Origin and Relationships

Origin and Relationships
Approximately 147 million years ago, a flying animal drowned and settled to the bottom of a shallow marine lagoon in what is now Bavaria, Germany. It was rapidly covered with a fine silt and eventually fossilized. There it remained until discovered in 1861 by a workman splitting slate in a limestone quarry. The fossil was approximately the size of a crow, with a skull not unlike that of modern birds except that the beaklike jaws bore small bony teeth set in sockets like those of reptiles (Figure 29-1). The skeleton was decidedly reptilian with a long bony tail, clawed fingers, and abdominal ribs. It might have been classified as a reptile except that it carried the unmistakable imprint of feathers, those marvels of biological engineering that only birds possess. Archaeopteryx lithographica (ar-kee-op´ter-ix lith-o-graf´e-ca, Gr., meaning “ancient wing inscribed in stone”), as the fossil was named, was an especially fortunate discovery because the fossil record of birds is disappointingly meager. The finding was also dramatic because it proved beyond reasonable doubt the phylogenetic relatedness of birds and reptiles.
Archaeopteryx
Figure 29-1
Archaeopteryx, a 147-million-year-old ancestor of modern birds. A, Cast of the second and most nearly perfect
fossil of Archaeopteryx, which was discovered in a Bavarian stone quarry. Seven specimens of Archaeopteryx
have been discovered, the most recent one in 1992. B, Reconstruction of Archaeopteryx.

Zoologists had long recognized the similarity of birds and reptiles. The skulls of birds and reptiles abut against the first neck vertebra by a single balland- socket joint, the occipital condyle (mammals have two condyles). Birds and reptiles have a single middle ear bone, the stapes (mammals have three middle ear bones). Birds and reptiles have a lower jaw composed of five or six bones, whereas the lower jaw of mammals has one mandibular bone, the dentary. Birds and reptiles excrete their nitrogenous wastes as uric acid whereas mammals excrete theirs as urea. Birds and reptiles lay similar yolked eggs with the early embryo developing on the surface by shallow cleavage divisions.

The distinguished English zoologist Thomas Henry Huxley was so impressed with these and many other anatomical and physiological affinites that he called birds “glorified reptiles” and classified them with a group of dinosaurs called theropods that displayed several birdlike characteristics (Figures 29-2 and 29-3). Theropod dinosaurs share many derived characters with birds, the most obvious of which is the elongate, mobile, S-shaped neck. As shown in the cladogram (Figure 29-3), theropods belong to a lineage of diapsid reptiles, the archosaurians, that includes crocodilians and pterosaurs, as well as the dinosaurs. There is now overwhelming evidence that Huxley was correct: birds’ closest phylogenetic affinity is to the theropod dinosaurs. The only anatomical feature required to link bird ancestry with the theropod dinosaurs was feathers, and this was provided by the discovery of Archaeopteryx. Recent discoveries of Cretaceous bird fossils in Spain, Madagascar, and China provide new data on bird ancestry. All these new discoveries, however, still link early birds with theropod reptiles.
Evolution of modern birds
Figure 29-2
Evolution of modern birds. Of 27 living bird orders, 9 of the largest are shown. The earliest known bird,
Archaeopteryx, lived in the Upper Jurassic, about 147 million years ago. Archaeopteryx uniquely shares many
specialized aspects of its skeleton with the smaller theropod dinosaurs and is considered to have evolved within
the theropod lineage. Evolution of modern bird orders occurred rapidly during the Cretaceous and early Tertiary
periods.

Cladogram of the Archosauria
Figure 29-3
Cladogram of the Archosauria, showing the relationships of several archosaurian groups to modern birds. Shown
are a few of the shared derived characters, mostly those related to flight, that were used to construct the genealogy.
The outgroup is Lepidosauria (see Figure 28-2)

strangest birds
Figure 29-4
One of the strangest birds in
a strange land, the flightless
cormorant, Nannopterum
harrisi, of the Galápagos
Islands dries its wings after a
fishing forage. It is a superb
swimmer, propelling itself
through the water with its
feet to catch fish and
octopuses. The flightless
cormorant is an example
of a carinate bird (having a
keeled sternum) that has lost
the keel and the ability to fly.
Living birds (Neornithes) are divided into two groups: (1) Paleognathae (Gr. palaios, ancient, + gnathos, jaw), the large flightless ostrichlike birds and the kiwis, often called ratite birds, which have a flat sternum with poorly developed pectoral muscles, and (2) Neognathae (Gr. neos, new, + gnathos, jaw), flying birds that have a keeled sternum on which powerful flight muscles insert. This division originated from the view that flightless birds (ostrich, emu, kiwi, rhea) represented a separate line of descent that never attained flight. This idea is now completely rejected. Ostrichlike paleognathids clearly have descended from flying ancestors. Furthermore, not all neognathous birds can fly and many of them even lack keels (Figure 29-4). Flightlessness has appeared independently among many groups of birds; the fossil record reveals flightless wrens, pigeons, parrots, cranes, ducks, auks, and even a flightless owl. Penguins are flightless although they use their wings to “fly” through water. Flightlessness almost always has evolved on islands where few terrestrial predators are found. Flightless birds living on continents today are the large paleognathids (ostrich, rhea, cassowary, emu), which can run fast enough to escape predators. The ostrich can run 70 km (42 miles) per hour, and claims of speeds of 96 km (60 miles) per hour have been made.

Characteristics of Class Aves
  1. Body usually spindle shaped, with four divisions: head, neck, trunk, and tail; neck disproportionately long for balancing and food gathering
  2. Limbs paired with the forelimbs usually modified for flying; posterior pair variously adapted for perching, walking, or swimming; foot with four toes (2 or 3 toes in some)
  3. Epidermal covering of feathers and leg scales; thin integument of epidermis and dermis; no sweat glands; oil or preen gland at base of tail; pinna of ear rudimentary
  4. Fully ossified skeleton with air cavities; skull bones fused with one occipital condyle; skull diapsid with antorbital fenestra; each jaw covered with a horny sheath, forming a beak; no teeth; ribs with strengthening processes, the uncinate process attaching ribs with one another; tail not elongate; sternum well developed with keel or reduced with no keel; single bone in middle ear
  5. Nervous system well developed, with brain and 12 pairs of cranial nerves
  6. Circulatory system of fourchambered heart, with the right aortic arch persisting as the dorsal aorta; reduced renal portal system; nucleated red blood cells
  7. Endothermic
  8. Respiration by slightly expansible lungs, with thin air sacs among the visceral organs and skeleton; syrinx (voice box) near junction of trachea and bronchi
  9. Excretory system of metanephric kidney; ureters open into cloaca; no bladder; semisolid urine; uric acid main nitrogenous waste
  10. Sexes separate; testes paired, with the vas deferens opening into the cloaca; females with left ovary and oviduct only; copulatory organ in ducks, geese, paleognathids, and a few others
  11. Fertilization internal; amniotic eggs with much yolk and hard calcareous shells; embryonic membranes in egg during development; incubation external; young active at hatching (precocial) or helpless and naked (altricial); sex determination by females (females heterogametic)

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