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  Section: Anatomy of Vertebrate Animals » The Classification and the Osteology of the Reptilia
 
 
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The Pterosauria

 
     
 

The flying Reptiles, which belong to this group, and are commonly known as Pterodactyls, are, and long have been, extinct, their remains occurring only in Mesozoic rooks, from the Lias to the Chalk inclusively.

They are all remarkable for their proportionally long heads and necks, and for the great size of the anterior limb, the ulnar finger of which, enormously elongated and devoid of a claw, appears to have supported the outer edge of an expansion of the integument, like the patagium of a Bat (Fig. 79).

The vertebral column is distinctly divided into cervical, dorsal, sacral, and caudal regions, the cervical vertebrae being, as in Birds, the stoutest of all. The atlas and axis are anchylosed together, at least in the eretaceous species. The other cervical vertebrae, apparently not more than six or seven in number, have low, or obsolete, spinous processes; and, like the vertebrae of the rest of the spine, are procoelous, and have the neuro-central suture obliterated. The existence of cervical ribs is doubtful. From fourteen to sixteen vertebrae intervene between the cervical and the sacral regions; and not more than one or two of the hindermost of them, if any are devoid of ribs. The number of vertebrae auchylosed together to form the sacrum, is not fewer than three, nor more than six.

The tail is very short in Pterodactylus, and, in this genus, all the vertebrae are movable upon one another; but in Rhamphorhynchus, it is extremely long, and the vertebrae are immovably fixed by what appear to be ossified ligamentous fibres.

The vertebral ribs are slender, and the anterior ones, at any rate, have distinct capitula and tubercula. There are ossified sternal ribs, and splint-like abdominal ribs. The sternum is broad, and, unlike that of other Reptilia, is very completely ossified, and bears a strong median crest on the anterior part of its ventral surface. No median posterior prolongation has been observed in connection with it.

The brain-case is more rounded and bird-like than in the other Reptilia, and, in many other respects, the skull approaches that of birds. Thus, the occipital condyle is on the base of the skull, not on its posterior face; the cranial bones anchylosed very early; the orbits are very large, and the external nares are situated close to them. The premaxillae are very large, the maxillae slender, and the dentary pieces of the mandible are fused together into one bony mass, without any trace of a symphysial suture.

The resemblance to birds is still further increased, in some species, by the presence of wide lachrymo-nasal fossae between the orbits and the nasal cavities, and by the prolongation of the extremities of the premaxillae and of the symphysial part of the mandible into sharp, beak-like processes, which appear to have been covered with horny sheaths. But the reptilian type is kept up by the presence of a distinct post-frontal, which unites with the squamosal and thus gives rise to a supratemporal fossa. The post-frontal and the jugal unite behind the orbit, in Lacertilan fashion; and both the upper and the lower jaws contain teeth. The sclerotic is supported by a ring of bones, as in many other Sauropsida.

The scapula and the coracoid are wholly unlike these structures in any other Sauropsida, but are extremely similar to the same parts in birds, and indeed to the shoulder-girdle of the less reptilian Carinatae. The scapula is slender and blade-like, and its long axis is inclined, at less than a right angle, to that of the coracoid. The glenoidal surface is cylindroidal, concave from above downward, convex from side to side. The coracoid, elongated and comparatively narrow, is devoid of fontanelle, epicoracoid, or procoracoid.
The nearly entire skeleton of Pterodactylus spectabilis (Von Meyer), as shown by the two halves of a split block of lithographic slate, a, the left pre-pubic bone on the right side this bone is not shown, and the ilium is exposed
Fig. 79. - The nearly entire skeleton of Pterodactylus spectabilis (Von Meyer), as shown by the two halves of a split block of lithographic slate, a, the left pre-pubic bone on the right side this bone is not shown, and the ilium is exposed.

No trace of any clavicle has been discovered. The humerus has a great deltoid ridge or process. The radius and ulna are equal in size and separate. There are four distinct metacarpal bones, that on the ulnar side being very much stronger, though not longer, than the others. Another styliform bone attached to the carpus does not appear to have belonged to the metacarpal series. The radial metacarpal bears two phalanges; the second, three; the third, four, so that these represent the pollex and the succeeding digits of the Lizard's manus. The terminal phalanx of each of these digits is strong and curved, and was doubtless ensheathed in a horny claw. The fourth, like the corresponding digit in the Crocodile, has four phalanges, the last of which is straight and bears no nail. But these phalanges are enormously elongated and of great relative strength. A strong process projects from the dorsal side of the proximal end of the first phalanx, and doubtless gave attachment to the tendon of a correspondingly powerful extensor muscle. The articular surface below and behind it is concave, and plays over the convex distal pulley of the fourth metacarpal.

The pelvis is remarkably small. The ilia are elongated bones, produced both anteriorly and posteriorly, as in Birds; but the rest of the pelvis is not at all ornithic. The flat and broad ischia appear to be united with the pubes into wide bony plates, which pass, at right angles with the ilia, to their median ventral symphysis. A large spatulate bone articulates with each pubis near the symphysis, and seems to be an exaggeration of the pre-pubic process of Lacertilia and Chelonia. Or it may be (though I do not think this very probable) that the broad flat plates correspond almost altogether to the ischia, and that the spatulate ossifications are the pubes; in which case the structure of the pelvis would be a sort of extreme exaggeration of that observed in the Crocodilia.

The hind-limb is small compared with the fore-limb. The fibula is imperfect, and appears to coalesce with the tibia at its distal end. The structure of the tarsus requires further elucidation. In some Pterosauria there seem to be only four digits, with, perhaps, a rudiment of a fifth, in the pes; but others, such as Rhamphorhynchus Gemmingi, have five digits in the foot. Where there are only four, each digit is terminated by a curved and pointed ungual phalanx, and the number of the phalanges from the tibial to the fibular side is 2, 3, 4, 5. These digits, therefore, are the hallux, and the three which immediately follow it; and the rudimentary digit is the fifth.

The long bones of the Pterosauria have thin walls, enclosing a large cavity, which appears to have contained air, as in many birds; and pneumatic foramina are visible on the sides of the vertebrae.

The remains of more than twenty species of Pterosauria have been discovered. Some of them are exquisitely preserved in the fine matrix of the lithographic stone of Solenhofen.

They are thus grouped into genera:

  1. With two joints in the ulnar digit of the manus.
  2. Ornithopterus.
  3. With four joints in the ulnar digit.
    1. The jaws strong, pointed, and toothed to their anterior extremities. The tail very short. The metacarpus usually longer than half the length of the antebrachium.
    2. Pterodactylus.
    3. The extremities of the jaws produced into toothless beaks, probably ensheathed in horn. The tail very long. The metacarpus shorter than half the length of the antebrachium.
      1. All the mandibular teeth similar.
      2. Rhamphorhynchus.
      3. The posterior teeth for the most part very short. The anterior long.
      4. Dimorphodon.

I am much inclined to suspect that the fossil upon which the genus Ornithopterus has been founded, appertains to a true Bird.
 
     
 
 
     



     
 
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