Crinoids became moderately abundant at various times, but in many ways different from their relatives of the preceding era. Some of the largest known species, with stems estimated as fifty feet long, have been found in lower Jurassic rocks. A great abundance of microscopic life is indicated by the frequent occurrence of chalk in the Cretaceous formations. Corals of the modern reef-building type (hexacorals) were common in the warm seas of a large part of the world.
The insects of the early Mesozoic are represented by few fossils although it is evident some new forms were becoming established. The warm climates prevailing throughout much of the world appear to have been a favorable factor in the progress of insect life. In addition to the older cockroach and dragon-fly types may be mentioned the arrival of grasshoppers, cicadas, caddis-flies, beetles, and ants.
Several hundred species are found in Jurassic rocks, and by the end of the Cretaceous period most of the insect families now known to us were probably in existence. The record is seriously obscured by the erosion of rocks which so frequently marks the end of a period, also by the small size of the subjects, and by the exceptional conditions required for the production of such delicate fossils. Among the last of the familiar insects to appear were the bees and butterflies. These evidently came in with the more advanced types of flowering plants that produce the nectar on which many insects feed. It is probable, too, that without the arrival of these insects and their service in the pollination of flowers, the floras of today would be rather different from what they are.
EXTINCT BIRDS
It is not surprising to find that birds made their first appearance in the Mesozoic era, for of all animals they are most like the reptiles as a class. Feathers are about the only dependable characteristic of the entire group, nearly every other feature being matched by some reptilian creature of great antiquity. The nesting habit, which includes care of the young as well as the eggs, is a matter of progress which relates to flight and to warmer body temperature. It appears to have been developed by forest-dwelling types living among trees and nesting there in comparative safety from enemies prowling on the ground.
The oldest known prehistoric birds were found in lithographic stone of Upper Jurassic age. Archaeopterix was discovered in 1861 at Solenhofen, Germany. Sixteen years later a similar bird in a better state of preservation was found in Bavaria. The latter was named Archaeornis. These Jurassic fossils are regarded as true birds by some authorities, while others believe them to be more nearly related to the reptiles, the opinions being based on careful studies of the skulls and other skeletal features. Both birds had teeth of reptilian character, and it is evident that there was no beak, for the jaws were covered with scaly skin. The bony construction of the long tail would suggest lizards rather than birds, were it not for the presence of feathers which were attached at each side. Head, neck, and parts of the body were covered with scales. Wings were well provided with stout feathers but the skeletal framework indicates that the birds were gliders rather than true flyers. Claws on the wings served like fingers to aid in climbing among the branches of trees, a practice which is occasionally noted among the young of living birds. In adult birds of today, however, the claw-like appendages of the fore limbs are greatly reduced and of little service.
The next fossil birds of importance have been found in Cretaceous rocks of Kansas, both of them fishers of the seas instead of forest inhabitants. Ichthyornis was a small bird, standing about eight inches in height, a powerful flyer with reptilian jaws and teeth. Hesperornis was built for diving and swimming, like the loon, but was somewhat larger and provided with teeth. Its wings were too poorly developed to be of use in flying.
Toothed birds became extinct with the close of Cretaceous time, and the ancestors of modern types were in existence before the Age of Mammals, but fossil remains are few and poorly preserved. Large ostrich-like birds, however, are known to have lived in North America during the Eocene period. One of these, named Diatryma, stands nearly seven feet tall in the reconstructed skeleton. Its legs are heavy, wings greatly reduced, beak massive. In its relation to modern birds it is possibly nearer to the cranes than the ostriches.
Flightless birds of large size are known from many parts of the world and seem to have been prominent throughout the Cenozoic era, as they are today in the southern hemisphere. Aepyornis lived in Madagascar during the Pleistocene period and may have become extinct quite recently. Its eggs are the largest known among fossils, several times the size of an ostrich egg. Also in this period the moas were living in New Zealand where their remains are still abundant. One of the largest, known as Dinornis, had about the same form as Diatryma but the neck was longer, head and beak smaller, legs better fitted for running, height about eleven feet.
A much smaller flightless bird, the dodo, became extinct in modern time. This former inhabitant of Mauritius and other islands of the Indian Ocean was related to the doves and pigeons, and had lost its power of flight through disuse of the wings. It was a clumsy, defenseless bird weighing possibly as much as fifty pounds. Actual remains are few and incomplete, and descriptions published by the explorers who knew the bird two centuries ago are not entirely trustworthy. In the Pleistocene Rancho la Brea beds of California the largest of all prehistoric flying birds has been found, a vulture bearing the name of Teratornis. Re-assembled skeletons show them to be slightly bigger than existing condors.