Last post, I mentioned two groups, the anapsids and the diapsids. I think I’ll clarify that a bit.
First, some definitions are needed: a taxon is any group of two or more populations of an organism or organisms, no matter how arbitrary. If, and only if, the taxon includes an organism, all of its descendant organisms, and only those organisms, it is a clade. Only clades are considered scientifically valid groupings.
If a taxon is composed of arbitrarily chosen organisms picked at random from the “family tree” of life, it is called polyphyletic.
If it consists only of the descendants of a common ancestor, but has some of this ancestor’s descendants left out, it is paraphyletic. Reptiles are paraphyletic, since they include all descendants of a first, unknown proto-reptile, but leave out the birds, which are descended from the dinosaurs, which are classified as reptiles.
Finally, if a taxon includes all the organisms descended of a common ancestor, and only those organisms, it is monophyletic. Mammals, birds and modern amphibians all fit this description. Obviously, only monophyletic taxa are considered valid clades.
In the picture below, we see examples of all three types of taxa: warm-blooded animals (birds and mammals) are polyphyletic; reptiles, which leave out their descendants the birds, are paraphyletic. By including birds as reptiles, the latter group becomes monophyletic.
Now, vertebrate classification can be broken down, a bit simplistically, as such: all animals with an internal skeleton of some sort are vertebrates. Those vertebrates with distinct limbs adapted for land-bound movement, or descended from animals matching that description, are known as tetrapods—essentially, every vertebrate excluding fish and certain ancient transitional forms, the “fishapods”. Finally, those tetrapods that lay their eggs outside of the water or retain them within the mother are known as amniotes. Traditionally, amniotes were classified based on the number and location of their temporal fenestrae. These are openings in the skull, located behind the eye sockets, serving as anchoring points for the jaw muscles, and are generally associated with high metabolism. This system of classification split the amniotes in four clades: synapsids, diapsids, anapsids, and euryapsids.
Synapsids are distinguished by the presence of a single fenestra on each side of the skull, placed fairly low. A monophyletic group, they first appeared as lizard-like animals in the Carboniferous, the period directly before the Permian, and diversified fairly quickly. Notable synapsids include the sail-backed Dimetrodon of the early Permian, the warm-blooded theriodonts I devoted an earlier post to, and of course the mammals. In modern humans, the fenestra is still present, although it has migrated forwards—it can be seen in the picture below where the zygomatic and temporal bones meet.
Diapsids have two fenestra on each side, and are extremely diverse—examples include lizards, snakes, crocodiles, pterosaurs, and dinosaurs, including birds. They remained small and lizard-like for most of the Carboniferous and Permian, until the Permian Extinction left open a plethora of niches, which the diapsids filled in short order. The diapsids begat several successful lineages: the dinosaurs, obviously, which dominated most land niches for a great deal of the Mesozoic and, as the birds, rule modern skies; the crocodiles, which were surprisingly diverse in the past, including armored, land-bound herbivores, the seagoing, finned Thalattosuchids, several fully terrestrial hunters, and of course the modern amphibious forms; and the ever prolific lizards.
Those amniotes with a single fenestra placed high in the skull were traditionally classified as euryapsids. Entirely extinct, they would have included several sets of seagoing reptiles—the ichthyosaurs, the plesiosaurs, the nothosaurs (a group considered ancestral to the plesiosaurs), and the turtle-like placodonts. Nowadays, they are not considered a valid clade, and are instead viewed as unrelated (with the exception of nothosaurs and plesiosaurs) diapsids whose only common features were that they shared an aquatic lifestyle and that their lower fenestrae had closed up—they were a valid clade in the same way that whales, manatees, and seals are—namely, they weren’t.
Finally, there are the anapsids. Officially, the anapsids are those amniotes with no temporal fenestrae, the only surviving members of which are the turtles, although a wide variety of Carboniferous and Permian reptilian creatures are traditionally classified as anapsids. Effectively, they serve as a dumping ground for various basal amniote lineages with no fenstrae, or for more derived amniotes whose fenstrae closed up later on, and are paraphyletic. They are not considered a valid group by most modern taxonomists, and are generally classified as diapsids—turtles, for instance, are often considered relatives of either lizards or plesiosaurs—although some attempts have been made to redefine them as a valid clade, consisting of turtles and any creatures more related to them than to other surviving amniotes, consisting mostly of a Carboniferous and Permian group of reptiles known as the captorhinids, resulting in a far smaller, if more scientifically valid, group than the one previously recognized.
In light of these facts, we can form a crude mental picture of the amniote family tree: A reasonably flourishing side limb represents the synapsids, flourishing during the Permian, suffering a massive pruning at its end, and growing back to full size with time. A thinner side branch stands in for the anapsid clade, enduring with reasonable stability through the ages. The vast majority of the tree, of course, is taken up by the prolific synapsids, who have been thriving since the Triassic and who today include every amniote that isn’t a mammal or, arguably, a turtle.
