Zoology of Chordates - Textbook - J. V. Tsaryk - 2013

Chapter 5. SUPERCLASS TETRAPODS (TERRESTRIAL VERTEBRATES) TETRAPODS, seu QUADRIPEDA.

ANAMNIOTS (ANAMNIA) AND AMNIOTES (AMNIOTA)

5.3. CLASS BIRDS AVES

5.3.2. Origin and Evolution of Birds

Paleontological Materials concerning THE ORIGIN OF birds are scarce, which is why our understanding of the Cytology/cytology/16.html">Early stages of evolution in this animal Class exists primarily in the form of hypotheses. According to the most widely accepted views, birds diverged from archosaurs—a numerous and diverse group of diapsid reptiles that existed during the Mesozoic era. The direct ancestors of birds are considered to be pseudosuchians, the earliest group of archosaurs and the ancestral stock from which other archosaur lineages arose. Pseudosuchians were small, lizard-like animals that led a terrestrial lifestyle. In some species, the hind limbs were slightly longer than the forelimbs, and during locomotion, these animals evidently relied exclusively on their hind limbs.

There are several hypotheses regarding how the evolution of pseudosuchians proceeded toward birds. According to the first hypothesis, certain pseudosuchians gradually transitioned to an arboreal lifestyle, and flight evolved through leaping and gliding using all four limbs. The elongation of epidermal scales along the sides of the body, the tail, and the posterior margin of the limbs allowed for extended jumps from branch to branch.

According to the second hypothesis, the capacity for flight in birds is linked to bipedal ancestors that were strong runners and jumpers, eventually transitioning to flight from the ground level. Further specialization and Selection led to The formation of the musculoskeletal Structure, The Development of feathered wings, and corresponding transformations in other Organ Systems, enabling first gliding and subsequently active flight. Body plumage may have initially evolved as an adaptation for thermal insulation, later providing aerodynamic streamlining. It formed even before the capacity for flight appeared. Some pseudosuchians possessed elongated scales with a longitudinal ridge and fine transverse Ribs; it is possible that feathers originated from these through splitting.

If the first hypothesis is followed, the reconstruction of how flight emerged in birds is easy to visualize. However, in this case, the reasons behind the appearance of such a unique Respiratory system and integument lack a convincing explanation.

Based on the second hypothesis, the plausible pathways for The Emergence of flight can be envisioned differently. It is known that already among archosaurs, which were globally distributed from the beginning of the Triassic period, there were representatives that actively colonized open spaces. This is evidenced by the remains of forms adapted for running and jumping exclusively with the aid of their hind limbs. Such a mode of locomotion would have been most advantageous in open landscapes. In any case, The connection between birds and bipedal, ground-dwelling, and jumping reptiles is unquestionable.

According to the third hypothesis, the ancestors of birds were tree-dwelling reptiles that retained The ability to run. In them, adaptations for climbing trees and the transformation of forelimbs into wings developed simultaneously.

There is also a concept that combines the previous hypotheses. According to this view, the ancient ancestors of birds were bipedal reptiles that transitioned to an arboreal lifestyle and subsequently to gliding flight.

Aside from these concepts, yet another hypothesis regarding the formation of avian Organization has been put forward. It is known that the transition to bipedalism (locomotion on hind limbs) causes a reduction of the forelimbs. Such reduction could not lead to the emergence of wings—an organ capable of lifting an animal's body off the ground in a flat landscape. However, in mountainous terrain, the forelimbs of reptiles, which were partially reduced during the transition to bipedalism, could have initially been used for balancing while running down mountain slopes. Similar balancing during running is utilized by A large number of modern birds, including ostriches, bustards, cranes, and snowcocks. In particular, snowcocks frequently employ balancing when running up steep slopes. Later, such balancing could have been used during downward gliding, which subsequently gave rise to powered flight driven by flapping Movements of the forelimbs. This type of flight required an increased metabolic rate, enhanced gas exchange, and, consequently, the further Progressive development of the respiratory and circulatory Organs. Running down slopes while balancing with the forelimbs led to the reduction of the tail, which became a redundant counterweight during such movement. Meanwhile, the hind limbs did not undergo reduction at this stage as flight capabilities developed.

To protect against elevated solar radiation and low temperatures in the mountains, running and gliding bipedal reptiles required a lightweight and effective body covering. The appearance of such a covering was made possible by the action of increased ultraviolet radiation on their Organism, acting as a strong mutagen, which resulted in the formation of epidermal scales. The mutagenic effect in high-altitude conditions could have been amplified by the combined action of elevated doses of ultraviolet and Background gamma radiation.

