Even though diapsid phylogeny has been intensively studied, relatively few computer-assisted studies of early diapsids have been published. The large-scale phylogenetic analyses of Benton (1985), Evans (1988), and Gauthier et al. (1988) focused on lepidosauromorphs and archosauromorphs. More recently, phylogenies based on data matrices incorporating several diapsid terminal taxa (more than 20) were presented. Therefore, only simplified versions of these phylogenies are presented here (the node names are modified to be consistent with the recommended usage and do not necessarily follow the published versions).
This page deals mostly with the phylogeny between the various stem-diapsids, Lepidosauromorpha and Archosauromorpha. However, some taxa, such as ichthyosaurs and sauropterygians, have been variously thought to be stem-diapsids, lepidosauromorphs, or archosauromorphs, so their affinities are briefly discussed below. Turtles are usually not considered diapsids, but a few paleontological studies have suggested that they were closely related to sauropterygians (and to lepidosauromorphs). The results of these studies are also briefly exposed below.
Benton (1985) presented the following phylogeny:
===================================== Araeoscelidia | | === Younginiformes =====| ==Lepidosauromorpha=| ==Neodiapsida=| === Lepidosauria | ======================= Archosauromorpha
Evans (1988) studied more stem-diapsids, but her main conclusions are similar to those of Benton (1985) and Gauthier et al. (1988):
============================================ Araeoscelidia | | ========================================= Coelurosauravus | | | | ====================================== Sauropterygia | | | | | |===================================== Heleosaurus | | | =====| | |===================================== Galesphyrus ===| | ===| === Heleosuchus | ===| | ==Lepidosauromorpha=| === Younginiformes ====Sauria==| | | ====== Lepidosauria | ========================== Archosauromorpha
Both of these studies, as well as Gauthier et al. (1988) suggested that younginiforms were lepidosauromorphs. Laurin (1991) argued that younginiforms were not saurians, and this conclusion has been accepted by most scientists (Rieppel, 1993, 1994; Gauthier, 1994). The phylogeny suggested by Laurin (1991) is as follows:
================================ Araeoscelidia | | ================== Coelurosauravus | | | | =============== Apsisaurus =====| | | ==Neodiapsida=| | ============ Younginiformes ===| | ===| === Lepidosauromorpha ==Sauria=| === Archosauromorpha
Rieppel (1994), Rieppel and deBraga (1996), and deBraga and Rieppel (1997) argued that turtles (Testudines) are closely related to sauropterygians (a group of aquatic diapsids from the Mesozoic), and that both groups are lepidosauromorphs. This phylogeny suggests that turtles are actually diapsids that have lost their temporal fenestrae. While this conclusion remains controversial, it deserves to be presented:
==================================================== Araeoscelidia | | ====================================== Claudiosaurus | | | | =================================== Younginiformes | | | ====| | | === Sauropterygia ==Neodiapsida=| | ===| ===| ==Lepidosauromorpha=| === Testudines ==Sauria=| | | ====== Lepidosauria | ========================== Archosauromorpha
Gauthier (1994) reviewed amniote phylogeny.
Caldwell (1996) did one of the first computer-assisted phylogenetic analyses of diapsids incorporating ichthyosaurs. He found that if turtles and ichthyosaurs were included in his matrix, ichthyosaurs were the sister-group of sauropterygians, and this large clade of aquatic diapsids was close to the base of Archosauromorpha, while turtles appeared to be lepidosauromorphs.
======================== Araeoscelidia | =====| ===================== Claudiosaurus | | ===| ================== Younginiformes | | ===| ====== Lepidosauria | ===| | | ====== Testudines ==Sauria=| | === Sauropterygia | ===| ===| === Ichthyosauria | ====== Archosauria
Without ichthyosaurs, the same matrix resulted in a topology in which turtles were the sister-group of sauropterygians, and in which this clade was part of Lepidosauromorpha (sauropterygians shifted from archosauromorphs to lepidosauromorphs).
======================== Araeoscelidia | =====| ===================== Claudiosaurus | | ===| ================== Younginiformes | | ===| ====== Lepidosauria | ===| | | | === Sauropterygia ==Sauria=| ===| | === Testudines | ========= Archosauromorpha
Finally, without turtles, ichthyosaurs were the sister-group of sauropterygians, and that clade was located outside Sauria (Sauria is identified with the wrong clade in Caldwell's figure).
===================== Araeoscelidia | =====| ================== Claudiosaurus | | ===| =============== Younginiformes | | ===| ========= Ichthyosauria | ===| | | ========= Sauropterygia ===| | === Lepidosauromorpha ==Sauria=| === Archosauromorpha
Motani et al. (1998) reassessed the affinities of ichthyosaurs using the data matrices of Rieppel and deBraga (1996) and of Caldwell (1996) by adding ichthyosaurs (in the first case) or recoding them by incorporating data that they had recently obtained about the early (Lower Triassic) ichthyosaur Utatsusaurus hataii. Using the matrix of Rieppel and deBraga (1996), they obtained the following topology:
======================== Araeoscelidia | =====| ===================== Claudiosaurus | | ===| ================== Younginiformes | | ===| =============== Ichthyosauria | | ===| === Lepidosauria | ===| ==Sauria=| === Sauropterygia | ====== Archosauromorpha
In that tree, turtles (not shown) were located outside diapsids, in their traditional location (along with other parareptiles). According to that tree, ichthyosaurs are stem- diapsids, and sauropterygians are lepidosauromorphs.
Using the data matrix of Caldwell (1996), Motani et al. (1998) obtained the following topology:
=========================== Araeoscelidia | =====| ======================== Claudiosaurus | | ===| ===================== Younginiformes | | ===| ================== Ichthyosauria | | ===| ====== Lepidosauria | ===| | | | === Testudines ==Sauria=| ===| | === Sauropterygia | ========= Archosauromorpha
In this tree, the position of ichthyosaurs and sauropterygians remains the same, but turtles move into lepidosauromorphs, as the sister-group of sauropterygians.
Finally, Merck (1997) suggests that ichthyosaurs are the sister-group of sauropterygians, and that this clade is part of Archosauromorpha.
The conflicting results obtained from various authors, and sometimes, by a single author (or group of authors) using different taxa suggest that more work is needed to assess the affinities of ichthyosaurs, sauropterygians, and turtles.