Evolution and paleontology ( bird )
Tagged: animal, animals, bird, cat, fish, insect, mammalArchaeopteryx
The earliest known fossil bird is Archaeopteryx lithographica, which was discovered in Upper Jurassic deposits in Bavaria. This bird was about the size of a magpie. It resembled some reptiles, however, and differed from Recent birds in many ways:
- the jaws contained teeth set in sockets;
- the articulations between the vertebrae were amphicoelous (concave at both ends);
- there were only six sacral vertebrae;
- the long tail was made up of a series of free vertebrae each bearing a pair of rectrices;
- the slender ribs lacked articulations with the sternum and uncinate processes (flat upward projections);
- ventral ribs (gastralia) were present behind (posterior to) the sternum;
- the sternum was short and not keeled;
- the bones were not pneumatic;
- the third metacarpal bone in the wing was fused to the carpals, but the first two metacarpals were free, resulting in three movable digits of the “hand,†all with functional claws;
- the fibula was as long as the tibia;
- the metatarsal bones were free;
- the cerebral hemispheres were elongated and slender, and the cerebellum lay behind the midbrain, not overlapping it from behind or crowding it downward.
The avian characteristics of Archaeopteryx included the possession of feathers, the elongated, backward-directed pubis, the furcula, and the opposable hallux. In the structure of the beak, eye, and jaw articulation, in the fusion of the third metacarpal with the carpals, and in the fusion of each of the distal tarsals with the corresponding metatarsal, Archaeopteryx represented an intermediate stage between reptiles and modern birds. The absence of a keel on the short sternum indicates that Archaeopteryx did not fly but glided. The opposable hallux, indicative of the perching type of foot, and the clawed digits of the hand point to an arboreal existence. From the arrangement of feathers on the wing and the number and arrangement of bones in the limbs, it appears that Archaeopteryx was near the main line of avian evolution. From the fact that the skull was diapsid (i.e., had two “windowsâ€) and from certain features of the limb bones, it appears that Archaeopteryx was descended from reptiles of the Triassic order Thecodontia.
By the Triassic Period (225,000,000 years ago) a group of small bipedal reptiles, the pseudosuchians, were well established. Their skulls had much in common with that of Archaeopteryx, although they had heavier jaws and smaller eyes. It is likely that one group of pseudosuchians became arboreal. The advantages of such a life would be safety from large terrestrial predators and an abundance of insect food. Once these reptiles were in the trees, selective pressures would favour mutations leading to many avian features. The swaying of branches would favour the evolution of the grasping foot. Use of the forelimbs in climbing from branch to branch would favour enlargement of the claws and elongation of the forelimb, which was short in the bipedal ancestors. Greater visual acuity and more effective coordination are of special advantage in arboreal animals. Natural selection thus favoured larger eyes, narrower snouts (permitting better forward vision), and greater development of the cerebral lobes and cerebellum of the brain. The smaller jaws may also indicate the advantages of lightness, balance, and a specialization for feeding on insects as opposed to the apparently more general carnivorous diet of the terrestrial ancestors. Perhaps most important was the development of homeothermy (internal temperature control). A warm-blooded insect eater has an enormous advantage in being able to capture insects when they are cold and slow to react. It is also advantageous in the wind-moved environment of the treetops. In addition to increased food intake and advanced respiratory and circulatory systems, however, homeothermy requires effective insulation. It is likely that feathers evolved to fill this requirement, although many authorities believe the origin of feathers was directly connected with flight. Just how feathers evolved from reptilian scales is unknown, but it is known that the two are similar in chemical composition and that some pseudosuchians had scales bearing an imprint of a feather-like pattern on their surface. Elongated feathers on the forelimb and tail may have evolved for balancing and for gliding to produce the Archaeopteryx stage.
Modern birds
In the evolution of modern birds from an Archaeopteryx-like form, the development of active flight must have occurred early. This meant an increase in size of the muscles moving the wing and the development of a keel on the sternum as an added area of attachment for these muscles. As the tail took on more of a steering function and less of a supportive one, it became shorter and more readily moved as a unit. Feathers became increasingly specialized for different functions, and at the same time, the trends in the development of the eyes, brain, and respiratory and circulatory systems associated with the evolution of the homeothermic, arboreal, gliding types continued. By the time birds became strong fliers, they were ready to radiate out into many new environments; and by the Cretaceous Period (136,000,000 to 65,000,000 years ago), they had begun to do so. This radiation has produced the large array of adaptive types known today. The lightness and pneumaticity of bird bones makes them poor candidates for fossilization. As might be expected, heavyboned diving birds and large flightless birds are disproportionally represented in the record.
One of the best known groups of fossil birds consists of Hesperornis and its relatives. These birds were highly specialized foot-propelled divers of the Upper Cretaceous. The known species of Hesperornis were up to six feet (1.8 metres) long and had completely lost the power of flight. The sternum lacked a keel; the humerus was small and weak; and the other, more distal, elements of the wing were missing. The pelvis and hindlimb had a strong but superficial resemblance to those of modern loons and grebes the pelvis was narrow; the femur short and stout with a hingelike articulation with the pelvis; the tibiotarsus long, with a long cnemial crest (a projection at its upper end); and the tarsometatarsus laterally compressed. Two major features (and several less obvious ones) indicate, however, that the resemblance was the result of convergent evolution: the ischia and pubes were free for most of their length, and the cnemial process was made up entirely of the patella; in the loon, this process is derived from the tibiotarsus. Hesperornis was remarkable for three features: it had teeth set in grooves, not sockets, in the maxilla and mandible; the phalanges of the stout fourth toe had a unique rotary ball-and-flange type of articulation; and the free tail vertebrae had broad lateral projections and limited vertical motion, indicating that the tail was somewhat beaver-like in its action. Baptornis, a contemporary relative of Hesperornis, was smaller and less strongly modified. While flightless, it had less reduced wings than Hesperornis, and it lacked the peculiar modifications of the fourth toe and caudal vertebrae.
