There is still some debate as to the exact relationship of pterodactyloids to rhamphorhynchoids. Figure 4-5 shows the current consensus but, as mentioned earlier, it is also possible that they branched off, together with the anurognathids. Everyone agrees, however, that pterodactyloids themselves form a single, unique, clearly defined group. This is because on the line leading to pterodactyloids, pterosaurs underwent a profound reshaping and rebuilding of their anatomy that affected almost every part of their bodies.
In the skull, the nostril and the antorbital fenestra merged to form a single large opening and the braincase was considerably expanded, developments that hint at some significant, but as yet only poorly understood, physiological and neurological changes. Elsewhere, there was a sharp reduction in the length of the tail and various modifications to the limbs, most notably the lengthening of the metacarpals, the bones between the wrist and the wing-finger, and the extreme reduction and eventual loss of the fifth toe in the foot. Because all these features were directly or indirectly involved in the flight apparatus, their modification must have had a significant impact on how pterodactyloids flew, although, surprisingly, this has yet to be investigated in any detail. What recent research has established, however, to which we will return in later chapters, is that these changes also had a profound impact on another aspect of pterosaurs' lives— it opened up a whole new world for them, on the ground.
Returning to the pterosaur tree, we find that pterodactyloids can be separated into four major branches. One of these, the ornithocheiroid clan, is composed of a group of species that are distinctly different from all other pterodactyloids, so they are shown branching off somewhat earlier. It is very difficult to establish exactly how the three remaining clans are related, so I have avoided this problem altogether by having them emerge simultaneously from the main trunk.
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