Diapsida

Shared Derived Characteristics

found only in New Zealand), and birds; extinct diapsids include dinosaurs as well as many other forms. Nobody really knows how many members of this clade have come and gone.

Diapsida is united by a suite of shared, derived features, including having two temporal openings in the skull roof, and an upper temporal fenestra and a lower temporal fenestra. The upper and lower temporal fenestrae are thought to have provided accommodation for the bulging of contracted jaw muscles, as well as increased the surface area for the attachment of these muscles.

Moving to the ultimate node in Figure 4.7, there are two major clades of diapsids. The first, Lepidosauromorpha (lepido - scaly; morphos - shape), is composed of snakes and lizards and the tuatara (among the living), as well as a number of extinct lizard-like diapsids;5 the second, Archosauromorpha (archo - ruling), brings us within striking distance of dinosaurs.

Archosauromorpha

Archosauromorpha is supported by many important, shared, derived characters (Figure 4.11). Within archosauromorphs are a series of basal members that are known mostly from the Triassic. Some bear a superficial resemblance to large lizards; others look like beefed up crocodiles; a few even look like reptilian pigs (see Figure 13.4).

A subset of archosauromorphs possesses a number of significant evolutionary innovations (Figure 4.11), most notably an opening on the side of the snout, just ahead of the eye, called the antorbital fenestra (Figure 4.12). This is the key character that unites Archosauria, the group that contains crocodilians, birds, and dinosaurs. It is ironic that, for all its phylo-genetic importance, the function of the antorbital fenestra is still uncertain; it may have contained a large air sac, or a salt gland.

Crocodilians and their close relatives belong to a clade called Crurotarsi (cruro - shank; tarsos - ankle) about which we won't be too concerned here; dinosaurs and their close relatives constitute a clade called Ornithodira (ornitho - bird; dira - neck; see Figure 4.11).

Ornithodira brings us quite close to the ancestry of dinosaurs. This group is composed of two monophyletic groups, Dinosauria (deinos - terrible) and Pterosauria (ptero - winged;

5. Two marine groups, ichthyosaurs and plesiosaurs (see Figure 15.9) have also been placed within Diapsida.

Saurischia (including Aves)

Saurischia (including Aves)

Dinosauria Pterosauria

Figure 4.11. Cladogram of Archosauromorpha. Derived characters include: at 1, teeth in sockets, elongate nostril, high skull, and vertebrae not showing evidence of embryonic notochord; at 2, antorbital fenestra (see Figure 4.12), loss of teeth on palate and new shape of articulating surface of ankle (calcaneum); at 3, a variety of extraordinary specializations for flight, including an elongate digit IV; at 4, erect stance (shaft of femur is perpendicular to head; upper part of hip socket is thickened or has a ridge; ankle has a modified mesotarsal joint), perforate acetabulum; at 5, predentary and rearward projection of pubic processes (see introductory text for Part II: Ornithischia; at 6, asymmetrical hand with distinctive thumb, elongation of neck vertebrae, and changes in chewing musculature (see introductory text for Part III: Saurischia).

Figure 4.11. Cladogram of Archosauromorpha. Derived characters include: at 1, teeth in sockets, elongate nostril, high skull, and vertebrae not showing evidence of embryonic notochord; at 2, antorbital fenestra (see Figure 4.12), loss of teeth on palate and new shape of articulating surface of ankle (calcaneum); at 3, a variety of extraordinary specializations for flight, including an elongate digit IV; at 4, erect stance (shaft of femur is perpendicular to head; upper part of hip socket is thickened or has a ridge; ankle has a modified mesotarsal joint), perforate acetabulum; at 5, predentary and rearward projection of pubic processes (see introductory text for Part II: Ornithischia; at 6, asymmetrical hand with distinctive thumb, elongation of neck vertebrae, and changes in chewing musculature (see introductory text for Part III: Saurischia).

Figure 4.13). That pterosaurs are unapologeti-cally Mesozoic archosaurs has led to their being called "dinosaurs"; that they had wings and flew has led some to mistake them for birds; but in fact they were something utterly different from either dinosaurs or birds. They were unique, magnificent, and now, sadly, very extinct.

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