Discussion of Phylogenetic Relationships
Phylogeny modified from Laurin & Reisz (1995) and Lee(1995); the position of Mesosauridae follows Modesto (1999). Node names follow Gauthier et al. (1988b) and Gauthier (1994). The position of turtles (Testudines) is uncertain; some authors place them approximately in the position shown above (Laurin & Reisz, 1995; Lee, 1993, 1995), while others place them among Diapsida (deBraga & Rieppel, 1996, 1997; Rieppel & Reisz, 1999; Hedges & Poling, 1999; Mannen & Li, 1999).
Generations of systematists have studied amniote phylogeny at diverse genealogical levels, and until a few years ago, its broad outlines were thought to be reasonably well understood. Indeed, recognition of the major living clades, such as mammals, turtles and birds, antedates the Theory of Descent. Relations among these taxa, and especially the connections of various fossils to them, have been contentious in post-Darwinian times. Much of that controversy can, however, be attributed to the fact that during the first two-thirds of this century, there was little thought given to what constituted evidence for phylogenetic relationships. The origins of the major extant lines of Amniota have become clearer in the post-Hennigian era. Nevertheless, the precise relations of a number of clades, most notably the turtles among extant forms and the aquatic and highly divergent ichthyosaurs and sauropterygians among extinct forms, remain contentious.
Early phylogenetic analyses placed turtles outside of the remaining amniotes (only crown-clade names are listed to simplify the trees):
* Major living amniote clades after Gaffney (1980).
Gauthier et al. (1988a, b, and c) later placed turtles as the sister clade to Sauria (crown-diapsids), and this topology has now gained wide acceptance, at least among morphologists and paleontologists:
However, Rieppel (1994, 1995), Rieppel & deBraga (1996) and deBraga & Rieppel (1997) have suggested that turtles may be the sister clade to lepidosaurs. This requires that turtles are saurians who have lost both the upper and lower temporal fenestrae (holes in the skull associated with jaw muscles) so diagnostic of diapsid reptiles:
The three trees presented above include only extant taxa, and many phylogenetic analyses of amniotes have ignored extinct taxa. However, it is important to bear in mind that discovering the globally most parsimonious tree requires the inclusion of extinct taxa in a phylogenetic analysis (Gauthier et al., 1988b). Without fossils, the best-supported tree for amniotes inferred from morphological data is the following (although only one more step is required to switch the positions of lepidosaurs and turtles):
* Tree based on living amniotes only (after Gauthier et al., 1988b).
Recent molecular evidence for amniote relationships conflicts with paleontological and morphological evidence. Initially, some gene sequences suggested a close relationship between birds and mammals, although never with strong statistical support (e.g., Bishop & Friday, 1987; Goodman et al., 1987; Hedges et al., 1990). More recently, a study of the molecular evidence for the origin of birds (15 genes; 5280 nucleotides, 1461 amino acids) discovered strong support (100% bootstrap P value, BP) for a close relationship between birds and crocodilians (Hedges, 1994). A smaller data set of 11 transfer RNA genes (686 sites) also resulted in a bird-crocodilian grouping (Kumazawa & Nishida, 1995). A basal position for mammals was supported (99% BP) by analysis of a 3 kilobase portion of the mitochondrial genome containing the two ribosomal RNA genes (Hedges, 1994). In the same study, a Sphenodon-squamate relationship also was found, but support for that grouping and for the position of turtles was not very strong.
The most recent molecular phylogenies have generally placed turtles among archosauromorphs, and often within archosaurs (Mannen et al., 1997; Mannen & Li, 1999; Hedges & Poling, 1999). The latter placement is the least compatible with the morphological evidence, and no convincing explanation has been found so far to explain this discrepancy.
Many gene sequences of birds and mammals exist, but the relatively small number of sequences from representatives of other amniote lineages, especially tuataras (Sphenodon) and turtles, has hindered the estimation of a robust molecular phylogeny for all major groups of living amniotes. This is reflected by the low resolution of the molecular phylogeny obtained by Hedges & Poling (1999) when Sphenodon (using only sequences of genes available in sphenodon) was included:
Without Sphenodon and using the greater number of sequences available for other taxa, Hedges & Poling obtained the following fully resolved phylogeny, in which turtles are the sister-group of crocodilans:
If extinct amniotes are considered, the phylogeny is much more complex and controversial. Formerly, captorhinids were believed to be closely related to turtles (Gauthier et al., 1988b, c), but more recently, procolophonids (Reisz & Laurin, 1991; Laurin & Reisz, 1995), pareiasaurs (Lee, 1993, 1994, 1995, and 1996), and even sauropterygians (a group of Mesozoic diapsids) have been suggested to represent early relatives of turtles (Rieppel, 1994, 1995; Rieppel & deBraga, 1996; deBraga & Rieppel, 1997). The linked page Phylogeny and Classification of Amniotes provides information about the phylogenies incorporating extinct amniote taxa, and provides a detailed classification of the relevant groups. The linked page Temporal Fenestration and the Classification of Amniotes discusses how temporal fenestration has been used to classify amniotes, and how tempororal fenestration evolved.
No one has provided updates yet.