Living on the same seas as Hesperornis and Baptornis was a group of flying birds known as Ichthyornis and Apatornis. Although not related to gulls, these birds resembled them superficially and may well have been their ecological counterparts. It was long believed that Ichthyornis had teeth, like Hesperornis; but it is now thought that the toothed jaws formerly thought to belong to Ichthyornis were really those of a small mosasaur, a marine reptile. After the extinction of the dinosaurs and before large carnivorous mammals evolved, two groups of large flightless birds evolved to fill a similar niche. From the upper Paleocene to the middle Eocene, Diatryma and its relatives were major predators in the Northern Hemisphere. The largest species stood over two metres (seven feet) tall and had stout hooked beaks. They are of uncertain relationships but may have been distantly related to cranes and rails. The second group, that of Phororhacos and related genera, had a long history (from the lower Oligocene to the middle Pliocene) in South America, which was without large carnivores until relatively late. Fragmentary Pleistocene material from Florida has also been assigned to this group. The Phororhacos line evidently evolved from cariama-like stock and radiated into numerous genera and species, the largest of them (Onactornis) standing 2.5 metres (eight feet) tall and having a skull 80 centimetres (31 inches) long and 40 centimetres (16 inches) high.
Large grazing or browsing birds appear to have evolved several times. On continents where there are large predators, these birds have always been rapid runners (ostriches, rheas, emus), but on islands lacking such predators, they were slow-moving, heavy-bodied birds. Two such groups were the elephant birds of Madagascar and the moas of New Zealand, the largest in each group approaching 10 feet in height. Fragmentary fossil material from Eocene and Oligocene deposits in Egypt indicates that similarly adapted birds occurred there before the advent of large carnivores. Except for the Hesperornis line, teeth appear to have been lost very early in the history of birds, but fish-eating birds have evolved several toothlike structures for grasping their prey. Perhaps the most remarkable adaptation was that of Osteodontornis and its relatives, large, flying marine birds that flourished from the lower Eocene to the Miocene. In these birds, there were bony projections of the upper and lower jaws, which were covered by the ramphotheca, forming sharp, toothlike structures.
The fossil record
The fact that fewer bird fossils are found in earlier deposits is well illustrated by expressing the number of known species in a given geological period in terms of the duration of the period. About 35 species of birds are known from Cretaceous deposits, which were laid down over an estimated 71,000,000 years, giving a figure of 0.5 species per 1,000,000 years. The corresponding figure for the Paleocene is 1.2 (12 species in 10,000,000 years) and that for the Eocene, 5.4 (87 species in 16,000,000 years). Up-to-date figures for the later periods are not available, but estimates based on several recent sources are 7.9 per 1,000,000 years in the Oligocene, 12.3 in the Miocene, and 21.8 in the Pliocene. The Pleistocene, which lasted approximately 2,500,000 years, has yielded nearly a thousand species of fossil birds. From this it is evident that very little is known about the early avifaunas. It is known, however, that, as might be expected, the birds in the earlier periods differ more from Recent species than do those of the later periods. Of the 12 families of birds recorded from Cretaceous deposits, only two are still extant, whereas the majority of species recorded from the Pleistocene were structurally little, if at all, different from living forms. Thus the absence of a group in the fossil record, especially in the earlier periods, is rarely significant.
The major diversification of birds probably took place in the Cretaceous, which lasted longer than the sum of all subsequent periods, and it must have started early in that period because fragmentary material of foot-propelled divers (Enaliornis) and of an early relative of the flamingos (Gallornis) are known from Lower Cretaceous deposits of Europe. Upper Cretaceous deposits have yielded, besides Hesperornis and Ichthyornis and their relatives, diving birds similar to Enaliornis (Lonchodytes), other early flamingo-like birds, and species in the same suborders as gannets, ibises, rails, and shorebirds.
Paleocene deposits have yielded the earliest known loons, cormorants, New World vultures, and gulls. In addition, the large, flightless predatory birds culminating in Diatryma first made their appearance during this period. From the far richer Eocene deposits have come the earliest known rheas, penguins, albatrosses, tropic birds, anhingas, true flamingos, herons, storks, secretary birds, hawks, curassows, cranes, bustards, avocets, auks, sand grouse, cuckoos, owls, swifts, trogons, rollers, hornbills, and songbirds. Almost certainly all living orders and most living families of birds were in existence by the end of the Eocene period. One of the most interesting finds from this period was fossils of Neocathartes, a long-legged bird allied to the New World vultures. There are several anatomical similarities between this group of vultures and the storks, and the existence of this fossil lends support to the idea that the storks and New World vultures are more closely related to each other than each family is to the birds with which it is usually grouped.
Important Oligocene fossils include the earliest phororhacoids, one of the few groups of fossil birds that is known from enough material from over a long enough time span to show evolutionary trends, in this case, both in size and in bill form. Fossils of Miocene birds are numerous. Several early groups of pelecaniform, cranelike, and flamingo-like birds are known last from this period, and the first of the Mancallidae, superficially penguin-like auks, appeared. Otherwise, the avifauna was essentially modern. By Pliocene times, most modern genera were probably in existence. The Mancallidae continued on the California coast at least until the middle of the period. The appearance and extinction of large birds as well as mammals was a feature of the Pleistocene. Perhaps most notable were the teratorns, “super condors,†which were found in North America. These included Teratornis incredibilis, the largest known flying bird.
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