Baptanodon, of the Upper Jurassic epoch of Wyoming, was toothless, and was one of the six-toed forms.
Sub-Class IX.–PTEROSAURIA.
Mesozoic reptiles with fixed quadrate bones and with the anterior limbs transformed into wings, the enormously elongated ulnar finger carrying a patagium.
The skull bears a superficial resemblance to that of Birds. It articulates with the neck by a single condyle, at nearly a right angle. The interparietal foramen is absent, but there are five pairs of foramina on the surface of the skull, namely, the nostrils, orbits, supra- and infra-temporal and pre-orbital foramina. Most of the constituent bones of the cranium fuse with each other, and the composition of the various arches is therefore difficult to make out with certainty. The premaxillaries are fused together, and extend dorsally backwards between the nasals, which themselves diverge towards the prefrontals. The nostrils are bordered chiefly by the maxillaries, nasals, and prefrontals. The orbits are very large, mostly shut off in front from the pre-orbital foramina by a bridge, which is formed by descending processes of the prefrontals and ascending processes of the jugal. Above and behind, the orbits are bordered by the frontals, postfrontals, and possibly the quadrato-jugals. The whole temporal region is shortened from before backwards, but heightened dorso-ventrally, and the whole temporal fossa is divided into a supra- and infra-temporal portion by the junction of the postfrontal with the squamosal, the latter joining the parietal, thus closing the supratemporal fossa behind. This is conspicuous only in the older forms, e.g. Dimorphodon, but is very small in Pterodactylus, and quite abolished in Pteranodon. The infratemporal fossa is a narrow slit, slanting obliquely upwards and backwards, between the quadrate and the quadrato-jugal. A foramen of this kind occurs elsewhere only in the Rhynchocephalia. The quadrate is long, firmly fixed, and slants so far forwards that the mandibular joint lies on a level below the middle of the orbit. The pterygoids articulate with strong and long processes of the basisphenoid, touch the quadrate posteriorly, enclose an interpterygoid vacuity, and extend forwards as slender bones to the vomer, separating the palatines. The choanae are enclosed by the vomer, palatines, and maxillaries, and they lie in dorsal recesses above the level of the roof of the mouth. The teeth are alveolar, pointed, of variable size, and restricted to the jaws; in the Pteranodonts they are absent.
The brain is known from the natural cast of Scaphognathus, and shows some remarkably bird-like features, especially the width of the hemispheres, which touch the well-developed cerebellum, while the optic lobes lie on the sides of the cerebellum, with a pair of appendices, the so-called flocculi, elsewhere known in birds only.
The caudal vertebrae are still amphicoelous, while the presacral vertebrae are procoelous. Abdominal ribs are few in number and are very thin. The true ribs possess capitula and tubercula; those of the neck are very short and directed backwards; in the thoracic region they are long, and some are attached to a broad sternum with a keel and a median anterior process, on the sides of which latter articulate the coracoids. Precoracoids and clavicles are absent. The scapulae are long, sabre-shaped, and turned back as in birds; in Pteranodon they show the unique modification of articulating with special processes of the neural arches of several ankylosed thoracic vertebrae.
Fig. 116.–Pterodactylus crassirostris. × ⅓. (From Geikie.)
The hand possesses only four fingers; the four phalanges of the ulnar finger are very much elongated for the support of the patagium; the other fingers remain short and are provided with little claws. The ilia are expanded horizontally, and are firmly attached to from three to six vertebrae, which mostly fuse together into a sacrum. The ventral half of the pelvis consists of a pair of broad bones, which contain a small obturator-foramen; they form a ventral symphysis, and are usually fused with the ilium. These bones represent the conjoint ischia and pubes, while the so-called pubes, a pair of flat and club-shaped bones, are excluded from the acetabulum. The whole arrangement resembles that of the Crocodilian pelvis. The hind-limbs are bird-like in so far as the fibulae are reduced to splints, and attached to the proximal halves of the long and slender tibiae. The feet contain five separate toes with rather long metatarsals and short claws. Many of the bones are hollow.
The Pterosauria have no relationship with the birds, in spite of the number of apparently striking resemblances (e.g. choanae, pre-orbital foramina, brain, scapula, fibula, cervical vertebrae), which are, however, coincidences, cases of convergence, in conformity with the aerial life. The totally different plan of the wings is sufficient to show this. On the other hand, the real affinities of this group of flying reptiles are unknown. They turn up "fully fledged" in the Lower Lias, and they reach their highest specialisation in the Upper Cretaceous epoch, with which they have died out. In fact we do not know any forms through which to connect them with other extinct reptiles. The skull shows some Rhynchocephalian features; the pelvis, Crocodilian features; and this combination points back a long way.