eurasia. Mapping this onto the UCE phylogeny suggests cryptodires originated in the Jurassic of Eurasian. The emergence of cryptodires in Eurasia is complemented by the concurrent origin of pan-pleurodires in the Southern Hemisphere (Gondwana). Given the distribution of the clades and the timing of their origin, the geography of the cryptodire-pleurodire split can be plausibly linked to the breakup of the supercontinent Pangaea; a pattern common to other terrestrial vertebrates (e.g., placental vs. marsupial mammals).
Despite the Jurrasic origin of cryptodiran turtles they did not dominate the fauna of northern continents for 100 million years (in the Cenozoic). Instead, stem turtles mostly the Paracryptodira) were diverse and abundant in North America during the Cretaceous and into the Cenozoic. In the late Cretaceous cryptodires started to appear in North America invading via high latitude dispersal routes. The UCE phylogeny confirms one of the North American durocryptodire lineages, the Americhelydia, underwent a modest radiation and accounts for 38 living species.
Warm periods in the Paleogene are responsible for the dispersal of many organinsism into North America through high latitude dispersal routes, including a wave of testudinoids. Two are modest radiations, four species of Gopherus (Testudinidae); nine species of Rhinoclemmys (Geoemydidae). Previous studies suggested that these genera are sister taxa to all of the Old World members of their respective clades. The authors sequenced Gopherus, Rhinoclemmys, and representative divergent members of geoemydids and testudinids and confirm the basal position of these North American genera. This pattern links the overall diversification at the base of these clades with their intercontinental dispersal, which can logically be attributed to periods of warm climate.
Similar to the Americhelydia, short branches within the testudinoids also suggest a rapid adaptive radiation that coincides with high latitude intercontinental dispersal events. This pattern suggests that global climate change has a major impact on the diversity and distribution of turtles.
The end of the Paleogene (∼45–23 Ma) coincides with global environmental changes, with the climate becoming significantly cooler and drier, thus much less favorable to turtles. Many turtle lineages that inhabited the Western Interior, including the last stem cryptodires in North America, became extinct at this time. One testudinoid lineage took advantage of the subtropical southeastern portions of the continent and radiated into the diverse clade Emydidae (53 species).
The recent description of a fossil taxon on the stem of Platysternon megacephalum from the Eocene of North America raises possibility that the more inclusive Emysternia may also have an American origin. Depending on the resolution of that possibility, the UCE topology indicates that two dispersal events into North America led to the origin of 36–43% of the recognized families of turtles.
The entire article is available on-line.
CitationCrawford NG, Parham JF, Sellas AB, Faircloth BC, Glenn TC, Papenfuss TJ, Henderson JB, Hansen MH, and Simison WB. 2014 (2015). A phylogenomic analysis of turtles. Molecular Phylogenetics and Evolution (2014).