Traditionally turtles are considered anapsids. Recently some morphologists claim that turtles are really diapsids that have become secondarily anapsid. The solid skulls of the turtles are therefore, according to these morphologists, a reversal to the ancestral anapsid condition. In the past I have cited evidence from physiology to refute this claim. Specifically all known diapsids and their descendants (e.g. crocs, lizards, tuatara, snakes, birds) have irreversibly lost ureotelism, meaning that all of these diapsids cannot excrete nitrogenous waste as urea. All diapsids are uricotelic, meaning they excrete their nitrogenous waste primarily as uric acid. Turtles and mammals, however, retain the probably ancestral condition of ureotelism, and they excrete urea. All amniotes are capable of excreting uric acid. In humans, an excessive amount of uric acid can produce the painful condition known as gout. Hence turtles differ from all other diapsids in being ureotelic. It is certainly possible that turtles have undergone reversal in this character as well and returned to ureotelism because of the semi-aquatic habits of most turtles. But such a reversal seem unlikely since crocodilians, though semi-aquatic, has not undergone the same reversal to ureotelism. Crocs in fact still excrete uric acid, although they have also taken advantage of their aquatic habitats by excreting ammonia.

Physiology therefore argues strongly against turtles being diapsids. Dr. Michael Lee of the University of Queensland also claims that the characters used to unite the turtles and diapsids are convergences. Therefore it is surprising that a recently published paper by Rest et al. claims that turtles are diapsids using mtDNA data.

Rest, J.S., J.C. Ast, C.C. Austin, P.J. Waddell, E.A. Tibbets, J.M. Hay, D.P. Mindell. (2003). Molecular systematics of Reptilia and the tuatara mitochondrial genome. Molecular Phylogenetics and Evolution, 29:289-297

Rest et al.'s study is congruent with another claim by Dr. Lee: that snakes are closely related to varanoid lizards, therefore making it all the more surprising. Looking more carefully at their choice of outgroups, however, I believe I have found a potential problem in Rest et al.'s study. In their analysis of the relationship of snakes and lizards, Rest et al. correctly chose the tuatara as the outgroup since the tuatara is almost certainly not a member of the Squamata, to which snakes and lizards belong. Rest et al.'s choice of 2 mammals as the "outgroup" to their "Reptilia" is more problematic. They apparently are influenced by the current cladistic dogma that birds are "reptiles" but that mammals are not "reptiles" even though mammals of course are descendants of the synapsids (a group that is traditionally classified in Reptilia) but excluded from the cladists' "Reptilia" (sensu Gauthier). Monotremes, for example, retain many ancestral reptilian characters, including egg laying.

Rest et al. may also have erred by ASSUMING (again on the basis of cladistic dogma) that the ancestors of the mammals (namely the synapsids) are the first group to have branched off from the Amniota (the all inclusive group that includes birds, mammals and reptiles). Rest et al. therefore assumed that all amniotes other than the mammals and their ancestors form a holophyletic group. If, however, the turtles are in fact anapsids and if the anapsids giving rise to the turtles actually branched off earlier than the synapsids, then Rest et al. may have chosen a member of the ingroup as an outgroup. Evidence that shows they may have done just that is contained in their own paper. The two mammals they chose as "outgroup" form an unresolved polytomy with their "Reptilia." This is of course an anomaly because these two mammals should form a single lineage since all mammals evolved from a single therapsid, but the polytomy suggests that the marsupials and placental mammals evolved from different groups of synapsids!

Rest et al. would probably need to redo their analysis by including an amphibian and probably a fish as outgroups. Until then their data showing turtles as secondarily anapsid descendants of a diapsid reptile is problematic and not reliable. Their data in support of a close relationship between snakes and varanoid lizards, however, is much better because the correct outgroup (Sphenodon) was chosen.

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