Diapsids and Taxon Definitions

I. Diapsida: A major chunk of extant and fossil amniote diversity.

  1. Synapomorphies:
    1. Infratemporal and supratemporal fenestra.
    2. Snout longer than temporal region of skull.
    3. Limbs long and slender.
    4. Complex joint between tibia and astragalus (a proximal tarsal) that creates a relatively solid immobile articulation between the two.

  2. There are two general patterns here:
    1. The jaws are adapted to a faster but weaker bite.
    2. The creatures are adapted for faster locomotion.

slightly before the first diapsids during the Mississippian: Flying insects. Hard to catch, but relatively soft.

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

  1. Though not very diverse at first, they attained a number of ecological specializations including:
    1. small-bodied terrestrial.
    2. Invasion of fresh water.
    3. Gliding.

  • Sauria: The most recent common ancestor of living lizards, Sphenodon, crocodylians, and birds.

    There are two major lineages of saurians, one that accentuated the lightness of the skull and quickness of the bite to the maximum, the other that reevolved a strong solid bite within the framework of the diapsid skull pattern.

  • Thus, Sauria breaks down into two important monophyletic groups:
    1. Lepidosauria (Permian to recent) containing living lizards and Sphenodon

    2. Archosauria (Permian to recent). containing living crocs and birds.

    VI. 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.

    1. Definition vs. Diagnosis: The taxonomy of Diapsida contains some object lessons for the cladist. If we looked at a traditional taxonomic publication, we would see a taxon defined by the possession of characters. E.G. the presence of a supratemporal and infratemporal fenestra defined Diapsida as well as diagnosing it (i,e, allowing it to be recognized.) In phylogenetic systematics, this is not how it's done.
      1. Quick review. A class is characterized by attributes, an individual or system is characterized by a unique history. What are cladists concerned with? Individuals, not classes. That being the case, it would be inappropriate for us to define our taxa (which are hiostorical individuals) with their attributes. Instead we define them according to their ancestry.
      2. Definition: A statement of a taxon's historical origin. This can take a couple of forms:
        1. Node based: The most recent common ancestor of A and B and all of its descendants. (Crown group, in which A and B are living representatives, is a special case.)
        2. Stem based: Everything closer to A than to B.

      3. Diagnosis: The presumed synapomorphies of the group. Note that they can be reversed or obscured by evolutionary history, but the members are still members. The key point is that synapomorphies may help us identify members of a taxon, but they do not, in principle, define it.
        1. But how do we diagnose a stem-based taxon. Its diagnosis is, by definition, the same as that of the larger node-based group of which it is a member.

    2. Priority: Both traditional and phylogenetic systematists respect the notion that the first name to be applied to a group is its proper name and has priority over all subsequent names.

    3. Stability: We strive to coin taxon names and definitions that will not need confusing revisions.

    4. Mistakes were made: 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.
      1. Sloppy definitions that were vulnerable to being rendered meaningless by changing phylogenetic hypotheses. E.G. Lepidosauromorpha.
      2. Sloppy interpretations of definitions resulting in misleading literature.

    5. Discussion: Sometimes, taxa have been defined in ways that make instability likely.
      1. Problems with Sauria of Gauthier, 1984.
      2. Is it possible to say that two node based groups originated at the same time if we know they are sister taxa?
    The last ten years have seen significant efforts to establish strict criteria for the cladistic definition and diagnosis of taxa in order to limit this kind of confusion. Perhaps the most ambitious project is the Phylocode - a cladistic equivalent of the traditional Code of Zoological Nomenclature.