Tuesday, November 13, 2007

>>Radiation of modern birds

Radiation of modern birds

Modern birds are classified in the subclass Neornithes, which are now known to have evolved into some basic lineages by the end of the Cretaceous . The Neornithes are split into two superorders, the Paleognathae and Neognathae. The paleognaths include the tinamous of Central and South America and the ratites. The ratites are large flightless birds, and include ostriches, rheas, cassowaries, kiwis and emus (though some scientists suspect that the ratites represent an artificial grouping of birds which have independently lost the ability to fly in a number of unrelated lineages).
The basal divergence from the remaining Neognathes was that the Galloanserae, the superorder containing the Anseriformes (ducks, geese, swans and screamers), and the Galliformes (the pheasants, grouse, and their allies, together with the mound builders, and the guans and their allies). The dates for the splits are much debated by scientists. It is agreed that the Neornithes evolved in the Cretaceous and that the split between the Galloanseri from other Neognathes occurred before the K-T extinction event, but there are different opinions about whether the radiation of the remaining Neognathes occurred before or after the extinction of the other dinosaurs.This disagreement is in part caused by a divergence in the evidence, with molecular dating suggesting a Cretaceous radiation and fossil evidence supporting a Tertiary radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial.
The classification of birds is a contentious issue. Sibley and Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds, although frequently debated and constantly revised. A preponderance of evidence seems to suggest that the modern bird orders constitute accurate taxa. But scientists disagree about the relationships between orders; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.

>>Early evolution of birds

Early evolution of birds










Basal bird phylogeny simplified after Chiappe, 2007
During the Cretaceous Period, birds diversified into a wide variety of forms. Many of these groups retained primitive characteristics, such as clawed wings and teeth, though the latter was lost independently in a number of bird groups, including modern birds (Neornithes). While the earliest birds retained the long bony tails of their ancestors (birds such as Archaeopteryx and Jeholornis), more advanced birds shortened the tail with the advent of the pygostyle bone in the clade Pygostylia.
The first large, diverse lineage of short-tailed birds to evolve were the Enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was the reverse of the condition seen in modern birds. Enantirornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters. More advanced lineages also specialized in eating fish, like the superficially gull-like subclass of Ichthyornithes ("fish birds"). One order of Mesozoic seabirds, the Hesperornithiformes, became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic. Despite their extreme specializations, the Hesperornithiformes represent some of the closest relatives of modern birds.

Friday, November 2, 2007

>>Dinosaurs and the origin of birds

Dinosaurs and the origin of birds

Confuciusornis, a Cretaceous bird from China

Confuciusornis, a Cretaceous bird from China
There is significant evidence that birds evolved from theropod dinosaurs, specifically, that birds are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As more non-avian theropods that are closely related to birds are discovered, the formerly clear distinction between non-birds and birds becomes blurred. Recent discoveries in Liaoning Province of northeast China, demonstrating that many small theropod dinosaurs had feathers, contribute to this ambiguity.
The oldest known bird, the Late Jurassic Archaeopteryx, is well-known as one of the first transitional fossils to be found in support of evolution in the late 19th century, though it is not considered a direct ancestor of modern birds. Protoavis texensis may be even older although the fragmentary nature of this fossil leaves it open to considerable doubt whether this was a bird ancestor.
The dromaeosaurids Cryptovolans and Microraptor may have been capable of powered flight to an extent similar to or greater than that of Archaeopteryx. Cryptovolans had a sternal keel and had ribs with uncinate processes. In fact, Cryptovolans makes a better "bird" than Archaeopteryx which is missing some of these modern bird features. Because of this, some palaeontologists have suggested that dromaeosaurs are actually basal birds, and that the larger members of the family are secondarily flightless, i.e. that dromaeosaurs evolved from birds and not the other way around. Evidence for this theory is currently inconclusive, as the exact relationship among the most advanced maniraptoran dinosaurs and the most primitive true birds are not well understood.
Although ornithischian (bird-hipped) dinosaurs share the hip structure of birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs, and thus evolved their hip structure independently. In fact, the bird-like hip structure also developed a third time among a peculiar group of theropods, the Therizinosauridae.
An alternate theory to the dinosaurian origin of birds, espoused by a few scientists (most notably Larry Martin and Alan Feduccia), states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama, a theory which is contested by most palaeontologists and evidence based on feather development and evolution.

>>Evolution and taxonomy

Evolution and taxonomy

Archaeopteryx, the earliest known bird

Archaeopteryx, the earliest known bird
The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume, Ornithologiae. Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system still in use.Birds are categorised as the biological class Aves in Linnean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda. Aves and a sister group, the order Crocodilia, together are the sole living members of the reptile clade Archosauria. Phylogenetically, Aves is commonly defined as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.Archaeopteryx, from the Kimmeridgian stage of the Late Jurassic (some 155–150 million years ago), is the earliest known bird under this definition. Others have defined Aves to include only the modern bird groups, excluding most groups known only from fossils,in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.
Modern birds all sit within the subclass Neornithes, which is divided into two superorders, the Paleognathae (mostly flightless birds like ostriches), and the wildly diverse Neognathae, containing all other birds. Depending on the taxonomic viewpoint, the number of species cited varies anywhere from 9,800 to 10,050 known living bird species in the world.