The evolutionary position of turtles revised

Abstract

Consensus on the evolutionary position of turtles within the amniote phylogeny has eluded evolutionary biologists for more than a century. This phylogenetic problem has remained unsolved partly because turtles have such a unique morphology that only few characters can be used to link them with any other group of amniotes. Among the many alternative hypotheses that have been postulated to explain the origin and phylogenetic relationships of turtles, a general agreement among paleontologists emerged in favoring the placement of turtles as the only living survivors of the anapsid reptiles (those that lack temporal fenestrae in the skull). However, recent morphological and molecular studies have radically changed our view of amniote phylogenetic relationships, and evidence is accumulating that supports the diapsid affinities of turtles. Molecular studies favor archosaurs (crocodiles and birds) as the living sister group of turtles, whereas morphological studies support lepidosaurs (tuatara, lizards, and snakes) as the closest living relatives of turtles. Accepting these hypotheses implies that turtles cannot be viewed any longer as primitive reptiles, and that they might have lost the temporal holes in the skull secondarily rather than never having had them. Living turtles (order Testudines) are among the most striking and strangest tetrapods.

Haptoglobin, a hemoglobin-binding plasma protein, is present in bony fish and mammals but not in frog and chicken

“…Alternatively, Hp has been lost before the emergence of amphibians and has then reappeared independently in mammals and paleognaths. This possibility is less likely in view of the fact that we found a Hb-binding protein similar to the ␤-chain of mammalian Hp in serum of Chinese soft-shell turtle (data not shown); turtles seem to represent a separate reptilian class (27).…”

Going nuclear: gene family evolution and vertebrate phylogeny reconciled

“…This has been challenged by data placing turtles as the sister group to the lepidosaurs (Rieppel & deBraga 1996) and molecular data, which seem unanimously to place turtles as relatives of archosaurs (Hedges & Poling 1999;Rieppel 2000). A number of recent reviews (Rieppel 2000;Zardoya & Meyer 2001a) have concluded that relationships within the reptiles are still uncertain. The results of our analysis are unconventional in placing turtles as the closest relative of birds, but add to the molecular evidence placing turtles within crown-group diapsids.…”

Recent Advances in the (Molecular) Phylogeny of Vertebrates

“…More recent phylogenetic analyses based on morphological and fossil data agreed that synapsids-the mammals-(those with a single lower temporal hole in their skulls) are the sistergroup to the remaining amniotes, and they placed anapsids as sistergroup of the diapsids-tuatara, snakes and lizards, crocodiles and birds-(those that have, at least ancestrally, two fenestrae in the temporal region of the skull) (Gauthier et al 1988, Laurin & Reisz 1995, Lee 1997, Reisz 1997) ( Figure 6A). However, during the past decade several different amniote phylogenies have been proposed by both paleontologists (Rieppel & Reisz 1999) and molecular phylogeneticists (Zardoya & Meyer 2001a), most of which favor a more derived position for turtles within the reptiles ( Figures 6B,C,D).…”