Basal diapsids

Diapsida includes two big, diverse clades, Lepidosauromorpha (animals that are closer to lizards and snakes) and Archosauromorpha (animals that are closer to crocs and birds). It also includes a whole host of other things that are probably diapsids but difficult to place precisely in the Tree of Life. In some cases this is because the animals in question are small, poorly known, or primitive, and therefore have few clear features that tell us about their relationships. In other cases, the animals are so transformed that until we discover their less derived ancestors, it will be hard to determine where they came from.

Araeoscelidia is a clade containing the earliest and most primitive known diapsids. These small, superficially lizard-like animals flourished in the Late Carboniferous Period and the Early Permian Period. Global conditions were generally getting cooler during this time, and the great coal swamps were replaced by drier environments. Reptiles flourished, including the early diapsids, while amphibians and other basal tetrapods declined.

A reconstruction of Thalattosaurus alexandrae from the Upper Triassic limestones of Shasta County, CA.

Younginiformes is another clade of basal diapsids, with a very confused taxonomic history. For a long time, this group was known as "Eosuchia" and it was used as a taxonomic wastebasket for diapsids that did not clearly belong to either the Lepidosauromorpha or Archosauromorpha. Many of the Younginiformes, like Youngina itself, from the Permian of South Africa, were small, lizard-like animals that differed from the earlier Araeoscelidia only in the details of their anatomy. Others, such as the Tangasauridae, included freshwater swimming animals. Most of what we know of the Younginiformes comes from Permian and Triassic fossils from South Africa, Madagascar, and other parts of Gondwana.

Claudiosaurus is another aquatic basal diapsid from the Permian of Madagascar. It had a small head, large webbed feet, and a long tail. It might have resembled modern-day marine iguanas, and was about the same size as a small marine iguana (~2 feet long). We don't know yet whether Claudiosaurus is closely related to the aquatic tangasaurids or whether its adaptations for swimming represent convergent evolution.

Ichthyosaurs are superficially dolphin-like reptiles that were important marine predators from the Triassic Period through the middle of the Cretaceous Period. The earliest known ichthyosaurs are already quite derived, which makes untangling their evolutionary origins difficult. That doesn't mean that the transitional forms that led to ichthyosaurs didn't exist — we just haven't found them or recognized them yet. As the fossil histories of birds and whales show, the transitional fossils are out there, if only we can find them. For now, paleontologists think that ichthyosaurs are diapsids, but exactly where they fit in the diapsid tree is not yet known.

Thalattosauridae is yet another group of aquatic basal diapsids. Thalattosaurs were up to seven feet in total length, considerably larger than Claudiosaurus and the tangasaurids. All of the known thalattosaur fossils come from Late Triassic deposits along the west coast of North America, which suggests that this group was fairly limited in both space and time.

Coelurosauravids were lizard-like gliding reptiles, similar to the "flying lizard" Draco that lives today in the rainforests of Indonesia. However, coelurosauravids were not lizards, and they were not closely related to Draco or to the kuehneosaurids of the Late Triassic. All three of these groups independently evolved the ability to glide. In Draco and the kuehneosaurids, the gliding membrane is formed by long ribs. The ribs fold where they contact the vertebral column or backbone, just like your ribs move up and down when you breathe. Coelurosauravids were different. Their gliding membranes were also supported by long bony rods, but these were not formed by the ribs. Rather, the ribcage enclosed the body as in most animals, and the wing-rods attached to the side of the trunk. This is a good example of convergent evolution. Draco and coelurosauravids both evolved wings for gliding, but they did it using different parts of their bodies. Coelurosauravids must have ranged over much of the world. Their fossils have been found in England, Germany, and Madagascar.


At left, a nearly complete skeleton of Coelurasauravus jaekeli from the Late Permian of Germany. The many long rods probably supported a lateral gliding membrane. At right, the gliding lizard Draco.

The most important thing to remember about basal diapsids is that their evolution was more than a "lizard-factory" or "archosaur factory." These animals ranged all over the world in the Late Carboniferous, Permian, and Triassic Periods, and although they gave rise to the evolutionary lineages of lizards, crocs, and birds, they were also interesting and successful animals in their own right. In particular, the evolution of gliding forms like coelurosauravids and swimming forms like thalattosaurids, tangasaurids, and Claudiosaurus shows that basal diapsids were ecologically diverse and radiated into a variety of forms and lifestyles. Hopefully in the future we will find more and better fossils that will illuminate the history of this pivotal group of animals.

Sources

Evans, S.E. 1982. The gliding reptiles of the Upper Permian. Zoological Journal of the Linnean Society 76:97-123.
Evans, S.E. 1987. A review of the Upper Permian genera Coelurosauravus, Weigeltisaurus, and Gracilisaurus (Reptilia: Diapsida). Zoological Journal of the Linnean Society 90:275-303.
Ketchum, H.F., and P.M. Barrett. 2004. New reptile material from the Lower Triassic of Madagascar: Implications for the Permian–Triassic extinction event. Canadian Journal of Earth Sciences 44(1):1-8.
Bickelmann, C., J. Müller, and R.R. Reisz. 2009. The enigmatic diapsid Acerosodontosaurus piveteaui (Reptilia: Neodiapsida) from the Upper Permian of Madagascar and the paraphyly of "younginiform" reptiles. Canadian Journal of Earth Sciences 46(9): 651-661.

Text by Matt Wedel, 5/2007; Thalattosaurus reconstruction by Donna Sloan; Coelurosauravus image from Frey, E., H.-D. Sues, and W. Munk. 1997. Gliding mechanism in the Late Permian reptile Coelurosauravus. Science 275(5305):1450-1452 — reprinted with permission of AAAS; Draco photo by Arie van der Meijden (CC BY-NC-SA 3.0)