Autapomorphies of Diapsid Clades

Michel Laurin and Jacques A. Gauthier

A list of autapomorphies of Diapsida and Sauria appears on the Diapsida branch page, but no list of autapomorphies of the other clades was provided (for clades that are more inclusive than Sauria but less inclusive than Diapsida). Autapomorphies of these clades are given below.

Neodiapsida (all diapsids more closely related to saurians than to araeoscelidians) and Eosuchia (the last common ancestor of Coelurosauravus, Apsisaurus, younginiformes, lepidosaurs, and archosaurs and all its descendants) currently have approximately the same taxonomic content. The only neodiapsid that may not be an eosuchian is the poorly known genus Heleosuchus. Therefore, Neodiapsida and Eosuchia share the same list of autapomorphies:

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Figure 1. Diapsid skulls in lateral view. A, Petrolacosaurus, a Pennsylvanian araeoscelidian; B, Claudiosaurus, an Upper Permian neodiapsid; C, Youngina, an Upper Permian younginiform; D, Clevosaurus, a Late Triassic sphenodontid (a saurian). Redrawn from A, Reisz, 1981; B and C, Carroll, 1981; D, Fraser, 1988. Scale bar equals 1 cm. Copyright 2000 Michel Laurin.

Apsisaurus, Claudiosaurus, younginiforms, and saurians share the following synapomorphies:

Claudiosaurus, younginiforms, and saurians share the following synapomorphies:

Younginiforms, ichthyosaurs and saurians share the following synapomorphies:

Ichthyosaurs and saurians share the following synapomorphies (Motani et al., 1998):

Position of problematic taxa

Galesphyrus (Carroll, 1976a), Heleosaurus (Carroll, 1976b), and Heleosuchus (Carroll, 1987) are problematic because they are incompletely known. These taxa were placed on the most basal position on the tree that was consistent with the derived characters that could be identified.


Galesphyrus possesses the following derived characters:

Neodiapsid characters:

Synapomorphies shared with Apsisaurus, Claudiosaurus, younginiforms, and saurians:

Galesphyrus lacks the following derived characters:

Synapomorphies of Apsisaurus, Claudiosaurus, younginiforms, and saurians:

Saurian synapomorphies:

Very little evidence supports the placement of Galesphyrus in the diapsid phylogeny, but this taxon appears to be closely related to Apsisaurus, Claudiosaurus, younginiforms, and saurians. It may be more closely related to saurians than suggested in the main tree (in the Tree of Life page), but more data is required to test this hypothesis.


Heleosaurus possesses the following derived characters:

Neodiapsid characters:

Characters shared with Apsisaurus, younginiforms, and saurians:

Characters shared with Claudiosaurus, younginiforms, and saurians:

Heleosaurus apparently lacks the large retroarticular process characteristic of saurians.

The presence of the other characters in Heleosaurus cannot be assessed, but the available evidence suggests that it is more closely related to Claudiosaurus, younginiforms, and saurians than to Apsisaurus.


Heleosuchus retains an olecranon process, a structure lost in all other early neodiapsids except for Coelurosauravus. However, the status of the other characters cannot be unambiguously determined. The shape of the ectopterygoid described by Carroll (1987) suggests that a suborbital fenestra was present, and this would confirm the diapsid status of Heleosuchus. The position of this taxon will have to be reassessed when better material is discovered.


Carroll R. L. 1976a. Galesphyrus capensis, a younginid eosuchian from the Cistephalus zone of South Africa. Annals of the South African Museum 72: 59-68.

Carroll R. L. 1976b. Eosuchians and the Origin of Archosaurs. In: C. S. Churcher (ed.) Athlon-Essays on Palaeontology in Honour of Loris Shano Russell: 58-79. Toronto: Royal Ontario Museum. Life Sciences. Miscalleneous Publications.

Carroll R. L. 1981. Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society 293: 315-383.

Carroll R. L. 1987. Heleosuchus: an enigmatic diapsid reptile from the Late Permian or Early Triassic of southern Africa. Canadian Journal of Earth Sciences 24: 664-667.

Carroll R. L. & D. Baird. 1972. Carboniferous Stem-Reptiles of the Family Romeriidaed. Bulletin of the Museum of Comparative Zoology 143: 321-364.

Fraser N. C. 1988. The osteology and relationships of Clevosaurus (Reptilia: Sphenodontida). Philosophical Transactions of the Royal Society B321: 125-178.

Holmes R. 1977. The Osteology and Musculature of the Pectoral Limb of Small Captorhinids. Journal of Morphology 152: 101-140.

Motani R., N. Minoura, and T. Ando. 1998. Ichthyosaurian relationships illuminated by new primitive skeletons from Japan. Nature 393: 255-257.

Reisz R. R. 1981. A diapsid reptile from the Pennsylvanian of Kansas. University of Kansas Publications of the Museum of Natural History 7: 1-74.

About This Page

Michel Laurin
Muséum National d'Histoire Naturelle, Paris, France

Jacques A. Gauthier
Yale University, New Haven, Connecticut, USA

Page: Tree of Life Autapomorphies of Diapsid Clades Authored by Michel Laurin and Jacques A. Gauthier. The TEXT of this page is licensed under the Creative Commons Attribution License - Version 3.0. Note that images and other media featured on this page are each governed by their own license, and they may or may not be available for reuse. Click on an image or a media link to access the media data window, which provides the relevant licensing information. For the general terms and conditions of ToL material reuse and redistribution, please see the Tree of Life Copyright Policies.

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