Posted by: Dan | November 23, 2008

What Do We Know About the Evolution of Birds

Via Ball State University in Indiana, I found an informative article on the basis for current theories of avian evolution:

Comparative anatomists during the 16th through 19th centuries noticed that birds were very similar to traditional reptiles. In 1860, shortly after the publication of Charles Darwin’s The Origin of Species by Means of Natural Selection, a quarry worker in Germany spotted an unusual fossil in the limestone of the Solnhofer Formation (late Jurassic period). This fossil turned out to be the famous ‘London specimen’ of Archaeopteryx. It is an example of a “traditional form” between the two vertebrate groups of reptiles and birds. Archaeopteryx, generally accepted as being the oldest known bird, is an important link between birds and other coelurosaurs that has helped to illuminate the evolutionary history (phylogeny) of the group. It is a common misconception that it is the ancestor of all living birds. Recent expeditions in China, Mongolia, Madagascar, Argentina, and elsewhere may uncover dinosaurs that usurp the “urvogel” status of Archaeopteryx. An example of this would be the specimen of Protoavis.

Many scientists, including Thomas Henry Huxley (a supporter of Darwin), saw similarities between birds and the theropod dinosaurs (especially the coelurosaurs). Others since Huxley have also hinted at the striking resemblances. However, birds were still not well accepted as dinosaur descendants – such hypotheses as A. Walker’s “crocodylomorph” ancestor and G. Heilman’s “thecodont” ancestor held sway for most of the 19th and 20th century, or else birds were simply dismissed as originating from some unknown reptile that didn’t matter anyway. Dr. J.H. Ostom’s 1969 description of Deinonychus antirrhopus and its similarities to Archaeopteryx was the major step. His work since the 1970’s has provided the impetus for a paradigm shift in paleontologists visions of the origin of birds and the evolution of flight. In the mid-1980’s, Dr. Gauthier’s cladistic work provided the best analytical systematic support for the theory that birds are the descendants of dinosaurs. Several independent analyses by other scientists have repeatedly upheld Gauthier’s results. Today the controversial issue is specifically which dinosaurs are the closest relatives of birds.

The first birds shared the following major skeletal characteristics with many coelurosaurian dinosaurs:

  • Publis shifted from an anterior to a more posterior orientation.
  • Elongated arms and forelimbs and clawed manus.
  • Large orbits.
  • Flexible wrist with a semi-lunate carpal (wrist bone).
  • Hollow, thin-walled bones.
  • Three fingered opposable grasping manus, 4-toed foot
  • Reduced, posterior stiffened tail.
  • Elongated metatarsals.
  • “S” shaped curved neck.
  • Erect, digitgrade stance with feet positioned directly below the body.
  • Similar eggshell structure.
  • Teeth with a constriction between the root and the crown.
  • Functional basis for wing power stroke present in arms and pectoral girdle (during motion, the arms were swung down and forward, then up and backwards, describing a “figure-eight” when viewed laterally).
  • Expanded pneumatic sinuses in the skull.
  • Five or more vertebrae incorporated into the sacrum.
  • Straplike scapula.
  • Clavicles fused to form a furcula.
  • Hingelike ankle joint, with movement mostly restricted to the fore-aft plane.
  • Secondary bony palate (nostrils open posteriorly in throat).

Thinking about this knowledge got me thinking some more. What about other aspects of avian origins? What other things can we say we know?

Feathered Dinosaurs?

The possibility of feathered dinosaurs could further strengthen the contention that birds are descended from dinosaurs. And since the 1990’s, this is just what has been seen, with fossils from 20 dinosaur genera showing evidence consisting of feather impressions in the fossils, presence of quill-knobs in the fossils, and/or chemical evidence of beta-keratin. [Wiki]

Proto-feather types

The evolution of feather structures is thought to have proceeded from simple hollow filaments through several stages of increasing complexity, ending with the large, deeply rooted, feathers with strong pens (rachis), barbs and barbules that birds display today.

Fossil feather impressions are extremely rare; therefore only a few feathered dinosaurs have been identified so far. However, through a process called phylogenetic bracketing, scientists can infer the presence of feathers on poorly-preserved specimens. All fossil feather specimens have been found to show certain similarities. Due to these similarities and through developmental research almost all scientists agree that feathers could only have evolved once in dinosaurs. Feathers would then have been passed down to all later, more derived species (although it is possible that some lineages lost feathers secondarily). If a dinosaur falls at a point on an evolutionary tree within the known feather-bearing lineages, scientists assume it too had feathers, unless conflicting evidence is found. This technique can also be used to infer the type of feathers a species may have had, since the developmental history of feathers is now reasonably well-known. [Quarterly Review of Biology]

How did flight begin?

Two theories have dominated most of the discussion: the cursorial (“from the ground up”) theory proposes that birds evolved from small, fast predators that ran on the ground; the arboreal (“from the trees down”) theory proposes that powered flight evolved from unpowered gliding by arboreal (tree-climbing) animals. A more recent theory, “wing-assisted incline running” (WAIR), is a variant of the cursorial theory and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes, for example to escape from predators. [Essay]

And of course, Why Migrate?

Again, two theories have dominated most of the discussion: whether migration evolves towards new breeding areas or towards survival areas in the non-breeding season. Either way, the origins of bird migration probably were effected by factors similar to those that effect the habits of birds today.

The primary advantage of migration is energetic. The seasonal explosion of plants, insects and other types of food in temperate and Northerly regions provide greater opportunities for breeding birds to feed their young, whereas the scarcity of food in the same regions in winter demand that birds find food elsewhere. The first migrating birds were probably short-distance migrants, or simply vagrant/nomadic species that followed their food source(s) during the annual climatic cycle.

Migratory behavior continues to evolve because of the changing environment in which the birds live: If environmental conditions favor migration, the number of birds that migrate increases; if conditions permit the birds to stay in one place, the sedentary type predominates. For example: in the early 1940s, some House Finches from a nonmigratory population in California were released on Long Island, New York. Once established in the East Coast’s far more seasonally variable climate, the birds began to develop a migratory pattern. The eastern House Finch has since become partially migratory and has spread throughout the Northeast. Some individuals are resident the year around, while others regularly migrate back and forth to the Gulf States. [Atlas of Bird Migration]


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