Sauropsida - the reptilian half of Amniota


The basal sauropsid Mesosaurus by Nobu Tamura from Wikipedia
Sauropsid Phylogeny: All organisms more closely related to birds than to Synapsids.

Is this a total-group-based of node-based definition?



Parareptilia: (Permian - Triassic) This group contains a number of distinct fossil groups. Our reasons for covering them here will become apparant soon. Parareptilian skulls generally retain the anapsid condition of the anapsid skull, lacking temporal fenestration. Yet, like egg-laying in monotremes, this is a plesiomorphy, not a synapomorphy. We will return to them in a later lecture.

Diapsida: (Carboniferous to Rec.) Diapsids first appeared in the Late Carboniferous and still exist. Modern diapsids include birds, crocodylians, lizards and snakes, and Sphenodon. In this course, we have addressed enough radical changes caused by HOX gene duplications or other radical developmental transformations that we might start to think of these as the norm. It's interesting to encounter a vertebrate group whose evolutionary novelties seem to be genuine adaptations by natural selection.

We start, as usual, with a list of synapomorphies. If you aren't careful, you might construe this as a list of unrelated factoids, but actually, they form a coherent picture of a suite of related adaptations.


Jaws as levers. It is possible to get a hint of how an animal uses its jaws by looking at their physical properties as levers. In gnathostomes, the input lever is the distance between the insertion of the jaw muscles from the jaw joint. The output lever is the distance from the biting surface to the jaw joint. When the ratio of the input to output lever length is high, the bite is strong, but the output lever moves more slowly for every bit of jaw muscle contraction. When it is low, the jaw moves more quickly but with less force.

The diversity of primitive diapsids: These small (lizard-sized) creatures shared their world with anapsids and with the large-bodied primitive synapsids.


Sauria: (Late Permian - Rec.) The most recent common ancestor of living lizards, Sphenodon, crocodylians, and birds.

The first members of Sauria appear during the Permian. The end of the Permian (which was also the end of the Paleozoic) marked the largest extinction event in the rock record. Although it was primarily felt by marine creatures, it had repercussions on land. Among diapsids, only Sauria survived the Permian extinction and continued into the Mesozoic. After the Permian extinction, saurian diversity exploded while synapsids went into a slow decline.

Lepidosauria: (Triassic - Rec.) The most recent common ancestor of lizards and Sphenodon, and all of its descendants.

There are two living Lepidosaurian monophyletic groups.


Compare the rhynchocephalian Sphenodon (left) with the squamate Tupinambis (right)
Synapomorphies:

  • Squamata: Lizards and snakes. (Triassic - Rec.) Thousands of species. Many interesting adaptations. There is no way to cover exhaustively if we had an entire semester.


    Precision in taxon names. As the phylogenetic system of taxonomy has developed, its practitioners have become more sophisticated. In the early days, however, researchers were often were more exuberant than prudent. This failing often showed up in the way in which group names were defined.