The lower jaw of the mosasaur was jointed; the quadrate bone, which in all reptiles connects the bone of the lower jaw with the skull, was movable, and as in snakes the lower jaw could be used in thrusting prey down the throat.  The family became extinct at the end of the Mesozoic, and left no descendants.  One may imitate the movement of the lower jaw of the mosasaur by extending the arms, clasping the hands, and bending the elbows.

Flying reptiles.  The atmosphere, which had hitherto been tenanted only by insects, was first conquered by the vertebrates in the Mesozoic.  Pterosaurs, winged reptiles, whose whole organism was adapted for flight through the air, appeared in the Jurassic and passed off the stage of existence before the end of the Cretaceous.  The bones were hollow, as are those of birds.  The sternum, or breastbone, was given a keel for the attachment of the wing muscles.  The fifth finger, prodigiously lengthened, was turned backward to support a membrane which was attached to the body and extended to the base of the tail.  The other fingers were free, and armed with sharp and delicate claws, as shown in Figures 336 and 337.

These “dragons of the air” varied greatly in size; some were as small as sparrows, while others surpassed in stretch of wing the largest birds of the present day.  They may be divided into two groups.  The earliest group comprises genera with jaws set with teeth, and with long tails sometimes provided with a rudderlike expansion at the end.  In their successors of the later group the tail had become short, and in some of the genera the teeth had disappeared.  Among the latest of the flying reptiles was ORNITHOSTOMA (bird beak), the largest creature which ever flew, and whose remains are imbedded in the offshore deposits of the Cretaceous sea which held sway over our western plains.  Ornithostoma’s spread of wings was twenty feet.  Its bones were a marvel of lightness, the entire skeleton, even in its petrified condition, not weighing more than five or six pounds.  The sharp beak, a yard long, was toothless and bird-like, as its name suggests

Birds.  The earliest known birds are found in the Jurassic, and during the remainder of the Mesozoic they contended with the flying reptiles for the empire of the air.  The first feathered creatures were very different from the birds of to-day.  Their characteristics prove them an offshoot of the dinosaur line of reptiles.  Archaeopteryx (ancient bird) (Fig. 338) exhibits a strange mingling of bird and reptile.  Like birds, it was fledged with perfect feathers, at least on wings and tail, but it retained the teeth of the reptile, and its long tail was vertebrated, a pair of feathers springing from each joint.  Throughout the Jurassic and Cretaceous the remains of birds are far less common than those of flying reptiles, and strata representing hundreds of thousands of years intervene between Archaeopteryx and the next birds of which we know, whose skeletons occur in the Cretaceous beds of western Kansas.