It can be hypothesized that the unique abiotic factors of high altitudes—low partial pressure of oxygen and enhanced solar radiation—contributed to the emergence of specialized respiratory organs and integument.

It has been proven that among vertebrates, only birds exhibit high tolerance to the environmental stressors of high altitudes: reduced oxygen partial pressure, hypobaric conditions, hyperinflation, and high radiation. Birds are capable of reaching altitudes exceeding 11,000 meters above sea level.

It is believed that the divergence of birds from reptiles took place as early as the Late Triassic to Early Jurassic period (190–170 million years ago). In Jurassic deposits (dating back approximately 150 million years), feather impressions and five skeletal impressions of varying degrees of preservation and feathering have been found, belonging to the earliest known bird, Archaeopteryx (Archaeopteryx lithographica). In size, Archaeopteryx was roughly comparable to a Eurasian magpie. It is assigned to a distinct subclass of lizard-tailed birds, Archaeornithes, because, unlike modern birds, it possessed a long tail consisting of roughly 20 vertebrae. Paired feathers were attached to the lateral surfaces of each vertebra. The wing plumage was well developed, and the entire body was feathered. The humerus resembles that of modern birds, and three well-developed free digits terminate in sharp claws. The clavicles are fused into a furcula (wishbone), and the scapula is saber-shaped. The hind limb is of the avian type, yet retains primitive features—a developed Fibula and an incomplete Formation of the tarsometatarsus. Much like many reptiles, Archaeopteryx possessed abdominal ribs (gastralia). The Skull is of a reptilian type, though featuring a proto-beak, composed of enlarged bones, and possessing orbits. Teeth are located in sockets on both the upper and lower jaws. Evidently, Archaeopteryx was capable only of fluttering from branch to branch or moving from tree to tree via gliding flight. For locomotion within tree canopies, it likely utilized its well-developed, mobile wing digits. Despite a multitude of reptilian traits, Archaeopteryx is undoubtedly a bird. It is considered a primitive yet specialized side branch of ancient birds. The direct ancestors of modern birds were likely even more primitive lizard-tailed birds.

From the perspective of the mountain-origin theory of birds, Archaeopteryx can be considered an inhabitant of low-altitude mountain forests. If it possessed air sacs, they were likely in an underdeveloped state. It is suggested that this Lineage diverged after primitive plumage evolved in reptiles at significant mountain elevations, potentially occurring simultaneously with the development of a specialized respiratory system. Subsequently, these animals colonized the forests of lower mountain belts.

Modern birds belong to the subclass of true, or fan-tailed, birds (Neornithes): in these birds, the caudal region of THE Vertebral Column is drastically shortened, and the terminal tail vertebrae are fused into a single bone—the pygostyle—to which tail feathers are attached in a fan-like array. A typical avian wing Skeleton has formed, a keel is present on the Sternum, the formation of the tarsometatarsus is complete, and abdominal ribs have disappeared, among other features. Primitive fan-tailed birds of the Jurassic period have not yet been discovered, although they undoubtedly existed at that time. The oldest remains of fan-tailed birds have been unearthed in Cretaceous deposits (dating to approximately 80–90 million years ago). They are classified into two superorders. Hesperornis species (Hesperornis sp.) were large, up to 1 meter in length, aquatic and diving birds that resembled loons. They lost the power of flight: their forelimbs and Pectoral Girdle are heavily reduced, and the sternum lacks a keel. Ichthyornis species (Ichthyornis sp.) and others were pigeon-sized. Their wing skeleton is typically avian, and the sternum features a well-developed keel. These birds were unquestionably capable of flight. Cretaceous birds differed from modern ones by the presence of small teeth on the upper and lower jaws and a very small cranial capacity. These likely represent specialized side branches of primitive fan-tailed birds. At the end of the Cretaceous and during the Tertiary period of the Cenozoic era, approximately 70–40 million years ago, an intensive adaptive radiation of fan-tailed birds took place, giving rise to most modern orders. Unfortunately, paleontological remains from this period are scarce and do not provide a clear picture of The Nature and pace of avian evolution. The sharp increase in bird species diversity and the formation of modern orders coincided temporally with the intensive evolution of angiosperms and insects: the growth of potential food resources facilitated the establishment of new trophic links, driving intensive avian speciation.