The Traditional Reptilia
Paleontologists have traditionally assumed that all the animals we commonly refer to as “reptiles” (lizards, snakes, turtles, crocs, the tuatara and all their prehistoric ancestors) were monophyletic, descending from a single ancestor and forming a single clade, the Reptilia (Modesto and Anderson 2004). Birds were added to this group, having descended from the dinosaurian sisters of crocodile ancestors. In this hypothesis the mammals and their ancestors (collectively the Synapsida) were not considered reptiles because they were thought to have branched off the family tree earlier.
The Traditional Amniota
Traditionally, mammals (and their ancestors) joined birds, crocs, lizards, turtles (and their ancestors) in a monophyletic clade, the Amniota. All these taxa share a trait derived from a common ancestor, an amnion, an embryonic membrane that protects the embryo during development whether an egg shell is present or not.
Purported outgroup taxa
Various diadectids and microsaurs were said to nest just outside the Amniota. According to tradition, these “reptile-like amphibians” must have laid their eggs in water and produced tadpoles because they were thought to precede the development of the amnion. It didn’t seem to matter what sort of evolutionary mismatches resulted. Diadectids and microsaurs certainly do not share many traits with each other.
Now these traditions have been changed, according to the results of a very large cladistic analysis, unprecedented in scope.
Just like a larger telescope brings greater resolution to astronomical images, a larger cladistic analysis brings greater resolution to family trees. No one had ever created a cladistic analysis that included basal representatives from the gamut of the Amniota until now. All prior analyses used smaller inclusion sets based on assumption and tradition. Many recovered poorly resolved trees with poorly matched purported sisters sharing few traits.
The present analysis recovered a single tree from over (the number continues to grow) 235 specific and generic taxa with all reptiles (including synapsids) descending from the “pre-reptile” Gephyrostegus (see below). All sister taxa share many traits and greatly resemble one another. The tree solves many prior mysteries and nests several former enigmas.
A Big Surprise
The new tree produced two major reptilian branches before the advent of any known reptile fossils. Thus, there was not a single basal reptile (defined here as “without a discrete intertemporal bone”). These two major branches go by old names: the Lepidosauromorpha and the Archosauromorpha because one branch includes lepidosaurs and the other branch includes archosaurs. The second branch also includes mammals and their ancestors.
As Carroll (1970) predicted, the most basal known reptiles, Cephalerpeton and Casineria, were indeed tiny, but not as tiny as the last of the pre-reptiles (one of which would have been the sister to the last common ancestor of Cephalerpeton and Casineria, and thus would have been the first reptile/amniote).
Now several reptiles (including Casineria, the microsaurs, and at least four protosynapsids) precede the branching of the Synapsida. That means the Reptilia = the Amniota. Since the former term precedes the latter, the Amniota has now become redundant, no longer distinct from the Reptilia.
These results shift taxa around like branches on a fake Christmas tree.
Diadectids and microsaurs join the Reptilia. Caseids leave the synapsids. Mesosaurs join the ichthyosaurs. We have a new basal dinosaur family tree. We’ll talk about other details in future blogs, or you can read them for yourself now at reptileevolution.com.
What does this have to do with pterosaurs?
Everything. This is the study that nested pterosaurs with lizards, specifically with a new, previously unidentified third squamate clade, the Tritosauria, originating with Lacertulus in the Late Permian. Breaking paradigms left and right, this new tree invalidates such clades as the Ornithodira, the Avemetatarsalia and several others that included pterosaurs with dinosaurs in their definitions.
As always, I encourage readers to see the specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Brough MC and Brough J 1967. The Genus Gephyrostegus. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 252 (776): 147–165. doi:10.1098/rstb.1967.0006
Carroll RL 1970. The Ancestry of Reptiles. Philosophical Transactions of the Royal Society London B 257:267–308. online pdf
Jaeckel O 1902. Über Gephyrostegus bohemicus n.g. n.sp. Zeitschrift der Deutschen Geologischen Gesellschaft 54:127–132.
Modesto SP and Anderson JS 2004. The Phylogenetic Definition of Reptilia. Systematic Biology 53(5):815–821. DOI: 10.1080/10635150490503026